geochemistry of lavas from the garrett transform fault...

ELSEVIER Earth and Planetary Science Letters 173 (1999) 271–284www.elsevier.com/locate/epsl

Geochemistry of lavas from the Garrett Transform Fault:insights into mantle heterogeneity beneath the eastern Pacific

J. Immo Wendt a, Marcel Regelous a,Ł, Yaoling Niu a, Roger Hekinian b, Kenneth D. Collerson a

a Department of Earth Science, The University of Queensland, Brisbane 4072, Australiab IFREMER, Centre de Brest, 29287 Plouzane, France

Received 23 December 1998; accepted 22 September 1999

Abstract

Young intra-transform lavas erupted as a result of extension within the Garrett Transform Fault on the southern EastPacific Rise, are more porphyritic, less evolved, have lower concentrations of incompatible trace elements, and lower ratiosof more incompatible to less incompatible elements (e.g. low K=Ti and La=Sm) compared to lavas from the adjacentEast Pacific Rise ridge axis. Sr, Nd and Pb isotope compositions overlap with the depleted end of the field for Pacificmid-ocean ridge basalts, but extend to lower 87Sr=86Sr (0.702137), 206Pb=204Pb (17.462), 207Pb=204Pb (15.331), 208Pb=204Pb(36.831), and higher 143Nd=144Nd (0.513345) than any lavas previously reported from the Pacific. Peridotites from theGarrett Transform have Nd isotope compositions within the range of the intra-transform lavas. The unusual major and traceelement compositions of the Garrett lavas appear to be characteristic of other intra-transform lavas from elsewhere in thePacific. The chemical and isotopic features of the Garrett lavas can be explained by remelting, beneath the transform, atwo-component upper mantle which was depleted in incompatible element-enriched heterogeneities during melting beneaththe East Pacific Rise ridge axis (within the past 1 Ma). Our data place new constraints on the trace element and isotopecomposition of the depleted mantle component that contributes to magmatism in the Pacific, and show that this componentis heterogeneous, both on the scale of a single transform fault, and on the scale of an ocean basin. 1999 ElsevierScience B.V. All rights reserved.

Keywords: East Pacific Rise; transform faults; East Pacific; lava flows; geochemistry

1. Introduction

Geochemical studies of mid-ocean ridge basalts(MORB) have shown that the upper mantle beneaththe ocean basins is chemically and isotopically het-erogeneous [1–3]. In some cases, heterogeneity inMORB can be attributed to the presence of near-

Ł Corresponding author. Present address: Max-Planck-Institut furChemie, Postfach 3060, 55050 Mainz, Germany. Fax: C49 6131371051; E-mail: [email protected]

ridge mantle plumes (e.g. [4–7]). However, chemicaland isotopic heterogeneity is found in lavas eruptedat all mid-ocean ridges, including those far fromknown mantle plumes [3,8–10].

The magmas erupted at mid-ocean ridges are de-rived from relatively high degrees of partial meltingof relatively large volumes of mantle, and thesemelts undergo mixing during melt aggregation andfocusing towards the ridge axis, and also withinmagma chambers in the crust. As a result, it isdifficult to infer the chemical and isotopic com-

0012-821X/99/$ – see front matter 1999 Elsevier Science B.V. All rights reserved.PII: S 0 0 1 2 - 8 2 1 X ( 9 9 ) 0 0 2 3 6 - 8

272 J.I. Wendt et al. / Earth and Planetary Science Letters 173 (1999) 271–284

positions of mantle heterogeneities from studies oflavas erupted on ridge axes alone. Lavas eruptedon seamounts on the ridge flanks are derived frommelting of smaller volumes of mantle, and studiesof such rocks have given important insights intothe composition and scale of heterogeneities in theupper mantle. For example, the trace element andisotope variation within Pacific seamounts is greaterthan that seen in Pacific MORB [11–15]. This ob-servation has led to the idea that the chemical andisotopic heterogeneity observed in lavas erupted atspreading centers and on seamounts is the resultof melting a two-component mantle, consisting ofeasily melted, incompatible element-enriched veinsor ‘plums’ with relatively high K=Ti, high 87Sr=86Sretc., within a more refractory matrix which is rela-tively depleted in incompatible trace elements, andhas lower K=Ti and low 87Sr=86Sr [9–11,13–17].Several different depleted and enriched componentsappear to be necessary to explain the heterogeneityobserved in the lavas erupted on Pacific ridge axesand on Pacific seamounts [9,15,18,19]. However, be-cause all of these lavas are derived from differentdegrees of melting of variably enriched mantle, thecompositions of individual endmember componentsin the sub-Pacific upper mantle is not well known.

In this paper, we present new trace element andSr, Nd and Pb isotope data for young lavas erupted asa result of intra-transform extension within the Gar-rett Transform Fault, on the East Pacific Rise (EPR)in the eastern Pacific (Fig. 1). These intra-trans-form lavas include samples with lower 87Sr=86Sr,206Pb=204Pb, 207Pb=204Pb and 208Pb=204Pb and higher143Nd=144Nd than any other lavas previously reportedfrom the Pacific. Compared to lavas from the adja-cent EPR ridge axis, the Garrett lavas are more por-phyritic and less evolved, have lower concentrationsof incompatible trace elements, and lower ratios ofmore incompatible to less incompatible trace ele-ments (e.g. low K=Ti, La=Sm). We argue that theunusual trace element and isotope compositions ofthese lavas are the consequence of melting a two-component mantle beneath a transform fault. Thatis, the Garrett intra-transform lavas are derived froma mantle source which was depleted in the easilymelted, incompatible element-enriched components,as a result of partial melting beneath the EPR axis,within the last 1 Ma. Our data give new insights

into the composition of the depleted upper mantlebeneath the eastern Pacific.

2. Geological setting

The Garrett Transform Fault offsets the fast-spreading (14.5 cm=y) southern EPR axis at 13º280Sby 130 km (Fig. 1a), and is one of the few transformfaults in which active volcanism is known to occur.In the Pacific, systematic sampling of intra-transformvolcanism has only been carried out in the Siqueirosand Garrett transform faults [20–25]. Detailed de-scriptions of the structure and geology of the GarrettTransform have been published elsewhere [22–25].The transform fault zone is around 24 km in width,and the deepest parts of the transform valley are over5000 m in depth. Within the transform valley, threeNE–SW-trending ridges (named the Alpha, Beta andGamma Ridges, Fig. 1) lie at a depth of <3500m. Submersible observations have shown that recentvolcanism within the transform is largely restrictedto these ridges, but has also built small cones on thefloors of the eastern and western transform troughs[22,23]. Young flows also occur on the south wallof the Eastern Trough, in the East Valley, and onthe southern wall of Garrett Deep, in which uppermantle and lower crustal rocks are exposed [22,23].The intra-transform lavas were erupted as pillowlavas and as sheet flows, many of which consist offresh glass [22,23]. Of the samples analysed in thisstudy, sample GN13-1 was collected from a youngintra-transform lava flow on the floor of the EasternTrough (Fig. 1b). Samples GN12-1 and GN12-10are from the Alpha Ridge, and samples GN2-2 andGN2-5 were collected from young flows on the flankof the Gamma Ridge. The other samples were col-lected from older flows, or from talus piles, andmay therefore be from intra-transform lavas, or fromolder crust formed at the EPR ridge axis. Details ofthe sample locations are given in Tables 1 and 2.

3. Petrology and major element chemistry

The Garrett intra-transform lavas are dominantlyolivine and=or plagioclase phyric basalts and picriticbasalts [22,23]. On the EPR, porphyritic lavas such

J.I. Wendt et al. / Earth and Planetary Science Letters 173 (1999) 271–284 273

Fig. 1. Simplified tectonic maps adapted from [25] showing (a) the location of the Garrett Transform in the eastern Pacific, and (b) themajor structural features of the transform and locations of the samples studied.

as these are rare, and are generally only found closeto large transform faults.

Petrographic descriptions and major element anal-yses of the Garrett intra-transform lavas have beenreported previously [22–25]. Major element data forthe samples analysed in this study are given in Ta-ble 2. Compared to lavas erupted on the adjacent seg-ments of the southern East Pacific Rise ridge axis, theGarrett lavas are generally less evolved, and containlower concentrations of incompatible minor and traceelements. For example, the intra-transform lavas haveMg# in the range 58–71, and average K2O contents of0.04% [22,25], compared with values of 40–65 and0.14% for lavas from ridge segments of the south-ern EPR axis adjacent to the Garrett Transform [9].The Garrett lavas also have lower TiO2, FeO, Na2Oand P2O5, and higher CaO and Al2O3 compared to

lavas from the adjacent EPR. The range in major el-ement composition within the Garrett lavas can beexplained by fractionation of olivine š plagioclaseš clinopyroxene from a range of primary melt com-positions; however, the intra-transform lavas do notrepresent parental magmas to the more evolved lavaserupted at the EPR ridge axis [22,25]. Compared toMORB erupted at adjacent segments of the south-ern EPR ridge axis, the Garrett lavas were derivedfrom smaller degrees of melting (¾15%), of a mantlesource which was relatively depleted in incompatibleminor and trace elements [9,22,25,26].

The peridotites recovered from the Garrett Trans-form are variably serpentinised harzburgites, someof which have been locally refertilised, probably asa result of intra-transform magmatism [25]. Spinel,orthopyroxene and rare clinopyroxene in these peri-


Table 1Isotope data for peridotites and lavas from the Garrett Transform

Sample Latitude Longitude Depth 87Sr=86Sr 143Nd=144Nd εNd 206Pb=204Pb 207Pb=204Pb 208Pb=204Pb(ºS) (ºW) (m)

PeridotitesGN 3-9 13º28.3 111º32.1 4324 0:513222š 31 11.4GN 14-1 13º28.0 111º30.4 5050 0:513271š 12 12.4GN 14-2 13º28.0 111º30.4 5046 0:513276š 17 12.5GN 14-9 13º28.4 111º30.8 4660 0:513256š 42 12.1GN 14-14 13º28.7 111º31.1 4244 0:513250š 34 11.9GN 15-2 13º27.8 111º32.8 4647 0:513253š 8 12.0GN 15-5 13º27.8 111º32.8 4688 0:513311š 9 13.1

LavasGN 2-2 13º28.1 111º22.2 3451 0:702319š 10 0:513345š 17 13.8 17.830 15.363 37.121GN 2-5 13º28.1 111º22.2 3257 0:702298š 10 0:513247š 8 11.9 17.817 15.374 37.121GN 4-1 13º28.2 111º30.1 5062 0:702268š 10 0:513269š 8 12.3 17.462 15.341 37.025GN 4-11 13º26.9 111º29.5 3720 0:702459š 10 0:513270š 9 12.3 17.903 15.444 37.327GN 10-3 13º25.9 111º49.9 4434 0:702211š 10 0:513272š 28 12.4 17.469 15.379 36.831GN 11-4 13º27.5 111º49.9 3806 0:702219š 11 0:513259š 18 12.1 17.545 15.346 36.895GN 12-1 13º24.2 111º56.5 3740 0:702354š 13 0:513286š 8 12.5 17.621 15.418 37.079GN 12-10 13º24.2 111º56.5 3478 0:702207š 13 0:513289š 9 12.7 17.655 15.363 37.004GN 13-1 13º26.7 111º35.4 4528 0:702156š 8 0:513295š 7 12.8 17.697 15.353 36.980GN 13-6 13º28.1 111º35.3 3657 0:702392š 10 0:513225š 7 11.5 18.023 15.416 37.413GN 13-8 13º28.3 111º34.7 3764 0:702417š 11 0:513257š 9 12.1 18.075 15.425 37.468GN 15-1 13º27.3 111º32.9 4801 0:702137š 6 0:513292š 7 12.8 17.768 15.331 37.028

dotites are extremely depleted in Al2O3 comparedto those in peridotites from slow-spreading ridges,suggesting that the Garrett peridotites are residues ofvery high degrees of melting [25,27].

4. Analytical methods

Trace element and isotope analyses of intra-trans-form lavas were carried out on hand-picked chips offresh glass and crystalline basalt which were leachedin 5% HCl–10% H2O2 in an ultrasonic bath for10 min, and washed thoroughly with water beforedigestion. Sr and Nd isotope analyses were also car-ried out on aliquots of the same peridotite samplesfor which major and trace element data have beenpublished previously [25]. These samples were takenfrom the freshest peridotite, away from gabbro vein-lets [25]. 1–5 g of unleached sample powder wasdissolved for the analyses.

All isotope measurements were carried out on aVG 54-30 Sector multicollector mass spectrometer instatic mode. For Sr and Nd, exponential fractionation

corrections were applied using 86Sr=88Sr 0.1194 and146Nd=144Nd 0.7219. The NBS-987 Sr standard andan in-house Nd standard prepared from Ames Ndmetal gave 87Sr=86Sr 0:710261š 14 (n D 3, 2¦ ) and143Nd=144Nd 0:511973 š 12 (n D 10, 2¦ ), respec-tively, over the period of analysis. Data in Table 1are normalised to values of 0.710248 and 0.511972(the value of the in-house Nd standard relative to avalue of 0.511855 for the La Jolla standard). Pb wasanalysed at 1350ºC, and the data corrected for instru-mental mass fractionation using the values of Todt etal. [28] for the NBS-981 Pb standard. The total Pbprocedure blank for whole-rock analyses is between60 and 100 pg per analysis. Based on repeated anal-ysis of standards, the 2¦ external precision on the Pbisotope analyses is approximately 0.014, 0.018 and0.040 for 206Pb=204Pb, 207Pb=204Pb and 208Pb=204Pb,respectively.

Trace element analyses were carried out on aFisons Plasmaquad II inductively coupled plasmamass spectrometer at The University of Queensland;details of sample preparation and analytical condi-tions are given elsewhere [13].


Tabl

e2

Maj

oran

dtr

ace

elem

enta

naly

ses

ofla

vas

from

the

Gar

rett

Tra

nsfo

rm

Sam

ple

desc

ript

ion,

Gam

ma

Rid

gePi

llow

frag

men

t,W

estT

roug

hA

lpha

Rid

geO

utcr

opof

Pillo

wfr

agm

ent

Pillo

wfr

agm

ent,

Talu

s,C

entr

alge

olog

ical

setti

ng:

talu

s,E

astT

roug

hsh

eete

dla

vas

from

outc

rop

talu

sB

asin

GN

2-2

GN

2-5

GN

4-1

GN

4-11

GN

10-3

GN

11-4

GN

12-1

GN

12-1

0G

N13

-1G

N13

-6G

N13

-8G

N15

-1

SiO

250

.95

50.5

750

.78

50.1

349

.59

48.6

850

.58

50.4

750

.78

50.5

0T

iO2

1.21

1.18

1.25

1.11

1.28

1.35

1.16

1.73

2.05

2.45

Al 2

O3

14.6

215

.07

14.1

715

.40

15.9

716

.20

15.5

914

.73

13.4

814

.96

FeO

t9.

309.

329.

819.

078.

7010

.23

8.95

10.1

511

.76

10.0

7M

nO0.

250.

180.

190.

190.

170.

150.

160.

140.

200.

15M

gO8.

338.

458.

058.

958.

908.

168.

797.

576.

876.

83C

aO12

.41

12.2

312

.29

12.2

411

.83

11.1

512

.34

11.2

711

.31

10.2

6N

a 2O

2.32

2.41

2.23

2.18

2.60

2.78

2.37

2.67

2.74

3.71

K2O

0.03

0.04

0.04

0.03

0.03

0.04

0.04

0.11

0.20

0.19

P 2O

50.

090.

080.

060.

050.

000.

000.

100.

150.

180.

06

Sc40

.839

.136

.426

.433

.432

.538

.439

.733

.336

.439

.235

.9V

262

256

255

193

203

216

217

281

235

302

335

331

Cr

383

361

372

317

441

362

258

214

394

330

232

307

Co

46.9

47.6

48.2

39.8

43.2

46.6

49.1

44.4

46.6

45.8

46.4

50.8

Ni

67.8

68.9

67.0

92.0

130

194

148

73.6

129

102

89.1

153

Cu

95.0

92.9

89.2

74.7

65.6

67.9

95.4

68.3

74.9

74.3

70.9

73.1

Zn

69.6

67.7

70.7

64.1

53.8

66.0

71.2

78.3

68.8

243

101

349

Ga

15.6

15.2

15.7

15.4

13.8

14.9

15.5

16.5

15.8

17.5

17.8

18.9

Rb

0.23

30.

207

0.28

50.

200

0.43

20.

145

0.21

30.

238

0.29

02.

452.

290.

921

Sr79

.777

.062

.259

.062

.310

313

894

.563

.180

.284

.695

.1Y

24.3

23.4

22.6

18.3

15.8

23.2

24.8

29.3

22.6

34.2

40.1

45.6

Zr

55.9

52.8

49.1

36.0

28.0

62.2

67.8

74.5

46.3

98.1

117

131

Nb

0.89

20.

824

0.93

00.

570

0.44

70.

597

0.86

31.

100.

767

2.44

3.13

2.05

Ba

2.25

2.04

2.34

1.63

1.25

1.16

2.06

2.30

1.92

6.99

8.60

3.84

La

1.63

1.53

1.42

0.98

20.

775

1.63

2.04

2.17

1.24

3.18

3.99

3.61

Ce

5.90

5.57

5.01

3.55

2.84

6.40

7.41

7.87

4.63

10.6

13.0

12.9

Pr1.

151.

080.

982

0.71

90.

571

1.24

1.38

1.50

0.95

61.

972.

402.

48N

d6.

476.

155.

364.

103.

366.

897.

508.

325.

3910

.112

.213

.1Sm

2.48

2.40

2.05

1.64

1.43

2.58

2.76

3.14

2.15

3.54

4.24

4.68

Eu

0.97

20.

945

0.82

10.

664

0.60

50.

996

1.07

1.19

0.86

81.

261.

471.

55G

d3.

493.

362.

942.

392.

083.

423.

634.

263.

094.

815.

726.

39T

b0.

653

0.62

80.

546

0.44

30.

410

0.63

80.

683

0.80

70.

565

0.85

31.

021.

15D

y4.

484.

333.

823.

122.

894.

364.

625.

463.

965.

957.

128.

01H

o0.

982

0.93

80.

808

0.65

90.

637

0.93

71.

011.

190.

825

1.23

1.48

1.67

Er

2.81

2.72

2.40

1.95

1.85

2.67

2.91

3.42

2.42

3.66

4.40

4.91

Tm

0.41

80.

403

0.36

40.

298

0.27

80.

398

0.43

10.

505

0.36

50.

559

0.65

80.

745

Yb

2.68

2.59

2.30

1.86

1.80

2.56

2.78

3.26

2.31

3.51

4.16

4.69

Lu

0.40

70.

392

0.34

40.

278

0.28

00.

391

0.42

50.

498

0.34

40.

531

0.62

60.

701

Hf

1.70

1.63

1.43

1.11

0.94

91.

821.

882.

241.

472.

733.

303.

70Ta

0.06

790.

0655

0.07

730.

0585

0.03

450.

0507

0.07

060.

0876

0.06

540.

194

0.24

70.

177

Pb0.

562

0.24

10.

271

0.13

60.

275

0.30

90.

294

0.24

70.

454

0.53

20.

724

Th

0.04

830.

0430

0.04

440.

0303

0.02

180.

0334

0.04

930.

0588

0.03

670.

148

0.18

90.

111

U0.

0229

0.02

060.

0974

0.01

600.

0221

0.01

730.

0228

0.02

880.

0455

0.07

220.

290

0.66

2

Maj

orel

emen

tcon

cent

ratio

nsin

wei

ght%

,tra

ceel

emen

tcon

cent

ratio

nsin

ppm

.Maj

orel

emen

tana

lyse

sby

elec

tron

mic

ropr

obe

from

Hek

inia

net

al.[

19].


5. Results

5.1. Garrett intra-transform lavas

Sr, Nd and Pb isotope data for the intra-transformlavas, and Nd isotope data for the associated peri-dotites are given in Table 1. The Garrett lavas haveisotope compositions which overlap with the de-pleted end of the field defined by Pacific MORB, butextend to considerably lower 87Sr=86Sr, 206Pb=204Pb,207Pb=204Pb, 208Pb=204Pb, and higher 143Nd=144Ndthan any lavas previously reported from the Pacific,including lavas from the EPR axis immediately tothe north and to the south of the Garrett Transform

Fig. 2. Isotope compositions of Sr, Nd and Pb of the Garrett intra-transform lavas (ITL) analysed in this study (filled squares). 2¦ externalprecision on the isotope analyses is shown as a cross in bottom left corner of each figure. The bar in (a) represents range in 143Nd=144Ndof the Garrett peridotites (see Table 1). Open circles are data for lavas from segments of the EPR axis immediately to the north and southof the Garrett Transform [18]. Two previous analyses of Garrett lavas (open square, [18] and open triangle [2]) are also shown, togetherwith fields for Pacific, Indian and Atlantic MORB.

[18] (Fig. 2). Two previous isotope analyses of Gar-rett intra-transform lavas [2,18] lie within the rangeof our new data and are shown for comparison inFig. 2.

Previously, the lavas with the most depleted iso-tope signatures known from the Pacific (excludingthe two previous analyses of Garrett lavas [2,18])were samples from the western rift of the EasterMicroplate [6,29], and from the Lamont seamountsnear the EPR axis at 10ºN [15]. To our knowl-edge, sample GN2-2 from the Garrett Transformhas higher 143Nd=144Nd (0.513345) than any otherlava yet reported from the ocean basins. 87Sr=86Srratios of samples GN13-1, GN15-1, GN10-3 and


GN12-10 of between 0.702137 and 0.702211 arelower than the lowest value previously reported for alava from the Pacific Ocean (sample 1558-2014 fromthe Lamont Seamounts; 0.702218 [15]) and similarto the lowest values observed in lavas from the Mid-Atlantic Ridge at 2–7ºS (0.702125 to 0.702270 [30]).206Pb=204Pb ratios of samples GN4-1, GN10-3 andGN11-4 (17.462 to 17.545) are lower than those ofany other lavas from the Pacific and Atlantic Oceans,including sample EN113 26D from the West Rift ofthe Easter Microplate which has a 206Pb=204Pb ratioof 17.594 [6]. These 206Pb=204Pb values are not aslow as some MORB and ocean island basalts fromthe Indian Ocean (e.g. [31,32]), but the Indian Oceanbasalts differ as their low 206Pb=204Pb is often asso-ciated with high 87Sr=86Sr. Two of the Garrett lavas(GN13-6, GN13-8) have 87Sr=86Sr and 206Pb=204Pbvalues that overlap with the field defined by lavasfrom adjacent segments of the southern EPR, buthave higher 143Nd=144Nd (Fig. 2a).

The Garrett lavas have a large range in 207Pb=204Pbfor a given 206Pb=204Pb, compared to lavas from theadjacent spreading segments on the southern EPR(see Fig. 2b). The two previous Pb isotope anal-yses of Garrett lavas [2,18] have a similar rangein 207Pb=204Pb. In Pb isotope evolution diagrams(Fig. 3), the lavas scatter about the evolution curvesestimated for the depleted upper mantle [33], or havelower 207Pb=204Pb than the depleted mantle modelevolution curves. There is a positive correlation be-tween 87Sr=86Sr and 206Pb=204Pb (Fig. 2). The Garrettlava analysed by Mahoney et al. [18] has a 3He=4Heratio of 9.7 R=RA, which is significantly higher thanthe typical range for Pacific MORB (¾8.5 R=RA).

5.2. Garrett peridotites

The peridotites from the Garrett Transform havebeen extensively serpentinised, which has modifiedthe primary Sr isotope compositions of these rocks(87Sr=86Sr values of the peridotites are 0.7076–0.7093); however, Nd appears to have remained im-mobile during this process [25]. Nd isotope compo-sitions of the peridotites (0.51322–0.51331, see Ta-ble 1) lie within the range of the intra-transform lavas(vertical bar in Fig. 2a). The whole-rock analysesof the Garrett peridotites have significantly higher143Nd=144Nd than clinopyroxenes from abyssal peri-

Fig. 3. Pb isotope diagrams showing model evolution curvefor the depleted upper mantle [33], and data for the Garrettintra-transform lavas. Data are from this study (filled squares),Mahoney et al. [18] (open square) and Hamelin et al. [2] (opentriangle). Also shown are data for lavas from segments of theEPR axis immediately to the north and south of the GarrettTransform [18] (circles).

dotites in the Indian Ocean which have values in therange 0.51296–0.51320 [34].

6. Combined trace element and isotope variations

Trace element data for the samples we have anal-ysed for isotopes are listed in Table 2. Additionaltrace element analyses of Garrett intra-transformlavas are given in [25].

Compared to normal Pacific MORB, the Garrettintra-transform lavas have relatively low concen-trations of incompatible minor and trace elements[24,25]. For example, the Garrett lavas have Zr con-centrations of 28–130 ppm, compared to 80–350ppm for lavas from adjacent segments of the EPRaxis [18]. The Garrett lavas also have low ratiosof more incompatible to less incompatible elements


Fig. 4. K=Ti, Rb=Sr, Nb=Zr and Nd=Sm for Garrett intra-trans-form lavas (data from [2,18] and this study), and MORB fromthe adjacent segments of the EPR axis [18]. Symbols as in Fig. 2.The Garrett lavas have low ratios of more incompatible to lessincompatible trace elements, compared to Pacific MORB.

(for example low K=Ti, Nb=Zr, Nd=Sm, and Rb=Sr)compared to lavas from adjacent ridge segments ofthe southern EPR (Figs. 4 and 5). In addition, traceelement ratios such as Nb=Ta (9.74–13.1) and Zr=Hf(29.5–36.1), which are generally little fractionateddue to the similar chemical behaviour of these el-ement pairs, are low compared to MORB from theEPR and chondrites (Nb=Ta ¾17; Zr=Hf ¾38), andare similar to the lowest values observed in somelavas from Pacific seamounts [13].

In incompatible trace element ratio-isotope dia-grams (Fig. 5a–i), the Garrett lavas overlap the fielddefined by lavas from the adjacent ridge segments ofthe southern EPR (Fig. 5), but extend to lower K=Ti,

Rb=Sr, 206Pb=204Pb, 87Sr=86Sr, and higher Sm=Ndand 143Nd=144Nd.

7. Discussion

7.1. Comparison with other intra-transform lavas

Intra-transform volcanism is known to occur atonly a few locations in the Pacific, and geochemicalanalyses have been carried out only on intra-trans-form lavas from the Siqueiros, Raitt and GarrettTransforms.

Lavas erupted at small spreading centers withinthe Siqueiros Transform are more primitive (9.5–10.6% MgO) and more porphyritic, than lavas fromthe nearby EPR ridge axis [20,21,35]. Siqueiros in-tra-transform lavas also have lower K=Ti and La=Smthan MORB erupted at the nearby ridge axes (Fig. 6),and have 87Sr=86Sr compositions (0.70235 to 0.70260[21,35]) which extend to values that are lower thanthose observed in lavas from the EPR at 9º300N and10º300N (0.70248 to 0.70259 [36–38]). However, pi-critic basalts from the Siqueiros Transform, whichhave the lowest incompatible trace element abun-dances, have 87Sr=86Sr values similar to those of lavasfrom adjacent segments of the EPR [35].

Two intra-transform lavas recovered from theRaitt Transform on the Pacific–Antarctic Ridge at54º260S [39] are also relatively primitive and havelow concentrations of incompatible trace elements,low K=Ti (0.052–0.054), and La=Sm (0.51–0.54)relative to MORB from adjacent ridge axis segments(Fig. 6). Pb isotope compositions of the intra-trans-form lavas are within the range of MORB fromelsewhere on the Pacific–Antarctic Ridge, whereas87Sr=86Sr is higher, and 143Nd=144Nd lower, thanother Pacific–Antarctic Ridge lavas [39].

Thus on the basis of the available data, intra-trans-form lavas are in general more porphyritic, moreprimitive, contain lower concentrations of incompat-ible trace elements, and have lower ratios of moreincompatible to less incompatible trace elements,compared to normal MORB (Fig. 6). However, therelatively low 87Sr=86Sr and 206Pb=204Pb, and high143Nd=144Nd ratios of the Garrett lavas are not acharacteristic of all intra-transform lavas. In the fol-lowing section, we discuss a petrogenetic model


Fig. 5. Variation in Sr, Nd and Pb isotope composition with Rb=Sr, Sm=Nd and K=Ti, for the Garrett intra-transform lavas, and MORBfrom adjacent segments of the southern EPR. Symbols as in Fig. 2.

for intra-transform magmatism that can account forthese observations.

7.2. Petrogenesis of Garrett intra-transform lavas

The petrological differences between the intra-transform lavas and those erupted at the ridge axismay reflect the different physical conditions beneaththe transform [20,22]. Thus, the relatively primitiveand porphyritic nature of the Garrett lavas may bethe result of rapid transport of magmas to the sur-face, without extensive cooling and fractionation incrustal magma chambers [22].

Hekinian et al. [22] argued that the relatively lowconcentrations of incompatible trace elements, andthe low ratios of more incompatible to less incompat-ible trace elements, could also be explained in termsof processes operating within the crust. In this model,although more enriched lavas are produced duringmantle melting beneath the transform, these are noterupted at the surface, but instead freeze within thelithosphere. This model thus requires a unique pro-cess, restricted to transform faults, whereby enrichedand depleted melts from the same source remain sep-arate, with the more enriched melts freezing beforethey are erupted [22].


Fig. 6. K=Ti and La=Sm for intra-transform lavas (ITL) from theSiqueiros and Raitt Transform Faults in the Pacific, comparedwith MORB from the adjacent segments of the northern EPRand Pacific–Antarctic Ridge (PAR). Data are from [21,37,39].

Alternatively, the depleted incompatible trace ele-ment signatures of the Garrett and other intra-trans-form lavas may have been derived directly from theirmantle source. We suggest that the unusual isotopeand trace element compositions of the Garrett lavasmay simply be a consequence of melting a two-component mantle beneath a transform fault [24].As discussed in Section 1, geochemical studies oflavas from Pacific ridge axes and Pacific seamountshave shown that the upper mantle beneath the re-gion consists of incompatible element-enriched, eas-ily melted heterogeneities which occur as veins or‘plums’ within a depleted matrix [10,11,13,16]. Ma-honey et al. [18] and Sinton et al. [9] speculatedthat the Garrett sample they analysed was derivedfrom mantle that had been depleted in incompati-ble element-enriched heterogeneities as a result ofmelting beneath the adjacent ridge axis. The uppermantle material currently melting beneath the GarrettTransform has previously undergone partial meltingbeneath the EPR axis within the last 900 ka (theage offset at the transform). During upwelling andmelting of mantle beneath the ridge axis, the en-riched, most fertile components would have been thefirst to melt. Asthenospheric mantle that underwentonly limited upwelling and melting will thereforehave lost the enriched, easily melted component, but

will have remained sufficiently fertile to undergodecompression melting during lithospheric extensionwithin the Garrett Transform [24] (Fig. 7).

Peridotites from the Garrett Transform are highlydepleted harzburgites with clinopyroxene being es-sentially absent (<1 vol.%), and residual mineralmodes and chemistry indicate that they represent theresidues of very high degrees of melting (¾25%)beneath the EPR axis. The Garrett harzburgites aremelting residues of mantle that ascended to shallowdepths and melted most beneath the EPR [25,27](see Fig. 7b). In contrast, the intra-transform lavasare inferred to result from decompression melting,beneath the Garrett Transform, of mantle that hadpreviously undergone only limited melting and meltextraction beneath the EPR axis, which depleted thesource of the Garrett lavas only in the easily melted,incompatible element-enriched components (Fig. 7).The fact that the peridotites and the intra-transformlavas have a similar range in 143Nd=144Nd (Fig. 2a)suggests that the mantle currently melting beneaththe transform was entirely stripped of the enrichedcomponent during partial melting at the EPR axis.Assuming that the Garrett peridotites are residuesof ¾25% melting (estimated from the compositionsof residual spinel, orthopyroxene and clinopyroxene[25]) indicates that the enriched component com-prises significantly less than 25% of fertile mantle(see Fig. 7b).

Our model can account for the low concentrationsof incompatible elements, the low ratios of more in-compatible to less incompatible trace elements (e.g.K=Ti, La=Sm) in lavas from the Garrett, Siqueirosand Raitt Transforms, and the relatively depleted Sr,Nd and Pb isotopic signatures of the Garrett lavas.The model may also explain the differences in iso-tope compositions of the Garrett, Siqueiros and Raittintra-transform lavas. For example, whereas Garrettlavas have low Rb=Sr and low 87Sr=86Sr compared toMORB from the adjacent ridge segments, Siqueirosintra-transform lavas have low Rb=Sr, but many have87Sr=86Sr values similar to lavas from the nearbyridge axis [21,35], and intra-transform lavas fromthe Raitt Transform have lower Rb=Sr, but higher87Sr=86Sr than lavas erupted at the Pacific AntarcticRidge axis [39]. It is possible that these differencesreflect variations in the scale of mantle heterogeneityin the upper mantle beneath the Pacific ridges. If


mantle heterogeneities are small enough that meltscan equilibrate with bulk mantle during the meltingprocess, then after a small degree of melting beneaththe EPR axis, the residue will have low K=Ti, Rb=Sr

etc, but isotope ratios remain the same [11,40]. Incontrast, partial melting of mantle containing largerheterogeneities (>1–10 m) will yield a residue withlow K=Ti, Rb=Sr, but also lower 87Sr=86Sr and higher143Nd=144Nd. Variations in the age of these hetero-geneities may be an important factor in determiningthe contrast in isotope composition between enrichedheterogeneities and depleted matrix, and hence be-tween the lavas erupted at the ridge axis, and withintransforms. The low 207Pb=204Pb ratios of the Garrettlavas compared to MORB from the EPR (Figs. 2 and3) suggest that the mantle heterogeneities beneaththis part of the southern EPR are probably severalGa old. A third possibility is that the differences inSr, Nd and Pb isotope composition between intra-transform lavas from the Garrett, Siqueiros and RaittTransforms and MORB from the adjacent ridge axesmay be related to variations in spreading rate alongthe EPR. Spreading rate has an important influenceon the mean extent of melting at mid-ocean ridges[27], and thus on the degree of depletion of themelting residues, which determines the nature of thesources for subsequent intra-transform magmatism.The degree of residual mantle depletion is expectedto be most extreme beneath the fast-spreading south-ern EPR (spreading rate 14–16 cm=y), and leastbeneath the Pacific–Antarctic Ridge (8–9 cm=y).Further studies of other intra-transform lavas fromelsewhere in the Pacific are needed to test thesepossibilities.

Our model for intra-transform magmatism mayalso explain the relatively high 3He=4He ratio (9.7R=RA) of the Garrett lava analysed by Mahoney etal. [18]. This sample has a He concentration which

Fig. 7. Schematic diagrams showing processes by which highlydepleted lavas are generated in the Garrett Transform. (a) Mapshowing location of sections in (b) and (c). (b) Section across theEPR. Mantle upwelling directly beneath the ridge axis (path x)will melt to the greatest extent, leaving a highly depleted residue(abyssal peridotite). In contrast, mantle upwelling away from theridge axis (path y) will experience only limited decompressionmelting and melt extraction, leaving a residue (cross-hatchedregion) which is preferentially depleted in the enriched hetero-geneities. (c) Section across the Garrett Transform. Residualmantle that has been stripped of enriched heterogeneities duringpartial melting beneath the EPR axis, is the source for subsequentintra-transform magmatism.


is similar to that of lavas from the nearby ridgeaxis (J.J. Mahoney, pers. commun., 1999), but theGarrett lavas have generally lower U and Th concen-trations. 3He=4He ratios higher than those of MORBare often taken as evidence for a contribution from a‘primitive’, undegassed mantle source. Alternatively,if U and Th are more incompatible than He, ashas previously been speculated [41], then the rela-tively high 3He=4He and low (U C Th)=He of theGarrett lava can be explained by our model. TheGarrett lavas may have been inherited from a sourcefrom which easily melted heterogeneities (enrichedin U, Th and 4He) were removed during meltingbeneath the EPR axis, leaving a residue with low Uand Th, moderate He concentration, and relativelyhigh 3He=4He, which became the source for subse-quent intra-transform magmatism beneath the Gar-rett Transform. This suggestion could be tested withHe isotope measurements on other intra-transformlavas.

7.3. Implications for mantle heterogeneity

The lavas erupted at Pacific ridge axes, on sea-mounts, and on oceanic islands, all consist of vari-able mixtures of incompatible trace element de-pleted matrix and enriched heterogeneities [10–13,16,17,42]. Earlier studies of oceanic lavas there-fore attempted to estimate the isotope composition ofthe depleted mantle endmember(s), either by inver-sion of the available isotope data [43], or by arbitrar-ily placing this endmember beyond the most depletedend of the MORB array (the DMM component [19]).The fact that Garrett peridotites and Garrett intra-transform lavas have similar 143Nd=144Nd (Fig. 2)suggests that the Sr, Nd and Pb isotope compositionof the depleted mantle endmember that contributes tomagmatism beneath this region of the southern EPRis similar to that of the most depleted Garrett lavas.However, lavas from the Garrett Transform and fromthe southern EPR do not lie upon simple two-compo-nent mixing lines in trace element-isotope diagrams(Fig. 5), which suggests that the depleted mantlematrix itself is heterogeneous, both on the scale ofa few km beneath the Garrett Transform, and onthe scale of an ocean basin (the Raitt intra-trans-form lavas have 87Sr=86Sr values that do not overlapwith the data for Garrett lavas). Variations in mantle

composition on this scale may reflect the complexityof the processes causing mantle heterogeneity, andthe fact that these processes have occurred through-out geological time (e.g. [44]). As discussed earlier,mantle heterogeneity beneath the southern EPR maybe several Ga old.

Insights into the composition of the DMM end-member have also come from studies of abyssalperidotites. Clinopyroxene separates from abyssalperidotites from the Indian Ocean have 143Nd=144Ndvalues that are similar to, or higher than, MORBfrom the nearby segments of the ridge axes [34].This was interpreted as the effects of melting a het-erogeneous mantle, with the peridotites representingresidual mantle from which an enriched componentwith lower 143Nd=144Nd was removed during melt-ing beneath the ridge axis [34]. The Indian Oceanperidotites have lower 143Nd=144Nd than the Gar-rett peridotites. However, the primary Sr and Pbisotope compositions of abyssal peridotites cannotbe determined easily, because of the effects of ser-pentinisation. Intra-transform lavas are therefore animportant source of information on the nature andorigin of heterogeneity in the upper mantle.

8. Conclusions

(1) Lavas erupted within the Garrett TransformFault are more primitive and more porphyritic, havelower concentrations of incompatible trace elements,and lower ratios of more incompatible to less incom-patible elements, compared to lavas from adjacentsegments of the southern EPR axis.

(2) The unusual petrology and major and traceelement compositions of the Garrett lavas appear tobe characteristic of intra-transform lavas from theSiqueiros and Raitt Transforms.

(3) The relatively primitive and porphyritic na-ture of these intra-transform lavas may be a resultof the different physical conditions that exist be-neath the transform. In contrast, the unusual traceelement compositions of intra-transform lavas is in-herited from the mantle source of these lavas, whichhas undergone prior melt extraction beneath the EPRaxis, depleting an originally heterogeneous mantlein incompatible element-enriched, easily melted het-erogeneities.


(4) The Garrett lavas have Sr, Nd and Pb isotopecompositions which overlap with the depleted endof the array defined by Pacific MORB, but extendto lower 87Sr=86Sr, 206Pb=204Pb and 207Pb=204Pb, andhigher 143Nd=144Nd values than any other lavas fromthe Pacific Ocean.

(5) 143Nd=144Nd for peridotites from the GarrettTransform are similar to values for the intra-trans-form lavas, indicating that the source of the Gar-rett lavas has been entirely stripped of the enrichedheterogeneities that exist in the uppermost mantlebeneath much of the eastern Pacific.

(6) Studies of intra-transform lavas can give im-portant insights into the composition of the uppermantle, and in particular can be used to estimate theSr and Pb isotope composition of the DMM compo-nent, which is difficult to determine from studies ofabyssal peridotites. The DMM component appearsto be heterogeneous, both on the scale of a singletransform fault, and on the scale of an ocean basin.

Acknowledgements

We thank Alan Greig for help with the ICPMSanalyses. Reviews by K. Haase and B.B. Hanan im-proved the paper, and we are grateful for commentsby J.J. Mahoney and J.-G. Schilling on an earlierversion of the manuscript. [FA]

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