0012-821X/$ - see front matter D 2005 Elsevier B.V. All rights reserved.

doi:10.1016/j.epsl.2005.02.031

T Corresponding author. Tel.: +1 217 244 6293; fax: +1 217 244 4996.

E-mail address: lundstro@uiuc.edu (C. Lundstrom).

www.elsevier.com/locate/eps

l

Plume–ridge interaction studied at the Galapagos spreading

center: Evidence from 226Ra–230Th–238U and 231Pa–235U

isotopic disequilibria

Thomas Find Kokfelta, Craig Lundstromb,T, Kaj Hoernlea,Folkmar Hauff a, Reinhard Wernerc

aLeibniz Institute for Marine Sciences (IFM-GEOMAR), Wischhofstraße 1-3, 24148 Kiel, GermanybDepartment of Geology, University of Illinois at Urbana Champaign, 1301 W Green St., Urbana, IL 61801, USA

cTethys Geoconsulting GmbH, Wischhofstraße 1-3, 24148 Kiel, Germany

Received 23 February 2004; received in revised form 7 February 2005; accepted 18 February 2005

Available online 12 April 2005

Editor: B. Wood

Abstract

New 238U–230Th–226Ra and 231Pa–235U disequilibria data measured by TIMS are presented for ridge-centered MORB

glasses dredged during the R/V Sonne 158 cruise at the Galapagos or Cocos-Nazca Spreading Center (GSC) between 86.08Wand 92.38W. The application of U-series isotopes to the GSC region, situated a few hundred kilometres to the north of the

Galapagos hotspot, allows assessment of fundamental questions related to the dynamics of plume–ridge interaction. These

include (1) the relationship between long-lived source variations, U-series disequilibria and extent of differentiation, (2) partial

melting during solid upwelling, and (3) the nature and rates of plume–ridge mass transfer. The along axis U-series disequilibria

variation show gradational patterns that locally are correlated with geochemical and isotopic parameters such as La/Sm, Tb/Yb,206Pb/204Pb and 143Nd/144Nd as well as major element compositions. The correlation of (230Th)/(238U) with radiogenic isotopes

and Tb/Yb provides constraints on the plume source influence on the melting process, reflecting an increase in the amount of

melting at depth in the presence of garnet or aluminous clinopyroxene. Moreover, the correlation between U-series signatures,

radiogenic isotopes, incompatible element abundance and MgO content indicates a causative relationship between the melting

of plume source materials and how these lavas differentiate at shallow depths. We speculate that this involves loss of alkalis

from ascending melts to shallow peridotite and crustal gabbro, resulting in increased olivine fractionation from the magmas. The

U-series data place stringent constraints on the timing of plume–ridge mass transfer and thus distinguish whether mass transfer

occurs by movement of melts or solid mantle. Within the likely conditions proposed by the model of (Braun and Sohn [EPSL

213 (2003): 417–430] and with knowledge of (231Pa)/(235U) and (230Th)/(238U) observed in Galapagos Islands lavas [A. Saal,

personal communication], we show that all 226Ra excess will be lost and the initial 231Pa and 230Th excesses will be largely

Earth and Planetary Science Letters 234 (2005) 165–187

T.F. Kokfelt et al. / Earth and Planetary Science Letters 234 (2005) 165–187166

decayed. Therefore, we conclude that the plume influence on the GSC lavas results from a solid mantle flow process instead of

through migration of plume-derived melts to the ridge.

D 2005 Elsevier B.V. All rights reserved.

Keywords: Galapagos hotspot and spreading center; Cocos-Nazca Spreading Center; plume–ridge interaction; melt transport; uranium series

disequilibria

1. Introduction

A handful of hotpots around the globe are situated

in the vicinity of actively spreading ridges. These

include the Galapagos, Azores, Iceland, Easter,

Ascension, Bouvet, and Louisville hotspots. The

distances over which these hotspots interact with their

respective adjacent ridges vary from zero to several

hundreds of kilometres, signifying not only the

strength of the driving force, but also the potential

significance of the process in modifying the compo-

sition of normal Mid-Oceanic Ridge Basalts (MORB).

Because mantle plumes are typically geochemically

enriched compared to the ambient upper mantle

through which they travel, geochemical and isotopic

studies are particularly useful in unravelling the

details about the morphology of plume–ridge inter-

action. Traditionally long-lived isotopic tracers have

been applied in such studies; however, growing

attention has been devoted to the unique information

passed on from studies of short-lived isotopic

disequilibria within the uranium series (U-series:238U–230Th–226Ra and 235U–231Pa). Because the short

half-lives of these isotopes (226Ra: 1600 yr, 231Pa: 32

kyr, 230Th: 75 kyr) overlap the time scale of melting

and melt extraction, U-series isotopes can provide

information about the rates of processes such as

partial melting, mantle upwelling and melt transport in

the upper mantle and crust. However, existing U-

series studies focusing on plume–ridge interaction

have all concentrated on the North Atlantic region

such as the Azores hotspot/Azores MAR [1] and

Iceland hotspot/Reykjanes and Kolbeinsey ridges

[2,3].

The Galapagos hotspot and adjacent Galapagos or

Cocos-Nazca Spreading Center (GSC) in the central

east Pacific is a classic location for studying hotspot

activity and plume–ridge interaction [4–6]. The

Galapagos plume–ridge system has become a focus

area for studying plume–ridge interaction, involving

detailed morphological, geophysical and geochemical

studies (e.g., [4–20]). Here we contribute to the

growing database by presenting a comprehensive U-

series data set on recently dredged glasses from the

GSC between 868W and 928W for which major and

trace elements as well as Sr–Nd–Pb isotopes are

being reported [21,22]. The key issues that we

address include: (1) the relationship between long-

lived source variations, U-series disequilibria and

extent of differentiation; (2) the role of source

composition on the depth and process of melting;

and (3) the mechanism and time scales for plume–

ridge mass transfer. Specific questions include: Is

plume material fed to the ridge as a melt [23] or is

transport dominated by much slower solid mantle

flow [2,17]? To what extent do crustal processes

(e.g., differentiation, crustal contamination and pro-

longed magma chamber residence times) influence

the GSC lavas and their disequilibria systematics? To

answer these questions, this study focuses on the U-

series systematics of fresh basalt glasses sampled

along the GSC adjacent to the Galapagos hotspot but

also includes off-axis seamounts in the region to

constrain a slightly larger spatial and temporal

window.

2. Geological background, sample details and

analytical methods

The Galapagos hotspot is believed to be the

surface expression of a mantle diapir or plume

[24,25] of intermediate strength (~1 Mg yr�1; [26]).

Lavas erupted in the western and southern part of the

archipelago (e.g. Fernandina Island) have some of the

most extreme 3He/4He ratios (up to 30 Ra; [8,27,28])

reported in ocean island basalts (OIB) worldwide.

Seismic tomography data suggest an origin below the

410 km discontinuity [16] to at least 1000 km [25].

The center of the Galapagos plume at depths of z100

T.F. Kokfelt et al. / Earth and Planetary Science Letters 234 (2005) 165–187 167

km is presumably beneath the island of Fernandina at

the western leading edge of the Galapagos archipe-

lago, some 100–200 km to the south of the GSC

[8,9,16,27] (Fig. 1). Some geophysical evidence

however suggests that the plume is bent towards the

northeast at levels b100 km [29]. The Galapagos

archipelago and associated seamounts can be divided

into a geochemically enriched horseshoe enclosing

depleted MORB-like material [11,12,30–32] (Fig. 1).

Based on distinct Sr–Nd–Pb–Hf isotopic composi-

tions, the enriched horseshoe-shaped region can be

further subdivided into dnorthernT, dcentralT and

dsouthernT domains, indicating that the Galapagos

plume is compositionally zoned [10,32] (Fig. 1). The

hotspot track on the Cocos and Nazca plates exhibit

isotopic zonation patterns very similar to that of the

Galapagos Islands today, suggesting that the spatial

distribution of plume components has persisted for

the last 20 Ma [10,18]. The compositional zonation of

the Galapagos plume provides a unique opportunity

Eastern

Central

Wolf

Darwin

Isabela

92°W 91°W 90°W 8

1°S

1°N

0°

Plume Center(>100km

depth)

Pinta

50-1

47a-1

46a-1

41a-1

30-5

2°N

~110

km

Southern

(<100km depth)

Northern

~200km

Fernandina

Plume Center

Galápagos Sprea

91°W Transform

Fig. 1. Map of the Galapagos archipelago and neighbouring Galapagos Sp

expression of a deep-rooted mantle plume that currently resides beneath the

indications from seismic tomography that the plume is bending to the NE a

plume-lithosphere impingement SWof Pinta (stippled outline) [29]. The Ga

domains (labelled Northern, Eastern, Central and Southern), presumably ref

section of the GSC contains an overlapping spreading center at ca 87.3

approached from the east, the ridge axis gets progressively shallower and at

ridge type morphology. The increasing depth and change in ridge morpholo

plume with increasing distance from the plume. Insert figure shows the tra

Galapagos plume for at least 20 Ma. Arrows indicate the relative movem

for studying plume–ridge interaction pathways in

detail.

The GSC has an intermediate full spreading rate of

~46–64 mm/yr and is influenced structurally, morpho-

logically and compositionally by the neighbouring

Galapagos hotspot [33]. The studied section of the

GSC includes an overlapping spreading center (OSC)

at 87.38W and a major dextral transform fault at

90.88W (918W TF) that offsets the ridge about 100

km. To the west of the 918WTF, the GSC is intersected

by a series of seamount chains of which the Wolf-

Darwin Lineament (WDL) is the most westerly and

pronounced. Previous work on the 838W to 1058WGSC has revealed a broadly symmetric gradational

pattern around the main point of plume influx at

~918Won the GSC [5,20]. The along-axis gradation is

reflected in a more enriched radiogenic isotopic sig-

nature closer to the plume (i.e., higher 87Sr/86Sr,206Pb/204Pb and lower 143Nd/144Nd), as well as a shift

in ridge morphology and axial ridge depth. As the

9°W 88°W 87°W 86°W

Cocos Plate

Nazca Plate

GSC hotsp

ot tra

ck

GalápagosIslands

Off-axis seamountsRidge samples

ding Center (GSC)

OverlappingSpreading Center (OSC)

0 50 100

km

reading Center (GSC). The Galapagos hotspot is thought to be the

island of Fernandina at depths greater than 100 km. There are some

t depths shallower than 100 km, indicated by a possible location for

lapagos Islands’ lavas may be divided into four distinct geographical

lecting spatial zonation of the Galapagos plume [10,32]. The studied

8W and a transform fault at ~918W. As the Galapagos hotspot is

the same time, the spreading ridge changes from axial valley to axial

gy has been attributed to a diminishing influence from the Galapagos

cks formed by the Galapagos hotspot, indicating the existence of the

ent of the plates.

Table 1

U–Th–Pa–Ra data by TIMS on Galapagos spreading center

Sample MgOa

(wt.%)

Th

(Ag/g)U

(Ag/g)

�238U

232 Th

�b �234U238 U

� � 230Th232 Th

� �230Th238 U

� 226Ra

(fg/g)

�226Ra230 Th

� 231Pa

(fg/g)

�231Pa235 U

�

Eastern sector

DR3-1 6.36 0.345 0.104 0.915F5 – 1.058F15 1.157F17 38.6F1.7 – –

DR3-1* 0.352 0.109 0.945F5 1.008F8 – – 39.9F0.7 0.981F23 – –

DR6-6 6.01 0.724 0.226 0.949F4 1.007F6 1.036F6 1.091F6 – – 116F7 1.57F10

DR7-1 8.4 0.128 0.041 0.963F5 – 1.065F16 1.106F17 21.1F0.2 1.389F26 – –

DR8-1 6.8 0.293 0.093 0.957F4 0.998F8 1.047F11 1.094F12 42.6F0.6 1.246F21 49.2F2.0c 1.64F07

DR9-3 0.238 0.075 0.958F6 0.989F21 1.079F43 1.127F45 29.7F0.3 1.036F42 – –

DR10-2 7.07 0.271 0.0846 0.949F5 1.020F11 1.014F13 1.068F15 30.5F0.4 0.998F17 – –

East-central sector

DR13-14 6.71 0.390 0.118 0.914F4 1.004F8 1.045F8 1.143F9 43.6F0.9 0.962F21 – –

DR14-1 7.08 0.313 0.095 0.919F3 1.013F7 1.034F7 1.125F8 34.6F0.9 0.963F27 43.3F2.3 1.41F07

DR17-2 6.24 0.496 0.157 0.959F5 1.015F5 1.059F8 1.104F8 64.9F1.3 1.111F24 74.9F2.4 1.47F05

DR19-1 6.60 0.721 0.213 0.896F5 1.010F8 0.991F7 1.106F9 79.9F1.2 1.004F17 – –

DR21-1 8.23 0.486 0.153 0.956F5 0.993F10 1.047F10 1.095F11 64.7F0.8 1.143F19 – –

DR21-4 5.93 0.495 0.156 0.958F9 1.003F20 – – 68.8F1.4 – – –

DR22-1 0.392 0.123 0.947F4 1.000F10 1.046F8 1.104F9 45.9F1.6 1.006F36 – –

DR23-1 6.53 0.002 0.145 0.954F8 1.016F26 1.045F17 1.095F20 54.9F0.4 1.024F18 – –

DR24-1 7.00 0.486 0.149 0.931F5 1.011F12 1.030F7 1.106F9 60.3F0.6 1.079F20 – –

Central sector

DR25a-1 7.94 0.220 0.0720 0.995F4 0.999F11 1.044F14 1.050F15 26.9F0.3 1.055F19 – –

DR25a–8 7.94 0.218 0.0729 1.017F5 1.009F10 1.084F10 1.066F11 26.9F0.4 1.027F19 – –

DR26-2 7.71 0.299 0.101 – – – – 41.5F1.5 – – –

DR27a-1 7.57 0.423 0.135 0.973F5 1.003F8 1.053F91 1.085F94 55.6F1.3 1.12F10 – –

DR29a-6 7.13 0.637 0.198 0.944F3 1.009F8 1.038F6 1.099F7 84.2F1.7 1.144F24 92.7F3.2c 1.44F05

DR31-1 7.15 0.486 0.156 0.973F5 1.015F8 1.034F18 1.062F19 – – – –

DR31-1* 7.15 0.484 0.155 0.971F7 1.003F13 1.053F14 1.085F16 61.5F0.8 1.092F21 107F5c 2.11F09

DR32-1 7.11 0.607 0.194 0.971F4 1.010F5 – – 75.9F1.4 – 102F4 1.63F07

DR32-1* 7.11 0.604 0.193 0.969F5 1.015F9 1.053F12 1.087F13 84.9F1.2 1.199F21 – –

DR33-2 7.02 0.628 0.200 0.965F4 1.012F4 1.045F11 1.083F12 84.4F0.9 1.160F23 – –

DR34-1 6.53 0.790 0.248 0.952F5 0.996F10 1.042F8 1.094F10 112F1 1.217F17 – –

DR35-1 4.01 1.972 0.601 0.925F5 1.010F6 1.044F8 1.129F9 – – – –

DR35-1* 4.01 – 0.597 – – – – 279F8 1.219F34 – –

DR36-1A 3.09 3.634 1.043 0.871F4 1.000F12 1.014F8 1.164F10 372F6 0.908F17 468F9 1.38F03

DR36-5 5.50 1.364 0.407 0.905F4 1.003F10 1.038F7 1.147F8 157F2 0.997F13 179F4 1.35F03

DR37-5 7.76 0.445 0.139 0.944F3 1.001F8 1.058F9 1.121F10 55.3F1.0 1.055F21 – –

DR38-1 6.04 1.057 0.325 0.934F4 1.009F3 – – 125F2 – 132F6 1.25F06

DR38-1* 6.04 1.068 0.326 0.926F11 0.988F25 1.044F10 1.127F17 124F1 1.004F10 – –

Transform fault

DR43-1 6.42 0.0761 0.0252 1.003F4 0.998F13 1.112F24 1.109F24 – – – –

DR43-6 6.42 0.0780 0.0260 1.013F4 1.004F11 1.111F11 1.097F11 12.8F0.2 1.323F23 20.4F0.8c 2.41F09

Western sector

4D-2 0.271 0.084 0.936F11 – – – – – – –

DR48-6 7.27 0.436 0.132 0.919F4 0.997F8 1.049F14 1.142F15 69.1F0.4 1.357F19 – –

12D-2 1.736 0.506 0.884F5 1.003F9 1.055F8 1.193F10 222F4 1.088F21 – –

DR49-1 5.57 1.227 0.352 0.870F4 1.000F8 1.026F6 1.180F8 133F2 0.952F13 – –

DR49-10 5.83 1.224 0.344 0.853F4 0.997F7 1.027F9 1.204F11 140F3 0.998F23 177F6 1.58F05

17D-4 1.081 0.321 0.902F8 1.013F25 1.027F11 1.139F15 126F1 1.024F13 – –

DR53-1 6.46 1.123 0.315 0.850F4 1.007F8 1.032F13 1.214F15 – – 189F11 1.85F11

T.F. Kokfelt et al. / Earth and Planetary Science Letters 234 (2005) 165–187168

Table 1 (continued)

Sample MgOa

(wt.%)

Th

(Ag/g)U

(Ag/g)

�238U

232 Th

�b �234U238 U

� � 230Th232 Th

� �230Th238 U

� 226Ra

(fg/g)

�226Ra230 Th

� 231Pa

(fg/g)

�231Pa235 U

�

DR53-1* 6.46 1.117 0.310 0.841F5 1.009F10 1.017F7 1.208F10 155F1 1.228F14 197F6c 1.94F06

19D-1 1.625 0.454 0.848F4 1.004F6 1.005F9 1.185F11 221F3 1.219F21 – –

DR54-7 7.64 0.437 0.120 0.835F3 1.004F11 1.010F10 1.210F12 63.3F0.4 1.290F14 70.9F1.1 1.81F03

25D-1 1.176 0.333 0.859F4 1.005F5 1.018F9 1.186F11 133F4 0.996F34 – –

Off–rift seamounts

DR30-5 8.12 0.091 0.033 1.091F6 0.995F14 1.075F39 0.985F36 13.7F0.4 1.252F56 – –

DR41a-1 0.614 0.187 0.925F5 – 0.934F11 1.010F12 64.2F1.0 1.006F19 – –

DR46a-1 1.078 0.329 0.925F4 1.005F7 0.957F11 1.035F12 – – – –

DR47a-1 8.43 0.281 0.093 1.002F6 1.020F9 1.047F11 1.045F11 – – – –

DR47a-1* 8.43 0.275 0.091 1.007F6 1.000F12 1.072F13 1.064F14 33.0F0.9 1.007F28 48.9F2.3c 1.64F08

DR50-1 0.698 0.204 0.886F6 1.003F16 0.918F10 1.036F12 70.7F0.9 0.991F16 – –

Rock standard

TMLd (n=6) 29.74 10.69 1.090F6 1.008F7 1.088F8 0.998F9 3536F28 0.991F15

a Major elements are measured on glass by electron micro-probe. Italic numbers are measured on different glass sample but from same dredge

(see [21]).b Decay constants used: E(226Ra) = 4.3322�10�4; E(230Th) = 9.1577�10�6; E(231Pa) = 2.116�10�5; E(232Th) = 4.9475�10�11;

E(234U)=2.8263�10�6; E(238U)=1.55125�10�10. Errors are 2 S.D. analytical errors.c Individual digestions for Pa analysis only. Treplicate analyses.d Table Mountain Latite, split powder acquired from A. Heumann (Uni. Gfttingen).

T.F. Kokfelt et al. / Earth and Planetary Science Letters 234 (2005) 165–187 169

hotspot is approached from the east and west, the GSC

undergoes a transition from an axial valley-and-rift

morphology at relatively deep axial depths (e.g., ~2450

m below sea level (m.b.s.l.) at ~86, 8W) to an axial high

morphology at progressively shallower depths (e.g.,

~1500 m.b.s.l. just east of the 918WTF) [13,19,21,34].

The 918W TF marks a dramatic depression in

bathymetry with axial depths reaching 3300 m.b.s.l.

The gradational patterns along the ridge are believed to

reflect an unusually strong mantle temperature and

compositional gradient along the GSC related to the

Galapagos plume (e.g. [13,20,22] with increased

magma production closer to the plume. During the R/

V Sonne 158 cruise, a detailed mapping and dredging

program was carried out on the GSC, whereby basaltic

glass was collected from ~40 locations over 700 km of

ridge, between 86802W and 92832W [21,35] (Fig. 1).

The average spacing of the on-axis dredge hauls was

~17 km. The SIMRAD EM120 multi-beam sonar

system onboard the R/V Sonne was used to generate a

detailed map of the seafloor topography, facilitating the

location of the present-day spreading activity, as well

as, of several off-axis seamounts [21, 22]. This

mapping revealed a slightly off-axis seamount at

89839VW, which was sampled by dredging (sample

DR30-5), along with several seamounts situated to the

west of the 918W transform fault, also sampled by

dredging (samples DR41a, DR46a, DR47a-1 and

DR50-1). In addition to the Sonne 158 samples, we

also include five samples from the recent Ewing cruise

EW0004 to the westernmost part of the study area

[13,15,19]. Appendix Table A contains a description

of the glasses included in this study.

The U–Th–Ra data were obtained by TIMS

analysis at the GEOMAR Research Center (now:

Leibniz Institute for Marine Sciences: IFM-GEO-

MAR) while Pa data were measured at the University

of Illinois at Urbana Champaign (UIUC). All work on

the submarine samples involved mm-sized glass chips

that were broken off basaltic pillow rims and hand-

picked under a binocular microscope. Typically 0.2–

0.7 g of glass chips were leached for 10 min with a 1:1

mixture of 2.5 M HCl and 30% hydrogen peroxide

while immersed in an ultra-sonic bath at room

temperature and thereafter rinsed repeatedly with

Milli-Q water before digestion. The procedures for

acid treatment, spiking and column chemistry for U,

Th and Ra follow Kokfelt et al. [3] while Pa chemical

purification generally followed that in [36]. Ten out of

sixteen Pa analyses were carried out on the same

T.F. Kokfelt et al. / Earth and Planetary Science Letters 234 (2005) 165–187170

digested sample as U–Th–Ra while the remaining

(six) involved individual sample digestions (see Table

1), following a modified chemistry procedure adopted

from Regelous et al. [37]. The TML rock standard

gave ( 2 3 0Th ) / ( 2 3 2Th) = 1 .088F8 , ( 2 3 0Th ) /

(238U)=0.998F9 and (226Ra)/(230Th)=0.991F15

(all errors being 2r standard deviation; Table 1),

indicating radioactive equilibrium as expected for

N1 Myr old material. The (230Th)/(232Th) is slightly

higher than what has been published for earlier

batches of the TML (1.072; [38]; 1.068–1.092 [39]).

Based on the fact that our TML data are in secular

equilibrium, we ascribe the higher (230Th)/(232Th) to

heterogeneity between the different batches of TML

powder (see also [39]). Duplicate analyses of five

individually leached and digested GSC glasses show

good reproducibility (Table 1). We estimate the

uncertainty on Th/U and (230Th)/(232Th) to be better

than 1.0% and 1.5%, respectively (2r standard devia-

tion). Radium concentrations by isotope dilution are

reproducible to within 2% based on TML and six

duplicate GSC glasses (Table 1). The reproducibility

of the Pa concentrations by isotope dilution mass

spectrometry is only ~5%, reflecting problems with

low ionization efficiency on these relatively low

concentration samples. Agreement between U-series

disequilibria and U–Th concentrations in MORB glass

A2392-9 with measurements made in other laborato-

ries [40] indicates the accuracy of spike calibrations

and analytical methods at UIUC [41].

3. Results

The new U-series data on the GSC and selected

seamounts from the region are reported in Table 1.

Most glasses are moderately differentiated with MgO

contents ranging between 6.0 and 8.5 wt.% with a

few ranging from 3.0 to 6.0 wt.% (Table 1). U and

Th concentrations and Th/U span a wide range

(Th=0.076–3.63 ppm, U=0.025–1.04 ppm, Th/

U=2.78–3.64) with complete overlap between the

ridge axis basalts and the seamount samples (Table

1). The overall (230Th)/(232Th) and (230Th)/(238U)

(note that parentheses indicate activities throughout

this paper) range from 0.92 to 1.11 and from 0.99 to

1.21, respectively. Most samples, however, have

rather constant (230Th)/(232Th) ranging between

1.00 and 1.05 (Table 1). The (226Ra)/(230Th) range

from 0.91 to 1.36 with 18 out of 41 samples

measured having radium at or close to secular

equilibrium, possibly indicating post-eruptional age-

ing in these lavas. Sample DR36-1A, which is one of

the most evolved samples (MgO=3.1%), has 9%226Ra deficit, but at the same time has 16% 230Th

excess. The overall (231Pa)/(235U) disequilibria range

is between 1.25 and 2.41, but when excluding

samples with radium in equilibrium (or deficit), this

range narrows slightly to 1.44–2.41 (Table 1). (234U)/

(238U) for these glasses cluster tightly around 1.00

indicating that significant incorporation of seawater

or seawater-altered materials has not occurred.

3.1. Longitudinal variation of geochemical and long-

lived isotopic tracers

The longitudinal variation of the axial depth

profile and selected geochemical and isotopic

tracers is shown in Fig. 2 (see also [21,22]. Ratios

of Rare Earth Elements (REE) such as Tb/Yb and

La/Sm, as well as Pb and Nd isotopic ratios, show

generally consistent gradational along-axis varia-

tions with enrichment peaks near the WDL-GSC

junction (~928W) and just east of the 918W TF

(~90.5–90.78W), corresponding with two of the

shallowest parts of the GSC ridge. Corresponding

depletion peaks occur at the 918W TF and at 89.28W.

Smaller enrichment peaks also occur at ~89.58Wand at

~88.38W, although the sample at 88.38W is not zero-

aged (see below) and therefore may not relate directly

to the present situation on the ridge (Fig. 2).

3.2. Longitudinal variation of U-series isotopes

Fig. 3 shows Th/U, (230Th)/(232Th), (230Th)/(238U),

(226Ra)/(230Th) and (231Pa)/(235U) vs. longitude. The

variation of Th/U, (230Th)/(232Th), (230Th)/(238U), and

to some extent also (226Ra)/(230Th), show general

similarities to the gradational patterns observed for the

REEs and Pb–Nd isotopes (Fig. 2). The highest

(230Th)/(238U) (1.21), (231Pa)/(235U) (1.9) and Th/U

(3.64) of ridge samples occur at ~928Wnear the WDL-

GSC junction. Between 928W and the 918W TF,

(230Th)/(238U) and Th/U decrease and (230Th)/(232Th)

and possibly (226Ra)/(230Th) and (231Pa)/(235U)

increase, as the lavas become progressively more

Western longitude

-3000

-2500

-2000

-1500

Axi

al d

epth

(m

.b.s

.l.)

91°W

Transf

orm

OSC

0.0

0.8

1.0

1.2

1.4

0.0

0.5

1.0

1.5

La/S

mN

0.51295

0.51300

0.51305

0.51310

143 N

d/14

4 Nd

Tb/Y

b N

18.4

18.6

18.8

19.0

206 P

b/20

4 Pb

WDL

8586878889909293 91

30-5

(a)

(b)

(c)

(d)

(e)

19-1

47a-1

41a-146a-1

50-1

30-5

30-5

30-5

47a-1

37-5

41a-1

46a-1

50-1

Western lavas Eastern lavas

East (86.0°-87.2°W)

Central-East (87°-89.0°W)

Central bulge (89°0-90.7°W)

91°W Tranform fault

West (91°-92°W)Off-axis seamounts

GS

C

Fig. 2. Longitudinal variation along GSC of (a) axial depth, (b) Tb/Yb, (c) La/Sm, (d) 206Pb/204Pb and (e) 143Nd/144Nd. Two enrichment peaks

are observed; one at the WDL-GSC junction around 928W, and one immediately east of the 918W Transform at ca 90.58W. Sample DR19-1 may

represent a third possible enrichment peak at 88.28W. DR30 is a depleted off-axis seamount; DR43 is depleted glass from a volcano situated in

the 918W Transform trench. Data from [18,21,22].

T.F. Kokfelt et al. / Earth and Planetary Science Letters 234 (2005) 165–187 171

1.00

1.20

1.40

1.60

1.80

2.00

2.20

2.40

0.80

0.90

1.00

1.10

1.20

1.30

1.40

2.9

3.1

3.3

3.5

3.7

3.9

0.90

0.95

1.00

1.05

1.10

1.15

1.20

1.25

8586878889909293 91

Western longitude

0.90

0.95

1.00

1.05

1.10

2.7

(226 R

a)/(

230 T

h)

Th

/U(23

0 Th

)/(23

8 U)

(230 T

h)/

(232 T

h)

(231 P

a)/(

235 U

)

(a)

(b)

(c)

(d)

(e)

36-1A

19-1

43-143-6

30-5

19-110-236-1A

25a-8

31-1

47a-1

41a-1

50-1

46a-1

47a-1

30-5

30-5

41a-1

46a-1

50-1

47a-1

41a-146a-1

50-1

30-5

Western lavas Eastern lavas

91°W

Transf

orm

OSCW

DL

47a-1

47a-141a-150-1

(226Ra/230Th) <= 1

T.F. Kokfelt et al. / Earth and Planetary Science Letters 234 (2005) 165–187172

Table 2

Correlation matrice for GSC central bulge lavas (89.78W–90.78W)

CorrCoef TiO2 FeO MgO CaO Na2O K2O P2O5 Th U Th/U� 230Th232 Th

� �230Th238 U

� 87Sr86 Sr

143Nd144 Nd

206Pb204 Pb

TiO2 1 0.959 �0.837 �0.772 0.784 0.791 0.663 0.673 0.690 0.792 �0.372 0.761 0.906 �0.897 0.843

FeO 1 �0.858 �0.796 0.773 0.806 0.687 0.710 0.723 0.769 �0.524 0.721 0.879 �0.823 0.747

MgO 1 0.980 �0.974 �0.992 �0.955 �0.958 �0.966 �0.859 0.723 �0.812 �0.855 0.790 �0.710

CaO 1 �0.973 �0.983 �0.967 �0.960 �0.967 �0.842 0.742 �0.770 �0.799 0.731 �0.646

Na2O 1 0.988 0.962 0.966 0.971 0.896 �0.706 0.845 0.845 �0.810 0.753

K2O 1 0.973 0.983 0.987 0.888 �0.751 0.835 0.853 �0.791 0.714

P2O5 1 0.980 0.983 0.799 �0.764 0.711 0.717 �0.650 0.569

Th 1 1.000 0.866 �0.820 0.800 0.791 �0.709 0.629

U 1 0.866 �0.811 0.801 0.797 �0.718 0.638

Th/U 1 �0.712 0.967 0.931 �0.892 0.842

(230Th/232Th) 1 �0.509 �0.574 0.424 �0.303

(230Th/238U) 1 0.924 �0.911 0.88487Sr/86Sr 1 �0.969 0.920143Nd/144Nd 1 �0.985206Pb/204Pb 1

T.F. Kokfelt et al. / Earth and Planetary Science Letters 234 (2005) 165–187 173

depleted in incompatible elements (Table 2). Lavas

from the 918W TF have the maximum (230Th)/(232Th)

(1.11) and (231Pa)/(235U) (2.4) and near maximum

(226Ra)/(230Th) (1.32) but low (230Th)/(238U) (1.10)

and Th/U (3.0). East of the 918W TF, (230Th)/(232Th)

remain constant at ~1.05, while Th/U and (230Th)/

(238U) vary slightly (2.97–3.48 and 1.05–1.16, respec-

tively) with slight peaks at 90.38Wand a slight trough

at 89.28W (Fig. 3). The 87.38W OSC is not associated

with a significant change in any of these ratios.

Three out of four seamount samples from west of

the 918W TF have Th/U that fall on the general GSC

trend (Fig. 3a), and as such, do not show indications

of relative enrichment compared to the ridge basalts.

Interestingly, sample DR47a-1, which plots slightly

below the general Th/U transition, is from a seamount

that is only slightly off-axis and the closest of the four

seamounts to the ridge (Fig. 1). This sample however

falls on the array in terms of (230Th)/(232Th) (Fig. 3b).

On the other hand, the remaining seamount samples

west of the 918W TF have significantly lower (230Th)/

(232Th) and plot close to the equiline, suggesting that

they are older, non zero-aged samples (see below).

DR30-5 is a highly depleted (U=33 ppb, Th/U=2.78)

slightly off-axis seamount sample from east of the

Fig. 3. Longitudinal variation along GSC of (a) Th/U, (b) (230Th)/(232Th),

well-defined gradations exist, best expressed in panels (a–c). From 928W(230Th)/(232Th) increases. In this segment interval, there are no clear grada

898W, Th/U, (230Th)/(238U) and (226Ra)/(230Th) form a peak at ca 90.58W

918W TF (Fig. 1). Because DR30-5 has significant226Ra excess, the (230Th)/(238U) most likely reflects its

value upon eruption, unless 226Ra excess were to

reflect post-eruption alteration. The sample, however,

appears very fresh in thin section, supporting an age

significantly younger than 8000 yr.

3.3. The U–Th equiline diagram

On the U–Th equiline diagram (Fig. 4), the

majority of GSC lavas define a sub-horizontal

(weakly positive) array with (230Th)/(232Th)=1.00–

1.11 and (238U)/(232Th)=0.83–1.09. Two geographi-

cal groups are indicated on Fig. 4. Lavas from the

918WTF andwest of it (dwestern lavasT) form a slightly

steeper array of samples with generally higher Th/U

compared to lavas from east of the 918W TF (deasternlavasT) (Fig. 4). The low Th/U (depleted) end-member

in the dwestern lavasT array is represented by the

geochemically depleted 918W TF samples (DR43-1,

DR43-6), whereas the depleted end-member of the

deastern lavasT array is defined by the 898W samples

(DR26-2, DR25a-1 and DR25a-8). The depleted

(b8000 yr) seamount sample (DR30-5) lies directly

on the extension of the eastern array having the lowest

(c) (230Th)/(238U), (d) (226Ra)/(230Th) and (e) (231Pa)/(235U). Local

to 918W, Th/U and (230Th)/(238U) decrease towards the TF, and

tional patterns for the 226Ra or 231Pa excesses. Between 90.78W and

, whereas (230Th)/(232Th) remains constant (symbols as for Fig. 2).

0.8

0.9

1.0

1.1

1.2

1.3

0.6 0.7 0.8 0.8 1.0 1.1 1.2

MORB

OIB

(238U)/(232Th)

(230 T

h)/

(232 T

h)

166kyr

43kyr

Western lavas

Eastern lavas

Galápagosarchipelago(A. Saal)

average 2σ error bar

138kyr

(226Ra/230Th) <= 1

radioactive decay

t = ∞

38kyr

Fig. 4. U–Th equiline diagram comparing the GSC with literature data for Hawaii, MORB and the Galapagos Archipelago [A. Saal, personal

communication]. The GSC data form a sub-horizontal (weak positive slope) array at (238U)/(232Th)=0.83–1.09 and (230Th)/(232Th)=1.01–1.11.

(230Th)/(238U) disequilibria range from unity to 21% 230Th excess. The western and eastern lava fields encircle samples to the west and east of

the 918W TF, respectively. Samples with (226Ra)/(230Th)=1, which fall below their respective arrays, are interpreted to have undergone post-

eruptional ageing, in proportion to the vertical displacement from the arrays. The inferred post-eruptional ageing range from some tens of

thousands in two ridge basalts (DR10-2 and DR19-1), to a few hundred thousand years for several of the seamount samples (DR41a-1, DR46a-1

and DR50-1). Data sources for the OIB and MORB fields are compiled from the PETDB database [60] (symbols as for Fig. 2).

T.F. Kokfelt et al. / Earth and Planetary Science Letters 234 (2005) 165–187174

Th/U and 230Th–238U in equilibrium despite the

presence of 226Ra excess. If the TF samples are not

included, the GSC data form a single array. 226Ra

excess in most of these samples eliminates any simple

age explanation for the array(s) on the U–Th equiline

diagram. Linear positive arrays are typical of ridges and

are often interpreted as mixing of melts derived from

variably enriched sources that have undergone partial

melting under various physical conditions (e.g., vari-

able depths, degrees and rates of melting [42].

The GSC data plot in the center of the global OIB

field, which is characterized by relatively low (230Th)/

(232Th) and (238U)/(232Th) compared to global MORB

(Fig. 4). Compared to data from the Galapagos

archipelago (A. Saal, personal communication), the

GSC show a similar range of (230Th)/(232Th), but with

a less extreme variation in Th/U (Fig. 4). The GSC

data are similar to other plume influenced MORB

such as those from the Azores or Reykjanes regions of

the mid-Atlantic Ridge [1,2].

3.4. Identification of the Galapagos plume

components on the GSC

In the 143Nd/144Nd vs. 206Pb/204Pb diagram (Fig.

5a), the GSC is compared to the geographically

distinct isotopic domains of the Galapagos archipe-

lago [31] (Fig. 1), as defined in [10,32]. The eastern

GSC lavas (868W–90.78W) form a tight negative

array that extends from the edge of the Central

domain through the Galapagos Eastern domain

towards DMM (Fig. 5a). In contrast, most of the

western GSC and corresponding seamounts and

918W TF samples have lower 143Nd/144Nd at similar206Pb/204Pb, within the Eastern domain but indicat-

ing mixing with the Northern domain (Fig. 5a). The

most eastern GSC lavas cluster at the depleted end

of the general Nd–Pb array indicating progressively

less Central Plume Component to the east (CPC;

Fig. 5a). Therefore at least two isotopically distinct,

enriched mantle components are required as mixing

endmembers on the GSC [20–22]. A Northern

Plume Component (NPC) is inferred to enter the

GSC at ~928W, whereas the CPC enters at ~90.78W(Figs. 2 and 3). There is no indication of the

involvement of the Galapagos Southern Plume

Component (SPC) in the GSC lavas or neighbouring

seamount samples (Fig. 5a). The spatial division

between the NPC and CPC on the GSC appears to

occur approximately at the 918W TF. The lack of

evidence for a CPC signature in seamounts or

islands situated between Isabella Island and the

R2 = 0.809

R2 = 0.946

R2 = 0.908

(b)

(a)

0.51285

0.51290

0.51295

0.51300

0.51305

0.51310

0.51315

0.51320

18.0 18.5 19.0 19.5 20.0 20.5206Pb/204Pb

143 N

d/14

4 Nd

208 P

b*/

206 P

b*

0.92

0.93

0.94

0.95

0.96

0.97

2.6 2.8 3.0 3.2 3.4 3.6 3.8

Geo

chro

n κ

= κ P

b

50-1

47a-1

46a-1

41a-1

30-5

50-1

47a-146a-1

41a-1

30-5

Effect of recentmelt extraction

232Th/238U (κ)

NPC

CPCSPC

EPC/DMM

48-6

48-6

Eastern GSC lavas

Western GSC lavas

Western GSC lavas

East of OSC

Central bulgelavas

Eastern Central

Southern

Northern

53-1

36-1A

Fig. 5. (a) 143Nd/144 Nd vs. 206Pb/204Pb and (b) 208Pb*/206 Pb* vs. 232Th/238U (measured kappa). The analytical errors are in both diagrams

smaller than the plot symbols. (a): New data for GSC glasses are compared to the four geographical domains within the Galapagos archipelago,

referred to as the Northern, Eastern, Southern and Central domains [10,32], which have formed by mixing of four distinct mantle components

[11,12,30–32]. We refer to the Galapagos components [11,12,30–32] as the Central Plume Component (CPC), Northern Plume Component

(NPC), Southern Plume Component (SPC) and Eastern Plume Component (EPC). The GSC data form a relatively tight negative array which

largely overlaps the Eastern domain but extends to the Central Domain, indicating mixing between the Central Plume Component and a depleted

MORB or plume component. The 91–928W, including the 918W Transform samples, tend to have lower 143Nd/144Nd for a given 206Pb/204Pb

ratio, consistent with the involvement of the Northern Plume Component in these lavas. Seamount sample DR46a-1 plots well within the

Central domain despite its westerly location near the GSC (see Fig. 1), indicating that Central domain plume material is currently present to the

west of the 918W TF, as well as beneath the central bulge. (b) On a segment scale the GSC display correlations between 208Pb*/206Pb*, or the

time-integrated Th/U of the source, with the measured Th/U in the lavas. The central bulge (between 89.28W and the 918W TF) and western

(between the 918W Transform and 928W) lavas form linear arrays to the left of and oblique to the Geochron. Pb and Nd isotope data from [22]

(symbols as for Fig. 2).

T.F. Kokfelt et al. / Earth and Planetary Science Letters 234 (2005) 165–187 175

GSC probably suggests that plume–ridge mass

transfer occurs at sub-solidus depths. One possibility

is that a plume arm extends at depth to the GSC or

alternatively that the main plume conduit is tilted to

the northeast such that CPC material reaches the

ridge at depth as might be suggested from seismic

data in the region [29].

The gradational along-axis variations could reflect

progressive mixing and dilution of the two enriched

plume components with local depleted MORB-like

T.F. Kokfelt et al. / Earth and Planetary Science Letters 234 (2005) 165–187176

mantle (DMM). Alternatively, the depleted mantle

component is a part of the Galapagos plume (i.e.,

Eastern Plume Component; EPC), possibly reflecting

either entrained upper mantle [31] or recycled oceanic

lower crust or lithospheric mantle [10]. In case of the

latter alternative, the observed gradual variations may

also reflect the progressive melting out of the more

fertile plume components as mixed plume material

enters and flows laterally along-axis.

In a 208Pb*/206Pb* vs. 232Th/238U diagram, the

central bulge (between 89.28Wand the 918W TF) and

the western (between the 918W TF and 928W) lavas

consist of two sub-parallel positive trends oblique to

and to the left of the Geochron. These correlations

indicate that the Th/U variations observed in these

lavas reflect a long-term source characteristic and do

not simply reflect fractionation of Th and U during the

ridge melting process.

In summary, the relationship between 143Nd/144Nd

vs. 206Pb/204Pb provides evidence for source mixing

taking place at different longitudinal intervals along

the GSC involving geochemically and isotopically

distinct plume sources [20–22]. The two major geo-

chemical enrichment peaks at ~928W and ~90.58W(Figs. 2 and 3) are likely to represent the positions of

largest influx of CPC and NPC on the GSC, consistent

with each location representing the maximum flux of

each component into the ridge system through either

melt or solid flow [20–22] (Fig. 5).

4. Discussion

The combination of major elements, trace elements

and long-lived isotopic tracers with U-series disequi-

libria allow assessment of (1) time scales and relative

timing of melting, differentiation, crustal interaction

and mixing, (2) melting depth, and (3) identification

of the processes of material transport.

4.1. Effects of post-eruption decay

Samples having magmatic 226Ra excess are con-

strained to be younger than ~8 kyr. Within this time,

the decay of unsupported 230Th (1–2%) and 231Pa (6–

8%) will be relatively minor compared to the absolute

range of observed disequilibria. On the other hand,

samples with (226Ra)/(230Th)=1 may have undergone

significant post-eruption decay of 231Pa and 230Th.

For instance, the fact that lower 231Pa excesses occur

in samples with (226Ra)/(230Th)=1 indicates that

post-eruption decay has been important for these

samples. Furthermore, several lavas including the

three seamount samples (DR41a-1, DR46a-1 and

DR50-1) and two ridge axis samples (DR-10-2 and

DR19-1) lie significantly below the general array of

samples in the U–Th equiline diagram (Fig. 4). The

displacement of these samples towards lower (230Th)/

(232Th), along with their (226Ra)/(230Th)=1, is con-

sistent with these samples having undergone various

amounts of ageing since the time of original

disequilibria production, presumably during the melt-

ing process. Assuming decay after eruption is solely

responsible for their position beneath the array, the

following eruption ages may be inferred for these

samples: seamount samples DR46a-1=~138 kyr,

DR50-1=~166 kyr and ridge samples DR10-2=~43

kyr and DR-19-1=~38 kyr (Fig. 4). The lack of a

clear systematic along-axis variation in 231Pa excess

doesn’t allow us to constrain possible eruption ages

from (231Pa)/(235U) further.

4.2. The U-series disequilibria–magma differentiation–

source variation relationship

The GSC lavas have generally lower MgO contents

than average MORB (PETDB database at http://

petdb.ldeo.columbia.edu/petdb/), leading to the sug-

gestion that many GSC lavas have undergone some

crustal differentiation and interaction [5]. The U-series

data from the GSC, however, suggest that prolonged

crustal differentiation or contamination does not play

an important role in shaping the disequilibria in the

majority of GSC lavas. First, (234U)/(238U) is in

secular equilibrium for almost all samples measured,

indicating that incorporation of significant amounts of

seawater-altered crust has not occurred. Second,

(230Th)/(232Th), (230Th)/(238U) and (226Ra)/(230Th)

should remain constant or decrease with increased

differentiation (i.e., form a positive correlation with

MgO). To the contrary, (230Th)/(238U), Th/U and

(226Ra)/(230Th) of eastern GSC lavas correlate neg-

atively with MgO (Fig. 6) with these systematics

being particularly clear in the central bulge lavas

(90.7–89.08W). Western lavas on the other hand show

no change in (230Th)/(238U) with decreasing MgO

(c)

(b)

(230 T

h)/

(238 U

)

(a)

Th

/U

MgO (wt.%)

1.00

1.05

1.10

1.15

1.20

2.8

3.0

3.2

3.4

3.6

0.9

1.0

1.1

1.2

1.3

1.4

1098765432

(226 R

a)/(

230 T

h)

Central bulgelavas

36-1A

35-1

Western lavas

2σ error

2σ error

Central bulgelavas

Central bulgelavas

2σ error

fractionalcrystallization

frac. cryst.

(226Ra/230Th) <= 1

Fig. 6. MgO versus (a) Th/U, (b) (230Th)/(238 U), (c) (226Ra)/(230Th) to investigate the effects of differentiation on U-series disequilibria. In (a),

(b) and (c), lavas from the central bulge of the GSC form broad inverse arrays, indicating that the lower MgO samples have higher Th/U,

(230Th)/(238U) and (226Ra)/(230Th). These systematics are opposite to what would be expected for both very short (several hundred of years or

less= flat arrays) and longer (more than several hundred years=positive arrays) differentiation times and/or assimilation of crust more than

~1000 yrs old, ruling out that these processes are important for these lavas. In (c), the western lavas form a positive array with the samples with

the lowest MgO also having the lowest (226Ra)/(230Th) (symbols as for Fig. 2), consistent with differentiation times of more than ~1000 yr and/

or assimilation of crust that is more than ~1000 yr.

T.F. Kokfelt et al. / Earth and Planetary Science Letters 234 (2005) 165–187 177

content. Furthermore, a majority of the samples have226Ra excess indicating that differentiation occurs

over time scales b8 kyr, presumably for all samples

(Fig. 6c). Although difficulty due to unconstrained

amounts of decay since eruption hinder interpreting

the trends of (226Ra)/(230Th), MgO positively corre-

lates with 226Ra excess in western lavas, possibly

reflecting differentiation times of about ~1600 yr for

differentiation from ~7.5 to ~4.5 wt.% MgO. Alter-

natively, such a trend could reflect mixing of two

melts with no time information provided.

Most notably, on an intra-segment scale, strong

correlations exist between long-lived (Sr–Nd–Pb)

isotope ratios, U-series disequilibria and major and

trace element concentrations (Table 2). The strong

geochemical correlations for the central bulge region

(90.7–89.08W) are consistent with two component

mixing being the principle control on the chemical

T.F. Kokfelt et al. / Earth and Planetary Science Letters 234 (2005) 165–187178

compositions of these samples. The endmember with

the lowest MgO (~5.5 wt.%) also has the lowest CaO

and 143Nd/144Nd but the highest FeO, Na2O, TiO2,

K2O, P2O5, Cr, Tb/Yb, La/Sm, La/Yb, Th/U,87Sr/86Sr, 206Pb/204Pb, (230Th)/(238U) and (226Ra)/

(230Th), whereas the endmember with the highest

MgO (~7.9 wt.%) displays the opposite set of

geochemical characteristics (Figs. 2, 3, 5–7; Table

2). Both radiogenic isotope and incompatible trace

element ratios, unaffected by magmatic differentia-

tion, indicate that the low MgO endmember (Figs. 2,

3, 5) represents a greater contribution from the

enriched Galapagos CPC. The high MgO more

depleted endmember, on the other hand, reflects either

a depleted plume component or the ambient depleted

MORB mantle or both [22].

The correlation between Th/U, (230Th)/(238U) and

long-lived isotopic tracers (Fig. 7) can be straightfor-

wardly attributed to an influence of source hetero-

geneity on the generation of U-series disequilibria (see

Section 4.3.). However, the relationship between

MgO (and other major elements) and either U-series

disequilibria or long-lived tracers demands a more

complex explanation, because the major element

(a)

(b)

0.0

0.2

0.4

0.6

0.8

0.0 2.0 4.0 6.0 8.0

MgO (wt.%)

K2O

(w

t.%

)

36-1A

35-1

50

100

150

200

250

300

Cr

(pp

m)

36-1A35-1

0.702

mixing

mixing

fractionalcryst.

fractionalcryst.

Fig. 7. MgO versus (a) Cr and (b) K2O and 87Sr/86Sr versus (c) MgO and (d87Sr/86Sr isotope ratios indicate mixing of two components to explain vari

mixing component with high MgO has high Cr and low K2O,87Sr/86Sr an

low Cr and high K2O,87Sr/86Sr and Th/U. Samples DR35-1 and DR36-1A

undergone differentiation after the mixing event.

compositions clearly do not reflect variations in

primary magma composition but instead reflect differ-

entiation at relatively shallow depths. Unlike OIB

magmas, which can ascend through cold lithosphere

without significant chemical interaction and thus

preserve melt compositions reflecting melting of the

source heterogeneity, MORB magmas are likely to

extensively interact with asthenospheric mantle during

ascent. This results in MgO contents that either reflect

equilibrium with Fo90 olivine at shallow mantle

depths or lowering of MgO due to olivine removal

during differentiation. The observed relationship

between radiogenic isotope ratios, U-series disequi-

libria and MgO content suggests that source hetero-

geneity, which presumably controls the melt

composition produced at depth, somehow affects

how the magmas differentiate.

Similar trends between MgO, (230Th)/(238U) and

Th/U have been observed previously in other ridge

settings. For instance, samples from the 98N EPR

region including the Siquieros Transform show

decreasing Mg# (Mg#=molar (MgO/(MgO+FeO)))

with increasing Th/U and 230Th excess and decreasing226Ra excess [40]. For tholeiitic MORB, MgO and

(c)

(d)

2.9

3.0

3.1

3.2

3.3

3.4

3.5

6 0.7028 0.7030 0.703287Sr/86Sr

36-1A

35-1

2.0

4.0

6.0

8.0

Th

/UM

gO

(wt.%

)

36-1A

35-1

mixing

mixing

fract.cryst.

) Th/U. Correlations of major elements and trace element ratios with

ations in major and trace elements of the central bulge samples. The

d Th/U, whereas the component with low MgO is characterized by

fall below the mixing array in (b) indicating that these samples have

T.F. Kokfelt et al. / Earth and Planetary Science Letters 234 (2005) 165–187 179

Mg# strongly correlate and can be used interchange-

ably as indicators of differentiation. A similar

behavior between MgO, Th/U and (230Th)/(238U)

occurs in samples from the Garrett Transform [41].

Thus, the relationship between Th–U disequilibria,

Th/U and MgO appears to be a relatively reproducible

observation from several Pacific ridges. However, in

these two studies, variations in MgO with long-lived

isotopic tracers is not clear, making the central bulge

of the GSC key to understanding the relationship

between source heterogeneity, ridge melting and

differentiation.

The cause of the relationship between MgO and

(230Th)/(238U) remains to be identified. Sims et al.

[40] attributed the correlation to different liquid lines

of descent for magmas generated at different depths.

This work assumed that primary melts from depth

(from 20 kb) had Mg# significantly less (~0.65) than

melts derived from shallow depth (3 kb; ~0.79) due to

differences in degrees of melting. It is not clear

however why a melt from greater depth should have

lower Mg# nor why it would end up with a lower final

MgO content despite much higher initial MgO

contents (as it should from higher pressure). We

postulate another possible reason for the observed

behavior. Alkali-rich plume melts ascending in a melt

conduit from depth may lose alkalis by diffusion to

the peridotite surrounding the conduit [43]. As a

plume melt loses alkalis by this process, its liquidus

and solidus temperatures rise resulting in over-

saturation in its liquidus mineral phase. For melts

derived from depth, olivine will preferentially precip-

itate presumably in the melt conduits in the shallow

mantle. This will lead to increased olivine fractiona-

tion from enriched melts from depth relative to more

depleted melts derived from depleted peridotite at

shallow depth.

As a rough indication of this effect, we can

calculate the effect of losing Na2O from an ascending

melt by comparing results of crystallization calcula-

tions using MELTS [44]. Assuming a partial melt of

KLB-1 at 1300 8C and 15 kbar [45] as the starting

melt, we compare crystallization of this melt and the

same melt with 1 wt. % less Na2O (remaining oxides

normalized to 100%), both at 1 kbar and 1200 8C.Because Na2O strongly affects mineral-melt equili-

bria, the amount of melt coexisting with olivine

changes from 80% liquid to 70% liquid in the lower

Na2O melt with the increased crystallization domi-

nantly reflecting olivine. Although a quantitative

assessment of this model is beyond the scope of this

work, we note that the Na2O content of average

Galapagos lavas from Volcan Ecuador [46], which is

one of the closest Central Galapagos Domain volca-

noes to the central bulge [10], is significantly higher

(3.2 wt.%) than either the average central bulge or

GSC lava (2.9 wt.%) or typical MORB (~2.5 wt. %) at

the same MgO (6.5 wt.% in both cases) or in

conjunction with alkali loss, volatile contents could

also control the magma fractionation behavior with

greater volatile loss in deeper, enriched plume-derived

melts favouring increased olivine fractionation. These

models provide a link between source enrichment and

extent of differentiation and allow significant differ-

entiation to occur prior to arrival into the crust.

Linear arrays between MgO and long- and short-

lived isotope systems whereby both end members

have non-primary MgO contents are difficult to

explain except by mixing of two melts after differ-

entiation. Although one could theoretically project the

trend to higher MgO contents to identify one

bprimitiveQ endmember, this would result in an

extremely depleted radiogenic isotopic composition

(for example, 87Sr/86Sr=0.7019–23 at MgO=10–12

wt.%), yet to be observed in either MORB or OIB

globally. Therefore we conclude that both central

bulge components represent two distinct melts derived

from different mantle sources that have undergone

separate differentiation histories prior to mixing.

There are two alternatives for how this occurs: (1)

differentiation occurs during ascent in the shallow

mantle before arrival in the crustal melt lens where

mixing occurs; (2) primary melts arrive in the crustal

melt lens, differentiate, then mix.

Our model of differentiation during ascent due to

alkali loss would provide an explanation for how

differentiation may occur prior to arrival at the crust.

However, there are uncertainties with such a model so

the second alternative of differentiation then mixing in

the crust should also be considered. If near-primary

magmas arrive at the crust, then the systematic

decrease in (226Ra)/(230Th) along the ridge between

(90.3–89.08W) and the correlation between MgO and

(226Ra)/(230Th) indicate that differentiation and then

mixing occurred over time scales that did not allow

appreciable decay of 226Ra excess (probably b1000

T.F. Kokfelt et al. / Earth and Planetary Science Letters 234 (2005) 165–187180

yr). One simple model to explain the correlation of

geochemistry along the ridge segment is for the

sources of the two components to enter the segment

at different ends, similar to the spatial zonation

observed within the Galapagos Archipelago [10,18,

30–32]. Thus, the enriched CPC material melts

beneath the shallowest part of the central bulge in

the west while the depleted component melts beneath

the east. Each melt undergoes differentiation to

different MgO contents and lateral flow mixes these

components creating the trends with longitude. Melt

flow along the ridge is likely to be from west to east

down the axial depth gradient. Given the geochemical

gradients observed, melt would travel ~150 km by

such a process, possibly stretching the physical

limitations of such model.

4.3. Variable depth of partial melting indicated by

U-series disequilibria and REE

Significant correlations between U-series disequi-

libria and radiogenic isotopes indicate that composi-

tional variations in the source material affect the

melting process. The issue of whether radiogenic

isotope variations indicate lithologic heterogeneity

(marble cake mantle: [47]) or simply enriched versus

depleted peridotite has been abundantly discussed in

the recent literature. However, discriminating these

two endmember models, particularly in light of the

clear occurrence of shallow differentiation processes

in these lavas, is virtually impossible. We therefore

focus our discussion on the relevant observations to

understanding process, not source, a strength of the U-

series disequilibria technique.

We have already shown that geochemical varia-

tions correlate with the physical characteristics of the

GSC, particularly within the central bulge. Tb/Yb of

GSC lavas closely follow the axial depth profile

across the region (Fig. 2a,b), a relationship that can be

explained by increasing amounts of garnet involved in

producing the melts making up the shallowest portion

of a ridge. Thus a longer melting column, reflecting

either more fertile sources, higher mantle temper-

atures, or both, leads to more crust produced along the

ridge and shallow axial depth. The generation of230Th excess is generally attributed to melting in the

presence of either garnet [48] or aluminous clinopyr-

oxene [49]. A global relationship between 230Th

excess and axial depth has also been observed arguing

for more melting of garnet beneath more shallow

ridges [50,51]. The observation that the highest (Tb/

Yb)N and 230Th excess occurs along the shallowest

part of the GSC (central bulge), where the CPC

reaches the spreading center, is consistent with this

general model.

Indeed, GSC lavas erupted at the two identified

points of plume inflow based on isotopic enrichment

have relatively high 230Th excesses and high middle

REE to heavy REE ratios. These locations represent

the shallowest axial water depth and presumably the

thickest crust but whether this reflects a longer

melting column or increased melting rates is not

clear. In this regard, increased melting rates due to

higher productivity (% melt/ km decompression) or

faster upwelling (as might be expected for proximity

to the plume) will result in lower (230Th)/(238U) and

(231Pa)/(235U), opposite to the variation observed. In

contrast, a longer melting column where melts

continuously equilibrate as they ascend during porous

flow [52] will promote larger (230Th)/(238U) and

(231Pa)/(235U) due to increased melting column

residence times [53]. Melting models which more

closely approximate fractional melting do not create

such a residence time effect with variable depth and

therefore cannot explain the observation. Thus,

greater depths of melt initiation in a melt column

where melts ascend and react during porous flow can

account for the observed spatial pattern of 230Th

excess along axis while variations in melting rate

appear to have little influence.

Previous models have shown that the linear arrays

of U-series disequilibria data for a given area of ridge

can be explained by partial melting of separate

enriched and depleted source compositions followed

by mixing of the endmember melts [42,54]. Generally,

the enriched endmember is inferred to have high Th/

U, likely reflecting small-scale heterogeneities which

occur locally and are intimately intermingled within

the depleted peridotite matrix having low Th/U. The

good relationship between nTh (232Th/238U) and208*Pb/206*Pb for the western and central bulge lavas

indicates that Th/U is primarily controlled by source

variations and not by the melting process. Thus, we

interpret the high Th/U endmembers of the western

and eastern equiline arrays (Fig. 4) to reflect the NPC

and CPC respectively (Fig. 5a; [22].

T.F. Kokfelt et al. / Earth and Planetary Science Letters 234 (2005) 165–187 181

5. Mechanisms of plume–ridge mass transfer

Mass transfer between a plume and a neighbouring

ridge may principally occur in either of two ways: (1)

as movement of melt or (2) as solid mantle flow

[17,23,55–57]. In the case of the former, melt is

envisaged to migrate in sub-lithospheric erosion

channels that are permeable due to sustained melt

porosity and that constitute narrow elongated features

connecting the hotspot with the ridge [23,55]. Given

the buoyancy force driving the melt along with

possible pressure forces due to solid deformation,

melt transport may conceivably proceed over long

distances (up to 1000 km) and require several

hundreds of thousands of years duration, depending

on the speed of migration and the distance travelled.

In contrast, transport by solid flow will be much

slower and more diffuse. Indeed, solid transport will

follow the general direction of upper mantle flow, not

necessarily the direction of flow towards the ridge

along the base of the lithosphere. Possible means of

distinguishing these alternatives have focussed on

numerical models and how they can simulate obser-

vations of the geometry of the plume–ridge system

(position and shape of the inferred transport pathways

in relation to the hotspot-ridge system). Systematic

differences in chemistry of in-situ plume melts,

compared to transported plume melts may also help

constrain the dominant transport process [17]. U-

series disequilibria can provide potential information

about the rate of the transfer process and therefore the

transport mechanism of the process.

5.1. Melt transport from plume to ridge

In the simplest view of melt transport, the original226Ra, 230Th and 231Pa excesses of melts produced

within the plume are unsupported and therefore decay

back to secular equilibrium as they flow from the

plume to ridge. A more complicated scenario that we

do not attempt to model is if disequilibrium is

supported by chromatographic processes during the

plume to ridge transport; however, such an effect is

not likely given that melt channels would likely be

composed of dunite with little ability to fractionate

nuclides by mineral-melt partitioning. The extent of

decay during transport will reflect the effective

transport time, which is a function of the distance

travelled divided by the melt velocity. Due to the

different half-lives of the nuclides studied, each

parent–daughter system will record this dageingTeffect differently. For transport times on the order of

10000 to 100000 yr (i.e. long compared to the half-

life of 226Ra, but short enough to maintain the initial231Pa and 230Th excesses), the resulting dagedT melt

should presumably have (226Ra)/(230Th)= 1, but

(231Pa)/(235U)N1 and (230Th)/(238U)N1. For progres-

sively longer transport times, the 231Pa and the 230Th

excesses will decrease and eventually be lost.

Braun and Sohn [23] (hereafter B and S) provide

estimates of the likely transport times for melt through

sub-lithospheric channels as a function of melt

channel porosity. Thus, the sensitivity of different

U-series parent–daughter pairs can be used to

constrain the timescale of melt transport. We begin

by assuming that Fernandina lies above the plume

center, because it has the highest 3He/4He [8] and is

located over the plume stem at depth [16,29]. Thus,

transport of melts from the central and northern plume

components occurs over ~110 km and ~200 km,

respectively (Fig. 1). For a maximum channel porosity

of 10% (B and S; their Fig. 10), the minimum required

transport times for these distances would be ~90 kyr

and ~200 kyr, respectively. Unsupported decay over

200 kyr would cause the northern plume component

(NPC) melts to have lost all of their initial 226Ra and231Pa excesses, as well as ~85% of the initial 230Th

excess. Similarly, 90 kyr of unsupported decay will

cause the central plume component (CPC) melts to

have lost all 226Ra excess, ~80% of the initial 231Pa

excess and ~60% of the initial 230Th excess.

In plots of (230Th)/(238U) versus (231Pa)/(235U) and

(226Ra)/(230Th) (Fig. 8), the GSC samples from east

and west of the 918W TF constitute two broadly

parallel negative arrays, with the latter being shifted to

higher (230Th)/(238U) and (231Pa)/(235U). The arrays

could be explained by mixing between enriched OIB

and depleted MORB (or plume) type melts. The

intersection of these two arrays with (226Ra)/

(230Th)=1, indicates that the (230Th)/(238U) disequi-

libria in the dagedT NPC melts reaching the western

GSC are ~1.26 and the CPC melts reaching the

eastern GSC are ~1.14 (Fig. 8b). The (230Th)/(238U)

estimated for the NPC melts agrees well with the

average (230Th)/(238U) of 1.27 estimated for the NPC

in the northern Galapagos domain samples (A. Saal,

(2 30Th )/(23

0.80

0.90

1.00

1.10

1.20

1.30

1.40

1.50

1.60

1.70

(226 R

a)/(

230 T

h) t=0

2ka

10

1ka

30ka 3ka

5ka

10ka

t=0

90ka

2ka

1ka

3ka

5ka

0.90 1.00 1.10 1.20 1.30 1.40 1.50 1.60

(230Th)/(238U)

1.20

1.40

1.60

1.80

2.00

2.20

2.40

2.60

2.80

3.00

(231 P

a)/(

235 U

)

MORB

OIB

90ka 90ka

(a)

C*

N*

30-5

43-6

C*

N*

30ka50ka70ka

30-543-6

CPC(Saal)

NPC (Saal)

20ka30ka

10ka

t=0

70

30ka

40ka50ka

10ka

t=0

70ka

(c)

0.85

0.90

0.95

1.00

1.05

1.10

1.15

1.20

1.25

0.70 0.80 0.90 1.00 1.10

20ka

60ka

t=0

30ka

60ka

t=0

90ka

150ka

equiline

Galápagosarchipelago43

MORBC*N*

(238U)/(232Th)

(230 T

h)/

(232 T

h)

30

40ka

90ka

50ka

90ka

50ka

20ka

Zero-agedMORB melts

Decay duringmelt transport

Melt mixingbeneath ridge

Aged/transportedplume melts

Zero-agedplume melts

(226Ra/230Th) <= 1

GSC west of 91°W TransformGSC east of 91°W Transform

T.F. Kokfelt et al. / Earth and Planetary Science Letters 234 (2005) 165–187182

T.F. Kokfelt et al. / Earth and Planetary Science Letters 234 (2005) 165–187 183

personal communication and [58]). Although the CPC

melts from the eastern part of the GSC have higher

(230Th)/(238U) than the average of 1.08 observed in

the central Galapagos domain samples, the eastern

GSC lavas overlap the range observed in the central

domain lavas (A. Saal, personal communication and

[58]). The average (231Pa)/(235U) for the northern

(1.32) and central (1.17) Galapagos domains (A. Saal,

personal communication and [58]) are either similar or

lower than the enriched ends of the western and

eastern GSC arrays respectively (Fig. 8a). Therefore,

the average (230Th)/(238U) and (231Pa)/(235U)

observed presently in the northern Galapagos domain

and a composition within the range observed in the

central Galapagos domain could serve as the enriched

OIB-type endmembers for the western and eastern

GSC lavas, respectively.

The modelled curves in Fig. 8 show the effect of

unsupported radioactive decay, indicated for 90 kyr for

both the northern and eastern enriched plume melts.

The initial 231Pa and 230Th excesses calculated for

transport times of 90 kyr clearly deviate from the

compositions observed in b10,000 yr samples from

the Galapagos Archipelago and plot at the upper edge

of the global OIB array for the eastern GSC lavas and

well beyond the global OIB array for the western GSC

lavas (Fig. 8a). The U–Th equiline diagram (Fig. 8c)

most clearly shows the relative inadequacy of a melt

transport model as discussed by B and S to explain the

GSC data, because such a model would have required

the existence of in-situ plume melts with higher

(230Th)/(232Th) than has actually been measured (A.

Saal, personal communication and [58]). This diagram

also suggests that the maximum time for sustainable

Fig. 8. The effects of different plume-ridge transport mechanisms are mode

(238U) vs. (226Ra)/(230Th), (c) (230Th)/(232Th) vs. (238U)/(232Th). In (a): The

in both diagrams that extend from the global OIB to global MORB field

enriched plume components (labelled dCT for Central and dNT for Northesource, respectively), which is consistent with the conclusions reached ba

(231Pa)/(235U) for DR30-5 was assumed. The effects of ageing during melt

filled stars and labelled dC*T for the Central and dN*T Northern componen

the plume to the ridge). The open stars represent the composition of the plu

possible composition of situ plume melts that are generated by partial melti

plume melts could also have compositions lying along the mixing array wi

and lower (230Th)/(238U). Data sources are given in caption to Fig. 4. (c)

Galapagos hotspot to the GSC. For reference the field for Galapagos ar

(personal communication). The modelling indicate that melt transport from

minimum transport times would indicate initial (230Th)/(232Th) (and 230Th e

stem in presumably zero-aged plume melts.

melt transport is ~35 kyr (allowing melt transport of

~65 km) for the central and northern Galapagos plume-

type magmas, which would still be consistent with the

range found within the archipelago lavas for samples

with appropriate Th/U (A. Saal, personal communica-

tion and [58]). Therefore, melt transport from the

plume center located beneath Fernandina can be ruled

out, because Fernandina is ~110 km from the central

bulge on the eastern GSC and ~200 km from the Wolf

Darwin lineament intersection with the western GSC.

On the other hand, if the plume center at shallow

mantle levels was situated closer to the ridge as

suggested by seismic tomography [29], shorter trans-

port times would be required and might make melt

transport feasible. Seismic tomographic data shows a

broad low velocity zone extending NE towards the

GSC at depths shallower than 100 km, suggesting that

the Galapagos plume stem may be tilted to the NE and

that it currently impinges on the base of the litho-

sphere at a position just SW of the island of Pinta,

only 60 km from the GSC (Fig. 1) (Toomey, personal

communication, 2003). Impingement of plume mate-

rial at the base of the lithosphere in the vicinity of

Pinta is consistent with the proposal that the region

around Pinta is being actively uplifted [19]. Thus, for

an inclined plume stem (or deflection of central plume

material beneath a viscous layer) and accepting the

most favourable conditions of the B and S model

(10% channel porosity), this would indicate that melt

transport to the central bulge could be accomplished

within time scales that would be consistent with the

GSC U-series data.

Even with this alternative location of the plume

center, melt transport to the western GSC (~175 km

lled in diagrams of (a) (230Th)/(238U) vs. (231Pa)/(235U), (b) (230Th)/

eastern and western GSC lavas define distinct broad negative arrays

s. The trends indicate geographically controlled mixing of distinct

rn) with depleted sources (Eastern plume component and MORB

sed on the 206Pb/204Pb vs. 143Nd/144Nd diagram (Fig. 5). Note that

transport is simulated for 90 kyr for the two plume melts denoted by

ts (d*T Indicating the composition of the melts before transport from

me melts after transport for 90 kyr from the plume to the ridge or the

ng of plume material at the ridge after transport by solid flow. In situ

th MORB (or depleted plume) type melts with higher (231Pa)/(235U)

U–Th equiline diagram shows the effects of melt transport from the

chipelago lavas are shown based on unpublished data of A. Saal

plume to ridge is unlikely to be the dominant process, as the required

xcesses) which are far higher than what is observed above the plume

T.F. Kokfelt et al. / Earth and Planetary Science Letters 234 (2005) 165–187184

away, Fig. 1) would however still require periods of

time (~160 kyr) that are inconsistent with the U-series

disequilibria observed in the western GSC lavas.

Nevertheless, it has been proposed that the Galapagos

plume is geochemically zoned such that the central

and northern plume components are spatially sepa-

rated with the northern component being located at the

northern edge of the plume [10,18,32]. Therefore the

distance that melts from the northern zone of the

plume would have to travel to the ridge could be

considerably less than 175–200 km (the distance from

the center of the plume). Melt transport to the

intersection of the WDL with the ridge at 928Wwould however only be possible if the radius of the

plume is z135 km, assuming a plume center beneath

Fernandina and z110 km assuming an inclined plume

that reaches the base of the lithosphere in the vicinity

of Pinta.

In conclusion, transport of melts from the Galapa-

gos plume to the GSC are constrained to be V65 km,

requiring an inclined Galapagos plume to the NE for

melt transport to the GSC east of the 918W TF and a

plume radius of z110 km for an inclined plume or

z135 km for a plume center beneath Fernandina for

lavas reaching the ridge at 928W.

5.2. Solid transport from plume to ridge

The alternative to melt transport plume–ridge

interaction by solid mantle flow has also been recently

investigated using numerical modelling [17,59]. In the

model of Ito et al. [59], the Galapagos mantle plume

was able to flow to the ridge despite only mild

temperature elevations of 50–100 8C. Hall and

Kincaid [17] incorporated the effects of melting and

dehydration during transport into their model. Their

model suggested that dehydration would cause a

drastic viscosity increase of the restitic plume mantle,

which should accrete to the overlying oceanic litho-

sphere and act as a plug forcing later mantle flow to

proceed at a deeper, sub-solidus levels. In turn this

model suggests that relatively unmodified (through

partial melting) plume mantle may reach the ridge and

start to partially melt beneath the ridge [17]. We note

that the model of Hall and Kincaid is also consistent

with a broad low-seismic-velocity anomaly extending

from beneath Fernandina towards the ridge. Thus, it

should be expected that a solid flow model should

produce plume flavored melts beneath the ridge; the

U-series disequilibria in such melts would reflect

recent melting beneath the ridge and not be drastically

affected by decay during melt transport. We however

recognize that melting beneath the ridge extends to

shallower depths (resulting in lower excesses) and at

slower rates (generating higher excesses) than above

the plume stem, and that therefore U-series excesses

don’t have to be identical to the Galapagos Archipe-

lago melts. In Fig. 8a, a solid flow model is illustrated

by binary mixing between a MORB melt and a OIB

melt (short-dashed line).

In summary, the distinct overlap between the GSC

and the Galapagos archipelago lavas in terms of

(230Th)/(232Th) and Th/U (Fig. 8) is generally most

consistent with a solid transport model. Assuming the

plume center is situated beneath the island of

Fernandina, a plume–ridge melt transport model

cannot readily account for the U-series observed on

the GSC as the maximum allowable transport rates

appear to be too fast to be accommodated in a Darcy

flow model, i.e., channel porosities in excess of 10%

would have to be inferred, which probably is

unrealistic. However, if plume–ridge mass transfer

can be accomplished within ~35 kyr, possibly owing

to a more proximal position of the plume to the ridge,

a melt transport model might provide a viable

alternative for transport to the dcentral bulgeT. An

important question in relation to a solid flow model is

to what extent the plume mantle partially melts during

the lateral transport to the ridge, and how this process

may reduce the potential for such restitic plume

mantle to subsequently generate melts at the ridge

with high U-series excesses.

6. Conclusions

The following conclusions may be drawn from this

study:

1. Combined trace element geochemistry and U-

series data indicate inflow of plume-derived

enriched material into the Galapagos Spreading

Center at two locations: (1) at the intersection of

the Wolf Darwin Lineament with the GSC at

~928W and (2) at the central bulge just east of the

918W transform fault. At these two locations, the

T.F. Kokfelt et al. / Earth and Planetary Science Letters 234 (2005) 165–187 185

plume material is geochemically distinct and

identifiable as the dNorthernT and dCentralT Gal-

apagos plume components, respectively.

2. In the U–Th equiline diagram, GSC glasses

inferred to be of zero age form a flat array at

relatively constant (230Th)/(232Th). A few ridge

samples and some seamount samples plot below

the main array and are inferred to have undergone

post-eruption decay for tens to hundreds of

thousands of years. The range in Th/U overlaps

global OIB and MORB fields, consistent with

plume–ridge interaction and indicates mixing of

variably enriched plume and depleted upper mantle

and/or plume sources along the GSC.

3. The along-axis variation of trace element, Sr–Nd–

Pb isotopic ratios and U-series data show similar

gradational patterns, indicating an influence from

source composition on the U-series data. Samples

from the central bulge segment of the ridge (89.0–

90.78W) show good linear correlations between

major elements (e.g. MgO), trace elements, radio-

genic isotope ratios and U-series disequilibria,

reflecting mixing of an enriched (Central Galapa-

gos plume) and a depleted (upper mantle and/or

Eastern Galapagos plume) component. Neither of

the endmembers has MgO expected of melts in

equilibrium with peridotite and thus are interpreted

to reflect mixing between melts that have already

differentiated. This differentiation could occur

during ascent in the shallow mantle due to the

diffusive loss of alkali elements to surrounding

peridotite [43].

4. In diagrams of (230Th)/(238U) versus (231Pa)/

(235U) and (226Ra)/(230Th), the western and east-

ern GSC lavas define fairly distinct negative

correlations. In principal such systematics could

arise from mixing between aged (through trans-

port) off-axis plume melts and local MORB (or

depleted plume component). Assuming transport

from a plume stem located beneath the island of

Fernandina, the transport times (~90–200 kyr)

required for these distances (110–200 km) by the

model of Braun and Sohn [23] would imply

unrealistically high initial excesses at the upper

edge or outside the global OIB range. Moreover, a

comparison of the GSC with preliminary U-series

data from the Galapagos Islands indicates that the

likely maximum melt transport times is ca. 35 kyr,

which according to the Braun and Sohn model

would correspond to maximum transport distance

of ~65 km. A melt transport model would

therefore either require faster transport rates,

which in turn would require unrealistically high

channel porosities (N10%) for a Darcy flow

model, or more plausibly, a possible location of

the plume center closer to the GSC and a

minimum plume radius of 110 km. The latter

option needs further examination.

Acknowledgments

We thank the captain and crew on the R/V Sonne

(SO158) for a well-conducted cruise leg at the

Galapagos Spreading Center. John Sinton and Robert

Detrick are thanked for kindly providing five samples

from the western GSC taken during the Ewing

EW0004 cruise. We warmly thank Alberto Saal (U-

series data) for making his unpublished data available

for this study. This paper benefited from excellent

reviews by Alberto Saal, Simon Turner and Bernard

Bourdon. This study was supported by the German

Ministry of Education and Research (BMBF; Grant

MEGAPRINT). CCL acknowledges NSF support

from OCE9910921.

Appendix A. Supplementary data

Supplementary data associated with this article can

be found, in the online version, at doi:10.1016/

j.epsl.2005.02.031.

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