ELSEVIER
Earth and Planetary Science Letters 173 (1999) 271–284
www.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 Hékinian b , Kenneth D. Collerson a
a Department
of Earth Science, The University of Queensland, Brisbane 4072, Australia
b IFREMER, Centre de Brest, 29287 Plouzané, 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 East
Pacific Rise, are more porphyritic, less evolved, have lower concentrations of incompatible trace elements, and lower ratios
of more incompatible to less incompatible elements (e.g. low K=Ti and La=Sm) compared to lavas from the adjacent
East Pacific Rise ridge axis. Sr, Nd and Pb isotope compositions overlap with the depleted end of the field for Pacific
mid-ocean ridge basalts, but extend to lower 87 Sr=86 Sr (0.702137), 206 Pb=204 Pb (17.462), 207 Pb=204 Pb (15.331), 208 Pb=204 Pb
(36.831), and higher 143 Nd=144 Nd (0.513345) than any lavas previously reported from the Pacific. Peridotites from the
Garrett Transform have Nd isotope compositions within the range of the intra-transform lavas. The unusual major and trace
element compositions of the Garrett lavas appear to be characteristic of other intra-transform lavas from elsewhere in the
Pacific. The chemical and isotopic features of the Garrett lavas can be explained by remelting, beneath the transform, a
two-component upper mantle which was depleted in incompatible element-enriched heterogeneities during melting beneath
the East Pacific Rise ridge axis (within the past 1 Ma). Our data place new constraints on the trace element and isotope
composition of the depleted mantle component that contributes to magmatism in the Pacific, and show that this component
is heterogeneous, both on the scale of a single transform fault, and on the scale of an ocean basin.  1999 Elsevier
Science 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 beneath
the ocean basins is chemically and isotopically heterogeneous [1–3]. In some cases, heterogeneity in
MORB can be attributed to the presence of nearŁ Corresponding author. Present address: Max-Planck-Institut für
Chemie, Postfach 3060, 55050 Mainz, Germany. Fax: C49 6131
371051; E-mail: [email protected]
ridge mantle plumes (e.g. [4–7]). However, chemical
and isotopic heterogeneity is found in lavas erupted
at all mid-ocean ridges, including those far from
known mantle plumes [3,8–10].
The magmas erupted at mid-ocean ridges are derived from relatively high degrees of partial melting
of relatively large volumes of mantle, and these
melts undergo mixing during melt aggregation and
focusing towards the ridge axis, and also within
magma chambers in the crust. As a result, it is
difficult 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 of
lavas erupted on ridge axes alone. Lavas erupted
on seamounts on the ridge flanks are derived from
melting of smaller volumes of mantle, and studies
of such rocks have given important insights into
the composition and scale of heterogeneities in the
upper mantle. For example, the trace element and
isotope variation within Pacific seamounts is greater
than that seen in Pacific MORB [11–15]. This observation has led to the idea that the chemical and
isotopic heterogeneity observed in lavas erupted at
spreading centers and on seamounts is the result
of melting a two-component mantle, consisting of
easily melted, incompatible element-enriched veins
or ‘plums’ with relatively high K=Ti, high 87 Sr=86 Sr
etc., within a more refractory matrix which is relatively depleted in incompatible trace elements, and
has lower K=Ti and low 87 Sr=86 Sr [9–11,13–17].
Several different depleted and enriched components
appear to be necessary to explain the heterogeneity
observed in the lavas erupted on Pacific ridge axes
and on Pacific seamounts [9,15,18,19]. However, because all of these lavas are derived from different
degrees of melting of variably enriched mantle, the
compositions of individual endmember components
in the sub-Pacific upper mantle is not well known.
In this paper, we present new trace element and
Sr, Nd and Pb isotope data for young lavas erupted as
a result of intra-transform extension within the Garrett Transform Fault, on the East Pacific Rise (EPR)
in the eastern Pacific (Fig. 1). These intra-transform lavas include samples with lower 87 Sr=86 Sr,
206
Pb=204 Pb, 207 Pb=204 Pb and 208 Pb=204 Pb and higher
143
Nd=144 Nd than any other lavas previously reported
from the Pacific. Compared to lavas from the adjacent EPR ridge axis, the Garrett lavas are more porphyritic and less evolved, have lower concentrations
of incompatible trace elements, and lower ratios of
more incompatible to less incompatible trace elements (e.g. low K=Ti, La=Sm). We argue that the
unusual trace element and isotope compositions of
these lavas are the consequence of melting a twocomponent mantle beneath a transform fault. That
is, the Garrett intra-transform lavas are derived from
a mantle source which was depleted in the easily
melted, 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 mantle
beneath the eastern Pacific.
2. Geological setting
The Garrett Transform Fault offsets the fastspreading (14.5 cm=y) southern EPR axis at 13º280 S
by 130 km (Fig. 1a), and is one of the few transform
faults in which active volcanism is known to occur.
In the Pacific, systematic sampling of intra-transform
volcanism has only been carried out in the Siqueiros
and Garrett transform faults [20–25]. Detailed descriptions of the structure and geology of the Garrett
Transform 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 over
5000 m in depth. Within the transform valley, three
NE–SW-trending ridges (named the Alpha, Beta and
Gamma Ridges, Fig. 1) lie at a depth of <3500
m. Submersible observations have shown that recent
volcanism within the transform is largely restricted
to these ridges, but has also built small cones on the
floors of the eastern and western transform troughs
[22,23]. Young flows also occur on the south wall
of the Eastern Trough, in the East Valley, and on
the southern wall of Garrett Deep, in which upper
mantle and lower crustal rocks are exposed [22,23].
The intra-transform lavas were erupted as pillow
lavas and as sheet flows, many of which consist of
fresh glass [22,23]. Of the samples analysed in this
study, sample GN13-1 was collected from a young
intra-transform lava flow on the floor of the Eastern
Trough (Fig. 1b). Samples GN12-1 and GN12-10
are from the Alpha Ridge, and samples GN2-2 and
GN2-5 were collected from young flows on the flank
of the Gamma Ridge. The other samples were collected from older flows, or from talus piles, and
may therefore be from intra-transform lavas, or from
older crust formed at the EPR ridge axis. Details of
the sample locations are given in Tables 1 and 2.
3. Petrology and major element chemistry
The Garrett intra-transform lavas are dominantly
olivine and=or plagioclase phyric basalts and picritic
basalts [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) the
major structural features of the transform and locations of the samples studied.
as these are rare, and are generally only found close
to large transform faults.
Petrographic descriptions and major element analyses of the Garrett intra-transform lavas have been
reported previously [22–25]. Major element data for
the samples analysed in this study are given in Table 2. Compared to lavas erupted on the adjacent segments of the southern East Pacific Rise ridge axis, the
Garrett lavas are generally less evolved, and contain
lower concentrations of incompatible minor and trace
elements. For example, the intra-transform lavas have
Mg# in the range 58–71, and average K2 O contents of
0.04% [22,25], compared with values of 40–65 and
0.14% for lavas from ridge segments of the southern EPR axis adjacent to the Garrett Transform [9].
The Garrett lavas also have lower TiO2 , FeO, Na2 O
and P2 O5 , and higher CaO and Al2 O3 compared to
lavas from the adjacent EPR. The range in major element composition within the Garrett lavas can be
explained by fractionation of olivine š plagioclase
š clinopyroxene from a range of primary melt compositions; however, the intra-transform lavas do not
represent parental magmas to the more evolved lavas
erupted at the EPR ridge axis [22,25]. Compared to
MORB erupted at adjacent segments of the southern EPR ridge axis, the Garrett lavas were derived
from smaller degrees of melting (¾15%), of a mantle
source which was relatively depleted in incompatible
minor and trace elements [9,22,25,26].
The peridotites recovered from the Garrett Transform are variably serpentinised harzburgites, some
of which have been locally refertilised, probably as
a result of intra-transform magmatism [25]. Spinel,
orthopyroxene and rare clinopyroxene in these peri-
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J.I. Wendt et al. / Earth and Planetary Science Letters 173 (1999) 271–284
Table 1
Isotope data for peridotites and lavas from the Garrett Transform
Sample
Latitude
(ºS)
Longitude
(ºW)
Depth
(m)
Peridotites
GN 3-9
GN 14-1
GN 14-2
GN 14-9
GN 14-14
GN 15-2
GN 15-5
13º28.3
13º28.0
13º28.0
13º28.4
13º28.7
13º27.8
13º27.8
111º32.1
111º30.4
111º30.4
111º30.8
111º31.1
111º32.8
111º32.8
4324
5050
5046
4660
4244
4647
4688
Lavas
GN 2-2
GN 2-5
GN 4-1
GN 4-11
GN 10-3
GN 11-4
GN 12-1
GN 12-10
GN 13-1
GN 13-6
GN 13-8
GN 15-1
13º28.1
13º28.1
13º28.2
13º26.9
13º25.9
13º27.5
13º24.2
13º24.2
13º26.7
13º28.1
13º28.3
13º27.3
111º22.2
111º22.2
111º30.1
111º29.5
111º49.9
111º49.9
111º56.5
111º56.5
111º35.4
111º35.3
111º34.7
111º32.9
3451
3257
5062
3720
4434
3806
3740
3478
4528
3657
3764
4801
87 Sr=86 Sr
0:702319 š 10
0:702298 š 10
0:702268 š 10
0:702459 š 10
0:702211 š 10
0:702219 š 11
0:702354 š 13
0:702207 š 13
0:702156 š 8
0:702392 š 10
0:702417 š 11
0:702137 š 6
dotites are extremely depleted in Al2 O3 compared
to those in peridotites from slow-spreading ridges,
suggesting that the Garrett peridotites are residues of
very high degrees of melting [25,27].
4. Analytical methods
Trace element and isotope analyses of intra-transform lavas were carried out on hand-picked chips of
fresh glass and crystalline basalt which were leached
in 5% HCl–10% H2 O2 in an ultrasonic bath for
10 min, and washed thoroughly with water before
digestion. Sr and Nd isotope analyses were also carried out on aliquots of the same peridotite samples
for which major and trace element data have been
published previously [25]. These samples were taken
from the freshest peridotite, away from gabbro veinlets [25]. 1–5 g of unleached sample powder was
dissolved for the analyses.
All isotope measurements were carried out on a
VG 54-30 Sector multicollector mass spectrometer in
static mode. For Sr and Nd, exponential fractionation
143 Nd=144 Nd
eNd
0:513222 š 31
0:513271 š 12
0:513276 š 17
0:513256 š 42
0:513250 š 34
0:513253 š 8
0:513311 š 9
11.4
12.4
12.5
12.1
11.9
12.0
13.1
0:513345 š 17
0:513247 š 8
0:513269 š 8
0:513270 š 9
0:513272 š 28
0:513259 š 18
0:513286 š 8
0:513289 š 9
0:513295 š 7
0:513225 š 7
0:513257 š 9
0:513292 š 7
13.8
11.9
12.3
12.3
12.4
12.1
12.5
12.7
12.8
11.5
12.1
12.8
206 Pb=204 Pb
207 Pb=204 Pb
208 Pb=204 Pb
17.830
17.817
17.462
17.903
17.469
17.545
17.621
17.655
17.697
18.023
18.075
17.768
15.363
15.374
15.341
15.444
15.379
15.346
15.418
15.363
15.353
15.416
15.425
15.331
37.121
37.121
37.025
37.327
36.831
36.895
37.079
37.004
36.980
37.413
37.468
37.028
corrections were applied using 86 Sr=88 Sr 0.1194 and
146
Nd=144 Nd 0.7219. The NBS-987 Sr standard and
an in-house Nd standard prepared from Ames Nd
metal gave 87 Sr=86 Sr 0:710261 š 14 (n D 3, 2¦ ) and
143
Nd=144 Nd 0:511973 š 12 (n D 10, 2¦ ), respectively, over the period of analysis. Data in Table 1
are normalised to values of 0.710248 and 0.511972
(the value of the in-house Nd standard relative to a
value of 0.511855 for the La Jolla standard). Pb was
analysed at 1350ºC, and the data corrected for instrumental mass fractionation using the values of Todt et
al. [28] for the NBS-981 Pb standard. The total Pb
procedure blank for whole-rock analyses is between
60 and 100 pg per analysis. Based on repeated analysis of standards, the 2¦ external precision on the Pb
isotope analyses is approximately 0.014, 0.018 and
0.040 for 206 Pb=204 Pb, 207 Pb=204 Pb and 208 Pb=204 Pb,
respectively.
Trace element analyses were carried out on a
Fisons Plasmaquad II inductively coupled plasma
mass spectrometer at The University of Queensland;
details of sample preparation and analytical conditions are given elsewhere [13].
39.1
256
361
47.6
68.9
92.9
67.7
15.2
0.207
77.0
23.4
52.8
0.824
2.04
1.53
5.57
1.08
6.15
2.40
0.945
3.36
0.628
4.33
0.938
2.72
0.403
2.59
0.392
1.63
0.0655
0.562
0.0430
0.0206
36.4
255
372
48.2
67.0
89.2
70.7
15.7
0.285
62.2
22.6
49.1
0.930
2.34
1.42
5.01
0.982
5.36
2.05
0.821
2.94
0.546
3.82
0.808
2.40
0.364
2.30
0.344
1.43
0.0773
0.241
0.0444
0.0974
26.4
193
317
39.8
92.0
74.7
64.1
15.4
0.200
59.0
18.3
36.0
0.570
1.63
0.982
3.55
0.719
4.10
1.64
0.664
2.39
0.443
3.12
0.659
1.95
0.298
1.86
0.278
1.11
0.0585
0.271
0.0303
0.0160
50.13
1.11
15.40
9.07
0.19
8.95
12.24
2.18
0.03
0.05
GN 4-11
33.4
203
441
43.2
130
65.6
53.8
13.8
0.432
62.3
15.8
28.0
0.447
1.25
0.775
2.84
0.571
3.36
1.43
0.605
2.08
0.410
2.89
0.637
1.85
0.278
1.80
0.280
0.949
0.0345
0.136
0.0218
0.0221
GN 10-3
West Trough
32.5
216
362
46.6
194
67.9
66.0
14.9
0.145
103
23.2
62.2
0.597
1.16
1.63
6.40
1.24
6.89
2.58
0.996
3.42
0.638
4.36
0.937
2.67
0.398
2.56
0.391
1.82
0.0507
0.275
0.0334
0.0173
49.59
1.28
15.97
8.70
0.17
8.90
11.83
2.60
0.03
0.00
GN 11-4
38.4
217
258
49.1
148
95.4
71.2
15.5
0.213
138
24.8
67.8
0.863
2.06
2.04
7.41
1.38
7.50
2.76
1.07
3.63
0.683
4.62
1.01
2.91
0.431
2.78
0.425
1.88
0.0706
0.309
0.0493
0.0228
48.68
1.35
16.20
10.23
0.15
8.16
11.15
2.78
0.04
0.00
GN 12-1
Alpha Ridge
39.7
281
214
44.4
73.6
68.3
78.3
16.5
0.238
94.5
29.3
74.5
1.10
2.30
2.17
7.87
1.50
8.32
3.14
1.19
4.26
0.807
5.46
1.19
3.42
0.505
3.26
0.498
2.24
0.0876
0.294
0.0588
0.0288
GN 12-10
33.3
235
394
46.6
129
74.9
68.8
15.8
0.290
63.1
22.6
46.3
0.767
1.92
1.24
4.63
0.956
5.39
2.15
0.868
3.09
0.565
3.96
0.825
2.42
0.365
2.31
0.344
1.47
0.0654
0.247
0.0367
0.0455
50.58
1.16
15.59
8.95
0.16
8.79
12.34
2.37
0.04
0.10
GN 13-1
Outcrop of
sheeted lavas
36.4
302
330
45.8
102
74.3
243
17.5
2.45
80.2
34.2
98.1
2.44
6.99
3.18
10.6
1.97
10.1
3.54
1.26
4.81
0.853
5.95
1.23
3.66
0.559
3.51
0.531
2.73
0.194
0.454
0.148
0.0722
50.47
1.73
14.73
10.15
0.14
7.57
11.27
2.67
0.11
0.15
GN 13-6
Pillow fragment
from outcrop
Major element concentrations in weight %, trace element concentrations in ppm. Major element analyses by electron microprobe from Hekinian et al. [19].
0.0483
0.0229
40.8
262
383
46.9
67.8
95.0
69.6
15.6
0.233
79.7
24.3
55.9
0.892
2.25
1.63
5.90
1.15
6.47
2.48
0.972
3.49
0.653
4.48
0.982
2.81
0.418
2.68
0.407
1.70
0.0679
Sc
V
Cr
Co
Ni
Cu
Zn
Ga
Rb
Sr
Y
Zr
Nb
Ba
La
Ce
Pr
Nd
Sm
Eu
Gd
Tb
Dy
Ho
Er
Tm
Yb
Lu
Hf
Ta
Pb
Th
U
50.78
1.25
14.17
9.81
0.19
8.05
12.29
2.23
0.04
0.06
GN 4-1
50.57
1.18
15.07
9.32
0.18
8.45
12.23
2.41
0.04
0.08
GN 2-5
GN 2-2
50.95
1.21
14.62
9.30
0.25
8.33
12.41
2.32
0.03
0.09
Pillow fragment,
talus, East Trough
Gamma Ridge
SiO2
TiO2
Al2 O3
FeOt
MnO
MgO
CaO
Na2 O
K2 O
P2 O5
Sample description,
geological setting:
Table 2
Major and trace element analyses of lavas from the Garrett Transform
39.2
335
232
46.4
89.1
70.9
101
17.8
2.29
84.6
40.1
117
3.13
8.60
3.99
13.0
2.40
12.2
4.24
1.47
5.72
1.02
7.12
1.48
4.40
0.658
4.16
0.626
3.30
0.247
0.532
0.189
0.290
50.78
2.05
13.48
11.76
0.20
6.87
11.31
2.74
0.20
0.18
GN 13-8
Pillow fragment,
talus
35.9
331
307
50.8
153
73.1
349
18.9
0.921
95.1
45.6
131
2.05
3.84
3.61
12.9
2.48
13.1
4.68
1.55
6.39
1.15
8.01
1.67
4.91
0.745
4.69
0.701
3.70
0.177
0.724
0.111
0.662
50.50
2.45
14.96
10.07
0.15
6.83
10.26
3.71
0.19
0.06
GN 15-1
Talus, Central
Basin
J.I. Wendt et al. / Earth and Planetary Science Letters 173 (1999) 271–284
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J.I. Wendt et al. / Earth and Planetary Science Letters 173 (1999) 271–284
5. Results
5.1. Garrett intra-transform lavas
Sr, Nd and Pb isotope data for the intra-transform
lavas, and Nd isotope data for the associated peridotites are given in Table 1. The Garrett lavas have
isotope compositions which overlap with the depleted end of the field defined by Pacific MORB, but
extend to considerably lower 87 Sr=86 Sr, 206 Pb=204 Pb,
207
Pb=204 Pb, 208 Pb=204 Pb, and higher 143 Nd=144 Nd
than any lavas previously reported from the Pacific,
including lavas from the EPR axis immediately to
the north and to the south of the Garrett Transform
[18] (Fig. 2). Two previous isotope analyses of Garrett intra-transform lavas [2,18] lie within the range
of our new data and are shown for comparison in
Fig. 2.
Previously, the lavas with the most depleted isotope signatures known from the Pacific (excluding
the two previous analyses of Garrett lavas [2,18])
were samples from the western rift of the Easter
Microplate [6,29], and from the Lamont seamounts
near the EPR axis at 10ºN [15]. To our knowledge, sample GN2-2 from the Garrett Transform
has higher 143 Nd=144 Nd (0.513345) than any other
lava yet reported from the ocean basins. 87 Sr=86 Sr
ratios of samples GN13-1, GN15-1, GN10-3 and
Fig. 2. Isotope compositions of Sr, Nd and Pb of the Garrett intra-transform lavas (ITL) analysed in this study (filled squares). 2¦ external
precision on the isotope analyses is shown as a cross in bottom left corner of each figure. The bar in (a) represents range in 143 Nd=144 Nd
of the Garrett peridotites (see Table 1). Open circles are data for lavas from segments of the EPR axis immediately to the north and south
of the Garrett Transform [18]. Two previous analyses of Garrett lavas (open square, [18] and open triangle [2]) are also shown, together
with fields for Pacific, Indian and Atlantic MORB.
J.I. Wendt et al. / Earth and Planetary Science Letters 173 (1999) 271–284
GN12-10 of between 0.702137 and 0.702211 are
lower than the lowest value previously reported for a
lava from the Pacific Ocean (sample 1558-2014 from
the Lamont Seamounts; 0.702218 [15]) and similar
to the lowest values observed in lavas from the MidAtlantic Ridge at 2–7ºS (0.702125 to 0.702270 [30]).
206
Pb=204 Pb ratios of samples GN4-1, GN10-3 and
GN11-4 (17.462 to 17.545) are lower than those of
any other lavas from the Pacific and Atlantic Oceans,
including sample EN113 26D from the West Rift of
the Easter Microplate which has a 206 Pb=204 Pb ratio
of 17.594 [6]. These 206 Pb=204 Pb values are not as
low as some MORB and ocean island basalts from
the Indian Ocean (e.g. [31,32]), but the Indian Ocean
basalts differ as their low 206 Pb=204 Pb is often associated with high 87 Sr=86 Sr. Two of the Garrett lavas
(GN13-6, GN13-8) have 87 Sr=86 Sr and 206 Pb=204 Pb
values that overlap with the field defined by lavas
from adjacent segments of the southern EPR, but
have higher 143 Nd=144 Nd (Fig. 2a).
The Garrett lavas have a large range in 207 Pb=204 Pb
for a given 206 Pb=204 Pb, compared to lavas from the
adjacent spreading segments on the southern EPR
(see Fig. 2b). The two previous Pb isotope analyses of Garrett lavas [2,18] have a similar range
in 207 Pb=204 Pb. In Pb isotope evolution diagrams
(Fig. 3), the lavas scatter about the evolution curves
estimated for the depleted upper mantle [33], or have
lower 207 Pb=204 Pb than the depleted mantle model
evolution curves. There is a positive correlation between 87 Sr=86 Sr and 206 Pb=204 Pb (Fig. 2). The Garrett
lava analysed by Mahoney et al. [18] has a 3 He=4 He
ratio of 9.7 R=RA , which is significantly higher than
the typical range for Pacific MORB (¾8.5 R=RA ).
277
Fig. 3. Pb isotope diagrams showing model evolution curve
for the depleted upper mantle [33], and data for the Garrett
intra-transform lavas. Data are from this study (filled squares),
Mahoney et al. [18] (open square) and Hamelin et al. [2] (open
triangle). Also shown are data for lavas from segments of the
EPR axis immediately to the north and south of the Garrett
Transform [18] (circles).
dotites in the Indian Ocean which have values in the
range 0.51296–0.51320 [34].
6. Combined trace element and isotope variations
5.2. Garrett peridotites
The peridotites from the Garrett Transform have
been extensively serpentinised, which has modified
the primary Sr isotope compositions of these rocks
(87 Sr=86 Sr values of the peridotites are 0.7076–
0.7093); however, Nd appears to have remained immobile during this process [25]. Nd isotope compositions of the peridotites (0.51322–0.51331, see Table 1) lie within the range of the intra-transform lavas
(vertical bar in Fig. 2a). The whole-rock analyses
of the Garrett peridotites have significantly higher
143
Nd=144 Nd than clinopyroxenes from abyssal peri-
Trace element data for the samples we have analysed for isotopes are listed in Table 2. Additional
trace element analyses of Garrett intra-transform
lavas are given in [25].
Compared to normal Pacific MORB, the Garrett
intra-transform lavas have relatively low concentrations of incompatible minor and trace elements
[24,25]. For example, the Garrett lavas have Zr concentrations of 28–130 ppm, compared to 80–350
ppm for lavas from adjacent segments of the EPR
axis [18]. The Garrett lavas also have low ratios
of more incompatible to less incompatible elements
278
J.I. Wendt et al. / Earth and Planetary Science Letters 173 (1999) 271–284
Rb=Sr, 206 Pb=204 Pb,
and 143 Nd=144 Nd.
87
Sr=86 Sr, and higher Sm=Nd
7. Discussion
7.1. Comparison with other intra-transform lavas
Fig. 4. K=Ti, Rb=Sr, Nb=Zr and Nd=Sm for Garrett intra-transform lavas (data from [2,18] and this study), and MORB from
the adjacent segments of the EPR axis [18]. Symbols as in Fig. 2.
The Garrett lavas have low ratios of more incompatible to less
incompatible 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 of
the southern EPR (Figs. 4 and 5). In addition, trace
element ratios such as Nb=Ta (9.74–13.1) and Zr=Hf
(29.5–36.1), which are generally little fractionated
due to the similar chemical behaviour of these element pairs, are low compared to MORB from the
EPR and chondrites (Nb=Ta ¾17; Zr=Hf ¾38), and
are similar to the lowest values observed in some
lavas from Pacific seamounts [13].
In incompatible trace element ratio-isotope diagrams (Fig. 5a–i), the Garrett lavas overlap the field
defined by lavas from the adjacent ridge segments of
the southern EPR (Fig. 5), but extend to lower K=Ti,
Intra-transform volcanism is known to occur at
only a few locations in the Pacific, and geochemical
analyses have been carried out only on intra-transform lavas from the Siqueiros, Raitt and Garrett
Transforms.
Lavas erupted at small spreading centers within
the Siqueiros Transform are more primitive (9.5–
10.6% MgO) and more porphyritic, than lavas from
the nearby EPR ridge axis [20,21,35]. Siqueiros intra-transform lavas also have lower K=Ti and La=Sm
than MORB erupted at the nearby ridge axes (Fig. 6),
and have 87 Sr=86 Sr compositions (0.70235 to 0.70260
[21,35]) which extend to values that are lower than
those observed in lavas from the EPR at 9º300 N and
10º300 N (0.70248 to 0.70259 [36–38]). However, picritic basalts from the Siqueiros Transform, which
have the lowest incompatible trace element abundances, have 87 Sr=86 Sr values similar to those of lavas
from adjacent segments of the EPR [35].
Two intra-transform lavas recovered from the
Raitt Transform on the Pacific–Antarctic Ridge at
54º260 S [39] are also relatively primitive and have
low 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-transform lavas are within the range of MORB from
elsewhere on the Pacific–Antarctic Ridge, whereas
87
Sr=86 Sr is higher, and 143 Nd=144 Nd lower, than
other Pacific–Antarctic Ridge lavas [39].
Thus on the basis of the available data, intra-transform lavas are in general more porphyritic, more
primitive, contain lower concentrations of incompatible trace elements, and have lower ratios of more
incompatible to less incompatible trace elements,
compared to normal MORB (Fig. 6). However, the
relatively low 87 Sr=86 Sr and 206 Pb=204 Pb, and high
143
Nd=144 Nd ratios of the Garrett lavas are not a
characteristic of all intra-transform lavas. In the following section, we discuss a petrogenetic model
J.I. Wendt et al. / Earth and Planetary Science Letters 173 (1999) 271–284
279
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 MORB
from adjacent segments of the southern EPR. Symbols as in Fig. 2.
for intra-transform magmatism that can account for
these observations.
7.2. Petrogenesis of Garrett intra-transform lavas
The petrological differences between the intratransform lavas and those erupted at the ridge axis
may reflect the different physical conditions beneath
the transform [20,22]. Thus, the relatively primitive
and porphyritic nature of the Garrett lavas may be
the result of rapid transport of magmas to the surface, without extensive cooling and fractionation in
crustal magma chambers [22].
Hékinian et al. [22] argued that the relatively low
concentrations of incompatible trace elements, and
the low ratios of more incompatible to less incompatible trace elements, could also be explained in terms
of processes operating within the crust. In this model,
although more enriched lavas are produced during
mantle melting beneath the transform, these are not
erupted at the surface, but instead freeze within the
lithosphere. This model thus requires a unique process, restricted to transform faults, whereby enriched
and depleted melts from the same source remain separate, with the more enriched melts freezing before
they are erupted [22].
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J.I. Wendt et al. / Earth and Planetary Science Letters 173 (1999) 271–284
Fig. 6. K=Ti and La=Sm for intra-transform lavas (ITL) from the
Siqueiros and Raitt Transform Faults in the Pacific, compared
with MORB from the adjacent segments of the northern EPR
and Pacific–Antarctic Ridge (PAR). Data are from [21,37,39].
Alternatively, the depleted incompatible trace element signatures of the Garrett and other intra-transform lavas may have been derived directly from their
mantle source. We suggest that the unusual isotope
and trace element compositions of the Garrett lavas
may simply be a consequence of melting a twocomponent mantle beneath a transform fault [24].
As discussed in Section 1, geochemical studies of
lavas from Pacific ridge axes and Pacific seamounts
have shown that the upper mantle beneath the region consists of incompatible element-enriched, easily melted heterogeneities which occur as veins or
‘plums’ within a depleted matrix [10,11,13,16]. Mahoney et al. [18] and Sinton et al. [9] speculated
that the Garrett sample they analysed was derived
from mantle that had been depleted in incompatible element-enriched heterogeneities as a result of
melting beneath the adjacent ridge axis. The upper
mantle material currently melting beneath the Garrett
Transform has previously undergone partial melting
beneath the EPR axis within the last 900 ka (the
age offset at the transform). During upwelling and
melting of mantle beneath the ridge axis, the enriched, most fertile components would have been the
first to melt. Asthenospheric mantle that underwent
only limited upwelling and melting will therefore
have lost the enriched, easily melted component, but
will have remained sufficiently fertile to undergo
decompression melting during lithospheric extension
within the Garrett Transform [24] (Fig. 7).
Peridotites from the Garrett Transform are highly
depleted harzburgites with clinopyroxene being essentially absent (<1 vol.%), and residual mineral
modes and chemistry indicate that they represent the
residues of very high degrees of melting (¾25%)
beneath the EPR axis. The Garrett harzburgites are
melting residues of mantle that ascended to shallow
depths and melted most beneath the EPR [25,27]
(see Fig. 7b). In contrast, the intra-transform lavas
are inferred to result from decompression melting,
beneath the Garrett Transform, of mantle that had
previously undergone only limited melting and melt
extraction beneath the EPR axis, which depleted the
source of the Garrett lavas only in the easily melted,
incompatible element-enriched components (Fig. 7).
The fact that the peridotites and the intra-transform
lavas have a similar range in 143 Nd=144 Nd (Fig. 2a)
suggests that the mantle currently melting beneath
the transform was entirely stripped of the enriched
component during partial melting at the EPR axis.
Assuming that the Garrett peridotites are residues
of ¾25% melting (estimated from the compositions
of residual spinel, orthopyroxene and clinopyroxene
[25]) indicates that the enriched component comprises significantly less than 25% of fertile mantle
(see Fig. 7b).
Our model can account for the low concentrations
of incompatible elements, the low ratios of more incompatible to less incompatible trace elements (e.g.
K=Ti, La=Sm) in lavas from the Garrett, Siqueiros
and Raitt Transforms, and the relatively depleted Sr,
Nd and Pb isotopic signatures of the Garrett lavas.
The model may also explain the differences in isotope compositions of the Garrett, Siqueiros and Raitt
intra-transform lavas. For example, whereas Garrett
lavas have low Rb=Sr and low 87 Sr=86 Sr compared to
MORB from the adjacent ridge segments, Siqueiros
intra-transform lavas have low Rb=Sr, but many have
87
Sr=86 Sr values similar to lavas from the nearby
ridge axis [21,35], and intra-transform lavas from
the Raitt Transform have lower Rb=Sr, but higher
87
Sr=86 Sr than lavas erupted at the Pacific Antarctic
Ridge axis [39]. It is possible that these differences
reflect variations in the scale of mantle heterogeneity
in the upper mantle beneath the Pacific ridges. If
J.I. Wendt et al. / Earth and Planetary Science Letters 173 (1999) 271–284
mantle heterogeneities are small enough that melts
can equilibrate with bulk mantle during the melting
process, then after a small degree of melting beneath
the EPR axis, the residue will have low K=Ti, Rb=Sr
281
etc, but isotope ratios remain the same [11,40]. In
contrast, partial melting of mantle containing larger
heterogeneities (>1–10 m) will yield a residue with
low K=Ti, Rb=Sr, but also lower 87 Sr=86 Sr and higher
143
Nd=144 Nd. Variations in the age of these heterogeneities may be an important factor in determining
the contrast in isotope composition between enriched
heterogeneities and depleted matrix, and hence between the lavas erupted at the ridge axis, and within
transforms. The low 207 Pb=204 Pb ratios of the Garrett
lavas compared to MORB from the EPR (Figs. 2 and
3) suggest that the mantle heterogeneities beneath
this part of the southern EPR are probably several
Ga old. A third possibility is that the differences in
Sr, Nd and Pb isotope composition between intratransform lavas from the Garrett, Siqueiros and Raitt
Transforms and MORB from the adjacent ridge axes
may be related to variations in spreading rate along
the EPR. Spreading rate has an important influence
on the mean extent of melting at mid-ocean ridges
[27], and thus on the degree of depletion of the
melting residues, which determines the nature of the
sources for subsequent intra-transform magmatism.
The degree of residual mantle depletion is expected
to be most extreme beneath the fast-spreading southern EPR (spreading rate 14–16 cm=y), and least
beneath the Pacific–Antarctic Ridge (8–9 cm=y).
Further studies of other intra-transform lavas from
elsewhere in the Pacific are needed to test these
possibilities.
Our model for intra-transform magmatism may
also explain the relatively high 3 He=4 He ratio (9.7
R=RA ) of the Garrett lava analysed by Mahoney et
al. [18]. This sample has a He concentration which
Fig. 7. Schematic diagrams showing processes by which highly
depleted lavas are generated in the Garrett Transform. (a) Map
showing location of sections in (b) and (c). (b) Section across the
EPR. 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 the
ridge axis (path y) will experience only limited decompression
melting and melt extraction, leaving a residue (cross-hatched
region) which is preferentially depleted in the enriched heterogeneities. (c) Section across the Garrett Transform. Residual
mantle that has been stripped of enriched heterogeneities during
partial melting beneath the EPR axis, is the source for subsequent
intra-transform magmatism.
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J.I. Wendt et al. / Earth and Planetary Science Letters 173 (1999) 271–284
is similar to that of lavas from the nearby ridge
axis (J.J. Mahoney, pers. commun., 1999), but the
Garrett lavas have generally lower U and Th concentrations. 3 He=4 He ratios higher than those of MORB
are often taken as evidence for a contribution from a
‘primitive’, undegassed mantle source. Alternatively,
if U and Th are more incompatible than He, as
has previously been speculated [41], then the relatively high 3 He=4 He and low (U C Th)=He of the
Garrett lava can be explained by our model. The
Garrett lavas may have been inherited from a source
from which easily melted heterogeneities (enriched
in U, Th and 4 He) were removed during melting
beneath the EPR axis, leaving a residue with low U
and Th, moderate He concentration, and relatively
high 3 He=4 He, which became the source for subsequent intra-transform magmatism beneath the Garrett Transform. This suggestion could be tested with
He isotope measurements on other intra-transform
lavas.
7.3. Implications for mantle heterogeneity
The lavas erupted at Pacific ridge axes, on seamounts, and on oceanic islands, all consist of variable mixtures of incompatible trace element depleted matrix and enriched heterogeneities [10–
13,16,17,42]. Earlier studies of oceanic lavas therefore attempted to estimate the isotope composition of
the depleted mantle endmember(s), either by inversion of the available isotope data [43], or by arbitrarily placing this endmember beyond the most depleted
end of the MORB array (the DMM component [19]).
The fact that Garrett peridotites and Garrett intratransform lavas have similar 143 Nd=144 Nd (Fig. 2)
suggests that the Sr, Nd and Pb isotope composition
of the depleted mantle endmember that contributes to
magmatism beneath this region of the southern EPR
is similar to that of the most depleted Garrett lavas.
However, lavas from the Garrett Transform and from
the southern EPR do not lie upon simple two-component mixing lines in trace element-isotope diagrams
(Fig. 5), which suggests that the depleted mantle
matrix itself is heterogeneous, both on the scale of
a few km beneath the Garrett Transform, and on
the scale of an ocean basin (the Raitt intra-transform lavas have 87 Sr=86 Sr values that do not overlap
with the data for Garrett lavas). Variations in mantle
composition on this scale may reflect the complexity
of the processes causing mantle heterogeneity, and
the fact that these processes have occurred throughout geological time (e.g. [44]). As discussed earlier,
mantle heterogeneity beneath the southern EPR may
be several Ga old.
Insights into the composition of the DMM endmember have also come from studies of abyssal
peridotites. Clinopyroxene separates from abyssal
peridotites from the Indian Ocean have 143 Nd=144 Nd
values that are similar to, or higher than, MORB
from the nearby segments of the ridge axes [34].
This was interpreted as the effects of melting a heterogeneous mantle, with the peridotites representing
residual mantle from which an enriched component
with lower 143 Nd=144 Nd was removed during melting beneath the ridge axis [34]. The Indian Ocean
peridotites have lower 143 Nd=144 Nd than the Garrett peridotites. However, the primary Sr and Pb
isotope compositions of abyssal peridotites cannot
be determined easily, because of the effects of serpentinisation. Intra-transform lavas are therefore an
important source of information on the nature and
origin of heterogeneity in the upper mantle.
8. Conclusions
(1) Lavas erupted within the Garrett Transform
Fault are more primitive and more porphyritic, have
lower concentrations of incompatible trace elements,
and lower ratios of more incompatible to less incompatible elements, compared to lavas from adjacent
segments of the southern EPR axis.
(2) The unusual petrology and major and trace
element compositions of the Garrett lavas appear to
be characteristic of intra-transform lavas from the
Siqueiros and Raitt Transforms.
(3) The relatively primitive and porphyritic nature of these intra-transform lavas may be a result
of the different physical conditions that exist beneath the transform. In contrast, the unusual trace
element compositions of intra-transform lavas is inherited from the mantle source of these lavas, which
has undergone prior melt extraction beneath the EPR
axis, depleting an originally heterogeneous mantle
in incompatible element-enriched, easily melted heterogeneities.
J.I. Wendt et al. / Earth and Planetary Science Letters 173 (1999) 271–284
(4) The Garrett lavas have Sr, Nd and Pb isotope
compositions which overlap with the depleted end
of the array defined by Pacific MORB, but extend
to lower 87 Sr=86 Sr, 206 Pb=204 Pb and 207 Pb=204 Pb, and
higher 143 Nd=144 Nd values than any other lavas from
the Pacific Ocean.
(5) 143 Nd=144 Nd for peridotites from the Garrett
Transform are similar to values for the intra-transform lavas, indicating that the source of the Garrett lavas has been entirely stripped of the enriched
heterogeneities that exist in the uppermost mantle
beneath much of the eastern Pacific.
(6) Studies of intra-transform lavas can give important insights into the composition of the upper
mantle, and in particular can be used to estimate the
Sr and Pb isotope composition of the DMM component, which is difficult to determine from studies of
abyssal peridotites. The DMM component appears
to be heterogeneous, both on the scale of a single
transform fault, and on the scale of an ocean basin.
[7]
[8]
[9]
[10]
[11]
[12]
[13]
Acknowledgements
We thank Alan Greig for help with the ICPMS
analyses. Reviews by K. Haase and B.B. Hanan improved the paper, and we are grateful for comments
by J.J. Mahoney and J.-G. Schilling on an earlier
version of the manuscript. [FA]
[14]
[15]
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