Magmatic and tectonic effects of the interaction of the Juan de Fuca Mid-Ocean Ridge with the Cobb hotspot

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Magmatic and tectonic effects of the interaction of the Juan de Fuca Mid-Ocean Ridge with the Cobb hotspot
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MAGMATIC AND TECTONIC EFFECTS OF THE INTERACTION
OF THE JUAN DE FUCA MID-OCEAN RIDGE WITH THE
COBB HOTSPOT


By

DAVID JOHN CHADWICK


A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL
OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT
OF THE REQUIREMENTS FOR THE DEGREE OF
DOCTOR OF PHILOSOPHY

UNIVERSITY OF FLORIDA


2002













ACKNOWLEDGMENTS

I would like to thank Dr. Mike Perfit for his tremendous help on this

dissertation and for helping to prepare me for life as a research scientist. Mike

was helpful in finding the all-important funds that kept me from starving to

death. Without question, the most important choice graduate students make is

deciding who their advisor will be, and I chose very well. I thank him for

sending me out on high-seas adventures and to Papua New Guinea and Iceland.

I would also like to thank Drs. Paul Mueller, David Foster, Ann

Heatherington, and Willie Harris for serving on my committee, reading this

lengthy tome, and for offering help and guidance.

Much of the geochemical data in this study was provided by the analytical

wizardry of Ian Ridley, Ian Jonnassen, Petrus le Roux, and David Graham. I

offer many thanks to Marianne Kozuch for showing me the ropes on the mass

spectrometer and in the clean lab; she was a tremendous help.

Most importantly, I thank my family. I would not be able to spend my life

in this decadent, student-for-life fashion if it weren't for the constant moral and

occasional financial support of my parents. I thank the newest member of my

family, my fiance Claire, for her help with this dissertation, and for her constant

love, friendship, and support v.













TABLE OF CONTENTS
page

ACKNOWLEDGMENTS.............................................................................................. ii

A BST R A C T ...................................................................... ............................................... v

CHAPTERS

1 INTRODUCTION ............................................................................................... 1

2 MAGMATIC EFFECTS OF THE INTERACTION OF THE JUAN DE FUCA
RIDGE WITH THE COBB HOTSPOT.................................................................... 8

Regional Geology ............................................................. ................................ 8
Morphological Characteristics of the Axial Segment....................................... 15
Sample Collection and Petrographic Analyses ................................................. 17
Major Element Geochemistry ......................................................................... 19
Trace Element Geochemistry ....................................................................... 38
Isotope Geochemistry .........................................................................................51
D iscu ssion ............................................................................................................... 57

3 TEMPORAL GEOCHEMICAL VARIABILITY OF AXIAL
SEGMENT LAVAS REVEALED BY RIDGE CAPTURE.................................. 67

Introduction ............................................................................................................ 67
Sample Recovery and Geological Observations................................. ......... 76
R idge C apture .............................................................. ........................................ 82
Temporal Geochemical Variations....................................... .......................... 87
D iscu ssion ............................................................................................................. 101

4 THE COBB HOTSPOT: A FIXED MANTLE MELTING ANOMALY
WITH MORB-LIKE GEOCHEMICAL CHARACTERISTICS........................ 104

Introduction.......................................................... ................................................. 104
The Cobb Hotspot ................................................................................................ 106
D discussion .............................................................................................................107
Model Age of the Cobb Hotspot........................................................................ 111


iii








5 SUMMARY AND CONCLUSIONS.................................................................. 116

Magmatic Consequences of Ridge-Hotspot Interaction................................ 116
Capture of the Axial Segment by the Cobb Hotspot...................................... 119
The MORB-like Cobb Hotspot........................................................................... 120

APPENDIX: ANALYTICAL METHODS................................................................. 121

REFER EN C ES .............................................................................................................. 124

BIOGRAPHICAL SKETCH....................................................................................... 132





































iv













Abstract of Thesis Presented to the Graduate School
of the University of Florida in Partial Fulfillment of the
Requirements for the Degree of Doctor of Philosophy

MAGMATIC AND TECTONIC EFFECTS OF THE INTERACTION
OF THE JUAN DE FUCA MID-OCEAN RIDGE WITH THE
COBB HOTSPOT

By

DAVID JOHN CHADWICK

May 2002

Chair: Dr. Michael Perfit
Major Department: Geological Sciences

The Cobb hotspot has had a major influence on the Axial Segment, one of

seven segments of the Juan de Fuca Ridge and the one the hotspot is directly

interacting with. The Axial Segment has anomalous shallow bathymetry and is

dominated by volcanic landforms instead of the tectonic features found on the

other segments. Major element, trace element, and isotopic studies of the lavas

erupted along the Axial Segment reveal variable mixing of the indigenous mid-

ocean ridge basalt source with the Cobb hotspot magmas, with the proportion of

the mid-ocean ridge component increasing with distance from the hotspot.

The hotspot has also "captured" the Axial Segment, a process that is also

occurring where the Mid-Atlantic Ridge overlies the Iceland hotspot. Ridge








capture has caused the neo-volcanic zone to remain fixed over the hotspot for

about 0.4 Ma in spite of continued migration of the rest of the ridge system.

Ridge capture has led to the exposure of basalts of different ages along the Axial

Segment in a spatial framework, allowing for geochemical changes to be detected

between younger and older basalts. Younger basalts are more mafic and slightly

more enriched in some incompatible trace elements than older basalts,

suggesting a recent influx of primitive magmas into the Axial Segment system

that have a slightly higher proportion of the Cobb hotspot end-member.

The Cobb hotspot itself is anomalous in that it exhibits none of the

radiogenic isotopic enrichments observed at other hotspots, although it is clearly

a fixed mantle plume with a source below the mobile mantle. Although the

hotspot has major and trace element characteristics that are distinct from the

nearby Juan de Fuca ridge, these are within global mid-ocean ridge basalt

geochemical ranges. A set of 'model ages' calculated for Cobb-Eickelberg basalts

suggest that the hotspot is too young to be derived from oceanic crustal material

that has undergone subduction to the core-mantle boundary.













CHAPTER 1
INTRODUCTION

Plate tectonics and hotspot volcanism are two of the most dominant

geological processes that affect the Earth's surface. They build mountains, sever

continents to open ocean basins, and place immense volumes of molten rock

from the Earth's interior onto its surface. As powerful and global in scale as

these processes may be, they are merely a consequence of the cooling of the

planet, the surface manifestation of the removal of heat generated by radioactive

decay and heat remaining from the genesis of the planet 4.6 billion years ago.

The primary magmatic and tectonic features created by plate tectonic

processes are mid-ocean ridges, which comprise a planet-girdling mountain

chain where new oceanic lithosphere is created, and subduction zones, where

this lithosphere is recycled back into the mantle. Hotspots may complete the

cycle; subducted lithosphere is thought to be dense enough to sink into the deep

mantle, perhaps down to the D" layer at the core-mantle boundary. Here the

material may heat and become buoyant, rising as solid-state plumes back toward

the surface where they partially melt and form long hotspot chains like the

Hawaiian Islands and the Cobb-Eickelberg Seamount chain (Hauri and Hart,

1993; Albarede and Van der Hilst, 1999). Material in these hotspot plumes may

also replenish the asthenosphere, the upper mantle material tapped by mid-









ocean ridges, where the cycle begins anew (Yale and Phipps Morgan, 1998). This

pattern of convection in the Earth may explain why hotspots are more prevalent

near spreading centers than subduction zones (Marzocchi and Mulargia, 1993;

Weinstein and Olson, 1989).

Recent mid-ocean ridge research has largely been focused on how

magmatic, tectonic, and geochemical characteristics vary with rates of spreading,

and the nature and scale of ridge segmentation and mantle geochemical

heterogeneity (Perfit and Chadwick, 1998). Fast-spreading ridges like the East

Pacific Rise are robust magmatic systems, with relatively common volcanic

eruptions, widespread hydrothermal activity, and a broad, axial high

morphology that emulates a shield volcano (Macdonald, 1989). Slow spreading

ridges like the Mid-Atlantic Ridge are dominated by tectonism, with wide axial

valleys and extensive normal and detachment faulting, less volcanic and

hydrothermal activity, and generally larger constructional volcanic features.

Intermediate rate spreading ridges, such as the Juan de Fuca Ridge, generally

have features that are transitional between the ridges with fast and slow

spreading rates.

Recent hotspot research has involved the geochemical classification of

mantle plumes and determining the sources of radiogenic isotope and

incompatible trace element enrichments (Zindler and Hart, 1986; Saunders, 1988;

Hart, 1988); 'EMI' (enriched mantle) plumes such as Kerguelen have chemical









characteristics that possibly reflect enrichment by lower continental crust, 'EMII'

plumes like Samoa have characteristics that may result from mixing depleted

mantle with continental sediments, and 'HIMU' plumes such as St. Helena have

chemical features that suggest sources with a subducted oceanic crust

component. The Depleted Mantle ('DM') component is presumed to be derived

from the shallow mid-ocean ridge basalt (MORB) mantle source, and is thought

to mix with the other components.

Hotspots interact with mid-ocean ridges at 18 locations on the Earth,

including Iceland, the Galapagos Islands, Easter Island, and the Azores (see

http://triton.ori.u-tokyo.ac.jp/-intridge/hotspot.htm for a complete listing).

This dissertation research is an analysis of the magmatic, geochemical and

tectonic effects of one of these interactions between these two fundamentally

different tectonomagmatic features. The geographic area in this study is the Juan

de Fuca Ridge where it overlies the Cobb Hotspot in the northeast Pacific, about

400 km west of the North American coast.

The morphology of the Axial Segment of the Juan de Fuca Ridge has been

acutely affected by the presence of the Cobb hotspot. It has shallow bathymetry

and an inflated appearance, standing much higher than the other segments on

the ridge (Figure 1). It is also dominated by constructional volcanic features, a

morphology that is very different from the common tectonic features found on

the other Juan de Fuca segments. These observations suggest that the hotspot



















Figure 1-1. Perspective view of the central portion of the Juan de Fuca Ridge.
The Axial Segment is the linear ridge bisected by Axial Seamount,
the large edifice near the center of the image. Axial Seamount has a
large (3 x 8 kmn) caldera and its summit is about 1415 meters below
sea level. Brown Bear Seamount is the previous volcanic center built
by the Cobb hotspot. The Vance Segment of the Juan de Fuca Ridge
extends toward the south, and the CoAxial Segment extends toward
the north. Multibeam sonar imagery from Pacific Marine
Environment Laboratory (PMEL), courtesy of Robert Embley.












magmas have infiltrated the mid-ocean ridge magmatic system, possibly mixing

with indigenous Axial Segment magmas. In chapter 2, the results of an effort to

elucidate the magmatic consequences of the ridge hotspot interaction are

presented, using the major element, trace element, and radiogenic isotope

compositions of rock samples collected from along the Axial Segment.

Comparisons with the geochemical characteristics of the other ridge segments

show the effects of the hotspot on the Axial Segment lavas. An analysis of spatial

geochemical trends along the segment has clarified the importance of magmatic

mixing, migration, and eruption processes in this ridge-hotspot interaction.

The Axial Segment has an asymmetrical cross-sectional profile, with the

shallowest bathymetry on its eastern side, and the most recent lava flows,

freshest-looking basalts, and thinnest sediment cover are also found on the

eastern side of the segment. These observations suggest that the Axial Segment

is not operating as a normal, symmetrically spreading mid-ocean ridge. In

chapter 3, evidence is presented for the "capture" of the segment by the Cobb

hotspot, which has effectively arrested its northwesterly migration for at least the

past 0.4 my. This process is also occurring at Iceland, where westward migration

of a portion of the Mid-Atlantic Ridge has been halted by the Iceland hotspot

(Hardarson et al., 1997). With an understanding of the spatial variability in

eruptive ages on the Axial Segment caused by ridge capture, geochemical









differences between the younger lavas from the east side and older lavas from

the west side of the segment are examined.

Chapter 4 is an investigation of the anomalous geochemistry of the Cobb

hotspot, which has created the Cobb-Eickelberg (C-E) seamount chain. The chain

extends from Axial Seamount, the current locus of the hotspot, to the 33 Ma

Patton Seamount near the Aleutian Trench (Keller et al., 1997). 4Ar-39Ar and K-

Ar ages from volcanoes in the C-E chain show a clear progression from Axial

Seamount to the northwest, consistent with plate motion vectors for the north

Pacific and a fixed hotspot plume origin for the chain (Karsten and Delaney,

1989). The C-E chain exhibits the temporal and spatial characteristics of other

hotspots, but its lavas do not have typical ocean island basalt (OIB) isotopic or

incompatible trace element characteristics (Desonie and Duncan, 1990). C-E

basalts have a modest enrichment in LREE, Sr, and K, distinguishing them from

the Juan de Fuca Ridge (See Chapter 2), but they fall within global MORB

geochemical ranges. The hypothesis of deep mantle plume origins for hotspots is

largely supported by the enriched isotopic and trace element geochemistry of

OIB relative to MORB (Schilling, 1973; Hart et al. 1973). A fixed, deep mantle

plume with the geochemical characteristics of MORB presents a vexing problem

in light of the hotspot geochemical paradigm.













CHAPTER 2
MAGMATIC EFFECTS OF THE INTERACTION OF THE JUAN DE FUCA
RIDGE WITH THE COBB HOTSPOT

Regional Geology

Axial Seamount is located on the Juan de Fuca Ridge, approximately 400

km west of the western North American coast, and is the volcanic edifice

currently being built by the Cobb hotspot. The volcano is associated with a

linear system of "rift zones" that extend about 50 km to the north and south from

its flanks. These comprise the northern and southern halves of the Axial

Segment (a.k.a the Axial Seamount or Axial Volcano Segment; Hammond and

Delaney, 1985; Johnson and Holmes, 1989; Embley et al., 1990), one of seven

ridge segments that together comprise the Juan de Fuca Ridge (JdFR) system

(Figure 2-1). The north and south rift zones and Axial Seamount will collectively

be referred to as the Axial Segment in this dissertation.

Axial Seamount rises to about 2100 m above the surrounding seafloor and

its summit shallows to about 1415 meters below sea level. With a basal diameter

of about 30 km. Axial has dimensions that are similar to the subareal portion of

Kauai in the Hawaiian Islands. To the northeast of the caldera, the flank of the

volcano drops precipitously from about 1400 m down to 2300 m depth in the

Helium Basin over a distance of less than 2 km. Side-scan and Sea Beam sonar

surveys have shown that Axial Seamount has a unique morphology, with a 3 x 8



















Figure 2-1.


The 500-km-long Juan de Fuca Ridge is divided into seven segments
(labeled in the inset), and the Axial Segment directly overlies the
Cobb hotspot. The Axial Segment is in an overlapping relationship
with the neighboring Vance and CoAxial Segments. The outline of
Axial Seamount is denoted by the 1600 m isobath contour, and
shows the approximate position of the change from contours that are
concentric around the seamount to those that are linear and parallel
to the trend of the ridge. Locations of basalt samples from the
NeMO cruises that have been geochemically analyzed are denoted
with a'+' symbol.





10


130* 128' 126" 124' 122*







(f t Portland
Soab .' ..e tl


Newport

,. ....................... ........... . .....
41




.................................... .......................... ..... .... ........... .. .. ... ... .... ........ ... .. .. .............. .


Brw ++
Bear
SScamount Helium
'V I .-^-/^K ( Basin
.. .. . ........... ....................... . . . .. .. . . . . . . .. . . . ..... ................ .......... . . .



AXIAL
SEAMOUNT j
................. ....... ........ ........................... .....' ........ .. .. ........







W+1E
............... .............. ............................. ... ..... .... ... ...... .. ................................... ........... ...................... .



N o
.w E ... ............................... ..... .. ......... ........... ....., <,.. .. ................... ............ ...................

s <0

0 5 10
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km, roughly rectangular caldera that opens to the southeast (Embley et al., 1990).

The northeasterly trend of volcanic structures in the rift zones and the

northwesterly trend of the caldera's long axis give the system a sigmoidal

appearance in map view, similar to Long Valley Caldera in California. The en-

echelon offset between the north and south rift zones that occurs across the

caldera may be due to an overlapping spreading center offset (Embley et al.,

1990).

The Cobb hotspot has produced the major NW-SE-trending Cobb-

Eickelberg (C-E) seamount chain in the north Pacific (Smoot, 1985; Karsten and

Delaney, 1989; Desonie and Duncan, 1990), which extends from Axial Seamount

to the 33-million-year-old Patton Seamount near the Aleutian Trench (Keller et

al., 1997), and is roughly parallel to the trend of the Hawaiian chain. Axial

Seamount is connected by a low saddle to the next volcano in the chain to the

northwest, Brown Bear Seamount. The magmatic activity of the hotspot has

apparently been variable over time, with the comparatively smaller Anger, Sloth,

Gluttony, and Lust seamounts created during a period of reduced magmatic

output between about 2.5 and 4.5 Ma, and larger seamounts reflecting more

robust magmatic activity over the past 2.5 Ma (Desonie and Duncan, 1986;

Karsten and Delaney, 1989). Son of Brown Bear and Thompson Seamounts,

located to the east of Axial Seamount on the Juan de Fuca Plate, may have

formed off-axis during periods of higher magmatic output of the Cobb hotspot as

the ridge approached it (Desonie and Duncan, 1986; Karsten and Delaney, 1989).








However, a 3 Ma date for a sample from Thompson Seamount (Karsten and

Delaney, 1989) indicates that it was formed during the presumed period of

reduced activity.

The JdFR comprises the central 490 km of the boundary between the

Pacific and Explorer-Juan de Fuca-Gorda plates. Axial Seamount lies near its

center and is the shallowest portion of it (Figure 2-2). The region was part of a

classic mid-ocean ridge magnetic anomaly study (Raff and Mason, 1961; Vine

and Matthews, 1963), and the tectonic evolution and spreading rates of the ridge

have been studied in detail (Hey and Wilson, 1982; Wilson and Hey, 1984;

Wilson, 1993). The JdFR has a strike of about 20 and intersects the Blanco

fracture zone at its southern terminus, which offsets the spreading axis by -400

km from the Gorda Ridge. The JdFR is bounded to the north by a complex triple

junction with the Sovanco fracture zone and left-lateral Nootka fault (Hyndman

et al., 1979; Riddihough, 1984).

The number and names of the individual second-order ridge segments of

the JdFR have changed over time with improvements in mapping, and

descriptions of the segments and the ridge in general have been published in

previous studies (Delaney et al., 1981; Embley and Chadwick, 1994; Embley et al.,

2000; Johnson and Embley, 1990; Johnson and Holmes, 1989; Kappel and Ryan,

1986; Malahoff et al., 1982; Smith et al., 1994; Van Wagoner and Leybourne,

1991). The seven segments most recently identified are, from south to north

(previously used names of segments are given in parenthesis for comparison









-1500


-2000





-2500
Cleft Coaxial
Segment Segment West
Valley
Vance Cobb Segment
Segment Segment S
Endeavor /
-3000 Segment

0 100 200 300 400 500

Distance (kin)

Figure 2-2. Bathymetric profiles along the Juan de Fuca Ridge segments. The Axial Segment is the shallowest
of the entire ridge, attesting to the oversupply of magmas to the ridge from the hotspot, and allowing
the hotspot and MORB-source magmas to mix.








with older papers): Cleft (Southern Symmetrical Ridge), Vance (Segment "B" or

the Shingle Ridge Segment), Axial (Axial Seamount or Axial Volcano), CoAxial

(part of the Northern Symmetrical Ridge and later considered part of the Cobb

Segment), Cobb (part of the Northern Symmetrical Ridge), Endeavor and West

Valley (Figure 2-1). Offsets between these segments are generally characterized

by overlapping spreading centers, with the ends of each of the segments curving

toward each other. The JdFR is a medium spreading-rate system, with a full rate

of about 6 cm/year (Vine and Wilson, 1965; Johnson and Embley, 1990). The

entire plate boundary system is migrating to the NW at 3.1 cm/yr (Desonie and

Duncan, 1986; Karsten and Delaney, 1989).

The age of the interaction between the JdFR and Cobb hotspot is

constrained by the offset location of the Axial Segment in the paleomagnetic

lineation pattern. The center of the Brunhes normal-polarity crust pattern lies

about 15 km to the east of the Axial Segment, collinear with the Vance and

CoAxial segment trends (Tivey and Johnson, 1990). This indicates that active

spreading linked the Vance and CoAxial segments in early Brunhes time, but

ceased when the section of ridge between them was abandoned and spreading

moved westward to the Cobb hotspot, creating the Axial Segment between 0.2

and 0.7 Ma (Delaney et al., 1981; Karsten and Delaney, 1989; Tivey and Johnson,

1990). This ridge "jump" trapped small areas of Matuyama-age crust (e.g.

Helium Basin) between the abandoned and current spreading ridges. A study of

ridge morphological and bathymetric characteristics and age distribution of








basalts on the Axial Segment suggest that the segment has been captured by the

hotspot (see Chapter 3), essentially arresting its migration in a situation similar to

the capture of the Mid-Atlantic Ridge by the Iceland hotspot.

Morphological Characteristics of the Axial Segment

The general characteristics of the Axial Segment have been described in

previous studies based on side-scan sonar imagery (Delaney at al., 1981; Embley

et al., 1990; Johnson and Holmes, 1989; Johnson and Embley, 1990). The gross

bathymetric and detailed morphologic characteristics of the Axial Segment are

distinct from the rest of the JdFR segments due to the presence of the Cobb

hotspot. The segment is, however, an integral part of the JdFR spreading system

and not simply a volcanic rift system similar to the one associated with Kilauea

volcano in Hawaii (Delaney et al., 1981; Hammond and Delaney, 1985; Delaney,

1986; Johnson and Holmes, 1989). It accommodates spreading in the 60-km gap

between the southern end of the CoAxial segment and the northern end of the

Vance segment (Figure 2-1), and it overlaps these segments in the manner of an

overlapping spreading center (Embley et al., 2000). The complex interaction

between seafloor spreading and hotspot volcanism has led to the creation of the

ridge-centered Axial Seamount, and elongate, inflated rift zones that have the

highest-standing bathymetry on the entire JdFR, with average depths that are

250-750 m shallower than the other segments (Figure 2-2). The other JdFR

segments are characterized by a predominance of tectonic features typical of

medium spreading rate ridges (e.g. Karsten et al., 1986; Johnson and Holmes,








1989; Van Wagoner and Leybourne, 1991, Smith et al., 1994; Embley et al., 2000),

but the Axial Segment is dominated by constructional volcanic features

(Appelgate, 1990). It has a corrugated appearance, composed of parallel-

trending sets of thin, linear ridges that appear to be dike-fed volcanic constructs.

They are typically about 300 to 500 m wide, up to 50 m high, and can be up to 15

km long. Interspersed among this linear pattern are small (typically <1 km)

seamounts that represent more focused, localized higher effusion, and these

features are more prevalent closer to the northern and southern ends of the

segment.

The linear ridges are larger and more numerous close to Axial Seamount,

suggesting that they are fed by lateral dike intrusion from the large magma

chamber detected using seismic tomography under the seamount (West et al.,

2001). Lateral migration of magmas along the segment from the vicinity of the

hotspot is also implied by the locations of waterborne T-wave foci generated by

the eruption of Axial Seamount during January of 1998, detected by the Sound

Surveillance System (SOSUS) hydrophone network (Dziak and Fox, 1998,1999).

The activity migrated progressively southward from Axial's caldera -50 km into

the South Rift Zone during the first few days of the 11-day period of activity

(Embley et al., 1999). Similar migration of seismic activity during eruptive events

has been observed in Icelandic and Hawaiian diking events (Dziak and Fox,

1998). The bathymetric and morphologic features of the Axial Segment suggest

an "oversupply" and lateral migration of magmas from the hotspot focus at








Axial Seamount, along the entire length of the ridge segment; a hypothesis that

should be testable using petrologic and geochemical methods.

The January, 1998 eruption produced extensive lava flows to the southeast

of the caldera from at least two collinear fissure sources. The geological,

hydrothermal, and biological effects of this eruption have been the focus of

NOAA's New Millennium Observatory (NeMO) project

(http://www.pmel.noaa.gov/vents/nemo/index.html). The field work in

support of this research has included high-density rock sampling using wax

corers and the Remotely Operated Platform for Ocean Science (ROPOS)

submersible. In this paper we discuss the results of this geologic research, in an

effort to elucidate the magmatic consequences of the Cobb-JdFR interaction using

the major element, trace element, and radiogenic isotopic compositions of rock

samples collected on the Axial Segment. Comparisons with the geochemical

characteristics of the other segments on the JdFR allow us to better constrain the

effects of the hotspot on any Axial Segment MORB lavas, and analysis of spatial

geochemical trends has improved our understanding of mixing, migration, and

eruption processes that have occurred in response to this ridge-hotspot

interaction.

Sample Collection and Petrographic Analyses

A total of 254 rock samples were collected on Axial Seamount, the Axial

Segment rift zones, and the Cleft and Vance Segments of the JdFR during the

1998-2002 NeMO cruises (Figure 2-1). Samples were collected mostly by wax








coring on the Axial Segment, and the ROPOS submersible was used primarily in

the Axial Seamount caldera region, particularly around the 1998 eruption area.

The wax core samples were collected in a high density grid on the ridge within

25 km of Axial Seamount, allowing for trends and small-scale differences in

geochemistry to be identified. An additional 28 samples were collected in the

study area in 1986 by the Pisces IV submersible, and 42 samples from the

northern caldera and Axial Seamount flanks were collected using a grab-sampler

with integrated video camera (TV-grab) during the 1996 cruise of RV Sonne, and

geochemical analyses of these supplement the study where noted. Natural glass

samples were analyzed by electron microprobe, X-ray fluorescence spectroscopy,

and thermal ionization mass spectrometry for major element, trace element, and

isotopic compositions (See Appendix for analytical methods). Thin-sections of

representative samples were also petrographically examined to provide phase

chemical information.

Basalt flow morphologies in the Axial caldera region include sheet flows,

chaotic jumbled flows, pillow basalts, and lobate flows (Embley et al., 1990). The

freshest samples have thick (>1 cm) glass rinds that are friable and easily

exfoliated, while progressively older samples have thinner rinds and increasing

Mn alteration. Most of the basalt samples collected in the Axial Seamount

caldera and on the flanks are sparsely to moderately phyric, (<5%) and have

mineral assemblages dominated by plagioclase with rare olivine phenocrysts.

The plagioclase phenocrysts commonly occur as clots of crystals with dimensions








up to several mm. Samples from a small cone in the central part of the caldera

(Pisces sample XL-2087-5) are highly plagioclase phyric (~40%) with minor

olivine. Complex zoning and disequilibrium compositions of some plagioclase

and more rare, partially resorbed olivine crystals in basalts retrieved from the

caldera indicate magma mixing has taken place. Most samples from the adjacent

ridge portion of the Axial Segment are moderately plagioclase phyric (up to

15%), and the abundance of crystals generally increases with distance from Axial

Seamount. Plagioclase phenocrysts are typically less than 1 mm and euhedral to

subhedral. Microphenocrysts and microlites of olivine, plagioclase,

clinopyroxene and opaques commonly occur in the groundmass. Lavas typically

have a variolitic texture, with varioles composed of plagioclase and

clinopyroxene. Most of the basalts in the study area are sparsely vesicular (< 5%

by volume), although four andesite samples collected in the north rift zone and

one basalt from the caldera region (R494-3) are moderately to highly vesicular.

One basalt sample containing a small gabbroic (olivine-plagioclase-

clinopyroxene) xenolith was collected just to the south of the Axial Seamount

caldera (R483-6).

Major Element Geochemistry

Major and minor element compositions of 226 glass samples collected on

Axial Seamount and the adjacent rift zones were analyzed by electron

microprobe and are presented in Table 2-1 and plotted in Figure 2-3. The basalts

exhibit typical tholeiitic differentiation trends, with increasing concentrations of










Table 2-1. Major element concentrations (wt. %) for Axial Segment samples, analyzed by electron microprobe1.
Sample Latitude Latitude SiO2 A12z TiO2 FeO MnO MgO CaO K20 Na2O P20 5s Total
98-RC-01 -129.9970 45.8922 50.3 14.4 1.52 10.7 0.21 7.15 11.7 0.19 2.95 0.14 99.3
98-RC-02 -129.9758 45.8535 50.0 14.1 1.65 11.3 0.21 6.94 11.6 0.23 2.96 0.15 99.0
98-RC-03 -130.0130 45.8287 50.0 14.1 1.59 11.1 0.21 7.00 11.6 0.20 3.03 0.16 99.0
98-RC-04 -130.0117 45.8325 50.2 14.2 1.61 11.2 0.20 7.10 11.7 0.19 2.86 0.15 99.4
98-RC-05 -130.0053 45.8327 49.9 14.9 1.37 10.2 0.20 7.66 11.9 0.14 2.82 0.12 992
98-RC-06 -130.0097 45.8363 50.0 14.1 1.59 11.1 0.21 7.02 11.6 0.21 2.96 0.16 99.1
98-RC-07 -130.0597 45.7867 50.0 14.0 1.57 11.2 0.21 6.99 11.8 0.16 2.98 0.15 99.0
98-RC-09 -130.0575 45.8012 50.2 14.2 1.58 11.1 0.23 7.10 11.7 0.17 2.99 0.16 99.4
98-RC-10 -129.9633 45.9615 50.1 14.8 1.41 10.5 0.20 7.49 11.6 0.16 2.83 0.14 99.2
98-RC-11 -130.0062 45.8505 50.1 14.3 1.52 10.6 0.20 7.16 11.9 0.20 2.94 0.13 99.0
98-RC-12 -130.0088 45.8400 50.2 14.2 1.63 11.2 0.22 6.98 11.6 020 3.02 0.18 99.5
98-RC-13 -130.0268 45.8440 50.1 14.1 1.50 10.9 0.21 7.15 11.8 0.19 2.86 0.15 98.9
98-RC-15 -130.0318 45.8943 50.3 14.3 1.60 11.2 0.22 6.92 11.7 0.20 2.86 0.16 99.5
98-RC-16 -130.0258 45.7923 50.2 14.0 1.48 11.3 0.20 7.08 11.7 0.15 2.87 0.14 99.1
98-RC-17 -130.0283 45.7983 50.4 13.9 1.57 11.5 0.21 6.95 11.6 0.16 2.95 0.15 99.3
98-RC-18 -130.0193 45.8087 50.0 14.0 1.65 11.3 0.21 6.91 11.6 0.21 3.07 0.15 99.1
98-RC-21 -130.0813 45.6367 482 16.6 131 10.1 0.18 8.76 112 0.09 2.75 0.10 99.4
98-RC-22 -130.0467 45.8707 50.4 14.1 1.63 11.3 0.22 6.84 11.7 0.20 2.95 0.17 99.4
98-RC-23 -130.0417 45.8803 50.3 14.3 1.50 10.9 0.21 7.17 11.9 0.19 2.87 0.13 99.6
98-RC-24 -130.0380 45.8527 50.1 14.0 1.62 11.5 0.22 6.85 11.5 0.21 3.00 0.16 99.1
98-RC-26 -130.0485 45.8343 48.7 16.3 1.07 9.9 0.20 8.53 11.8 0.11 2.56 0.11 99.4
98-RC-27 -130.0258 45.8342 49.9 14.2 1.53 10.9 0.21 7.18 11.9 0.17 2.98 0.14 99.2
98-RC-28 -130.0250 46.0075 50.6 14.5 1.52 10.5 0.20 7.09 11.3 0.24 3.07 0.17 99.2
98-RC-30 -130.0375 45.8212 49.9 13.8 1.66 11.7 0.23 6.71 11.5 0.20 3.01 0.14 98.9
98-RC-31 -130.0277 45.8167 50.0 14.3 1.71 11.8 0.23 6.58 11.1 0.24 3.17 0.18 99.3
98-RC-32 -130.0130 45.8133 50.1 14.2 1.62 11.0 0.21 7.13 11.9 0.20 2.93 0.16 99.4
98-RC-33 -130.0315 45.8437 50.2 13.9 1.65 11.5 0.22 6.84 11.7 0.20 2.92 0.15 99.2
98-RC-34 -130.0475 45.8403 50.0 13.7 1.74 11.9 0.22 6.54 11.3 0.23 3.03 0.18 99.0
98-RC-35 -130.0113 45.8612 50.2 143 1.45 10.6 0.20 7.32 12.0 0.17 2.83 0.13 99.1
98-RC-36 -129.9638 45.8263 49.5 15.2 1.34 10.6 0.20 7.69 12.0 0.14 2.80 0.12 99.5
98-RC-39 -130.0028 45.8608 50.1 14.4 1.51 10.5 0.20 7.27 11.9 0.19 2.85 0.14 99.1
1 Analyzed by the U.S. Geological Survey, Denver, Colorado.









Table 2-1. Continued.
Sample Latitude Latitude SiO2 A120l3 TiO2 FeO MnO MgO CaO K20 Na2O P205 Total
98-RC-40 -130.0700 45.8400 50.1 13.9 1.67 11.6 0.22 6.79 11.4 0.18 3.13 0.16 99.2
98-RC-41 -130.0290 45.8893 50.4 14.4 1.54 10.8 0.21 7.21 11.8 0.19 2.91 0.15 99.6
98-RC-42 -129.9965 46.0227 50.1 14.3 1.62 10.8 0.22 7.14 11.7 0.21 2.97 0.18 99.3
98-RC43 -130.0475 45.8310 50.3 13.9 1.65 11.4 0.23 6.88 11.4 0.19 2.98 0.15 99.2
98-RC-44 -130.0500 45.8275 50.2 13.9 1.59 11.5 0.23 6.86 11.4 0.18 2.99 0.15 99.0
98-RC-45 -130.0602 45.8053 49.9 14.0 1.59 11.3 0.22 6.97 11.7 0.21 2.91 0.17 99.0
98-RC-46 -130.0308 45.7483 47.8 17.2 1.01 9.8 0.16 9.27 11.4 0.07 2.52 0.05 99.3
98-RC-47 -129.9828 46.0488 50.0 14.2 1.49 10.8 0.20 7.25 11.9 0.18 2.84 0.13 99.1
98-RC-48 -129.9678 46.0660 50.1 14.1 1.55 11.0 0.22 7.07 11.8 0.20 2.88 0.15 99.1
98-RC49 -129.9630 46.0623 57.7 12.3 1.79 13.6 0.32 1.82 5.8 0.90 2.29 0.70 97.2
99-RC-50 -130.0000 45.8810 49.5 15.5 1.35 10.0 0.17 8.05 12.1 0.12 2.85 0.12 99.7
99-RC-51 -130.0150 45.8790 50.1 14.1 1.59 11.1 0.22 6.93 11.7 0.17 2.88 0.16 99.1
99-RC-52 -130.0210 45.8760 49.9 14.5 1.50 10.9 0.21 7.15 12.1 0.15 2.88 0.15 99.3
99-RC-53 -130.0300 45.8620 49.9 14.4 1.53 10.9 0.21 7.40 12.0 0.15 2.85 0.15 99.6
99-RC-54 -130.0780 45.6560 48.7 16.9 1.25 9.3 0.17 8.47 11.8 0.09 2.78 0.09 99.5
99-RC-55 -130.0720 45.6870 50.0 14.1 1.69 11.4 0.22 6.78 11.9 0.18 2.91 0.13 99.2o
99-RC-56 -130.0820 45.6920 50.0 13.8 1.77 12.3 0.23 6.35 11.3 0.18 3.13 0.17 99.3
99-RC-57 -130.0820 45.7010 50.0 13.7 1.79 12.3 0.22 6.37 11.4 0.18 3.14 0.17 99.3
99-RC-58 -130.0640 45.7570 49.9 14.0 1.71 11.6 0.24 6.71 11.8 0.18 2.97 0.16 99.2
99-RC-59 -130.0630 45.7660 50.0 14.1 1.62 11.3 0.20 6.82 11.8 0.16 3.01 0.14 99.2
99-RC-60 -130.0630 45.7770 49.9 13.9 1.65 11.4 0.21 6.70 11.9 0.19 2.99 0.16 98.9
99-RC-61 -130.1380 45.7530 50.0 13.8 1.64 11.4 0.22 6.70 11.5 0.19 2.99 0.17 98.7
99-RC-62 -130.0510 45.8670 50.1 13.7 1.72 11.4 0.21 6.64 11.5 0.21 2.95 0.18 98.6
99-RC-63 -130.0530 45.8570 49.9 14.0 1.57 11.0 0.20 6.94 12.0 0.18 2.88 0.15 98.7
99-RC-64 -130.0800 45.7270 50.0 13.8 1.71 11.4 0.23 6.62 11.6 0.21 2.97 0.19 98.7
99-RC-65 -130.0790 45.7190 50.0 13.8 1.73 11.7 0.23 6.51 11.7 0.20 2.94 0.16 99.0
99-RC-66 -130.0800 45.7090 50.2 14.0 1.51 11.0 0.20 6.91 11.9 0.13 2.90 0.13 98.8
99-RC-67 -130.1410 45.8080 49.7 14.2 1.71 10.8 0.21 6.85 11.9 0.22 3.06 0.19 98.8
99-RC-68 -130.0580 45.8360 49.9 13.7 1.70 11.5 0.22 6.73 11.6 0.20 2.92 0.18 98.7
99-RC-69 -130.0610 45.8420 50.0 14.3 1.51 10.7 0.21 7.20 12.2 0.19 2.67 0.13 99.1
99-RC-70 -130.0190 45.7340 50.0 14.4 1.45 10.5 0.21 7.21 12.2 0.13 2.80 0.14 98.9
99-RC-71 -130.0180 45.7440 50.1 14.2 1.51 10.7 0.22 7.04 12.2 0.14 2.82 0.14 99.1









Table 2-1. Continued.


SiO2 A1203 TiO, FeO MnO Mi~O CaO K~O Na.,O PO~ TAtRJ


Sample
99-RC-72
99-RC-73
99-RC-74
99-RC-75
99-RC-76
99-RC-77
99-RC-78
99-RC-79
99-RC-81
99-RC-82
99-RC-83
99-RC-84
99-RC-85
99-RC-86
99-RC-87
99-RC-88
99-RC-89
99-RC-90
99-RC-91
99-RC-92
99-RC-93
99-RC-97
99-RC-98
99-RC-99
99-RC-100
99-RC-101
99-RC-102
99-RC-105
99-RC-106
99-RC-107
99-RC-108
00-RC-113


Table 2-1. Continued.


Latitude
-130.0140
-130.0070
-130.0170
-130.0120
-130.0010
-130.0070
-129.9990
-130.0050
-130.0230
-130.0370
-130.0550
-130.0600
-130.0600
-130.0430
-130.0430
-130.0390
-130.0240
-130.0350
-130.0280
-130.0210
-130.0170
-130.0370
-130.0340
-129.9620
-129.9600
-129.9520
-130.0370
-130.0620
-130.0580
-130.0620
-130.0640
_110 Q011


Latitude
45.7600
45.7550
45.7530
45.7740
45.7700
45.7810
45.7760
45.7620
45.7690
45.7630
45.8460
45.8470
45.8530
45.7870
45.7980
45.8110
45.8250
45.8300
45.8050
45.8020
45.8030
45.7030
45.7160
46.0640
46.0560
46.0770
46.0770
45.8550
45.8620
45.8290
45.8170
45.9165


49.0 14.1 1.59 11.0 0.20 6.97 11.9 0.17 2.89
49.4 14.3 1.45 10.4 0.21 7.28 12.2 0.17 2.73
49.5 14.1 1.58 11.1 0.22 6.80 12.0 0.17 2.92
49.9 14.1 1.64 11.2 0.22 6.61 12.0 0.18 2.94
49.5 14.3 1.48 10.4 0.21 7.21 12.2 0.18 2.72
49.4 14.0 1.62 11.2 0.21 6.87 11.9 0.17 2.94
49.5 14.3 1.48 10.4 0.21 7.25 12.2 0.18 2.76
48.8 14.3 1.44 10.3 0.21 7.31 12.2 0.17 2.72
49.9 14.2 1.38 10.3 0.19 7.40 12.6 0.12 2.81
47.8 16.9 1.17 9.6 0.19 8.82 11.8 0.07 2.68
49.8 14.1 1.55 10.7 0.22 7.10 12.2 0.17 2.85
49.8 14.1 1.54 10.9 0.22 7.06 12.0 0.18 2.86
50.0 14.0 1.55 10.8 0.20 7.09 12.0 0.17 2.85
49.9 13.7 1.73 11.8 0.22 6.46 11.5 0.20 3.09
49.9 13.7 1.76 11.9 0.24 6.46 11.5 0.19 3.12
49.9 13.9 1.59 11.0 0.20 6.97 11.8 0.17 2.95
50.1 13.9 1.64 11.1 0.20 6.75 11.8 0.20 3.01
49.8 13.9 1.59 11.1 0.21 7.25 11.7 0.17 3.01
49.7 14.0 1.54 10.7 0.21 7.41 12.1 0.14 3.00
49.8 14.1 1.54 10.7 0.21 7.36 11.9 0.18 2.84
49.8 14.1 1.52 10.6 0.20 7.08 11.9 0.17 2.92
47.9 16.7 0.98 9.6 0.17 9.37 11.6 0.04 2.58
48.6 16.1 1.40 9.6 0.18 8.13 11.8 0.08 3.01
58.3 12.5 1.57 12.9 0.29 1.56 5.4 0.90 3.67
55.1 12.8 1.99 13.2 0.29 2.26 6.1 0.84 4.27
53.8 12.1 2.19 14.6 0.32 2.40 6.5 0.71 4.12
49.7 14.1 1.52 10.6 0.21 7.08 12.0 0.15 2.88
49.6 14.3 1.47 10.3 0.20 7.24 11.9 0.15 2.84
49.5 14.0 1.53 10.7 0.20 7.03 12.0 0.17 2.91
49.7 14.2 1.53 10.5 0.21 7.15 12.2 0.16 2.93
50.0 13.9 1.65 11.2 0.22 6.82 11.6 0.19 3.04
49.6 14.5 1.51 10.5 0.18 7.57 12.0 019 2RQ


49.6 14... .... ... ....... 0.8 .7 20 0 9 8


-- Ldw J. J a J


0.14 98.0
0.14 98.4
0.17 98.5
0.15 98.9
0.15 98.3
0.16 98.5
0.15 98.3
0.15 97.7
0.13 99.0
0.09 99.1
0.15 98.8
0.15 98.9
0.15 98.8
0.19 98.8
0.19 98.9
0.16 98.8
0.18 98.8
0.13 98.9
0.14 98.9
0.15 98.7
0.14 98.5
0.05 99.0
0.09 99.1
0.63 97.7
0.68 97.5
0.%96 97.7
0.13 98.3
0.12 98.2
0.14 98.1
0.13 98.7
0.13 98.7
0.15 99.1


S102 A12C T102 FeO MnO WO CaO K-,Q NaC) PQn Total









Table 2-1. Continued.


SiO2 AC1203 TiO2 FeO MnO MeO CaO K(O NagO P-XN Total


Sample
00-RC-116
00-RC-117
00-RC-118
00-RC-119
00-RC-120
00-RC-122
00-RC-123
R460-04
R460-06
R461-16
R461-25
R461-26
R462-08
R462-15
R464-06
R465-01
R465-02
R467-01
R471-04
R473-06
R473-18
R473-21
R474-02
R474-03
R476-02
R476-07
R478-08
R479-15
R483-02
R483-04
R483-06
R484-01


Latitude
-130.0317
-130.0168
-129.9684
-129.9928
-130.0093
-129.9848
-129.9983
-129.9855
-129.9817
-129.9883
-129.9798
-129.9798
-129.9823
-129.9818
-129.9818
-129.9862
-129.9863
-130.0163
-130.0137
-129.9848
-129.9822
-129.9830
-129.9780
-129.9815
-129.9847
-129.9850
-129.9815
-130.0140
-129.9823
-129.9823
-129.9823
-1 29QR'1


Latitude
45.8283
45.8190
45.9167
46.0562
46.0619
45.8403
45.8398
45.9438
45.9333
45.9227
45.9270
45.9270
45.9333
45.9333
45.9333
45.8693
45.8695
46.0188
45.9337
45.9455
45.9333
45.9287
45.9330
45.9360
45.9460
45.9463
45.9358
45.9333
45.9333
45.9333
45.9333
45.9333


49.9 14.0
50.0 14.4
49.7 14.6
47.0 17.2
49.3 15.5
48.2 16.7
48.8 15.8
50.0 14.4
50.0 14.5
50.0 14.4
50.1 14.9
50.0 14.4
49.9 14.5
50.0 14.5
50.1 14.6
50.1 14.2
50.2 14.2
50.1 14.4
49.9 14.6
50.0 14.5
50.0 14.5
49.9 14.5
50.0 14.5
50.2 14.4
50.0 14.3
50.1 14.5
50.0 14.5
49.9 14.6
49.5 14.3
49.5 14.2
49.5 14.2
49.6 14.3


49...6. ... ..... 1.4 10. 020 74 1 1 18 28


1.71 11.6 0.22 6.88 11.7 0.21 2.94
1.54 10.6 0.21 7.32 12.1 0.20 2.87
1.48 10.2 0.20 7.27 12.2 0.18 2.84
1.32 8.9 0.17 8.58 11.7 0.11 2.81
1.33 9.7 0.21 7.80 12.2 0.13 2.78
1.38 9.2 0.18 8.06 11.9 0.13 2.%
1.29 9.7 0.18 8.04 12.1 0.12 2.72
1.51 10.7 0.20 7.32 12.1 0.18 2.97
1.52 10.6 0.21 7.35 12.1 0.18 2.98
1.52 10.6 0.21 7.32 12.1 0.19 2.96
1.23 9.9 0.20 7.96 12.1 0.12 2.69
1.47 10.4 0.20 7.54 11.8 0.19 2.89
1.52 10.7 0.21 7.34 12.0 0.19 2.98
1.55 10.7 0.21 7.34 12.0 0.19 2.96
1.53 10.7 0.20 7.30 12.0 0.18 2.94
1.53 10.9 0.19 7.04 11.7 0.20 2.98
1.57 10.9 0.20 7.05 11.6 0.20 2.97
1.50 10.6 0.20 7.38 11.7 0.19 2.92
1.52 10.7 0.19 7.29 12.1 0.19 2.97
1.52 10.7 0.21 7.33 12.1 0.18 2.99
1.51 10.7 0.20 7.34 12.1 0.18 2.96
1.53 10.7 0.22 7.31 12.1 0.19 2.95
1.45 10.3 0.19 7.51 11.8 0.20 2.89
1.50 10.7 0.18 7.11 11.7 0.20 2.95
1.47 10.7 0.20 7.35 11.7 0.19 2.88
1.51 10.6 0.21 7.32 12.0 0.19 2.95
1.55 10.7 0.21 7.26 12.0 0.20 2.99
1.53 10.6 0.22 7.30 12.1 0.19 2.95
1.51 10.7 0.20 7.21 12.2 0.18 2.88
1.49 10.6 0.20 7.22 12.1 0.19 2.87
1.50 10.7 0.20 7.20 12.1 0.18 2.86
1.48 10.6 020 743 121 11R 2Rf


0.17 99.2
0.16 99.3
0.15 98.9
0.12 98.0
0.11 99.1
0.12 98.8
0.11 98.9
0.15 99.5
0.15 99.6
0.15 99.5
0.10 99.3
0.14 99.1
0.14 99.5
0.14 99.6
0.14 99.6
0.14 99.1
0.14 99.0
0.13 99.0
0.14 99.6
0.13 99.6
0.14 99.5
0.15 99.6
0.15 99.0
0.15 99.0
0.16 98.9
0.15 99.6
0.16 99.6
0.15 99.6
0.15 98.7
0.14 98.7
0.14 98.6
0.16 98.9









Table 2-1. Continued


SiO2 A1203 TiC2 FeO MnO M9O CaO K20 Na2O P205s Total


Sample
R488-15
R488-16
R488-05
R488-06
R488-FS
R491-21
R491-22
R491-25
R492-02
R492-09
R492-08
R492-14
R493-01
R494-01
R494-04
R494-05
R494-06
R494-08
R494-09
R495-35
R495-36
R496-03
R496-RU
R497-12
R497-20
R500-06
R501-10
R501-11
R501-12
R501-13
R501-14
R501-16


0.15 99.0


Latitude
-129.9818
-129.9818
-129.9823
-129.9823
-129.9819
-129.9817
-129.9816
-129.9833
-129.9886
-129.9919
-129.9919
-129.9893
-129.9952
-130.0042
-130.0041
-130.0041
-130.0041
-129.9958
-129.9929
-129.9904
-129.9904
-129.9883
-129.9840
-130.0272
-130.0163
-129.9833
-129.9823
-129.9823
-129.9876
-129.9886
-129.9886
-127q 9I9


Latitude
45.9334
45.9334
45.9333
45.9333
45.9332
45.9364
45.9344
45.9451
45.9188
45.9168
45.9168
45.9162
45.9134
45.8583
45.8624
45.8624
45.8624
45.8782
45.8894
45.9111
45.9111
45.9226
45.9303
45.9889
46.0188
45.9451
45.9240
45.9239
45.9240
45.9240
45.9240
45.9240


49.5 14.3 1.48
49.5 14.2 1.51
49.4 14.2 1.50
49.5 14.3 1.53
49.5 14.3 1.51
50.0 14.6 1.48
50.0 14.6 1.48
49.9 14.5 1.53
49.9 14.4 1.52
49.8 14.5 1.49
49.9 14.5 1.54
49.9 14.4 1.52
50.1 14.5 1.51
50.1 14.3 1.53
50.1 14.4 1.56
50.1 14.5 1.53
50.1 14.4 1.55
50.1 14.5 1.57
50.1 14.5 1.52
50.0 14.4 1.51
49.9 14.5 1.52
50.0 14.5 1.53
50.0 14.5 1.53
50.1 14.5 1.49
50.0 14.5 1.46
49.8 14.5 1.49
49.8 14.3 1.51
49.7 14.3 1.50
49.5 14.5 1.48
49.7 14.3 1.51
49.6 14.5 1.47
49.6 14.5 1.49


49.6 145 149


10.6 0.21 7.17 12.3 0.19 2.85
10.7 0.19 7.17 12.0 0.18 2.90
10.6 0.20 7.16 12.0 0.18 2.87
10.7 0.19 7.19 12.0 0.18 2.88
10.6 0.20 7.21 12.0 0.18 2.86
10.3 0.18 7.33 12.3 0.19 2.83
10.3 0.19 7.62 12.2 0.18 2.85
10.4 0.19 7.35 12.3 0.18 2.83
10.7 0.20 7.58 12.2 0.18 2.89
10.5 0.19 7.35 12.2 0.18 2.87
10.6 0.19 7.22 12.5 0.18 2.81
10.6 0.20 7.15 12.4 0.17 2.89
10.4 0.18 7.54 12.1 0.18 2.84
10.6 0.19 7.03 12.3 0.18 2.84
10.8 0.20 7.11 11.8 0.18 2.90
10.9 0.19 7.14 11.9 0.18 2.91
10.7 0.18 7.15 12.0 0.19 2.86
10.7 0.21 7.10 12.0 0.18 2.88
10.7 0.19 7.24 12.0 0.18 2.87
10.7 0.20 7.53 11.9 0.18 2.86
10.7 0.20 7.21 12.2 0.18 2.87
10.7 0.20 7.50 12.0 0.18 2.89
10.8 0.19 7.21 12.1 0.18 2.88
10.4 0.21 7.18 11.7 0.20 3.05
10.5 0.20 7.13 11.9 0.20 3.06
10.4 0.19 7.41 12.1 0.18 2.95
10.6 0.20 7.21 12.1 0.18 2.60
10.5 0.19 7.18 12.2 0.18 2.48
10.2 0.20 7.40 12.2 0.18 2.48
10.7 0.21 7.48 12.0 0.18 2.90
10.4 0.19 7.43 12.1 0.19 2.84
10.4 0.19 7.39 12.2 0.18 2.85


0.15 98.8
0.15 98.5
0.15 98.4
0.16 98.6
0.14 98.5
0.15 99.3
0.13 99.5
0.14 99.3
0.15 99.7
0.16 99.2
0.15 99.5
0.14 99.3
0.13 99.4
0.15 99.2
0.15 99.3
0.15 99.5
0.15 99.3
0.14 99.4
0.13 99.4
0.15 99.5
0.14 99.4
0.15 99.6
0.14 99.5
0.16 99.0
0.16 99.1
0.15 99.1
0.15 98.7
0.15 98.4
0.14 98.3
0.15 99.2
0.16 98.8
0.15 99.0










Table 2-1. Continued.
Sample Latitude Latitude SiO2 A1203 TiO2 FeO MnO MgO CaO K2) Na2O P205 Total
R501-02 -129.9924 45.9206 49.7 14.6 1.51 10.4 0.19 7.39 12.2 0.19 2.92 0.16 99.2
R501-03 -129.9914 45.9207 49.9 14.5 1.50 10.6 0.20 7.27 12.0 0.18 2.94 0.13 99.2
R501-04 -129.9899 45.9203 49.8 14.4 1.49 10.5 0.20 7.32 12.1 0.18 2.92 0.15 99.1
R501-05 -129.9877 45.9204 49.7 14.4 1.50 10.5 0.19 7.35 12.2 0.18 2.92 0.17 99.1
R501-06 -129.9867 45.9206 49.9 14.4 1.50 10.5 0.19 7.31 11.9 0.18 2.92 0.16 99.0
R501-07 -129.9818 45.9204 49.7 14.4 1.49 10.5 0.19 7.32 12.2 0.18 2.91 0.16 99.0
R501-08 -129.9777 45.9202 49.6 14.6 1.48 10.3 0.21 7.68 12.3 0.19 2.84 0.14 99.3
R501-09 -129.9777 45.9201 49.8 14.3 1.51 10.6 0.21 7.17 12.0 0.18 2.89 0.14 98.8
R502-34 -130.0163 46.0188 49.8 14.4 1.52 10.6 0.19 7.14 12.2 0.19 2.88 0.15 99.1
R543-01 -129.9817 45.9334 49.8 13.7 1.89 11.5 0.22 7.03 10.7 0.22 2.79 0.23 98.2
R543-08 -129.9823 45.9333 49.7 14.6 1.53 10.4 0.21 7.58 12.1 0.19 2.87 0.15 99.4
R545-02 -130.0136 45.9336 48.9 14.5 1.54 10.5 0.20 7.62 12.1 0.18 2.87 0.16 98.6
R545-05 -130.0141 45.9333 50.0 14.6 1.51 10.4 0.20 7.49 12.2 0.18 2.89 0.15 99.7
R546-02 -130.0037 45.8631 50.1 14.4 1.53 10.6 0.20 7.38 12.3 0.19 2.85 0.15 99.6
R547-12 -129.9823 45.9333 49.8 14.6 1.50 10.4 0.19 7.59 12.1 0.19 2.88 0.14 99.4
R547-23 -129.9819 45.9332 49.8 14.6 1.51 10.4 0.18 7.55 12.1 0.19 2.88 0.15 99.3
R547-31 -129.9816 45.9334 49.8 14.6 1.52 10.4 0.19 7.55 12.1 0.19 2.87 0.15 99.3
R547-35 -129.9799 45.9261 50.1 14.4 1.52 10.4 0.19 7.49 12.1 0.19 2.89 0.13 99.4
R547-37 -129.9805 45.9261 50.2 14.4 1.37 10.1 0.20 7.68 12.6 0.12 2.70 0.11 99.5
R548-01 -129.9849 45.9462 50.0 14.5 1.50 10.5 0.21 7.16 12.1 0.19 2.87 0.15 99.3
R548-02 -129.9894 45.9162 49.2 14.3 1.50 10.4 0.21 7.34 12.0 0.18 2.88 0.15 98.1
R548-07 -129.9892 45.9175 49.3 14.3 1.47 10.5 0.20 7.32 12.1 0.19 2.88 0.14 98.3
R548-09 -129.9886 45.9188 49.4 14.5 1.52 10.5 0.20 7.32 12.1 0.19 2.89 0.17 98.7
R548-10 -129.9886 45.9188 49.4 14.5 1.52 10.4 0.20 7.28 12.2 0.19 2.88 0.15 98.7
R548-12 -129.9886 45.9188 49.9 14.5 1.52 10.4 0.21 7.14 12.2 0.19 2.86 0.16 99.1
R549-05 -129.9816 45.9334 49.2 14.3 1.53 10.3 0.18 7.30 12.1 0.19 2.89 0.16 98.1
R549-13 -129.9818 45.9330 49.7 14.5 1.51 10.5 0.19 7.30 12.1 0.19 2.88 0.14 98.9
R549-16 -129.9830 45.9287 49.0 14.3 1.52 10.4 0.20 7.35 12.0 0.18 2.84 0.14 97.9
R551-13 -129.9816 45.9334 49.0 14.5 1.52 10.5 0.20 7.49 12.0 0.19 2.84 0.14 98.4
R551-17 -129.9882 45.9227 49.4 14.7 1.51 10.3 0.20 7.37 12.1 0.19 2.82 0.15 98.7
R551-23 -129.9892 45.9175 49.0 14.6 1.52 10.4 0.21 7.25 12.1 0.20 2.82 0.14 98.3
R551-34 -129.9931 45.9175 49.7 14.7 1.50 10.3 0.21 7.32 12.1 0.20 2.84 0.14 98.9









Table 2-1. Continued.
Sample Latitude
R552-01 -129.9882
R552-02 -129.9932
R552-03 -129.9898
R552-06 -129.9761
R552-07 -129.9777
R552-08 -129.9761
R554-02 -129.9982
R554-03 -129.9983
R554-04 -130.0003
R554-05 -130.0015
R554-06 -130.0015
xl 1720-1B wr -130.0139
xl 1721-2B -130.0140
xl 1722-3 -130.0158
xl 1723-1B -130.0142
xl 1723-1A -130.0142
xl 1724-4A -130.0142
xl 1726-1.8 -130.0139
xl 1726-2 -130.0083
xl 1726-6 -130.0138
xl 1727-8 -130.0167
xl 1728-4B -130.0142
xl 1728-6.7 -130.0140
xl 1729-1 -130.0135
xl 1729-8 -130.0135
xl 1730 2-3 -129.9834
xl 1730-4 -129.9839
xl 1730-6-7 -129.9827
xl 1730-MC 5 -129.9835
xl 1731- 6.7B2 -130.0284
xl 1731-4 -130.0283
xl 1731-6.7R1 -1TV 30R2


SiO2 Al2O3 TiO2 FeO MnO MWO CaO K20 Nal) P205 Total


Latitude
45.9227
45.9168
45.9174
45.9122
45.9191
45.9120
45.8739
45.8739
45.8686
45.8665
45.8665
45.9336
45.9337
45.9337
45.9333
45.9333
45.9333
45.9336
45.9250
45.9336
45.9375
45.9333
45.9333
45.9337
45.9337
45.9371
45.9409
45.9396
45.9427
45.9885
45.9900
45.9887


1.43 10.4 0.20 7.25 11.8 0.18 279 016 984


45.9887


500 142


49.6 14.6
49.6 14.6
49.7 14.4
49.6 14.5
48.8 14.8
49.7 14.6
49.9 15.0
48.5 14.4
49.3 14.5
49.2 14.4
49.1 14.3
49.9 14.7
50.1 14.6
50.2 14.5
50.2 14.5
50.1 14.5
49.9 14.6
50.1 14.5
49.2 14.6
50.2 14.5
49.1 16.6
49.7 15.5
50.0 14.4
50.0 14.5
50.2 14.6
50.1 14.5
50.0 14.6
50.1 15.0
50.0 14.5
49.7 14.3
50.0 14.5
50.0 14.2


1.50 10.4 0.20 7.19 12.0 0.19 2.89 0.15 98.8
1.49 10.3 0.20 7.31 12.1 0.19 2.84 0.15 98.8
1.49 10.5 0.20 7.17 12.0 0.19 2.91 0.17 98.8
1.53 10.5 0.20 7.12 12.1 0.19 2.87 0.14 98.8
1.46 10.1 0.20 7.41 12.1 0.19 2.79 0.13 98.0
1.46 10.1 0.21 7.32 12.1 0.18 2.81 0.13 98.5
1.32 10.0 0.20 7.85 12.4 0.13 2.73 0.13 99.7
1.55 10.6 0.20 7.76 12.6 0.15 2.77 0.14 98.7
1.53 10.6 0.19 7.58 12.1 0.19 2.86 0.15 98.9
1.53 10.5 0.21 7.54 12.2 0.19 2.84 0.16 98.8
1.52 10.6 0.18 7.54 12.0 0.19 2.84 0.16 98.5
1.41 10.4 0.20 7.45 12.2 0.17 2.84 0.12 99.4
1.52 10.5 0.20 7.24 12.1 0.20 2.93 0.14 99.5
1.50 10.6 0.20 7.35 11.8 0.19 2.92 0.15 99.4
1.53 10.6 0.20 7.35 11.7 0.19 2.90 0.15 99.3
1.53 10.5 0.22 7.33 12.1 0.20 2.90 0.14 99.5
1.53 10.6 0.22 7.37 12.0 0.19 2.92 0.16 99.4
1.54 10.6 0.20 7.32 12.1 0.19 2.90 0.14 99.5
1.52 10.6 0.22 7.42 11.8 0.18 2.94 0.15 98.7
1.51 10.6 0.21 7.37 11.8 0.20 2.90 0.15 99.4
0.91 8.6 0.17 9.36 12.0 0.09 2.31 0.07 99.2
1.20 9.7 0.18 8.29 11.9 0.13 2.66 0.12 99.4
1.50 10.4 0.21 7.32 11.7 0.19 2.92 0.16 98.9
1.55 10.5 0.20 7.31 12.1 0.19 2.94 0.16 99.5
1.49 10.5 0.21 7.39 11.7 0.19 2.91 0.14 99.3
1.51 10.5 0.22 7.47 11.7 0.19 2.92 0.16 99.4
1.55 10.6 0.22 7.25 12.1 0.19 2.93 0.15 99.5
1.23 9.9 0.20 8.12 12.0 0.11 2.64 0.11 99.4
1.51 10.5 0.19 7.38 12.1 0.18 2.95 0.13 99.5
1.48 10.5 0.20 7.22 11.7 0.19 2.85 0.15 98.3
1.49 10.5 0.21 7.32 12.1 0.19 2.91 0.14 99.5
1.43 10.4 0.20 7.25 11.8 0.18 2.79 016 984









Table 2-1. Continued.
Sample Latitude Latitude SiO2 A1203 TiO2 FeO MnO MgO CaO K20 Na20 P205 Total
xl 1731-6-1 -130.0281 45.9886 50.2 14.5 1.53 10.6 0.19 7.35 11.7 0.19 2.97 0.15 99.4
xl 1732-1-8A -130.0172 45.9323 50.3 14.5 1.51 10.5 0.20 7.30 11.7 0.19 2.92 0.14 99.2
x11732- 4.8B -130.0154 45.9312 50.0 14.4 1.54 10.8 0.22 7.01 11.9 0.11 3.02 0.15 99.0
xl 1732-1.8B -130.0170 45.9322 50.1 14.5 1.52 10.5 0.21 7.27 12.1 0.18 2.93 0.16 99.5
xl 1732-2.3A -130.0164 45.9318 49.9 14.6 1.43 10.3 0.20 7.35 11.7 0.18 2.80 0.15 98.6
x11732-MCB -130.0125 45.9314 50.5 14.2 1.63 11.3 0.22 6.93 11.3 0.22 3.18 0.17 99.8
xl 1733-4 -130.0138 45.9336 50.1 14.5 1.51 10.5 0.21 7.38 12.1 0.19 2.93 0.16 99.5




















Figure 2-3.


Major element variation for samples collected from the Axial
Segment during four NeMO cruises, with an additional 28 samples
collected in 1986 from the Axial Seamount caldera region by the
Pisces IV submersible. Comparisons with the Juan de Fuca Ridge
(JdFR) and Cobb-Eickelberg (C-E) fields shows that the study area
samples have the same relative enrichments and depletions as the C-
E seamounts, although they still generally reside within the JdFR
fields. High alkalis and K20/TiO2 are particularly diagnostic of the
C-E and Axial Segment lavas. Liquid lines of descent were generated
using the methods of Weaver and Langmuir (1990), which show that
fractional crystallization of a single mafic parental magma cannot
explain the observed major element variation. Data for the C-E
seamounts field are from Desonie and Duncan (1990), and for the
JdFR field: (Cleft Segment) this study; Eaby et al., 1984; Smith et al.,
1994, (Vance Segment) this study; Eaby et al., 1984; M. Perfit
unpublished data, (CoAxial Segment) Eaby et al., 1984; Smith, 1999
(Cobb Segment), Eaby et al., 1984; Van Wagoner and Leyboume.





















2.2

2.0

d 1.8
F
a 1.6

S1.4

1.2

1.0


A
'.* .' .

C-E '.
x x a x..


-4.5 x.. X'.





x

x 0\
x
B


==" x
x : *.

C-E "*. x


14

13 '.. "

12 :. .,
11
% ,

.0 ... .. ""'"
"8K, y
9 C-E : < "
.,..... x


6 7 8
Wt. % MgO


12.5

12.0


11.5


11.0

10.5




0.25


0.20

0.15


0.10

0.05


9 10


D

C-E "x """
... ... ..


. C-E E
0





%x xx
"*'. x^ **.. *.
a .* x i*..
xl x "
..,< Egul-.
0 II.0 x "
....... .;o^X

JdFR .



F

C-E '
I .K

8, xx'
x x ".


X JdFR
x x ,
**. *,', x


5 6


7 8
Wt % MgO


Z.Z


-i -j



















Figure 2-3. Continued.












1.00
G
0.95

0.90
^,, ,,.**sA-' X'
0 X



0.75j: ., ; JdPR
O.O "**-.......; *x.... \
0.65 "






H
2 0.0 C -E
:'.~ xx .






0 15.0 .. a R







Cl 10.0 *' x^ ^ x><
aE :. C X x x








5.0 ....* JdFR"" x
0.70 .................. ... x 0...


















I
4.5
H









20 4.0 C-E
( 15.0 '...x







Ix







0 3.5





2.0
5 6 7 8 9 10
Wt. % M O
5 .0" . . . . ..R . . .




4.5

,t4.0 C-E

o 3.5" x

+ 3.0
X
2.5 ". d R "
JdF... ...-
2.0
5 6 7 8 9 10
Wt. % MgO








FeO, TiO2, MnO, Na20, K20, and P205, and decreasing A1203 and CaO with

decreasing MgO (Figure 2-3). In spite of the potential influence of the Cobb

hotspot, most of the basalts recovered from the study area contain relatively low

K20 contents and can be classified as mid-ocean ridge basalts (MORB) (Rhodes et

al., 1990; Desonie and Duncan, 1990), with the exception of the four tholeiitic

andesites that were recovered from the eastern side of the north rift zone (RC-49,

-99, -100, and -101). These andesites are extremely rare along the JdFR, having

only been recovered in one other area, near the ridge-transform intersection at

the southern end of the Cleft Segment (Perfit et al., 2001).

Compositional fields for lavas from the Cobb-Eickelberg seamounts and

basalts from JdFR segments located to the south of the Cobb Offset (located

between the Endeavor and Cobb Segments) are shown in Figure 2-3, to evaluate

the geochemical influence of the Cobb hotspot on lavas erupted from the Axial

Segment. The Endeavor and West Valley Segments are not included in the JdFR

comparative group due to the presence of anomalously enriched lavas including

E-MORB on these northernmost segments (Karsten et al., 1990; Van Wagoner

and Leybourne, 1991). Axial Segment basalts generally fall within the JdFR

fields, but are among the most enriched compared with the Cleft, Vance,

CoAxial, and Cobb Segments, in CaO, Na20, and K20, and notably depleted in

FeO and TiO2 at comparable MgO contents. These departures from the JdFR

trends reflect the same relative enrichments and depletions at given MgO

contents that are observed for these elements in the C-E basalts, an indication of








the influence of the Cobb plume on the Axial Segment basalts (Figure 2-3). The

observed enrichment in total alkalis (Na20+K20) is one of the most prominent

geochemical expressions of the Cobb hotspot (Desonie and Duncan, 1990;

Rhodes et al., 1990), and Axial Segment lavas reflect this with values up to 3.4

wt.% (Figure 2-3).

Liquid line of descent (LLD) calculations (Weaver and Langmuir, 1990)

produce major element variations that follow the general trend of the data

(Figure 2-3). The models predict olivine plus plagioclase fractionation for even

the most mafic samples in the suite, and clinopyroxene is expected to fractionate

in the more evolved magmas. LLD models were run at 1 Kbar, an appropriate

pressure considering seismic tomographic data identified a large magma storage

body at about 2.25-3.5 km depth under Axial Seamount (West et al., 2001).

Similar fractional crystallization models have been proposed to explain major

element variations in basalts from other segments of the JdFR (Smith et al., 1994;

Sours-Page et al., 1999). Inflections observed in the diagrams for CaO variation

and CaO/A1203 (Figure 2-3) correspond to the appearance of clinopyroxene on

the liquidus, although this phase is absent in thin section except in the

groundmass. Clear control of basaltic liquid fractionation by clinopyroxene with

scarce representation of the phase in thin section is relatively common for

evolved basalts and has appropriately been termed the pyroxenee paradox' (e.g.

Grove et al., 1992; Maclennan, et al., 2001).








Although LLD calculations produce major element variations that

generally correspond to the observed trends, a single liquid line of descent

cannot explain the wide scatter in major element concentrations for the suite of

lavas from the study area (Figure 2-3). Instead, the basalts appear to have

originated and evolved from a broad range of parental magmas. This is

particularly evident in the variation observed in K20/TiO2 (Figures 2-3 and 2-5),

a ratio that will show only a minor increase with crystal fractionation in a

cogenetic suite, but varies from 4.54 to 15.82.

The basalts in Table 2-1 have been grouped based on their location, with

"Seamount Group" samples taken from the Axial Seamount caldera and flanks,

where bathymetric contours are roughly concentric to the caldera and lavas have

flowed downslope from the summit region (Figure 2-1). "Ridge Group" samples

are those taken from the rift zones to the north and south of Axial Seamount on

the Axial Segment, where the volcanic morphology and bathymetric contours are

clearly dominated by parallel-trending volcanic ridges. This grouping based on

morphology does not coincide with any abrupt changes in geochemistry, but

suffices to separate lavas that may have erupted directly from Axial Seamount

and those that were erupted along the "ridge" portion of the Axial Segment. The

boundary between the Seamount and Ridge Groups is roughly demarcated by

the 1600 m isobath contour.

Seamount Group lavas are generally more primitive than the Ridge

Group, with MgO values ranging from 6.7 to 9.4 wt.%, and the vast majority (139








of 155) falling in a small range of lower values between 7.0 and 7.6 wt. % These

basalts can be classified as ferrobasalts, with most FeO concentrations in excess of

10 wt. %. The largely restricted compositional variation (average MgO 7.36 wt.

%, s.d. 0.26) possibly reflects long-term storage and homogenization of magmas

in a large chamber or lens under Axial Seamount (Perfit et al., 1988; Rhodes,

1988; West, 2001). However, six mafic basalts (MgO > 7.75 wt.%) were collected

in and near the caldera, suggesting that some of the seamount magmas

circumvent this homogenization process. The low standard deviations and

implied geochemical homogeneity of the Seamount Group are likely due in part

to a sampling bias, as many of the basalts from Axial Seamount were collected by

submersibles in confined sampling areas, and numerous samples may come from

individual lava flows. However, when this bias is removed and only samples

from diverse parts of the caldera and flanks are examined, there is still very little

geochemical variation, and a high degree of homogeneity is still demonstrable.

Basalts from the Ridge Group have significantly higher compositional

variability than the Seamount Group, with similar ranges of MgO (9.37 to 6.35

wt. %,), but substantially higher standard deviations (e.g., average MgO 7.20; s.d.

0.57, not including the andesites) and a notable lack of homogeneity. At a given

distance from the seamount, however, the variability is similar to that observed

in the Seamount Group (Figure 2-4a). The overall greater chemical variability in

the Ridge Group is largely due to gradients in major element contents with

distance from Axial Seamount, with more primitive basalts found close to Axial


















Figure 2-4. (A) Spatial variation in MgO content, showing a distinct trend of
decreasing MgO with distance from the Axial Seamount caldera in
the rift zones of the Axial Segment. Correlative increases and
decreases in other major element concentrations suggest this is a
fractional crystallization trend that increases with distance from the
hotspot along the ridge. A few basalts have more mafic compositions
(>7.75%) and do not fall along the main trend. (B and C) The
anticipated enrichment in incompatible trace element compositions as
a result of this fractionation with distance is not observed. The ratios
of Zr/Y and Sr show decreases with distance from the hotspot. The
peaks in the trends appear to be slightly offset to the north of the
Axial Seamount caldera.








37



Suth RftZ Semo NomtLh Rift Zone

10.V




9




7 -.-*,
0
0
S 8 %~




S 7 .4 N 0
O.




6




4.0







3.5 0 0

0 008
o g
s 0 5* 5.^ -

N * .* 0













3.0 *
00



0



0
160
3*













0
0
00
00 00

00
150 0 0
e o
to- 0 00












3.0
140
0.
&.

130



120











110

45.6 45.7 45.8 45.9 46.0 46.1 46.2 46.3


Latitude








Seamount, and increasingly more evolved lavas at greater distance from the

hotspot (Figure 2-4a). The corresponding decreases observed in A1203 and CaO

and increases in TiO2, FeO, and K20 suggest that this spatial pattern is controlled

by fractional crystallization. Scattered among this gradient are a few higher-

MgO basalts (>7.75 wt. %) that clearly do not fall along the regional trend. Some

of these mafic samples were collected from small volcanic cones in the rift zones,

but others were collected in areas with no anomalous bathymetric features.

Trace Element Geochemistry

Trace element concentrations in glasses were measured by x-ray

fluorescence (XRF) spectroscopy and inductively coupled plasma mass

spectrometry (ICP-MS). The trace element concentrations in representative

basalt samples are presented in Table 2-2. The homogeneity observed in major

element chemistry of the Seamount Group is also observed in the trace element

compositions, with a largely confined grouping of points on trace element

diagrams (Figure 2-5). Sr concentrations range from 145 to 156 ppm Y ranges

from 27 to 32 ppm, Sr/Y ratios range from 5.5 to 4.6, CeN/YbN (chondrite

normalized) from 0.9 to 1.1, and K20/TiO2 ratios range from 6.9 to 15.8, all

generally intermediate between the compositions of the C-E Seamounts and the

most depleted compositions from the JdFR. Seamount Group Zr/Y ratios range

from 3.2 to 3.6, and Zr/Nb from 18.6 to 23.6, values that slightly higher than the

ranges of basalts from the JdFR (Figure 2-5), resulting from similar Zr and lower

Y concentrations in Axial Seamount basalts.










Table 2-2. Representative Seamount Group trace elements; analyzed by XRF1 and ICP-MS2.
Sample R460-06 R461-16 R461-25 R461-26 R462-08 R462-15 R467-01 R471-04 R473-21 R474-02 R474-03 R476-02
XRF (ppm)
Zr 99.1 98.5 71.1 97.1 98.7 98.4 98.8 98.5 97.0 94.7 100.0 98.8
Sr 149.6 150.0 116.8 149.6 149.9 148.5 150.1 151.2 146.7 148.0 149.7 150.2
Y 29.6 30.1 26.8 29.0 29.9 31.1 29.8 30.2 30.1 28.9 30.0 29.8
Nb 4.6 4.6 1.7 4.5 4.9 4.6 5.3 4.9 4.7 4.7 4.8 4.9
Sc 60.0 66.7 55.2 60.6 54.7 59.4 57.1 56.9 65.0 63.7 59.6 61.1

ICP-MS (Rppm)
Cr 317 279
Ni 61 46
Rb 1.1 2.1
V 263 295

La 2.8 4.7
Ce 8.7 13.6
Pr 1.5 2.1
Nd 8.2 11.0
Sm 2.7 3.5
Eu 1.0 1.3
Gd 3.9 4.7
Tb 0.7 0.8
Dy 4.7 5.2
Ho 1.0 1.1
Er 2.7 3.0
Tm 0.4 0.5
Yb 2.6 3.0
Lu 0.4 0.5

Sr/Y 5.1 5.0 4.4 5.2 5.0 4.8 5.0 5.0 4.9 5.1 5.0 5.0
Zr/Y 3.3 3.3 2.7 3.3 3.3 3.2 3.3 3.3 3.2 33 33 3.3
Y/Nb 6.5 6.6 15.9 6.5 6.1 6.7 5.6 6.1 6.5 6.1 6.2 6.1
Zr/Nb 21.7 21.5 42.0 21.7 20.2 21.2 18.6 20.0 20.8 20.1 20.7 20.2
CeN/YbN 0.8 1.0
1 XRF analyses performed at the University of Florida, Dept. of Geological Sciences.
2 ICP-MS analyses performed at the Geological Survey of Canada.










Table 2-2. Continued (Seamount Group).
Sample R479-15 R488-16 R491-22 R497-20 R501-02 R501-07 R501-09 R501-10 R501-12 R548-02 R552-02 R552-07


XRF (ppm)
Zr 93.3
Sr 145.4
Y 29.2
Nb 4.7
Sc 58.9

ICP-MS (ppm)
Cr 295
Ni 47
Rb 2.1
V 290


La
Ce
Pr
Nd
Sm
Eu
Gd
Tb
Dy
Ho
Er
Tm
Yb
Lu

Sr/Y
Zr/Y
Y/Nb
Zr/Nb
C.P /Yh.,


4.9
13.0
2.2
12.0
3.5
1.4
4.8
0.8
5.3
1.2
3.1
0.5
3.2
0.5

5.0
3.2
6.2
19.9
noQ


98.7
149.4
29.3
4.8
64.2


282
44
2.1
297

4.8
13.0
2.1
11.0
3.8
1.4
5.0
0.9
5.5
1.2
3.1
0.5
3.2
0.5

5.1
3.4
6.2
20.7
0.9


97.2
156.9
28.5
5.0
60.8


302
55
2.3
281

5.1
14.0
2.2
11.0
3.5
1.4
4.7
0.8
5.2
1.1
2.9
0.5
3.1
0.5

5.5
3.4
5.7
19.3
1.0


116.3
148.5
32.3
5.3
56.2


276
44
2.3
287

5.5
15.2
2.3
12.0
3.8
1.4
5.1
0.9
5.4
1.2
3.2
0.5
3.3
0.5

4.6
3.6
6.1
21.9
1.1


96.5
154.3
28.3
4.5
55.5


295
51
2.1
284

4.7
13.0
2.1
11.0
3.6
1.4
4.8
0.8
5.3
1.2
3.1
0.5
3.2
0.5

5.4
3.4
6.3
21.3
n.9


98.6
149.5
30.1
4.7
61.2


4.8
13.0
2.1
11.0
3.6
1.4
4.7
0.9
5.4
1.2
3.1
0.5
3.2
0.5

5.0
3.3
6.4
21.0
no


97.3
147.2
30.8
4.4


101.3
153.2
30.9
4.3
66.3


199
44
2.1
293

4.9
14.0
2.1
11.0
3.8
1.4
4.8
0.9
5.4
1.2
3.2
0.5
3.3
0.5


95.4
154.3
28.6
4.7
61.6


298
52
2.1
283

4.7
13.0
2.2
11.0
3.6
1.3
4.6
0.9
5.2
1.2
3.1
0.5
3.1
0.5


97.2

29.0
5.3


100.8 94.9


3.2 3.3 3.3 3.3 3.6 3.4
6.9 7.2 6.1 5.5 5.0 5.1
21.9 23.6 20.2 18.3 18.0 17.3
1 n 1 n


1.n 09










Table 2-2. Continued (Ridge Groupl).
Sample RC-11 RC-102 RC-107 RC-108 RC-55 RC-60 RC-70 RC-75 RC-79 RC-81 99-RC-85


XRF (ppm)
Zr
Sr
Y
Nb
Sc

ICP-MS (ppm)
Cr
Ni
Rb
V

La
Ce
Pr
Nd
Sm
Eu
Gd
Tb
Dy
Ho
Er
Tm
Yb
Lu

Sr/Y
Zr/Y
Y/Nb
Zr/Nb
CN,/YbN


96.5
150.0
29.2
4.5
58.8


309
53
2.0
291

4.8
13.0
2.1
11.0
3.4
1.3
4.8
0.8
5.1
1.1
3.0
0.5
3.1
0.5

5.1
3.3
6.5
21.3
1.0


97.1
146.7
30.1
3.8
63.9


98.0
147.9
29.7
4.1
53.1


331
42
2.2
295

4.8
13.2
2.1
11.3
3.4
1.3
4.7
0.8
53
1.2
3.1
0.5
3.1
0.5


4.3
12.7
1.9
10.8
3.5
1.3
4.8
0.9
5.3
1.2
3.2
0.5
3.2
0.5

4.9
3.2
8.0
25.7
0.9


94.9
143.5
30.3
4.1
64.5


308
55
1.7
286

4.3
12.0
2.0
11.0
3.4
1.3
4.7
0.8
5.2
1.1
3.0
0.5
3.1
0.5


29.2
5.2


78.0
110.2
27.2
3.6


89.2
141.8
28.6
3.5
57.8


282
48
1.8
276

4.3
12.0
2.0O
10.0
3.2
1.3
4.5
0.8
5.1
1.1
3.0
0.5
3.3
0.5

5.0
3.1
8.1
25.2
0.8


0.8 UvI ut


873
142.6
27.0
3.5
60.7


5.3
3.2
7.8
25.2


91.2
144.0
29.5
3.9
47.0


300
51
1.9
278

4.3
12.4
1.9
10.1
3.4
1.3
4.5
0.8
4.9
1.1
2.9
0.4
2.9
0.5


3.1
8.3
25.5


U.-f I II1


0.9










Table 2-2. Continued (Ridge Group).
Sample RC-86 RC-88 RC-89 RC-91 R465-01 R465-02 R494-04 R494-05 R494-08 R554-04 R554-06

XRF (ppm)
Zr 97.6 93.1 97.0 90.9 97.5 98.4 98.4 98.0 97.9 97.3 98.8
Sr 137.1 145.7 141.0 148.5 148.8 149.0 149.7 149.0
Y 31.5 30.8 29.6 29.2 29.9 30.2 29.4 30.2 29.9 29.1 29.5
Nb 5.5 4.3 4.8 3.6 4.7 4.6 4.8 4.3 4.4 5.2 5.6
Sc 57.6 58.7 55.0 60.1 53.2 61.6 62.6 67.4

ICP-MS (ppm)
Cr 262 287 330 285 280 282 281
Ni 48 44 53 47 44 45 45
Rb 2.0 2.2 1.6 2.0 2.1 2.1 2.0
V 289 296 273 290 294 297 294

La 4.3 5.1 4.0 4.6 4.8 4.9 4.9
Ce 12.6 13.7 12.0 13.1 13.0 14.0 13.3
Pr 2.0 2.2 1.9 2.1 2.2 2.2 2.1
Nd 10.5 11.3 10.2 10.7 11.0 11.0 11.4
Sm 3.5 3.5 3.3 3.3 3.7 3.5 3.6
Eu 1.3 1.3 1.2 1.4 1.4 1.3 1.4
Gd 4.9 4.9 4.5 4.6 4.8 4.8 4.8
Tb 0.8 0.8 0.8 0.8 0.9 0.9 0.9
Dy 5.2 5.4 5.0 5.2 5.4 5.4 5.4
Ho 1.1 1.2 1.1 1.1 1.2 1.2 1.2
Er 3.1 3.2 2.9 2.9 3.1 3.2 3.1
Tm 0.5 0.5 0.5 0.5 0.5 0.5 0.5
Yb 3.1 3.1 2.9 2.9 3.3 3.3 3.2
Lu 0.5 0.5 0.5 0.5 0.5 0.5 0.5

Sr/Y 4.5 4.9 4.8 5.0 4.9 5.1 4.9 5.0
Zr/Y 3.1 3.0 3.3 3.1 3.3 3.3 3.3 3.2 3.3 3.3 3.4
Y/Nb 5.7 7.2 6.1 8.0 6.3 6.5 6.1 7.0 6.8 5.6 5.3
Zr/Nb 17.8 21.7 20.1 24.9 20.7 21.2 20.5 22.6 22.4 18.7 17.6
CeM/YbN 0.9 1.0 0.9 1.0 0.9 1.0 0.9








Trace element compositions in basalts from the Ridge Group are similar to

those from the Seamount Group, but are generally less enriched in incompatible

elements (not including the 4 andesites), creating a trend in the Axial Segment

data (Figure 2-5) toward the generally more depleted JdFR basalt compositions.

Overall, Zr/Y ranges from 3.0 to 3.3, Zr/Nb ranges from 20 to 25, CeN/YbN

ranges from 0.9 to 1.0, and K20/TiO2 ratios range from 7.0 to 12.3. Although

there is clear evidence in the major element data for increased fractionation of

magmas in the rift zones with distance from Axial Seamount (Figure 24a), the

expected corresponding enrichment in incompatible element abundances and

ratios is not observed. Instead, there is a moderate decreasing gradient in

K20/TiO2, Zr/Y, and Sr/Y ratios with increasing distance from the hotspot in

the Ridge Group (Fig 4b).

The relative enrichments and depletions in trace element compositions in

Axial Segment lavas relative to the neighboring JdFR generally reflect the

enrichments and depletions of the C-E Seamounts, similar to the major element

behavior. C-E Seamount Sr and Nb concentrations are distinctly higher than

those from the JdFR, and Axial Segment basalts are intermediate between the

two (Rhodes et al., 1990; Figure 2-5). Conversely, concentrations of Y are lower

in C-E basalts than the JdFR, and Axial Segment Y values are likewise at an

intermediate value and lower. The only exception is Sc, as Axial Segment basalts

have higher Sc concentrations than both the JdFR and C-E Seamounts. The C-E

basalts are more evolved than those from the JdFR, and increased fractionation



















Figure 2-5. Axial Segment basalts are enriched in Sr, Sc, and Nb and depleted in
Y relative to the rest of the Juan de Fuca Ridge, reflecting the same
relative enrichments of the Cobb hotspot. The exception is Sc, which
is lower for the C-E Seamounts and may reflect more progressed
fractionation of clinopyroxene. (B) The Axial Segment basalts are
distinctly intermediate between the JdFR and C-E compositions.
However, some of the more mafic (MgO >7.75 wt. %) basalts fall
within the JdFR field, suggesting that these represent the indigenous
Axial Segment MORB.






























3:

'0



An


tidd 3s


2f 2 "n 4 A A R a


tudd A


utdd az


uodd jS



















Figure 2-5. Continued.




















s Ridge Group |
o Seamount Group |


S
C-E
*. S. .St.
.... .........

0
0 o.
1,,1.* *q o
~00
JdFR 0

0 0


" *- -.... ........ .. . .


1 2 3


4 5 6 7 8 9


Sr/Y


0.12




0.10


0.08




0.06




0.04




0.02








of clinopyroxene may have led to a substantial decrease in Sc, which is

compatible in this phase.

The high-MgO basalts found scattered along the Axial Segment in both

the Ridge and Seamount Groups exhibit distinct depleted trace element

compositions similar to JdFR MORB (Table 2-2). Representative samples (R461-

25 from the Seamount Group and RC-70 from the Ridge Group) have much

lower Sr, Zr, Nb and REE concentrations than the more common lower-MgO

basalts in suite, and have depleted LREE patterns (CeN/YbN = 0.76) typical of N-

MORB and similar to those from the CoAxial and Vance Segments (Figure 2-6).

The typical lower-MgO basalts in the suite have relatively flat REE patterns, and

average about 15 times chondritic abundances. The flat REE pattern in these

basalts results from an enrichment in LREE relative to N-MORB. Despite the

clear evidence for plagioclase fractionation in the suite, negative Eu anomalies

are observed only in the andesites, which have flat REE patterns and

concentrations that are 50 to 100 times chondritic. Some REE patterns show

minor depletions of the HREE heavier than Dy, suggesting the presence of

residual garnet in the melt source.

As a trace element compatible in olivine, Ni should show a decrease with

MgO content and fractionation in a cogenetic suite, but considerable scatter is

evident and only a general correlation exists (Table 2-2). Similarly, a general

decrease in Sc and Cr, along with decreasing CaO/A1203 supports the likelihood

of clinopyroxene fractionation in more evolved samples, but again scatter



















Figure 2-6. Axial Segment basalts have flat rare earth element patterns that are
distinctly enriched relative to the typical depleted MORB patterns
shown by basalts from the neighboring CoAxial Segment. The
MORB-like pattern displayed by the depleted basalt from the Axial
Segment (R461-25) suggest these depleted types have a higher
proportion of a MORB mixing end-member.









40

30



20 ......... ...


^I


u-
0 -- --g



u




Enriched C-E Seamount (DH10-7)
Axial Segment and Seamount
- - Vance Segment
- - CoAxial Segment

La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu


REE








suggests that fractionation is not the sole process controlling their abundances.

Sr is compatible in plagioclase but incompatible in olivine and clinopyroxene,

and should remain fairly constant during fractionation in cogenetic suites (D-1).

However, there is a clear decrease in Sr concentrations with MgO content in the

Axial Segment basalts, evidence for possible mixing between enriched and

depleted components (Figure 2-4c).

Isotope Geochemistry

To further constrain the mantle source characteristics and discriminate

between the compositions of possible mixing end-members for Axial Segment

lavas, 87Sr/86Sr, 2D6 pb/204pb, 207 pb /204pb, and 2m8 pb/204pb isotopic ratios of 11

basalts from the Seamount and Ridge Groups were measured (Table 2-3).

Radiogenic isotopic ratios are effective tools because they are not changed by

melting or most shallow crustal processes. Sr isotopic ratios for the Axial

Segment fall along a regional trend for the JdFR, which decreases from a high

value on the Cleft Segment, declines to a minimum on the CoAxial Segment,

then increases again toward the Cobb Offset (Eaby et al., 1984; Smith, 1999;

Embley, 2000). Axial Segment radiogenic isotopic ratios do not depart from the

regional JdFR trend largely due to the relatively unradiogenic and MORB-like

isotopic composition of the Cobb hotspot. Samples from the C-E Seamounts

have87 Sr /8Sr ratios ranging from 0.70245-0.7026 and 143/144Nd ranging from

0.513109-0.513204 (Hegner and Tatsumoto, 1987; Desonie and Duncan, 1990).

The Sr and Nd isotopic compositions for basalts from the Axial Segment (Eaby et











Table 3. Radiogenic Isotopic ratios for Axial Segment, Juan de Fuca Ridge, and
Cobb-Eickelberg Basalts. Sources noted in parentheses.
Location 8Sr/8srl 143Nd/144Nd 06pb/Mpi 20 Pb/2D4 Pb2 =M/204 p b2

Axial Segment (this study)
RC-11 0.702515
RC-55 0.7024%
RC-64 0.702538
RC-75 0.702504
RC-82 18.501 15.442 37.912
RC-89 0.702548
RC-98 18.565 15.404 37.991
RC-100 0.702614 18.548 15.448 37.925
R548-2 0.702510
R497-20 0.702509 18.629 15.457 37.990

Axial Segment (Eabv et al., 1984)
Y74-1-3-33 0.702530
Y74-1-3-41 0.702560
TT152-77-6 0.702440
T1152-77-7 0.702520
Y74-1-1-7 0.702450
Y74-1-1-34 0.702520
TT152-55-25 0.702570
TT170-5 0.702460
TT152-53-6 0.702500
TT152-61-1 0.702490

Axial Segment (Rhodes. 1990)
Axial D55-1 0.702370 0.513170
Axial 47-3 0.702430 0.513120
Axial SRZ D77-7 0.702330 0.513240

Cobb-Eickelberg Seamounts (Desonie and Duncan, 1990; Church and Tatsumoto, 1975)
Eikelberg DHl-1 0.702467 0.513109 18.663 15.477 37.895
Eikelberg DH2A 0.702507 0.513122 18.624 15.491 37.985
Eilkelberg DH28 0.702518 0.513128 18.609 15.521 38.028
Cobb CB-1 0.702454 0.513135 18.823 15.494 38.233
Warwick DH4-4 0.7025% 0.513109 18.620 15.498 37.985
Cobb C-1 0.702490 0.513132 18.809 15.479 38.140
Cobb C-1 0.702560 0.51319 18.377 15.475 37.940
Cobb C-3 0.702570 0.513204 18.423 15.480 37.960

IdFR (this study)
RC-94 18.679 15.476 38.043

IdFR-Cleft Segment (Hegner and Tatsumoto. 1987)
alv 1455-2r 0.702524 0.513148 18.452 15.477 37.820
alv 1455-4r 0.702491 0.513152 18.461 15.491 37.866
alv 1457-lr-c 0.702479 0.513155 18.444 15.471 37.811
alv 1458-2r 0.702515 0.513199 18.435 15.458 37.764
alv 1459-lr 0.702451 0.513142 18.440 15.466 37.787
alv 1461-3r 0.7024% 0.513140 18.461 15.471 37.824
alvl461-7r 0.702498 0.513143 18.440 15.462 37.777
1 Long-term measurement of NBS 987 standard: 87Sr/&Sr value of [.710216 45x10-6 check]
2 NBS-981 standard during measurement 206pb/24pb 16.91518722 8.656 x 10-3; 20Tpb/24Pb
15.4492077 9.757 x 103; 2Npb/24Pb 36.5776344 3.196 x 10-2










Table 3. Continued.
Location 8"Sr/"Sr '4Nd/144Nd 26Pb/m0Pb 7 Pb/ O4 Pb 2pb /2)I Pb

NE Pacific MORB White et al.. 1987)
JDF VG768 0.702440 0.513030 18.766 15.547 38.226
JDF VG44 0.702580 0.513069 18.751 15.563 38.568
JDF D10-1 0.702560 0.513176 18.486 15.485 37.852
JDF VG348 0.702490 0.513250 18.470 15.457 37.867
JDF Average 0.702500 0.513150 18.488 15.480 37.897
Pacific N-MORB Average 0.702540 0.513140 18.437 15.489 37.891








al., 1984; Rhodes, 1990) are similarly unradiogenic in 87Sr/86Sr (0.7024-0.7026) and

radiogenic in 143Nd/144Nd (0.5130-0.5132). The MORB-like character of the Cobb

hotspot is anomalous for fixed mantle plumes (Desonie and Duncan, 1990;

Rhodes, 1990).

All of the Axial Segment isotopic ratios fall within MORB-like ranges, but

they are distinct from the C-E and basalts from the neighboring CoAxial Segment

(Figure 2-7) which is the most strongly depleted in 87Sr/86Sr (Smith, 1999; Embley

et al., 2000) on the entire ridge. Axial Segment basalts fall between the most

depleted basalts of the CoAxial Segment and the C-E basalts (Figure 2-7), similar

to the relationships observed in major and trace elements and supporting the

hypothesis that these are viable mixing endmembers. Covariation between

isotopic and incompatible trace element ratios indicates that much of the

incompatible element variability of Axial Segment basalts may be a consequence

of mixing between Axial Segment magmas that are similar to those from the

CoAxial Segment and C-E magmas. The limited isotopic variability of Axial

Segment basalts appears to correlate with trace element ratios such as Zr/Y

(Figure 2-7), suggesting that the trace element variations observed are related to

source compositional differences and varying mixing proportions from at least

two magmatic sources, and are not a result of differences in melting fractions of

the same source.



















Figure 2-7. Sr and Pb isotopic ratios in basalts from the Axial Segment are
generally indistinguishable from the C-E and other segments of the
JdFR, with only modest enrichments relative to the very depleted
CoAxial Segment. Axial Segment basalts have intermediate
radiogenic isotopic compositions between CoAxial and C-E basalts. A
correlation between trace element enrichment and radiogenic isotopic
enrichment suggests mixing between the ridge and hotspot end-
members.













0 Vance Segment |
0 CoAxial Segment
0 C-E Seamounts
* Axial Segment I


10



5








4.5




4




3.5




3




2.5 -
0.7022


0


0 0


0






0ee o ogee
$ .**.\ *


9. 0 *
'***;..;......... .***


0
0 0


0 .
6*


do


0.7023 0.7024 0.7025 0.7026 0.7027

"Se/MSr


15.5 15.55


Pb/'pb


0











o

o
00





* 0


01
15i


6










Discussion

The anomalous geomorphology, bathymetry, and geochemical gradients

observed on the Axial Segment provide insight into the physical and chemical

consequences of lateral magma migration, crystal fractionation, and mixing.

Similar geochemical gradients, albeit on a longer length scale, have been

identified on the Reykjanes Ridge and are attributed to the presence of the

Iceland hotspot and lateral migration of hotspot magmas along the ridge (e.g.

Hart et al., 1973, 1992; Schilling, 1973, Schilling et al., 1983, Elliot et al., 1991). The

oversupply of magma to the JdFR appears to be confined to the Axial Segment,

as the neighboring Vance and CoAxial Segments exhibit normal, axial rift

morphology and chemistry.

The decline in MgO, CaO, and A1203 along with corresponding increases

in TiC)2, FeO, K20, and Na20 along the Axial Segment rift zones with increasing

distance from Axial Seamount indicate that more fractionated lavas erupt at

greater distances from the hotspot (Figure 2-4a). This is most likely attributed to

the fact that the crust becomes progressively thinner and cooler away from the

hotspot. Enhanced fractionation is likely due to the "cold edge effect" or

equivalent phenomenon (Langmuir and Bender, 1984), which is commonly

observed near ridge-transform intersections or at the end of overlapping ridge

segments. The abundance of phenocrysts in Ridge Group basalts relative to most








Seamount Group samples, and the general increase in phenocryst abundance

with distance from Axial Seamount further supports this hypothesis.

The expected enrichment in incompatible trace element contents resulting

from the progressive fractionation postulated in the rift zones is not observed.

Instead, there is a negative correlation between trace element and major element

gradients in the rift zones, with generally more evolved, but more depleted

basalts with distance from the hotspot (Figure 2-4). This observation suggests

that the proportion of the depleted mixing component increases with distance

from the hotspot, and this effect overprints the enrichment effects of

fractionation.

The high degree of scatter exhibited by major and trace element data

about calculated LLD, the inverse correlation between major and trace elements

in the rift zones, and the positive correlation between radiogenic isotope and

trace element ratios indicate that fractionation of a single parent cannot explain

the geochemical variability of the Axial Segment suite, and indicates the presence

of more than one magmatic source. The high-MgO basalts that do not fall along

the regional gradient of the Ridge Group are also depleted in incompatible

elements relative to the rest of the suite, and have N-MORB-like REE patterns.

These high- MgO basalts show little of the geochemical enrichments observed in

the rest of the suite. This suggests that these basalts represent melts from the

indigenous Axial Segment MORB source (similar to the neighboring CoAxial and

Vance Segment MORB sources), which avoid significant fractional crystallization








and mixing with the Cobb hotspot component. The occurence of several

examples of the depleted, high-MgO basalt type on Axial Seamount suggests that

the magma chamber under the seamount may be transitory or smaller at times,

allowing a few MORB-source magmas to avoid mixing during ascent to the

surface (Rhodes, 1990).

The results of mixing calculations (Langmuir, 1978) between the most

enriched C-E basaltic compositions and the most depleted Axial and CoAxial

Segment compositions are shown in Figure 2-8. The results clearly indicate that

these are viable mixing end-members for producing the Axial Segment suite, and

show the range of hybrid magmas produced by mixing of these components. In

addition to mixing, low-pressure fractional crystallization of the hybrid parents

that form along the mixing lines is required to account for the remaining

geochemical variation observed in the entire Axial Segment suite (Figure 2-8).

The mixing relations suggest that the typical low-MgO basalts contain between

30 and 40% of the Cobb hotspot component. This result is in agreement with

geophysical studies that suggest a 30% increase in crustal production at Axial

Seamount due to the presence of the hotspot (Hooft and Detrick, 1995). The

more "enriched" basalts containing the highest proportion of the Cobb

component are more depleted than any recovered on the C-E chain (Desonie and

Duncan, 1990), a result of the thorough mixing with the more depleted end-

member at the ridge. Alkali and light rare earth element (LREE) enrichments

decrease with age in the C-E chain, suggesting an increasing influence of the



















Figure 2-8. Variable mixing between the Cobb hotspot magmas and the local
MORB source, and subsequent fractional crystallization of the hybrid
magmas can explain the geochemical variation observed on the Axial
Segment. Mixing relations between the Cobb-Eickelberg seamount
basalt composition and depleted CoAxial Segment basalts show that
mixing between the hotspot magmas and local MORB basalts can
explain the trace element variation on the Axial Segment. Thorough
mixing of the two end-members has prohibited any lavas with the
'pure' Cobb plume geochemical signature from erupting, and its
proportion in Axial Segment lavas is up to -40%.

















Axial Segment and Seamount CB-1
E C-E Seamounts Im
A Vance and Coaxial Segments
+ Mixing Lines DH1-1
LowPLLD + EB




+
+ +
+ D
+
50% +
+ 50%
+

R461-25

.^^A

267-4
2670-4 A


2.0





1.6





1.2





0.8





0.4
50


150


200


250


300


Sr (ppm)


100








depleted MORB source as the ridge approached the hotspot (Desonie and

Duncan, 1990).

The gradients in incompatible trace elements along the Axial Segment

indicate that the proportion of the MORB mixing component generally increases

with distance from the hotspot. One possible explanation for this observation is

that the Cobb plume is zoned or has entrained upper mantle materials, and has a

more enriched core and progressively more of the depleted component toward

its periphery (Figure 2-9a). Close to Axial Seamount, magmas with the highest

proportion of the Cobb hotspot component are sampled, but further along the

rift zones, progressively more of the depleted material is tapped. The MORB-

source material may reside in a mixed sheath around the plume core or as

discrete masses entrained in the plume (e.g. Campbell, 1990). A second possible

explanation for the gradients is that crustal dikes that emanate from the Axial

Seamount magmatic system interact with shallow MORB magma chambers as

they propogate down the rift zones during tectonic events (Figure 2-9b). The

dikes would initially contain a high proportion of the Cobb end-member, but

may become progressively more contaminated with distance down the rift zone

due to mixing.

The notable homogeneity of the basalts, both from Axial Seamount and at

any given distance from it in the Ridge Group (Figure 2-4a), suggests that they

have been homogenized in a large magmatic storage system prior to eruption

(West, 2001). It is not clear if all of the lower-MgO magmas originate in the same




















Figure 2-9. Models to explain the spatial geochemical trends observed on the
Axial Segment. The 'Cold Edge Effect' and smaller magma chambers
may lead to enhanced fractional crystallization and more evolved
lavas with increasing distance from the hotspot. Trace element trends
may result from: (A) tapping into a geochemically zoned plume, with
progressively more of the depleted MORB component with increasing
distance from the hotspot. The plume may also entrain discrete
masses of the MORB-source. (B) The mixing may take place at
shallow crustal levels, with propagating dikes penetrating MORB
magma lenses as they migrate along the ridge, becoming more
contaminated with the depleted component with distance.






64




A




MgO MgO








AXIAL
SEAMOUNT
SOUTH BW ZONE ^ ^





MORB SOURCE ** n. I ,





Mixedsheath or 0
entrained mantle



Material


AXIAL
SEAMOUNT


SOUTH 1ff ZONE


MORB SOURCE








large magma chamber under Axial Seamount and migrate laterally from it and

down the length of the Axial Segment rift zones. The clear correlations in major

and trace element gradients in the basalts along the rift zones suggests this may

be the case, although it is not possible for the progressive effects of fractionation

and mixing described above to operate in a closed system. The lateral transport

of the hybrid magmas away from the plume and along the rift zones is implied

by the shallow bathymetry and the dominance of volcanic features along the

length of the Axial Segment, in addition to the spatial gradients in major

elements. Moreover, the migration of SOSUS T-phase epicenters supports the

hypothesis that magmas can migrate great distances in shallow crustal dikes

from the Axial Seamount area.

Rhodes and others (1990) suggested that Axial magmas are derived from

the same mantle source as the JdFR, and that differences between the two were

based on the depth of melting rather than source differences. Melt initiation at

greater depths would account for a higher magma production rate at Axial and

many of the subtle geochemical differences between Axial and the ridge. They

suggested that the Axial melting column may originate in the spinel peridotite

stability field (where Sr is an incompatible element), whereas ridge lavas

originate in the shallower plagioclase peridotite stability field, where Sr is more

compatible. This would account for the higher Sr content of Axial Seamount

lavas. Higher pressure melts are expected to be less saturated in silica and

possess higher Na20 and MgO contents than low-pressure melts (Kushiro, 1968;








Presnall and Hoover, 1987). For the upwelling mantle to intersect the solidus at a

deeper level in this model, the mantle supplying Axial must be hotter than at the

JdFR, consistent with a thermal plume model. Batiza and Vanko (1983,1984) and

Zindler and others (1984) propose a model in which a smaller degree of partial

melting of the upper mantle source that is producing MORB lavas would result

in melts that are slightly enriched in alkalis and incompatible elements, but result

in essentially a MORB-like isotopic composition. This study has shown

differences in incompatible trace element enrichments and particularly

radiogenic isotopic ratios that require more than one mantle sources for Axial

Segment lavas, and that the most likely additional source is the Cobb hotspot

plume.













CHAPTER 3
TEMPORAL GEOCHEMICAL VARIABILITY OF AXIAL SEGMENT
LAVAS REVEALED BY RIDGE CAPTURE

Introduction

The Juan de Fuca Ridge (JdFR) is the 490-km-long boundary between the

Pacific and Juan de Fuca plates, located in the northeast Pacific off the west coast

of North America (Figure 3-1). The JdFR is a medium spreading rate ridge (~2.8

cm/yr half rate; Tivey, 1994), which is migrating to the northwest at 3.1 cm/yr

(Karsten and Delaney, 1989). The ridge is divided into seven segments,

extending from the Blanco fracture zone in the south where it connects to the

Gorda Ridge, to the Nootka fault at its northern terminus where the plate

boundary is called the Explorer Ridge.

The Axial Segment is a 100-km-long central segment (Figure 3-1) of the

JdFR, which is offset to the west in an en-echelon arrangement with the

neighboring Vance and Coaxial Segments. Its southern extension is bifurcated

into the Southwest and South Axial Rift Zones (Appelgate, 1990). The Axial

Segment is dominated by Axial Seamount, a 2100-m-high volcanic edifice

currently being created by the Cobb hotspot. The hotspot has created the Cobb-

Eickelberg (C-E) seamount chain, which extends from Axial Seamount to the 33

million year-old Patton Seamount near the Aleutian Trench (Karsten and

Delaney, 1989; Desonie and Duncan, 1990; Keller et al., 1997). The seamount


















Figure 3-1.


The Axial Segment of the Juan de Fuca Ridge is the 100-km-long
segment that is currently interacting with the Cobb Hotspot. The
interaction has created the large volcanic edifice of Axial Seamount,
and a broad, shallow ridge segment dominated by constructional
volcanic features. The North and South Rift Zones are the portions
of the ridge adjacent to Axial Seamount, and the entire structure is
collectively referred to as the Axial Segment. Overlapping spreading
centers mark the second-order ridge offsets with the neighboring
CoAxial and Vance Segments. Brown Bear Seamount is the hotspot
volcano that formed prior to Axial Seamount.








69





130" 128' 12 124' 122


.. ......... .. ............ .. ............... . .. .




Brown
Bear
\Sarmunt \A








chain lies along a trend that is consistent with predicted absolute motions of the

Pacific Plate indicating that the hotspot is a fixed mantle melting anomaly. The

chain is parallel to the trend of the Hawaiian seamount chain, and C-E

seamounts become progressively younger to the southeast as they approach the

hotspot. Axial Seamount is anomalous in that it erupts relatively depleted,

MORB-like lavas without the distinct elemental and isotopic enrichments found

in lavas from other hotspot-related seamounts and islands (Rhodes et al., 1990;

Desonie and Duncan, 1990).

The complex interaction between seafloor spreading and hotspot

volcanism has created distinct bathymetric and morphological features on the

Axial Segment. It has the highest-standing bathymetry on the entire JdFR, with

average depths that are 250-750 m shallower than the other segments (Figure 3-

2). The morphology of the Axial Segment changes with distance from the

hotspot; close to Axial Seamount, it is shallowest and has an inflated, corrugated

appearance, composed of parallel-trending sets of thin, linear ridges that are

likely dike-fed volcanic constructs (Appelgate, 1990). These ridges are similar to

volcanic ridges on the submarine extension of the Kilauea rift zone (Puna Ridge),

which are inferred to have erupted along linear vents where dikes intersect the

surface (Fornari et al., 1978). Near it's north and south distal ends, the Axial

Segment deepens and is characterized by small (typically <2km in diameter)

volcanic cones and mounds that are superimposed on more irregular volcanic

ridges. No structurally defined spreading axis or major tectonic features of any











-1600
SNVZ
-1800

-2000
Axial Segment
0-< ,/ NVZ\
S-2200


-2400 VanceSegment

-2600 \
0 5.0 10.0 15.0
Distance (kin)

Figure 3-2. Topographic cross-sections of the Axial and Vance Segments. Note the symmetrical profile of the Vance
Segment, with a centralized axial volcanic ridge identifying the neo-volcanic zone (NVZ) within a well-
defined axial graben. The Axial Segment has much shallower bathymetry due to magmatic oversupply
from the hotspot, and its NVZ is off-center to the east. Bathymetry gradually increases from west to east
as well, reflecting ridge capture.








kind have been identified in high-resolution sonar imagery of the ridge

(Appelgate, 1990). The dominance of volcanic features is atypical for the JdFR, as

the rest segments are characterized by a majority of tectonic features, including

well-defined axial graben or valleys. (Johnson and Holmes, 1989; Van Wagoner

and Leybourne, 1991; Smith et al., 1994; Perfit and Chadwick, 1998; Embley et al.

2000). In spite of the medium spreading rate on the JdFR (6 cm/yr full rate;

Johnson and Embley, 1990), the morphology of the Axial Segment is similar to

the broad, inflated axial high that is more commonly observed at fast spreading

ridges (Phipps Morgan and Chen, 1993), which is likely an effect of excess

magma supply from the hotspot. Axial high morphology has also developed on

the Kolbeinsy and Reykjanes Ridges in proximity to the Iceland hotspot, in spite

of the slow spreading rate of the Mid-Atlantic Ridge (Searle and Laughton, 1981).

The morphology of intermediate spreading rate ridges (2-3 cm/yr) may be

particularly sensitive to magma supply, varying between the rift morphology

typical of slow spreading ridges to the axial high typical of fast spreading ridges.

The transition in axial morphology of medium rate ridges depends on the

balance between the heat supplied by intrusion and its removal by hydrothermal

circulation (Phipps Morgan and Chen, 1993; Hooft and Detrick, 1995). The

shallow bathymetry of the Axial Segment suggests that hotspot magmas are

channeled laterally along the segment (Vogt and Johnson, 1975; Yale and Phipps

Morgan, 1998). The infiltration of the hotspot magmas leads to an oversupply








along the ridge and allows hotspot and shallow MORB magmas to mix (see

Chapter 2).

Although the characteristics of the Axial Segment are very different from

the rest of the JdFR due to its interaction with the hotspot, it is likely an integral

part of the JdFR spreading system and not an isolated volcanic rift system similar

to the Kilauea volcano rift zones in Hawaii (Hammond and Delaney, 1985). The

segment fills the approximately 60-km gap between the southern end of the

Coaxial segment and the northern end of the Vance segment (Figure 3-1), and it

overlaps these segments in the manner of an overlapping spreading center, with

the ends of the segments curving toward each other (Lonsdale, 1985; Embley et

al., 1990).

Migration of the JdFR system brought it to the vicinity of the Cobb

hotspot during the Brunhes magnetization period (Figure 3-3a). The Vance and

Coaxial segment trends are collinear with the centerline of the normal-polarity

Brunhes pattern, but the Axial Segment is offset to the west by about 15 km

(Tivey and Johnson, 1990). This indicates that active spreading linked the Vance

and Coaxial segments in early- to mid mid-Bruhnes time, but ceased when the

section of ridge between them was abandoned and spreading began on the Cobb

hotspot, creating the Axial Segment (Figure 3-3b). Although the JdFR was still 15-

20 km to the west of the hotspot, magnetic anomaly data reveal that a central

section of the ridge "jumped" to the location of the hotspot 0.2-0.7 m.y. ago

(Delaney et al., 1981; Hammond and Delaney, 1985; Karsten and Delaney, 1989;



















Figure 3-3. History of the interaction of the Juan de Fuca Ridge with the Cobb
Hotspot. (A) Prior to about 0.4 Ma, the ridge was approaching the
hotspot from the southeast, migrating at about 3 cm/yr. Brunhes
crust is shown in gray, Matuyama crust in white. (B) A central
portion of the ridge was abandoned, and spreading was initiated
over the Cobb hotspot, creating the Axial Segment. Small islands of
Matuyama crust were isolated between the new and old spreading
centers. (C) In the current situation, capture of the Axial Segment has
led to the production of abandoned volcanic ridges to the west of the
neo-volcanic zone (NVZ), leading to the apparent asymmetrical
properties of the segment. Continued migration of the Juan de Fuca
Ridge will re-link the Vance and CoAxial Segments with the Axial
Segment in about 0.5 my.






75




OC


- I ..


S.............








Tivey and Johnson, 1990). The age of the interaction is revealed by the off-center

location of the Axial Segment on the Brunhes magnetic pattern, as well as the

presence of Matuyama negative-polarity crust (e.g. Helium Basin) within the

Brunhes region age crust (Figure 3-3b), trapped between the abandoned and

current spreading ridges. This relocation event, and similar events at other

ridge-hotspot interactions (Small, 1995; Hey and Vogt, 1977), suggests that as

migrating ridges approach a hotspot, the regional extensional stresses on the

plates may transfer spreading to a position over the hotspot due to the thermal

and mechanical weakness imposed on the lithosphere by the plume.

This paper presents the results of a study of regional age relationships,

ridge morphology, and basalt geochemistry on the Axial Segment, in an effort to

understand the tectonic and magmatic processes that have taken place since the

interaction of these two major geological features began. The Axial Segment

provides a unique opportunity to study the first few hundred thousand years of

a ridge-hotspot interaction, and can offer insight into the universality of

paradigms developed from studies of the well-documented Iceland-Mid-Atlantic

ridge interaction.

Sample Recovery and Geological Observations

The goals of this study were to identify the location of the active plate

boundary on the Axial Segment using basalt ages, ridge bathymetry and

morphology, and to investigate the tectonic processes and geochemical

variability resulting from the JdFR-Cobb hotspot interaction. To these ends, an








extensive basalt sampling campaign was conducted as part of the 1998-2002

NOAA-sponsored New Millennium Observatory (NeMO) research program,

which was devoted to geological, biological, and hydrothermal studies of the

results of the 1998 eruption of Axial Seamount (NeMO Cruise reports, 1998-

2002). Basalt samples have been collected from the Axial Seamount caldera and

adjacent flanks by the Remotely Operated Platform for Ocean Science (ROPOS)

submersible during the NeMO cruises, operated from the RV Ronald Brown and

RV Thomas Thompson. For this study, a more regionally extensive sampling

campaign was accomplished along the North and South Rift Zones of the Axial

Segment with wax corers during the NeMO project as well (Figure 34). To

ascertain intra-segment variations in basalt ages and geochemistry, and to

elucidate the relative roles of Cobb hotspot and local MORB magmas in Axial

lavas, 139 wax core samples were acquired in a high-density pattern both along

and across the strike of the Axial Segment. Out of logistical necessity, the

majority of these were acquired in the South Rift Zone within 20 km of the 1998

eruption site.

The lavas in the 1998 eruption issued from a dike or dike system that

extended along the South Rift Zone about 10 km from the southeastern comer of

the Axial Seamount caldera, along the eastern side of the Axial Segment. The

basalts collected from these flows were all very fresh, glassy, and free of

associated sediments as would be expected from such a recent eruption, but wax

core samples collected elsewhere on the Axial Segment exhibit varying degrees



















Figure 3-4. Age relationships on the Axial Segment are revealed by the
glassiness, manganese coating, and sediment volume associated with
wax core samples. The freshest basalts (Age 1) were recovered
exclusively on the eastern side of the ridge, and a general
progression of older samples to the west indicate the neo-volcanic
zone (NVZ) is offset to the east from the centerline of the
topographic ridge. This hypothesis is supported by bathymetric and
sonar reflectivity data, as well as the asymmetric location of the 1998
lava flow. A few of the most weathered/altered samples (Age 3)
were recovered on the extreme eastern side of the ridge in the South
Rift Zone, indicating older lavas to the east of the NVZ.
Intermediate-age samples (Age 2) overlap the entire sampling
area, and are not shown.


















AX L







SS
-%r.sPA ." O







......:J .i ..../ .... .... -.9


.... .,^ ., .y ..; ...





" "",--, "'"^ i^ ; ," ?.. I ..
* .. .. . "..

... .,, .. ... ... .
ii " ":"" " ': 0
... .. . .,, .. ..... .:; ;;. ... : : .. ..

I .\":' ": " I.
oS- ... : 4, ..; -


* Agel
.. Age 3
















........... <. .
. . .. .
Is f s0j1
aA e





8FL0i:9 S::: : ..


...... I








of freshness, likely reflecting variations in their eruption age. The wax core

samples retrieved from the Axial Segment were given a qualitative age

classification (1-3) upon retrieval of the sampler from the ocean floor (Figure 3-4);

a "1" denoting the freshest, glassiest basalts with little weathering and/or Mn-

coating and the least amount of associated pelagic sediment (including sediments

embedded in the coring wax), a "2" for intermediate-age samples, and a "3" for

basalts with the least glass, thickest Mn coating and greatest amounts of

associated sediment, suggesting the longest time period since their eruption.

Similar qualitative age classification schemes have been used in previous studies

of basalt ages at Axial Seamount (Embley et al., 1990; Hammond, 1990), and it is

used here to gain insight into the spatial and temporal evolution of eruptive

activity on the Axial Segment.

Basalts retrieved from the eastern part of the Axial Segment (North and

South Rift Zones) are the glassiest and freshest, whereas samples from

progressively further west across the strike of the ridge are generally less glassy

and associated with thicker Mn-coatings and more sediment cover (Figure 3-4).

Wax core sampling attempts on the westernmost part of the ridge met with

limited success due to thick sediment cover, also implying an older surface.

These relative age relations suggest that the basalts are youngest on the eastern

side of the North and South Rift Zones of the Axial Segment, becoming

progressively older toward the west (perpendicular to the strike of the ridge).

Although the majority of the basalt samples were collected in the South Rift Zone








of the Axial Segment (Figure 3-4), the age pattern is also clearly present in the

North Rift Zone as well. This asymmetrical pattern is inconsistent with a normal

spreading ridge but is compatible with the east-of-center locations of the 1998

eruption and a recent eruption documented in the north rift zone, as well as the

off-center location of the Axial Seamount caldera (Figure 3-4). This age

interpretation is supported by high-resolution sonar mapping, which has

revealed high backscatter lavas interpreted to be fresh constructional volcanic

ridges and flows in the eastern half of the South Rift Zone, and low backscatter,

more sediment-covered and older terrain to the west (Appelgate, 1990). These

differences in sonar backscatter intensity are interpreted to reflect changes in

sediment thickness and age, and the boundary between younger and older

surfaces is gradational and indistinct. Tectonic structures are largely absent in

the South Rift Zone in sonar imagery, but post-eruptive faults with trends

mimicking the volcanic ridges are prevalent in the SW Basin and on the SW Rift

Zone. These observations suggest that the eastern sides of the rift zones are the

only areas exhibiting recent volcanic activity, and that the plate boundary of the

Axial Segment is located there. The absence of an unequivocal structural

expression of the plate boundary is likely due to high rates of volcanism that

buries large faults as fast as they form.

Mid-ocean ridges typically have a symmetrical cross-section, with a

centrally located neovolcanic zone (NVZ), either within a structurally-defined

central rift or graben (e.g. the Vance Segment, Figure 3-2), or atop a symmetrical








broad volcanic arch (axial high) in the case of fast-spreading ridges (Perfit and

Chadwick, 1998). In addition to the anomalous shallow bathymetry of the Axial

Segment, it also has an asymmetric across-axis bathymetric profile, with the

shallowest bathymetry on the eastern part of the ridge, corresponding with the

locations of the freshest basalt flows (Figure 3-2). This association indicates the

thermal and/or isostatic uplift associated with the NVZ is also off-center on the

eastern side of the Axial Segment. This is likely a consequence of thermal

buoyancy from a narrow column of buoyant material (e.g. Madsen et al. 1984,

Wilson, 1992), horizontal viscous shear stress driven by plate separation (Eberle

and Forsythe, 1998), or accretional bending stresses (Buck, 2001). The

bathymetric gradient to the west is similar to the gradients observed at all mid-

ocean ridges, and likely results from subsidence due to the gradual conductive

cooling of the lithosphere as it moves away from the NVZ (Cochran and Buck,

2001).

Ridge Capture

Several mid-ocean ridge systems that are currently interacting with

hotspots (e.g. Iceland, Discovery, Galapagos) show evidence of discrete ridge

jumps in the direction of the hotspot (Ward, 1971; Saemundsson, 1974; Hey and

Vogt, 1977; Small, 1995). The most likely cause for this is thermal weakening of

the ridge flank by the plume, resulting in the relocation of the axis of spreading

back over the plume. Wax-model study studies (Leary et al., 1991) have also

shown that sections of ridges jump to and reside over hotspots as the ridge









migrates toward a fixed plume. "Capture" of the segment continues for some

time after the adjoining sections of the ridge pass by, as is the case at Iceland

(Hardarson, 1997).

The repeated eastward relocation of the active rift zone over the Iceland

hotspot in response to WNW migration Mid-Atlantic Ridge has been a

significant geological process on the island for at least the last 15 m.y.

(Hardarson, 1997). Two previous rift zones can be identified in western Iceland;

the Northwest Iceland Rift and the Snaefellsnes Rift, each abandoned by periodic

rift relocations to the east. Rift relocation is a complex process, with volcanic and

tectonic activity initiated on the new rift several million years before the old rift

becomes extinct (Hardarson, 1997). The current neovolcanic zone in Iceland is

itself in the process of relocating, which will ultimately lead to the abandonment

of the still active Western Volcanic Zone and fully transfer activity to the Eastern

and Northern Volcanic Zones (Oskarsson et al., 1985), 100 km to the east over the

center of the hotspot.

The asymmetries in morphologic, volcanic and age properties of the Axial

Segment presented in the previous section support the hypothesis that it has

been captured by the Cobb hotspot. Active spreading first jumped from the JdFR

to the northwest to lie directly over the hotspot, and has since remained fixed

relative to the hotspot as the rest of the plate boundary has continued to migrate

to the northwest (Figures 3-3c and 3-5). This has led to an apparent eastward

migration of volcanism (relative to the center of the Axial Segment) over time,



















Figure 3-5.


Capture of the Axial Segment by the Cobb Hotspot has led to the
asymmetrical growth of the segment, and the majority of the new
crust produced since the capture is transferred to the Pacific Plate to
the west. The result of ~3 cm/yr (half-rate) ridge spreading (TIA)
and -3 cm/yr migration (TIB) after a given time period leads to the
removal of most of the new crust away from the hotspot. The
captured neo-volcanic zone remains over the hotspot or jumps back
to it, however, and spreading persists there (T2). Continued
spreading and migration (T3) has lead to the creation of an
asymmetric ridge segment. The shallowest bathymetry on the ridge
is also on the eastern side, due to progressive cooling of the older
crust to the west.

















Cobb
Hotspot


Axial
Segment .'ag igfy / ^ / //


Plan views
Cross-sections


4 Ridge
Migration


TIA


TIB


T2


T3








due to the formation of successive rift zones that migrate to the west with the

Pacific Plate when they are abandoned. This would lead to progressively older

basalt eruption ages from east to west, and is consistent with the geological

observations discussed above. The similarity in the magnitude of the plate

migration and spreading vectors (Karsten and Delaney, 1989), coupled with a 20

azimuthal difference results in the transferal of most of the new crust produced

by Axial Segment spreading to the Pacific Plate when the zone of active

spreading migrates or jumps back over the hotspot (Figure 3-5). A small

amount, however, remains on the Juan de Fuca Plate, and may be represented by

several "Age 3" samples on the extreme east side of the South Rift Zone (Figure

3-4). Calculations using plate motion vectors predict this sliver of crust

produced by the Axial Segment over the 0.4 my due to ridge capture should be

just over 1 km wide, and much of it may be buried by younger lavas.

The ridge relocation process appears to occur more rapidly at Axial

Seamount than on Iceland, leading to the development of a single, wide ridge on

the northern half of the segment instead of isolated rift zones. The southern half

of the Axial Segment is bifurcated (Southwest and South Axial Rift Zones of

Appelgate, 1990), suggesting a 3-4 km jump of the spreading center, creating the

Southwest Basin between the two arms of the ridge (Figures 3-1 and 3-4). It is

not clear why a similar feature did not form on the northern half of the segment,

but the 12-km total width of the two arms of the South Rift Zone is the same as








the uniform North Rift Zone, and was likely produced in the same amount of

time.

An understanding of the modified crustal accretion mechanism due to

capture of the Axial Segment can allow for a reassessment of the age of its

interaction with the hotspot. Assuming a 3 cm/yr half-spreading rate (azimuth

315), 3.1 cm/yr migration rate (azimuth 335), and a 12-km-wide ridge, the

captured Axial Segment produces ~2.8 cm/yr of new Pacific crust (Figure 3-5),

and results in a net Pacific Plate motion to the NW of 4.4 cm/yr (Desonie and

Duncan, 1986; Karsten and Delaney, 1989). About 430,000 years (12 km/2.8

cm/yr = 428,571 yrs) is required for the captured Axial Segment to produce 12

km of overthickened crust to the west of the current neovolcanic zone. This age

for the initiation of ridge-hotspot interaction is consistent with ages suggested by

previous workers (Delaney et al., 1981; Hammond and Delaney, 1985; Karsten

and Delaney, 1989; Tivey and Johnson, 1990) based on magnetic anomaly data.

Temporal Geochemical Variations

The capture of the Axial Segment and the resulting spatial segregation of

basalts with different ages provides an opportunity to examine the geochemical

variability of the Cobb hotspot-JdFR magmatic system over the history of their

interaction. Major and trace element abundances of samples collected by wax

core and ROV on the ridge were determined (Table 3-1) to investigate regional

geochemical variations. Electron microprobe and X-ray fluorescence (XRF)










Table 3-1. Age classifications for rock samples from the Axial Segment. Major element analyses from electron
microprobe1.
Sample Longitude Latitude Age Class SiO2 A1203 TiO2 FeO MnO MgO CaO K20 Na2, P220s5 Total
98-RC-01 -129.9970 45.8922 1 50.3 14.4 1.52 10.7 0.21 7.15 11.7 0.19 2.95 0.14 99.3
98-RC-03 -130.0130 45.8287 1 50.0 14.1 1.59 11.1 0.21 7.00 11.6 0.20 3.03 0.16 99.0
98-RC-04 -130.0117 45.8325 1 50.2 14.2 1.61 11.2 0.20 7.10 11.7 0.19 2.86 0.15 99.4
98-RC-05 -130.0053 45.8327 1 49.9 14.9 1.37 10.2 0.20 7.66 11.9 0.14 2.82 0.12 99.2
98-RC-06 -130.0097 45.8363 1 50.0 14.1 1.59 11.1 0.21 7.02 11.6 0.21 2.96 0.16 99.1
98-RC-11 -130.0062 45.8505 1 50.1 14.3 1.52 10.6 0.20 7.16 11.9 0.20 2.94 0.13 99.0
98-RC-12 -130.0088 45.8400 1 50.2 14.2 1.63 11.2 0.22 6.98 11.6 0.20 3.02 0.18 99.5
98-RC-15 -130.0318 45.8943 1 50.3 14.3 1.60 11.2 0.22 6.92 11.7 0.20 2.86 0.16 99.5
98-RC-23 -130.0417 45.8803 1 50.3 14.3 1.50 10.9 0.21 7.17 11.9 0.19 2.87 0.13 99.6
98-RC-26 -130.0485 45.8343 1 48.7 16.3 1.07 9.9 0.20 8.53 11.8 0.11 2.56 0.11 99.4
98-RC-28 -130.0250 46.0075 1 50.6 14.5 1.52 10.5 0.20 7.09 11.3 0.24 3.07 0.17 99.2
98-RC-35 -130.0113 45.8612 1 50.2 14.3 1.45 10.6 0.20 7.32 12.0 0.17 2.83 0.13 99.1
98-RC-36 -129.9638 45.8263 1 49.5 15.2 1.34 10.6 0.20 7.69 12.0 0.14 2.80 0.12 99.5
98-RC-39 -130.0028 45.8608 1 50.1 14.4 1.51 10.5 0.20 7.27 11.9 0.19 2.85 0.14 99.1
98-RC-46 -130.0308 45.7483 1 47.8 17.2 1.01 9.8 0.16 9.27 11.4 0.07 2.52 0.05 99.3
98-RC-49 -129.9630 46.0623 1 57.7 12.3 1.79 13.6 0.32 1.82 5.8 0.90 2.29 0.70 97.2
99-RC-100 -129.9600 46.0560 1 55.1 12.8 1.99 13.2 0.29 2.26 6.1 0.84 4.27 0.68 97.5
99-RC-101 -129.9520 46.0770 1 53.8 12.1 2.19 14.6 0.32 2.40 6.5 0.71 4.12 0.96 97.7
99-RC-50 -130.0000 45.8810 1 49.5 15.5 1.35 10.0 0.17 8.05 12.1 0.12 2.85 0.12 99.7
99-RC-81 -130.0230 45.7690 1 49.9 14.2 1.38 10.3 0.19 7.40 12.6 0.12 2.81 0.13 99.0
99-RC-90 -130.0350 45.8300 1 49.8 13.9 1.59 11.1 0.21 7.25 11.7 0.17 3.01 0.13 98.9
99-RC-91 -130.0280 45.8050 1 49.7 14.0 1.54 10.7 0.21 7.41 12.1 0.14 3.00 0.14 98.9
99-RC-92 -130.0210 45.8020 1 49.8 14.1 1.54 10.7 0.21 7.36 11.9 0.18 2.84 0.15 98.7
99-RC-93 -130.0170 45.8030 1 49.8 14.1 1.52 10.6 0.20 7.08 11.9 0.17 2.92 0.14 98.5
99-RC-97 -130.0370 45.7030 1 47.9 16.7 0.98 9.6 0.17 9.37 11.6 0.04 2.58 0.05 99.0
99-RC-99 -129.9620 46.0640 1 58.3 12.5 1.57 12.9 0.29 1.56 5.4 0.90 3.67 0.63 97.7
00-RC-113 -129.9825 45.9165 1 49.6 14.5 1.51 10.5 0.18 7.57 12.0 0.19 2.89 0.15 99.1
00-RC-117 -130.0168 45.8190 1 50.0 14.4 1.54 10.6 0.21 7.32 12.1 0.20 2.87 0.16 99.3
00-RC-118 -129.9684 45.9167 1 49.7 14.6 1.48 10.2 0.20 7.27 12.2 0.18 2.84 0.15 98.9
00-RC-119 -129.9928 46.0562 1 47.0 17.2 1.32 8.9 0.17 8.58 11.7 0.11 2.81 0.12 98.0
1 Microprobe analyses performed at USGS, Denver, Colorado
Samples with no geochemical data: no recovery, whole rock samples with no glass, or high analytical errors









Table 3-1. Continued.


Sample
R465-01
R465-02
R494-01
R494-04
R494-05
R494-06
R494-08
R546-02
R554-02
R554-03
R554-04
R554-05
R554-06
01-RC125a
01-RC127
01-RC129
01-RC130
01-RC131
01-RC132
01-RC133
0 1-RC134
01-RC135
01-RC136
01-RC138
98-RC-02
98-RC-10
98-RC-13
98-RC-18
98-RC-21
98-RC-22
98-RC-24
98-RC-27
98-RC-30
98-RC-31


Table 3-1. Continued.


Longitude
-129.9862
-129.9863
-130.0042
-130.0041
-130.0041
-130.0041
-129.9958
-130.0037
-129.9982
-129.9983
-130.0003
-130.0015
-130.0015
-130.0368
-129.9615
-130.0260
-129.9928
-129.9795
-129.9943
-129.9790
-130.0232
-130.0248
-130.0432
-129.9547
-129.9758
-129.9633
-130.0268
-130.0193
-130.0813
-130.0467
-130.0380
-130.0258
-130.0375
-130.0277


A1203 TiO2 FeO MnO MgO CaO K20 Na2O P2205 Total
14.2 1.53 10.9 0.19 7.04 11.7 0.20 2.98 0.14 99.1
14.2 1.57 10.9 0.20 7.05 11.6 0.20 2.97 0.14 99.0
14.3 1.53 10.6 0.19 7.03 12.3 0.18 2.84 0.15 99.2
14.4 1.56 10.8 0.20 7.11 11.8 0.18 2.90 0.15 99.3
14.5 1.53 10.9 0.19 7.14 11.9 0.18 2.91 0.15 99.5
14.4 1.55 10.7 0.18 7.15 12.0 0.19 2.86 0.15 99.3
14.5 1.57 10.7 0.21 7.10 12.0 0.18 2.88 0.14 99.4
14.4 1.53 10.6 0.20 7.38 12.3 0.19 2.85 0.15 99.6
15.0 1.32 10.0 0.20 7.85 12.4 0.13 2.73 0.13 99.7
14.4 1.55 10.6 0.20 7.76 12.6 0.15 2.77 0.14 98.7
14.5 1.53 10.6 0.19 7.58 12.1 0.19 2.86 0.15 98.9
14.4 1.53 10.5 0.21 7.54 12.2 0.19 2.84 0.16 98.8
14.3 1.52 10.6 0.18 7.54 12.0 0.19 2.84 0.16 98.5


50.0 14.1 1.65 11.3 0.21 6.94 11.6 0.23 2.96 0.15 99.0
50.1 14.8 1.41 10.5 0.20 7.49 11.6 0.16 2.83 0.14 99.2
50.1 14.1 1.50 10.9 0.21 7.15 11.8 0.19 2.86 0.15 98.9
50.0 14.0 1.65 11.3 0.21 6.91 11.6 0.21 3.07 0.15 99.1
48.2 16.6 1.31 10.1 0.18 8.76 11.2 0.09 2.75 0.10 99.4
50.4 14.1 1.63 11.3 0.22 6.84 11.7 0.20 2.95 0.17 99.4
50.1 14.0 1.62 11.5 0.22 6.85 11.5 0.21 3.00 0.16 99.1
49.9 14.2 1.53 10.9 0.21 7.18 11.9 0.17 2.98 0.14 99.2
49.9 13.8 1.66 11.7 0.23 6.71 11.5 0.20 3.01 0.14 98.9
50nn 143 171 11 R 0n3 6S 11 1 0.274 3117 0.18 99.3


Latitude Age Class SiO2
45.8693 1 50.1
45.8695 1 50.2
45.8583 1 50.1
45.8624 1 50.1
45.8624 1 50.1
45.8624 1 50.1
45.8782 1 50.1
45.8631 1 50.1
45.8739 1 49.9
45.8739 1 48.5
45.8686 1 49.3
45.8665 1 49.2
45.8665 1 49.1
45.7632 1
45.9190 1
46.0062 1
46.0553 1
46.0367 1
46.0282 1
46.0248 1


46.0232
46.0013
46.0000
45.9548
45.8535
45.9615
45.8440
45.8087
45.6367
45.8707
45.8527
45.8342
45.8212
45. 167










Table 3-1. Confinned_


Sample
98-RC-32
98-RC-33
98-RC-34
98-RC-41
98-RC-42
98-RC-44
98-RC-45
98-RC-47
98-RC-48
99-RC-51
99-RC-52
99-RC-53
99-RC-56
99-RC-58
99-RC-60
99-RC-63
99-RC-68
99-RC-69
99-RC-70
99-RC-71
99-RC-73
99-RC-76
99-RC-77
99-RC-78
99-RC-79
99-RC-84
99-RC-85
99-RC-89
99-RC-98
00-RC- 116
00-RC-120
00-RC-122
00-RC-123
OR-Rr-14


Latitude Age Class SiO2 A a1203 TiO2 FeO MnO MgO CaO K20 Na20 P2205s Total


Longitude
-130.0130
-130.0315
-130.0475
-130.0290
-129.9965
-130.0500
-130.0602
-129.9828
-129.9678
-130.0150
-130.0210
-130.0300
-130.0820
-130.0640
-130.0630
-130.0530
-130.0580
-130.0610
-130.0190
-130.0180
-130.0070
-130.0010
-130.0070
-129.9990
-130.0050
-130.0600
-130.0600
-130.0240
-130.0340
-130.0317
-130.0093
-129.9848
-129.9983
- 1 'lnnl9n


45.8133
45.8437
45.8403
45.8893
46.0227
45.8275
45.8053
46.0488
46.0660
45.8790
45.8760
45.8620
45.6920
45.7570
45.7770
45.8570
45.8360
45.8420
45.7340
45.7440
45.7550
45.7700
45.7810
45.7760
45.7620
45.8470
45.8530
45.8250
45.7160
45.8283
46.0619
45.8403
45.8398
45.9408


50.1
50.2
50.0
50.4
50.1
50.2
49.9
50.0
50.1
50.1
49.9
49.9
50.0
49.9
49.9
49.9
49.9
50.0
50.0
50.1
49.4
49.5
49.4
49.5
48.8
49.8
50.0
50.1
48.6
49.9
49.3
48.2
48.8


14.2
13.9
13.7
14.4
14.3
13.9
14.0
14.2
14.1
14.1
14.5
14.4
13.8
14.0
13.9
14.0
13.7
14.3
14.4
14.2
14.3
14.3
14.0
14.3
14.3
14.1
14.0
13.9
16.1
14.0
15.5
16.7
15.8


1.62
1.65
1.74
1.54
1.62
1.59
1.59
1.49
1.55
1.59
1.50
1.53
1.77
1.71
1.65
1.57
1.70
1.51
1.45
1.51
1.45
1.48
1.62
1.48
1.44
1.54
1.55
1.64
1.40
1.71
1.33
1.38
1.29


11.0
11.5
11.9
10.8
10.8
11.5
11.3
10.8
11.0
11.1
10.9
10.9
12.3
11.6
11.4
11.0
11.5
10.7
10.5
10.7
10.4
10.4
11.2
10.4
10.3
10.9
10.8
11.1
9.6
11.6
9.7
9.2
9.7


0.21
0.22
0.22
0.21
0.22
0.23
0.22
0.20
0.22
0.22
0.21
0.21
0.23
0.24
0.21
0.20
0.22
0.21
0.21
0.22
0.21
0.21
0.21
0.21
0.21
0.22
0.20
0.20
0.18
0.22
0.21
0.18
0.18


7.13
6.84
6.54
7.21
7.14
6.86
6.97
7.25
7.07
6.93
7.15
7.40
6.35
6.71
6.70
6.94
6.73
7.20
7.21
7.04
7.28
7.21
6.87
7.25
7.31
7.06
7.09
6.75
8.13
6.88
7.80
8.06
8.04


11.9 0.20 2.93
11.7 0.20 2.92
11.3 0.23 3.03
11.8 0.19 2.91
11.7 0.21 2.97
11.4 0.18 2.99
11.7 0.21 2.91
11.9 0.18 2.84
11.8 0.20 2.88
11.7 0.17 2.88
12.1 0.15 2.88
12.0 0.15 2.85
11.3 0.18 3.13
11.8 0.18 2.97
11.9 0.19 2.99
12.0 0.18 2.88
11.6 0.20 2.92
12.2 0.19 2.67
12.2 0.13 2.80
12.2 0.14 2.82
12.2 0.17 2.73
12.2 0.18 2.72
11.9 0.17 2.94
12.2 0.18 2.76
12.2 0.17 2.72
12.0 0.18 2.86
12.0 0.17 2.85
11.8 0.20 3.01
11.8 0.08 3.01
11.7 0.21 2.94
12.2 0.13 2.78
11.9 0.13 2.96
12.1 0.12 2.72


0.16
0.15
0.18
0.15
0.18
0.15
0.17
0.13
0.15
0.16
0.15
0.15
0.17
0.16
0.16
0.15
0.18
0.13
0.14
0.14
0.14
0.15
0.16
0.15
0.15
0.15
0.15
0.18
0.09
0.17
0.11
0.12
0.11


Table 3-1 C ntinued


99.4
99.2
99.0
99.6
99.3
99.0
99.0
99.1
99.1
99.1
99.3
99.6
99.3
99.2
98.9
98.7
98.7
99.1
98.9
99.1
98.4
98.3
98.5
98.3
97.7
98.9
98.8
98.8
99.1
99.2
99.1
98.8
98.9










Table 3-1. Continued.


Sample
98-RC-1 9
98-RC-29
01-RC124
01-RC126
01-RC128
01-RC137
98-RC-07
98-RC-09
98-RC-16
98-RC-17
98-RC-40
98-RC-43
99-RC-102
99-RC-105
99-RC-106
99-RC-107
99-RC-108
99-RC-54
99-RC-55
99-RC-57
99-RC-59
99-RC-61
99-RC-62
99-RC-64
99-RC-65
99-RC-66
99-RC-67
99-RC-72
99-RC-74
99-RC-75
99-RC-82
99-RC-83
99-RC-86
99-RC-87


Latitude Aee Class SiO2 A1203 TiCO2 FeO MnO MO CaO K20 Na2O P220s Total


Table 3-1. Continued.


Longitude
-130.0413
-130.0075
-130.0307
-130.0317
-129.9197
-129.9708
-130.0597
-130.0575
-130.0258
-130.0283
-130.0700
-130.0475
-130.0370
-130.0620
-130.0580
-130.0620
-130.0640
-130.0780
-130.0720
-130.0820
-130.0630
-130.1380
-130.0510
-130.0800
-130.0790
-130.0800
-130.1410
-130.0140
-130.0170
-130.0120
-130.0370
-130.0550
-130.0430
-130.0430


45.6920
45.9947
45.7482
45.9265
45.9085
45.9875
45.7867
45.8012
45.7923
45.7983
45.8400
45.8310
46.0770
45.8550
45.8620
45.8290
45.8170
45.6560
45.6870
45.7010
45.7660
45.7530
45.8670
45.7270
45.7190
45.7090
45.8080
45.7600
45.7530
45.7740
45.7630
45.8460
45.7870
45.79RO80


1.57
1.58
1.48
1.57
1.67
1.65
1.52
1.47
1.53
1.53
1.65
1.25
1.69
1.79
1.62
1.64
1.72
1.71
1.73
1.51
1.71
1.59
1.58
1.64
1.17
1.55
1.73
1.76


11.2
11.1
11.3
11.5
11.6
11.4
10.6
10.3
10.7
10.5
11.2
9.3
11.4
12.3
11.3
11.4
11.4
11.4
11.7
11.0
10.8
11.0
11.1
11.2
9.6
10.7
11.8
1 .9


0.21
0.23
0.20
0.21
0.22
0.23
0.21
0.20
0.20
0.21
0.22
0.17
0.22
0.22
0.20
0.22
0.21
0.23
0.23
0.20
0.21
0.20
0.22
0.22
0.19
0.22
0.22
0Q24


6.99
7.10
7.08
6.95
6.79
6.88
7.08
7.24
7.03
7.15
6.82
8.47
6.78
6.37
6.82
6.70
6.64
6.62
6.51
6.91
6.85
6.97
6.80
6.61
8.82


11.8 0.16 2.98
11.7 0.17 2.99
11.7 0.15 2.87
11.6 0.16 2.95
11.4 0.18 3.13
11.4 0.19 2.98
12.0 0.15 2.88
11.9 0.15 2.84
12.0 0.17 2.91
12.2 0.16 2.93
11.6 0.19 3.04
11.8 0.09 2.78
11.9 0.18 2.91
11.4 0.18 3.14
11.8 0.16 3.01
11.5 0.19 2.99
11.5 0.21 2.95
11.6 0.21 2.97
11.7 0.20 2.94
11.9 0.13 2.90
11.9 0.22 3.06
11.9 0.17 2.89
12.0 0.17 2.92
12.0 0.18 2.94
11.8 0.07 2.68


50.0
50.2
50.2
50.4
50.1
50.3
49.7
49.6
49.5
49.7
50.0
48.7
50.0
50.0
50.0
50.0
50.1
50.0
50.0
50.2
49.7
49.0
49.5
49.9
47.8
49.8
49.9
49-9


0.15
0.16
0.14
0.15
0.16
0.15
0.13
0.12
0.14
0.13
0.13
0.09
0.13
0.17
0.14
0.17
0.18
0.19
0.16
0.13
0.19
0.14
0.17
0.15
0.09


99.0
99.4
99.1
99.3
99.2
99.2
98.3
98.2
98.1
98.7
98.7
99.5
99.2
99.3
99.2
98.7
98.6
98.7
99.0
98.8
98.8
98.0
98.5
98.9
99.1


14.0
14.2
14.0
13.9
13.9
13.9
14.1
14.3
14.0
14.2
13.9
16.9
14.1
13.7
14.1
13.8
13.7
13.8
13.8
14.0
14.2
14.1
14.1
14.1
16.9
14.1
13.7
137


0.15 98.8
0.19 98.8
019 98.9


7.10 12.2 0.17 2.85
6.46 11.5 0.20 3.09
6.46 115s 019 3112










Table 3-1. Continued.


Lnn~itud~ Latitude Acre C1a~q SiO., AbO~ TiC), FeO MnO McrO CaO KO Na9O P,20s Total


3 49.9 13.9 1.59 11.0 0.20 6.97
3
3
3
3
3
3
3
3
3
3
3
3
3
3


11.8 0.17


2.95 0.16 98.8


SamnIp


99-RC-88
98-RC-08
98-RC-20
98-RC-25
98-RC-37
98-RC-38
99-RC-103
99-RC-104
99-RC-109
99-RC- 110
99-RC-80
00-RC-114
00-RC-115
00-RC-121
01-RC139


-130.0390
-130.0593
-130.0550
-130.0463
-129.9247
-130.0375
-130.0590
-130.0690
-130.1430
-130.1450
-130.0130
-129.8576
-129.8603
-130.0287
-129.9695


45.8110
45.7975
45.6717
45.8433
45.7897
45.7625
46.0800
46.0650
45.8220
45.8330
45.7670
45.9017
45.8902
46.0700
459877


Loncritude Latitude Acrt- Class SiO, A]-t),, TiQ, FeO MnO MgO CaO KO Na,)O P,)20-i Total
Sample


01-RC139 -129.9695 45.9877 3








spectroscopy were used in these analyses. See Appendix for a description of the

analytical methods used in this study.

The intermediate (Age 2) samples are found in the areas between the age 1

and 3 samples, but they also almost completely overlap the older and younger

basalts. Therefore, only the samples classified as youngest (Age 1) and oldest

(Age 3) were used in the geochemical analysis. Basalts may remain very fresh

and glassy for a relatively short period of time before they 'age' to the

intermediate category, and they may likewise be in the Age 3 category for a

relatively short time before they are buried by sediments and not retrievable by

the wax corer. They may remain in the intermediate state for the majority of

their exposure time on the ocean floor, which would explain the spatial

prevalence of Age 2 basalts. Using only the Age 1 and 3 samples allows for two

relatively discrete spatial (and therefore temporal) groups of basalts to be

examined for geochemical change in Axial Segment magmas.

The interaction of the Cobb hotspot and Juan de Fuca ridge has led to

mixing of the magmas produced by the two systems and the production of

hybrid magmas. Axial Segment lavas are more enriched in incompatible

elements closer to Axial Seamount due to a higher proportion of the Cobb

hotspot magmatic endmember, and they become progressively more depleted

due to a higher proportion of the MORB endmember as distance from Axial

Seamount increases along the ridge. In addition, Axial Segment lavas show

increasing effects of fractional crystallization with increasing distance from the








hotspot, suggesting that ridge magma chambers become progressively smaller

and cooler. These two processes overprint one another, leading to more

primitive and enriched basalts near Axial Seamount, and more depleted and

evolved basalts near the distal ends of the Axial Segment. Figure 3-4 shows that

the younger basalts were sampled more heavily in areas closer to the hotspot,

whereas the older basalts were more heavily collected further to the south, and

these spatial gradients prevent a direct comparison between the two groups. To

eliminate this bias, only those samples collected in an area of overlap between

latitude 45.76 and 45.88 were used to allow for a direct geochemical

comparison. The results are shown in Figure 3-6, which clearly shows the

younger basalts are generally more mafic than the older samples for a given

distance from the hotspot.

Major element Harker diagrams show that the younger (Age 1) basalts

are, in addition to being more mafic, slightly more enriched in K20 for a given

MgO content (Figure 3-6), and the K20/TiO2 ratio is clearly higher for the

younger basalts as a result (Figure 3-7). Concentrations of other major elements

are not significantly different between the two groups, however (Table 3-1).

Incompatible trace element concentrations and ratios are also enriched in the

more recent lavas (Figure 3-8). Elevated K20/ TiO2, along with elevated Sr and

total alkali abundances, are prominent geochemical signatures of the Cobb

hotspot relative to JdFR MORB (Rhodes et al., 1990), and an increase in these