JOURNAL OF PETROLOGY
VOLUME 43
NUMBER 1
PAGES 171–198
2002
A Platinum Group Element and Re–Os
Isotope Investigation of Siderophile Element
Recycling in Subduction Zones: Comparison
of Grenada, Lesser Antilles Arc, and the
Izu–Bonin Arc
S. J. WOODLAND1∗, D. G. PEARSON1 AND M. F. THIRLWALL2
1
DEPARTMENT OF GEOLOGICAL SCIENCES, UNIVERSITY OF DURHAM, SOUTH ROAD, DURHAM DH1 3LE, UK
2
DEPARTMENT OF GEOLOGY, ROYAL HOLLOWAY UNIVERSITY OF LONDON, EGHAM TW20 0EX, UK
RECEIVED AUGUST 15, 2000; REVISED TYPESCRIPT ACCEPTED AUGUST 6, 2001
The picritic (MgO >13·5%) lavas of Grenada provide a unique
opportunity to evaluate the platinum group elements (PGE) and
Os isotope compositions of primitive subduction-generated melts.
Compared with other arc lavas they have undergone very limited
crustal contamination (><2%) and only minimal fractionation
(earlier calculations indicate that <10% olivine removal from a
range of parental melts can explain all major element variations).
Despite their primitive nature, the picritic lavas contain very low
concentrations of PGE (<0·2 ppb Ir, 1–4 ppb Pd) compared with
other high-MgO lavas such as kimberlites, komatiites and plumerelated picrites. This is probably due to a combination of lower
degrees of partial mantle melting and early removal of PGE with
cumulus phases such as olivine, magnetite and sulphide in the
subduction systems studied here. Comparison of Grenada samples
with those from Izu–Bonin illustrates that although the major
element chemistries of Grenada and Izu–Bonin are different (alkalic
vs boninitic) their PGE signatures are very similar. Thus, on the
basis of the two systems studied, it would appear that melts generated
in a subduction regime may have common characteristics. In contrast,
Re is markedly depleted in the Grenada picrites compared with the
Izu–Bonin boninites, suggesting retention of Re by residual garnet
in the Grenada sub-arc mantle wedge. Generation above a subduction
zone does not appear to have any significant systematic effect on the
PGE signatures of resultant lavas. The potentially more fluidmobile elements, Os and Pd, do not show major enrichment in
relation to the other PGE compared with other tectonic environments.
Os isotope analyses of several Grenada picrites reveal variable
∗Present address: Institut für Isotopengeologie und Mineralische
Rohstoffe, Sonneggstrasse 5, ETH-Zentrum, NO CO61.1, CH-8092
Zürich, Switzerland. E-mail: [email protected]
187
Os/188Os signatures (0·134–0·164), which are even more
radiogenic than peridotite xenoliths previously measured from mantle
wedges modified by addition of subducted material. Crustal contamination appears unable to explain all of the Os isotope enrichment
and thus elevated 187Os/188Os may reflect the modification of the
Grenadian sub-arc mantle wedge by addition of small amounts
(>2%) of sediment and/or slab-derived fluids enriched in
radiogenic Os.
KEY WORDS: platinum
group elements; Re–Os isotopes; subduction zones;
Grenada; Izu–Bonin
INTRODUCTION
PGE geochemistry
Siderophile (iron-loving) elements are valuable geochemical tools because they have the potential to trace
mantle petrogenetic processes, such as degree of partial
melting and timing of S saturation, and thus provide
complementary information to the more commonly used
lithophile elements and their isotopes. On the basis of
their melting temperatures, the platinum group elements
(PGE) are classified into two groups: the iridium group
(IPGE, melting temperature >2000°C) consists of Os, Ir
 Oxford University Press 2002
JOURNAL OF PETROLOGY
VOLUME 43
and Ru, and the palladium group (PPGE, <2000°C)
consists of Rh, Pt and Pd. The PGE have the potential
to fractionate during geological processes, as a result of
their varying geochemical behaviour and their presence
within different mantle phases. The IPGE are thought
to occur within the mantle as discrete minerals or sulphides (i.e. osmiridium and laurite), often hosted within
silicate grains, whereas the PPGE are more likely to
occur as sulphides, often interstitial in nature and hence
more easily accessed during partial melting events (Alard
et al., 2000). As such, partial melting can substantially
fractionate IPGE from PPGE and hence low-degree
partial melts will have much higher Pd/Ir ratios than
high-degree partial melts.
Experimentally derived sulphide liquid–silicate melt
partition coefficients for the PGE are extremely high, i.e.
>104 (Peach et al., 1994; Naldrett, 1997). Therefore,
during partial melting, as long as residual sulphides
remain in the mantle the PGE also tend to remain within
mantle residua. Mantle sulphide will not be completely
consumed if the degree of partial melting is less than
>25% and the melts produced are S saturated (Hamlyn
et al., 1985; Keays, 1995). For example, primary midocean ridge basalt (MORB) liquids are S saturated and
as such a sulphide component enriched in PGE is thought
to be retained in the mantle residue during MORB
generation (Hamlyn et al., 1985). Once all mantle sulphide
is consumed, all PGE hosted within this residual sulphide
should be released into the melt, thus remelting of a
refractory source (e.g. MORB-depleted mantle during
boninite production) generates S-deficient, PGE-enriched
magmas (Hamlyn et al., 1985).
Another consequence of the high melting point and
sulphide–silicate Kd of PGE is that they crystallize very
early from a magma, particularly the IPGE. PPGE and
Re, which may remain in the melt longer, are rapidly
removed, however, when sulphide saturation occurs following fractionation of silicate phases. This explains the
common associations of Os, Ir and Ru with cumulus
minerals such as olivine and chromite, and the associations of Pt, Pd and Re with sulphide phases (Brügmann et al., 1987; Prichard et al., 1994).
PGE studies of arc-related rocks
Studies of lithophile elements and their isotopes have
demonstrated a significant transfer of many of these
elements from subducted oceanic crust into the source
regions of volcanic arcs (Pearce et al., 1995). The fate
of siderophile elements such as the PGE during the
subduction process is much less clear. Pelagic sediments
can contain high concentrations of siderophile elements,
e.g. Cu, Mo and PGE (Ravizza & Pyle, 1997), and so
might be expected to make significant contributions to
NUMBER 1
JANUARY 2002
arc magma sources if these elements are fluxed into such
regions from a subducting slab. PGE distributions in
harzburgitic arc xenoliths suggest mobilization of certain
PGE within the mantle wedge, perhaps linked to sediment
and/or fluid transfer from the subducting slab (Brandon
et al., 1996; Rehkämper et al., 1997). Understanding
the behaviour and potential fractionation of siderophile
elements in subduction zones is thus important for us to
assess whether residual material being returned to the
mantle via subduction might play a role in producing
large-scale siderophile element rich ore-bodies in volcanic
arcs.
PGE data for arc lavas are very scarce (except for Re
and Os) because of the analytical difficulties that have
existed to date. Boninitic lavas analysed by Hamlyn et
al. (1985) were generally found to have high Pd (6·9–35
ppb) and low Ir contents (<0·01–0·1 ppb). Analyses of
supra-subduction zone peridotites are more common.
Rehkämper et al. (1997) conducted PGE analyses on a
suite of residual harzburgites that had suffered repeated
episodes of melt extraction combined with re-enrichment
by silicic melts from a subduction zone. They discovered
that these samples were depleted in the more fluid-mobile
PGE, Pt and Pd, and thus had low Pd/Ir and Pt/Ru
ratios (Rehkämper et al., 1997). McInnes et al. (1999)
found PGE enrichment (in the order Pd > Pt > Re >
Os) in veined sub-arc mantle-wedge harzburgites metasomatized by slab-derived, oxidizing, hydrous fluids from
Lihir Island, Papua New Guinea. Both studies thus
suggest that Pd and Pt may be more readily mobilized
than the other PGE in the mantle wedge above a subduction zone. One aim of the present study is to examine
whether such inter-PGE fractionation persists during melt
transport through the mantle wedge and, if so, whether
it can be recognized within subduction-related lavas.
The Re–Os decay scheme and its use in
arc-genesis studies
Several groups have utilized the Re–Os decay scheme to
assess the siderophile element flux in subduction systems.
Although not a true PGE, Re is often considered in
conjunction with this group because of its chemical
similarity and because 187Os is derived by radioactive
decay of 187Re. Re behaves as a mildly incompatible
element during melting whereas Os is highly compatible.
This generates melts with much higher Re/Os ratios
than their source (Shirey & Walker, 1998). As such, old
crustal rocks have significantly higher 187Os/188Os ratios
than the mantle, making Os isotopes a powerful tool
in revealing crustal input to mantle-derived melts. A
summary of the current literature concerning Re–Os
behaviour in subduction systems is presented below, as
this provides a useful framework within which to consider
the PGE data obtained in this study.
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WOODLAND et al.
SIDEROPHILE ELEMENTS AND SUBDUCTION
Recent work on Sunda Arc lavas (basalts to rhyolites)
from Java (Alves et al., 1999) has shown that they have
low Os concentrations (0·1–17 ppt) and highly radiogenic
Os isotope ratios ( 187Os/188Os = 0·241–3·704). This
signature is attributed to mixing between unradiogenic
mantle peridotite and a radiogenic end-member: either
a crustal contaminant or recycled sediment in the magma
source. Identification of whether the crustal input occurs
in the mantle wedge or during late-stage crustal contamination, however, is not easy because of the low Os
content and the evolved nature of these rocks. Borg et
al. (2000) analysed primitive calc-alkaline rocks from
the Cascades Arc (Lassen area) and found higher Os
concentrations (>11–370 ppt) but still had difficulty in
unequivocally identifying recycled sediment in the
magma source region. They suggested that the radiogenic
Os isotope signatures ( 187Os/188Os = 0·12845–0·2829)
indicate a significant slab contribution, but that Re and
Os are retained within the mantle wedge by a phase
such as a sulphide, which is stabilized as a result of
fluxing with slab fluids (Borg et al., 2000).
Another difficulty that arises in assessing the role of
recycled components in generating the Os signatures of
arc lavas is uncertainty over the composition of the
mantle through which the arc lavas pass and whether
they inherit the signature of this mantle. Peridotites
thought to be representative of the Cascades and
Ichinomegata mantle wedges contain radiogenic Os
thought to be derived from either dehydration of oceanic
crust or from sediments (Brandon et al., 1996). Such crust
or sediment would need to have elevated 187Os/188Os but
low total Os (Brandon et al., 1996) compared with presentday mantle (present-day mantle Os = 1–5 ppb; Shirey
& Walker, 1998). Brandon et al. (1996) proposed that Os
behaves in a mildly incompatible manner during slab
dehydration or melting, possibly because of destabilization of Os-containing sulphide phases in the
oxidizing, Cl-rich regime of the subduction environment.
This therefore contradicts the model of Borg et al. (2000).
In contrast to the radiogenic Os isotope composition
of arc lavas and some arc peridotite xenoliths, mantle
peridotite xenoliths from both the Izu–Bonin forearc and
from Grenada have unradiogenic Os isotope compositions (Parkinson et al., 1998b). These xenoliths are
thought to represent residues from ancient melt depletion
events during which Re was depleted relative to the
PGE. Such a residue would thus, with time, develop an
unradiogenic Os isotope signature relative to undepleted
mantle. The low density of residual mantle compared
with fertile mantle may render it difficult to subduct and
hence residual material may survive mantle homogenization by convection. The presence of such old
depleted mantle in these modern arc systems has led
to the suggestion that subduction zones may act as
‘graveyards’ for old oceanic lithosphere (Parkinson et al.,
1998b). Arc melts rising through such depleted mantle
therefore have the potential to acquire unradiogenic
signatures by melt–solid interaction.
The varying and seemingly contradictory information
gained from these studies highlights the fact that our
knowledge of siderophile element behaviour in subduction zones is still rudimentary, as a result of a paucity
of coupled siderophile elemental and isotopic data for
the erupted magmatic products. To redress this situation
we have undertaken a combined PGE and Re–Os isotope
study on two suites of arc lavas. To minimize the potential
effects of fractionation and late-stage contamination, on
Re–Os isotope systematics in particular, we targeted
primitive arc rocks from two suites of inter-oceanic arc
lavas: Grenada, in the Lesser Antilles, and the Izu–Bonin
arc. The primitive nature of the lavas from both these
suites (picrites from Grenada and boninites from Izu–
Bonin) optimize the chances of seeing through the potential effects of late-stage crustal contamination from
the arc suprastructure, to gain insight into the processes
affecting siderophile element behaviour in the magma
source itself.
APPROACH
Sample selection
Grenada was selected to study the behaviour of PGE
in an arc-system because the elemental and isotopic
geochemistry of lavas on this island has been well characterized (Thirlwall et al., 1996) and hence well-constrained petrogenetic models exist to explain the
geochemical variations observed. Also, unlike other intraoceanic island arcs, highly magnesian (10–15% MgO)
picrites are common on Grenada (Arculus, 1973;
Thirlwall et al., 1996). Thus, the Grenadian picrites have
undergone significantly less fractionation (><10%) than
many other arcs. Furthermore, the occurrence of picrites,
basalts, andesites and cumulates on Grenada presents
an ideal opportunity to assess the effects of fractional
crystallization on PGE distribution.
Samples with the prefix ‘AMG’ were the subject of the
detailed study of Thirlwall et al. (1996) and were analysed
to supplement the samples (prefix ‘Gd’) collected by the
first author. Cumulate nodules Gd1, Gd2 and Gd3
were collected from the reworked volcanic rocks of SW
Grenada. Samples Gd8 and Gd10 were lava bombs
contained within a large scoriaceous deposit. All other
‘Gd’ samples are from ancient lava flows in central and
western Grenada (Woodland, 2000). All samples analysed
are from the western Grenada volcanic centres of Mt.
Granby–Fedon’s Camp and Mt. Maitland–Mt. Moritz,
as defined by Arculus (1973). The eruption ages of the
volcanic rocks at Mt. Maitland–Mt. Moritz are thought
to be between 1 and 2 Ma, and between 1 Ma and 50 ka
173
JOURNAL OF PETROLOGY
VOLUME 43
at Mt. Granby–Fedon’s Camp (Arculus, 1973). PGE are
generally considered to be immobile during weathering
processes (Barnes et al., 1985); however, ‘fresh’ samples
(i.e. in which olivines and feldspars appear unaltered
under a petrological microscope) were chosen for PGE
analysis. Thus, inter-element PGE fractionation can be
attributed to primary magmatic causes rather than to
secondary mobilization of PGE during weathering.
In addition to lavas from Grenada, several sediments
thought to be representative of material being subducted
below Grenada have been analysed. These samples were
obtained from the Ocean Drilling Program (ODP) core
store at the Lamont–Doherty Earth Observatory. There
is debate regarding the type and quantity of sediment
subducted beneath the Lesser Antilles Arc, as a large
décollement and accretionary prism exist in front of the
arc (White et al., 1985). However, strong similarities
between the Pb isotopic compositions of the Lesser Antilles volcanic rocks and sediments in front of the arc
provide strong evidence that sediments, derived from the
Archaean Guyana shield to the south, are contributing
to the Grenada arc source (White et al., 1985). Thus, we
analysed sediments from the Atlantic plate to the NE of
Grenada, collected during ODP Leg 78. Sediments being
subducted below Grenada probably consist of intercalated sandy silts (terrigenous material derived from the
Guyana shield), limestones, ash layers and minor Mnrich sediments. A Mn-rich clay was analysed to represent
a metalliferous high-PGE end-member sediment
(15°42′N, 58°39′W; ODP site 543a, Core 27; 27–28 cm)
and two sandy clays (ODP piston core samples RC13-175:
11°N, 57°75′W; 162–164 cm and V24-260: 12°80′N,
57°80′W; 643–646 cm) were analysed to represent ‘normal’ terrigenous sediments.
Samples from the Izu–Bonin arc region were recovered
during ODP Leg 125 at site 786b. Samples IB21 (a
rhyolite) and IB1 (an intermediate-Ca bronzite andesite)
are interpreted to be part of the basal sequence (>41 Ma)
of a volcanic edifice that makes up the oceanic forearc
basement (Murton et al., 1992; Pearce et al., 1992b).
Samples IB5, IB40 and IB67 are dykes of early Oligocene
age (>35 Ma) and intermediate-Ca to high-Ca boninitic
composition that bisect the basal sequence (Pearce et
al., 1992b). Thus, although the Izu–Bonin samples are
cogenetic they are related to distinct parental magmas
(Murton et al., 1992) and so cannot strictly be considered
a fractionation ‘suite’. Their use in this study is primarily
to illustrate how PGE abundances vary in relation to
degree of fractionation of the sample within boninitic arc
rocks and how they compare with the rocks of Grenada.
The Izu–Bonin samples, unfortunately, are highly serpentinized, but as stated above this should not affect
PGE abundances.
NUMBER 1
JANUARY 2002
Petrology of samples
The Grenada picrites and basalts are predominantly
composed of olivine, clinopyroxene, plagioclase and
spinel, with textures varying from microphyric to coarsely
porphyritic. The cumulate blocks are composed of variable proportions of plagioclase feldspar, amphibole, olivine, clinopyroxene and magnetite. The andesites are
extremely porphyritic with plagioclase feldspar, clinoand orthopyroxene, amphibole and magnetite being the
dominant phenocryst phases. High-silica andesites (e.g.
Gd25) may contain phenocrysts of quartz.
The Izu–Bonin intermediate-Ca and high-Ca boninites
(IB5, IB40 and IB67) typically consist of variable proportions of highly altered olivine, orthopyroxene, clinopyroxene, plagioclase and Cr-spinel (Pearce et al., 1992b).
The intermediate-Ca bronzite andesites (IB1) have the
same mineralogy but tend to contain higher proportions
of plagioclase and orthopyroxene. Rhyolites (IB21) are
characterized by absence of olivine and presence of
abundant magnetite phenocrysts and trace quartz (Pearce
et al., 1992b).
Descriptions of the petrology of the sediment layers
sampled were obtained from Deep Sea Drilling Project
(DSDP) Leg 78 Initial Reports. The sampled interval of
piston core V24-260 was a sandy layer made up of
medium-sized angular and sub-angular quartz, with some
chlorite and large planktonic foraminifera present. RC13175 was a slightly friable and homogeneous clayey sand,
pale yellowish brown and olive–grey in colour, with a
low carbonate content, containing abundant quartz,
mica, mafic minerals, rare foraminifera and siliceous
spicules. The Mn-rich sample from Site 543a, Core
27, was an orange-coloured, compacted, fine clay with
abundant dark brown or black Mn spots and layers.
Analytical techniques
Major elements were analysed using standard X-ray
fluorescence (XRF) fusion techniques; analytical precisions are given in Table 1. Trace elements were analysed
by inductively coupled plasma mass spectrometry (ICPMS) at Durham. Powders were dissolved using HF–
HNO3 with care being taken to ensure that no residual
fluorides remained. Before dilution with 3·5% HNO3,
samples were spiked with Rh, In and Bi, as internal
drift monitors. The resulting solutions were run on a
Perkin–Elmer–SCIEX Elan 6000 inductively coupled
plasma mass spectrometer using a cross-flow nebulizer
and Scott-type spray chamber. Oxide interferences were
corrected for by running standard solutions and commonly made up p2·5% of the total analyte signal.
Calibration was achieved using matrix-matched international rock standards and in-house reference materials.
Total procedural blanks were corrected for on-line and
174
WOODLAND et al.
SIDEROPHILE ELEMENTS AND SUBDUCTION
were negligible for all elements. Reproducibility is reported in Table 1 and is between 0·5 and 3% RSD for
most elements, but up to 6% for some elements with
atomic masses less than Ga. Washout times were usually
3 min between samples.
Samples with the prefix Gd, which have Sr-isotope
analyses, were first analysed for PGE and then aliquots
of the same powder were analysed for Sr at Royal
Holloway University of London by Thirlwall. The Sr
isotope data for samples prefixed AMG were obtained
from Thirlwall et al. (1996) and the reader is referred to
that work for the appropriate analytical techniques. PGE
and Os isotope analyses were conducted on aliquots of
these original sample powders.
PGE analytical details follow those recently presented
by Pearson & Woodland (2000). Briefly, samples are
spiked with a solution isotopically enriched in PGE (Pd,
Pt, Ru, Ir, Os) and Re. Sample and spike are then
digested and equilibrated in Carius tubes. Carius tubes
were first cleaned by double boiling in aqua regia. Os
was separated by solvent extraction and analysed by
negative thermal ionization mass spectrometry (N-TIMS;
Department of Terrestrial Magnetism, Carnegie Institution, Washington, DC), for isotopic analysis, or by
ICP-MS using a CETAC direct injection nebulizer (DIN)
for elemental concentration analysis. Following the Carius tube digestion and solvent extraction, the other PGE
were separated by anion-exchange chromatography and
analysed by ICP-MS using a cross-flow nebulizer. Total
procedural blanks are typically: Ir 1 pg, Os 1–7 pg (mean
>1·5 pg), Ru and Re 5 pg, Pd 10 pg and Pt <25 pg.
Procedural detection limits (3 × SD) of >3 pg/g for
Os and Ir, 5 pg/g for Re, Ru and Pd, and 15 pg/g for
Pt were obtained. These levels are adequate for most
silicate magmatic rocks and sediments and for the samples
analysed during this study.
RESULTS
Major and trace element data
Major and trace element data for both the Grenada and
Izu–Bonin samples are presented in Table 1.
Grenada
The Grenada picrites are characterized by high MgO
(>13·5 wt %), high Ni (>380 ppm), high Cr (>880 ppm)
and low SiO2 (<47 wt %; Table 1) and fall within the
calc-alkaline and alkalic suites of volcanic rocks. On the
basis of their chemistry and petrology the primitive rocks
of Grenada have been subdivided into two magmatic
series, an M-series (magnesian and olivine microphyric)
and a more evolved C-series (calcic and clinopyroxenephyric; Fig. 1a; Thirlwall & Graham, 1984). The Mseries picrites are parental to a suite of low-Ca M-series
basalts and both the M-series basalts and C-series basalts
evolve to more andesitic compositions (see Fig. 1a). In
addition, there is an M–C transitional series which is
generally basaltic, although Thirlwall et al. (1996) recognized one M–C picrite (which falls within the Mseries field in Fig. 1a). This group is represented by a
very limited number of samples, which occur only at the
Mt. Granby–Fedon’s Camp volcanic centre.
Grenada picrites are enriched in large ion lithophile
elements (LILE) and light rare earth elements (LREE)
and depleted in high field strength elements (HFSE)
relative to the LILE and LREE (Fig. 2a), a signature
characteristic of subduction-related lavas (Pearce et al.,
1995; Thirlwall et al., 1996). Variation of LREE/HREE
ratios within the M-series picrites has led to their subdivision into high- and low-La/Y subgroups (Thirlwall
et al., 1996; Fig. 2a). Thus, high- and low-La/Y groups
can also be recognized within basalts derived from these
picrites. Andesites always have high La/Y ratios because
of crystal fractionation.
In terms of their trace element signature the M–C
transitional series are very similar to the M-series picrites,
except that they are more enriched in the most highly
incompatible trace elements (Cs to U; Fig. 2b) and more
depleted in Ni. The C-series basalts contain higher LILE
and REE concentrations than the M-series picrites, but
lower Ni and Cr. The andesites also have low concentrations of highly compatible elements and in addition,
a pronounced depletion in Ti (see Fig. 2b).
Izu–Bonin
MgO contents of the Izu–Bonin samples decrease with
increasing fractionation in the order intermediate-Ca
boninite, high-Ca boninite, intermediate-Ca bronzite
andesite, rhyolite (Table 1 and Fig. 1b). The intermediateCa boninite (IB67) is the only Izu–Bonin sample of
picritic nature (13·1% MgO) and comparable MgO
content to the Grenada M-series picrites. SiO2 increases
in the order high-Ca boninite, intermediate-Ca boninite,
intermediate-Ca bronzite andesite, rhyolite (Table 1; Fig.
1c). Compared with MORB, the high-Ca boninite (Fig.
2c) is enriched in the most incompatible LILE, depleted in
HFSE, LREE and HREE, and enriched in the compatible
elements Cr and Ni. The intermediate-Ca bronzite andesite is markedly depleted in the compatible metals and
HREE compared with the boninite (Fig. 2c). Compared
with the bronzite andesite, the Izu–Bonin rhyolite contains even less Ni and Cr, and is also markedly depleted
in Ti and V (see Fig. 2c).
175
176
148
1270
38
75
91
81
19
17
700
Cr
Mn
Co
Ni
Cu
Zn
Ga
Rb
Sr
La
21·6
387
1·0
Cs
Ba
16·3
124
Nb
Zr
29·1
309
16·7
277
0·8
4·3
76·5
21·0
1216
17
17
70
75
64
42
1241
121
324
6117
50·2
813
1·0
19·9
82·1
18·0
995
109
21
103
32
11
8
290
10
98
2493
99·66
0·18
0·04
0·42
2·04
4·95
4·12
0·35
4·68
18·43
64·44
45.5
03.8
MMM
Andesite
Gd25
40·5
639
0·5
13·2
174
26·0
946
46
20
35
32
22
20
861
30
184
4330
100·06
0·24
0·11
0·69
1·94
4·22
7·43
2·95
6·65
17·24
58·61
45.3
06.95
MGF
Andesite
Gd17
2·5
77·0
0·01
3·3
33·2
21·7
236
2
13
58
22
294
64
1111
781
443
9994
100·15
0·03
0·15
1·65
0·42
2·48
13·39
14·87
11·68
13·15
42·34
45.6
11.6
MMM
Cum.
Gd1
2·7
40·3
0·01
2·1
27·1
16·7
601
1
19
65
14
59
48
1159
39
450
7151
99·86
0·02
0·16
1·15
0·14
1·29
15·77
7·62
13·54
18·10
42·08
45.6
11.6
MMM
Cum.
Gd2
5·1
73·2
0·2
1·4
25·7
9·2
1417
3
20
39
17
32
25
683
7
237
3627
99·52
0·06
0·11
0·71
0·18
1·67
15·91
4·20
9·23
23·36
44·11
45.6
11.6
MMM
Cum.
GD3
8·2
140·0
0·8
4·6
55·2
18·4
334
13
14
67
79
244
47
1309
804
262
5432
99·44
0·13
0·17
0·99
0·49
2·42
11·72
10·49
10·62
15·18
47·24
45.6
11.6
MMM
Basalt
M–C
Gd5
10·2
210
5·1
63·0
17·9
576
11
14
74
53
359
6792
956
262
4472
99·27
0·15
0·18
0·88
0·75
2·07
11·63
12·79
10·04
13·43
47·36
44.7
07.0
MGF
Picrite
M–C
AMG6157
0·003
0·135
0·01
0·043
0·005
0·047
0·018
0·031
0·072
1·047
0·042
9·075
1·963
0·929
3·18
2·27
2·39
0·75
0·72
0·78
2·39
2·32
3·49
6·43
5·5
5·69
5·62
4·78
4·46
2·6
3·12
0·01
0·005
0·01
0·01
0·03
0·09
0·11
0·08
0·17
0·2
error (%)
limit
NUMBER 1
Y
7282
V
100·13
0·19
0·17
1·00
0·98
1·97
14·81
7·15
10·49
16·38
46·98
45.0
07.7
MGF
Basalt
C
Gd21
Standard
Detection
VOLUME 43
Ti
0·31
100·16
1·34
K2O
Total
2·64
Na2O
P2O5
12·45
CaO
1·20
5·80
MgO
0·16
10·11
Fe2O3
MnO
17·59
TiO2
48·55
Al2O3
43.8
Long.:
SiO2
MGF
07.1
Lat.:
Rock type:
Centre:
C
Basalt
Series:
Gd18
Sample:
Grenada
Table 1: Grenada and Izu–Bonin major and trace element data compilation
JOURNAL OF PETROLOGY
JANUARY 2002
C
177
0·6
5·1
1·6
5·0
Eu
Gd
6·2
2·7
Th
U
0·2
0·8
3·0
Ta
Pb
2·0
0·3
2·8
Lu
Hf
0·3
2·6
4·5
2·0
0·2
1·6
0·4
2·1
Tm
1·7
0·7
3·4
37·1
45.0
Yb
0·8
2·2
Ho
Er
0·7
1·5
4·2
24·1
Nd
Sm
4·1
4·7
5·5
Tb
22·1
42·0
Ce
Pr
Dy
5·0
43.8
Long.:
MGF
07.7
MGF
07.1
Basalt
Lat.:
Rock type:
Gd21
Centre:
C
Basalt
Series:
Gd18
Sample:
Grenada
9·6
24·3
18·4
1·2
2·0
0·2
1·5
0·2
1·4
0·5
2·9
0·6
3·6
1·4
5·0
30·3
8·7
80·3
45.5
03.8
MMM
Andesite
Gd25
6·7
15·4
1·4
0·8
3·6
0·3
2·0
0·3
2·0
0·8
4·0
0·7
5·1
1·8
6·3
34·5
8·8
66·4
45.3
06.95
MGF
Andesite
Gd17
0·1
0·2
0·1
0·2
1·2
0·2
1·5
0·3
1·8
0·7
3·5
0·6
3·8
1·1
3·0
8·6
1·5
7·8
45.6
11.6
MMM
Cum.
Gd1
0·04
0·2
0·2
0·1
0·9
0·2
1·2
0·2
1·4
0·5
2·7
0·5
3·0
0·9
2·6
8·3
1·4
7·7
45.6
11.6
MMM
Cum.
Gd2
0·6
1·1
0·6
0·1
0·7
0·1
0·7
0·1
0·8
0·3
1·4
0·3
1·7
0·7
1·7
7·0
1·5
10·6
45.6
11.6
MMM
Cum.
GD3
0·9
2·2
2·0
0·3
1·5
0·3
1·6
0·3
1·7
0·6
3·2
0·5
3·2
0·9
2·9
11·0
2·4
17·2
45.6
11.6
MMM
Basalt
M–C
Gd5
1·9
3·0
1·7
0·2
1·6
1·8
3·1
3·6
1·1
3·5
14·3
23·0
44.7
07.0
MGF
Picrite
M–C
AMG6157
0·001
0·001
0·005
0·005
0·001
0·002
0·001
0·002
0·001
0·004
0·002
0·007
0·002
0·007
0·007
0·002
2·96
2·85
1·59
2·04
0·84
1·8
0·82
0·75
1·62
1·28
0·92
1·61
2·65
1·68
1·89
2·78
2·68
2·96
error (%)
limit
0·005
Standard
Detection
WOODLAND et al.
SIDEROPHILE ELEMENTS AND SUBDUCTION
0·39
178
68
15
6
256
Zn
Ga
Rb
Sr
18·6
78
Cu
Y
59
1256
Mn
421
974
Cr
Ni
267
0·17
18·3
255
8
14
66
80
435
60
1243
943
268
5245
99·58
0·10
19·5
244
9
15
65
81
429
59
1245
888
259
5292
99·75
0·11
0·17
0·86
0·40
1·92
10·80
14·06
10·17
18·5
513
25
15
70
95
390
58
1198
915
287
7021
99·56
0·29
0·17
1·15
1·03
1·78
12·51
13·66
10·35
13·26
45·36
45.2
05.8
MGF
Picrite
M
Gd12
18·5
252
8
15
69
87
432
60
1259
1145
262
5515
100·05
0·11
0·16
0·88
0·40
2·12
10·85
14·41
10·37
14·72
46·03
43.9
04.5
MMM
Picrite
M
Gd14
20·7
753
22
16
71
108
379
56
1553
643
271
5701
99·81
0·36
0·18
0·91
0·68
2·39
12·70
13·55
9·90
14·26
44·89
43.1
05.3
MMM
Picrite
M
Gd16
19·4
529
25
14
74
96
379
8783
967
285
6607
100·02
0·30
0·17
1·13
1·10
1·88
12·21
14·47
10·38
12·49
45·88
45.1
06.1
MMM
Picrite
M
AMG6078
17·4
679
11
14
72
86
404
7118
908
257
3219
99·45
0·23
0·18
0·92
0·54
1·93
12·30
15·28
10·45
12·53
45·10
40.4
06.8
SEM
Picrite
M
AMG6103
1R-1
ICBrzA
IB1
5·6
165·0
13·0
171
53
101
29
99
171
100·02
0·03
0·12
0·65
0·20
2·97
6·16
8·06
7·30
12·80
61·72
Core int. (cm): 75–79
Core name:
Magma type:
12·0
128·0
30·0
19
61
13
8
3
19
100·02
0·07
0·09
2·46
0·26
4·19
2·76
0·92
3·92
14·23
71·12
28–32
21R-2
Rhyolite
IB21
8·0
131·0
7·0
187
34
349
47
961
187
100·00
0·02
0·16
0·32
0·29
2·23
12·70
10·20
8·74
13·30
52·04
83–90
40R-2
HCB
IB40
13·0
147·0
6·0
237
63
170
40·7
333
237
99·99
0·1
0·11
1·30
0·37
2·56
11·34
7·99
8·90
15·00
52·32
69–71
5R-2
HCB
IB5
6·5
136·0
3·1
202
56
296
38·5
786
202
98·74
0·04
0·14
0·21
0·25
2·76
6·70
13·08
7·54
12·46
53·00
56–59
67R-1
ICB
IB67
NUMBER 1
Co
5352
Total
V
99·17
P2O5
0·85
10·88
14·15
10·30
14·59
46·68
45.1
03.7
MMM
Picrite
M
Gd11
Izu–Bonin
VOLUME 43
Ti
0·17
0·11
MnO
0·36
1·93
Na2O
0·86
10·94
CaO
K2O
13·86
MgO
TiO2
1·90
10·24
Fe2O3
14·34
14·36
46·50
46·35
45.1
Al2O3
45.1
Long.:
03.7
SiO2
MMM
03.7
Centre:
Lat.:
MMM
M
Gd10
Picrite
M
Series:
Rock type: Picrite
Gd8
Sample:
Grenada
Table 1: continued
JOURNAL OF PETROLOGY
JANUARY 2002
53·4
4·1
0·1
Zr
Nb
Cs
179
12·8
1·8
8·6
2·3
0·8
2·6
0·5
2·7
0·6
1·6
0·3
1·5
0·3
1·4
0·2
1·5
1·2
0·4
Ce
Pr
Nd
Sm
Eu
Gd
Tb
Dy
Ho
Er
Tm
Yb
Lu
Hf
Ta
Pb
Th
U
0·4
1·2
1·6
0·2
1·4
0·2
1·5
0·3
1·5
0·6
2·8
0·4
2·7
0·8
2·3
8·5
1·8
12·5
5·5
101
0·2
4·2
53·3
45.1
03.7
0·5
1·2
1·6
0·3
1·4
0·3
1·7
0·3
1·6
0·6
2·9
0·5
2·8
0·8
2·3
8·9
1·9
12·8
5·9
92
0·3
4·5
55·2
45.1
03.7
MMM
Picrite
M
Gd11
2·1
5·9
3·4
1·1
2·3
0·2
1·3
0·2
1·5
0·6
3·0
0·6
3·7
1·3
4·1
19·6
4·7
38·1
19·3
335
1·0
21·5
101·2
45.2
05.8
MGF
Picrite
M
Gd12
0·5
1·1
1·4
0·2
1·4
0·2
1·5
0·3
1·6
0·6
2·7
0·5
2·7
0·8
2·3
8·3
1·8
12·2
5·3
106
0·1
4·3
54·6
43.9
04.5
MMM
Picrite
M
Gd14
4·67
13·0
6·4
0·5
2·7
0·2
1·5
0·3
1·6
0·6
3·3
0·6
4·3
1·5
5·2
28·5
7·3
61·6
32·0
390
1·4
11·2
124·2
43.1
05.3
MMM
Picrite
M
Gd16
2·0
7·0
4·2
1·5
1·8
3·4
4·2
1·4
4·5
21·3
41·2
20·8
368
20·1
99·2
45.1
06.1
MMM
Picrite
M
AMG6078
2·0
5·5
2·2
0·2
1·5
1·7
3·2
3·8
1·2
4·0
18·4
35·9
18·0
268
6·7
66·4
40.4
06.8
SEM
Picrite
M
AMG6103
1R-1
ICBrzA
IB1
0·2
0·4
1·9
0·04
1·2
0·1
0·7
0·1
0·6
0·2
0·9
0·1
1·0
0·2
0·6
2·8
0·6
3·9
1·7
38·6
0·16
0·59
38·6
Core int. (cm): 75–79
Core name:
Magma type:
Izu–Bonin
0·4
0·8
3·5
0·1
2·4
0·2
1·4
0·2
1·3
0·4
1·9
0·3
2·0
0·4
1·5
6·1
1·3
9·4
3·9
68·0
0·7
1·1
79·0
28–32
21R-2
Rhyolite
IB21
0·1
0·1
0·9
0·02
0·7
0·1
0·9
0·2
0·8
0·3
1·3
0·2
1·2
0·3
0·7
2·8
0·6
3·0
1·3
25·0
0·3
0·4
23·0
83–90
40R-2
HCB
IB40
0·2
0·2
1·3
0·02
1·0
0·2
1·2
0·2
1·1
0·4
1·7
0·3
1·6
0·4
1·0
4·0
0·7
4·2
2·1
18·0
0.
0·4
34·0
69–71
5R-2
HCB
IB5
0·1
0·2
0·7
0·03
0·7
0·1
0·8
0·1
0·7
0·2
1
0·2
0·8
0·2
0·6
2·0
0·4
2·6
1·2
7·6
0·4
27·0
56–59
67R-1
ICB
IB67
For Grenada samples: Centre denotes the volcanic centre from which the sample originated: MMM ( Mt. Moritz); MGF ( Mt. Granby–Fedon’s Camp); SEM
(southeastern mountains). Major element data are presented as wt % oxides, trace element data in ppm. XRF error is expressed as the standard error in the
regression of observed results on recommended values for international standards. ICP-MS error is RSD of repeated standard analysis throughout the analytical
run.
5·6
La
95
45.1
Long.:
Ba
MMM
03.7
Centre:
Lat.:
MMM
M
Gd10
Picrite
M
Series:
Rock type: Picrite
Gd8
Sample:
Grenada
WOODLAND et al.
SIDEROPHILE ELEMENTS AND SUBDUCTION
JOURNAL OF PETROLOGY
VOLUME 43
NUMBER 1
JANUARY 2002
Fig. 1. (a) CaO vs MgO (wt %) illustrating the subdivision of the C (calcic, cpx-phyric) and M-series (magnesian, olivine-microphyric) of
Grenada. Large symbols are data acquired by Woodland (2000); small symbols are data from Thirlwall et al. (1996). (b) MgO vs SiO2 for
Izu–Bonin samples. (c) SiO2 vs CaO for Izu–Bonin samples [major element data from Pearce et al. (1992b)].
PGE data
Reproducibility
One limitation of Carius tube digestions is the small
sample size used (typically 2 g in this study). Thus, the
technique will be subject to greater sampling errors than
fusion techniques capable of digesting much larger sample
sizes. Carius tube digestion was preferred to fusion for
these rocks because of the much lower total procedural
blanks of the Carius tube method (Pearson & Woodland,
2000). As a result of the nature of PGE distribution
within rock samples as nuggets of sulphide or alloy, the
analytical reproducibility is more heterogeneous than for
lithophile elements. Care must therefore be taken to
ensure that sample aliquots are representative of the bulk
sample. To assess the intra-sample variation associated
180
with ‘nugget effects’, all Grenada samples in this study
were analysed in duplicate where possible and an average
of these is presented (Table 2) with the variability expressed as a percentage of twice the standard deviation
of the average. PGE analyses for the Izu–Bonin suite
were conducted in the same way as for the Grenada
suite; however, samples were not analysed in duplicate
(except for IB67), because of shortage of sample from
ODP reserves (Table 2). Considering the errors associated
with duplicate analyses (see Table 2), it is apparent that
reproducibility decreases as PGE concentration of the
sample decreases (Pearson & Woodland, 2000). There
also seems to be a link between sample grain size and
reproducibility, as PGE distribution within the coarsely
crystalline cumulates is more heterogeneous than within
the finer-grained picrites.
WOODLAND et al.
SIDEROPHILE ELEMENTS AND SUBDUCTION
Fig. 2. MORB-normalized trace element variation diagrams for Grenada (a, b) and Izu–Bonin samples (c). Data for samples AMG6103 and
AMG6157 from Thirlwall et al. (1996) and Izu–Bonin data from Pearce et al. (1992a). MORB normalization constants from Pearce & Parkinson
(1993).
Sediment PGE data
PGE concentrations of the sediments analysed are shown
in Table 2. The Mn-rich sample was analysed in duplicate
(Mn-1 and Mn-2) and reproducibility ranges from 3%
for Pt to 36% for Os (2 × SD of mean). No Os data
are available for the terrigenous sediments RC13-175
and V24-260, as Os was not quantitatively extracted
using the normal solvent extraction procedure. PGE
181
concentrations are an order of magnitude lower in the
terrigenous sediments, compared with the Mn-rich sediments (except for Re). Both terrigenous sediments analysed have steep positive CI-normalized PGE patterns
(see Fig. 3a), typical of evolved crustal rocks, although
RC13-175 is depleted in PGE (but not Re) relative to
V24-260. The Mn-rich sediment has a flatter chondritenormalized PGE pattern than the terrigenous sediments,
as a result of Os and Ir enrichment relative to Ru. Re
JOURNAL OF PETROLOGY
VOLUME 43
NUMBER 1
JANUARY 2002
Table 2: PGE data compilation
Sample
Rock type
Grenada andesites
Gd17
Gd25
Re
0·36
2
0·06
2·0
Grenada C-series basalts
Gd18
0·09
Gd21
0·10
Grenada M–C transitional series
Gd5
Basalt
0·12
AMG6157
Picrite
0·20
Grenada M-series low-La/Y picrites
Gd8
0·01
116·5
Gd10
0·01
38·4
Gd11
0·01
20·0
Gd14
0·01
177·0
Grenada M-series high-La/Y picrites
Gd12
0·03
4·4
Gd16
0·01
9·1
AMG6103
0·04
AMG6078
—
Grenada cumulates
Gd1
(Amphibole0·03
rich)
113·3
Gd2
(Plagioclase0·02
bearing)
20·2
Gd3
(Plagioclase0·09
bearing)
10·8
Izu–Bonin samples
IB21
R
0·42
IB1
ICBrzA
0·25
IB5
HCB
0·26
IB40
HCB
0·34
IB67A
ICB
0·20
IB67B
ICB
0·20
IB67
Average
0·20
3·72
Sediment samples
RC13-175
0·14
V24-260
0·06
Mn-1
0·04
Mn-2
0·05
Mn-1 and Mn-2 Average
0·05
31·43
Os
0·02
18·65
0·02
21·0
Ir
Ru
b.d.l.
0·09
b.d.l.
0·17
Pt
Pd
1·90
2·6
1·16
14·7
0·66
7·55
0·10
99·9
n
2
2
0·01
0·01
0·04
0·02
0·05
0·04
1·51
3·15
2·52
7·60
0·54
0·30
0·05
0·32
0·07
0·07
1·84
5·08
3·70
4·47
0·09
35·5
0·09
74·0
0·07
60·0
0·08
11·8
0·06
25·6
0·08
68·9
0·06
21·0
0·12
118·9
0·20
178·7
0·21
3·3
0·23
40·4
0·19
121·1
1·88
17·5
2·54
24·4
2·18
2·7
3·39
26·7
1·23
16·4
1·82
23·5
1·87
62·6
1·50
76·4
3
0·04
8·1
0·05
68·4
0·05
0·08
0·09
24·9
0·06
48·7
0·06
0·13
0·15
2·84
1·6
3·99
1·9
2·65
1·87
2
0·06
0·07
3·73
18·4
3·43
25·9
3·81
3·91
0·07
118·3
0·01
40·4
0·01
28·5
0·26
122·0
0·06
78·2
0·01
23·6
0·18
46·1
0·02
36·9
0·03
83·1
5·04
112·2
4·16
66·7
5·78
48·3
1·38
162·0
0·79
7·1
0·46
21·1
4
0·01
0·02
0·03
0·05
0·04
0·02
0·03
82·99
b.d.l.
0·03
0·10
0·11
0·08
0·07
0·08
11·98
0·02
0·24
0·12
0·25
0·15
0·20
0·18
34·50
0·18
3·31
1·89
1·54
5·77
6·27
6·02
11·81
1·02
7·22
1·19
3·68
3·53
3·44
3·49
3·60
2
b.d.l.
0·01
0·29
0·33
0·31
18·25
0·02
0·03
0·30
0·34
0·32
17·68
0·21
0·51
3·04
3·11
3·08
3·22
0·03
0·38
6·02
5·84
5·93
4·29
2
—
—
1·07
0·83
0·95
35·73
0·17
3
2
3
2
2
2
Concentration values presented are in ppb. b.d.l., below detection limit (i.e. <5 ppt Ir). Reproducibility (in italics) is expressed
as % 2 × SD of the average; replicates could not be run for all samples because of shortage of rock powders. For Grenada,
Lesser Antilles arc data (samples prefixed Gd were collected by the author and samples prefixed AMG are from Thirlwall’s
collection). Concentrations are averages of several replicate analyses (number of replicates denoted by n). For Izu–Bonin
(Prefix IB) IB67A and IB67B are replicate dissolutions of the same sample, analysed to assess reproducibility (expressed as
% 2 × SD of the average); R, rhyolite; ICBrzA, intermediate-Ca bronzite andesite; ICB, intermediate-Ca boninite; HCB, highCa boninite. Concentrations of PGE (ppb) in typical Atlantic plate sediments subducted below Grenada are also listed. Mn-1
and Mn-2 are replicate analyses of a single sample of Mn-rich sediment recovered from site 543a (ODP Leg 78) to the NNE
of Grenada. Samples RC13-175 and V24-260 are terrigenous sediments from ODP piston cores (Leg 78) to the east of
Grenada.
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SIDEROPHILE ELEMENTS AND SUBDUCTION
Fig. 3. (a) PGE patterns for Atlantic Ocean floor sediments analysed. It should be noted that Mn-1 and Mn-2 are replicates of a Mn-rich clay
(ODP Leg 78, site 543a) and samples RC13-175 and V24-260 are piston core samples of terrigenous sediments (ODP Leg 78). (b)–(d) PGE
patterns for Izu–Bonin arc lavas analysed. All PGE plots are chondrite normalized; CI values taken from Jochum (1996) and Naldrett (1997).
3c). Pt and Pd concentrations within the boninite samples
are far more variable, however, and the intermediateCa boninite (IB67) is enriched in Pt relative to the highCa boninites. Samples IB40 and IB67 (a high- and a
low-Ca boninite, respectively) contain similar high Pd
contents (>3 ppb), but the other high-Ca boninite (IB5)
contains less Pd (1·2 ppb). Therefore, Pd content does
not seem to be related to Ca content. In contrast to the
more evolved rocks, Ir is enriched relative to Os in
the boninites (Os/Ir = 0·3 for IB5), thus Os/Ir ratios
progressively increase as the rocks become more evolved.
within these Mn-rich clays is depleted relative to Pd (see
Fig. 3a). The Os/Ir ratios (2·5–3·6) and Pd/Pt ratios
(1·9–2) of the Mn-rich sediments analysed here overlap
with the Os/Ir ratios (2–3·5) and Pd/Pt ratios (1·8–4·8)
of reduced sediments from the continental margin of
Oman reported by Ravizza & Pyle (1997). Those workers
interpreted the high ratios as evidence of authigenic Os
and Pd enrichment under reducing conditions.
Izu–Bonin PGE data
Data reproducibility for the Izu–Bonin samples was assessed by analysing duplicate aliquots of IB67 and shows
good reproducibility for Re, Ir, Pt and Pd (<12% 2 ×
SD of mean). Reproducibility is worse, however, for Ru
(35%) and Os (83%; Table 2). Of the Izu–Bonin samples,
the rhyolite (IB21) has the lowest concentrations of all
PGE, but the highest concentration of Re (420 ppt; see
Fig. 3b). The PGE patterns for the rhyolite and the
intermediate-Ca bronzite andesite (IB1) have positive
slopes and are very similar in shape. The rhyolite is
depleted in Ir relative to Os (Os/Ir >5) but this is not
so pronounced in the andesite (Os/Ir = 1·6). Compared
with the boninites, the andesite (IB1) contains less Os
and Ir, similar Ru and Re concentrations, less Pt than
the intermediate-Ca boninite and more Pt than high-Ca
boninites (Table 2). Pd (7·22 ppb) is higher in the andesite
than in any of the other Izu–Bonin samples (Fig. 3d).
Os, Ir, Ru and Re concentrations are in the same
range for both the high- and low-Ca boninites (see Fig.
Grenada PGE data
Volcanic rocks. Chondrite-normalized PGE patterns for
the M-series picrites (see Fig. 4a and b) are characteristic
of mantle-derived partial melts, i.e. enriched in the more
incompatible PPGE (Pt and Pd) compared with the IPGE
(Ir, Os and Ru). An unusual feature of these picrites is
that they display a marked depletion in Re relative to
Pd. There is general similarity in shape of PGE patterns
within and between both the high- and low-La/Y subgroups of the M-series picrites (Fig. 4a and b). In terms
of overall PGE abundances, the high-La/Y group tend
to have slightly higher Pt and Pd concentrations (Table
2) and the low-La/Y group tend to have higher Ru
concentrations. Within the M-series, slopes of the PGE
patterns between Os–Ir, Ir–Ru, and Pt–Pd can be either
negative or positive, hence there is considerable variation
in inter-PGE ratios in this group. For example, Os/Ir
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Fig. 4. Chondrite-normalized ( Jochum, 1996; Naldrett, 1997) PGE patterns for Grenada volcanic rocks—all patterns shown are an average of
two or more replicates of the same sample, except for (g). (a) Low-La/Y M-series picrites; (b) high-La/Y M-series picrites; (c) M–C transitional
picrite (AMG6157) and M–C basalt (Gd5); (d) C-series basalts; (e) andesites; (f ) comparison of PGE patterns across a fractionation suite; (g) four
separate analyses of amphibole-rich cumulate Gd1; (h) plagioclase–amphibole cumulates.
varies between 0·6 (AMG6078) and 1·5 (Gd8), Ir/Ru
varies between 0·3 (Gd11) and 1·9 (AMG6078), and Pt/
Pd varies between 1·2 (Gd16) and 2·3 (Gd14).
Of all the Grenada rocks, the M-series picrites have
the flattest (least fractionated) PGE patterns (Fig. 4f ) and
PGE patterns become steeper as the rocks become more
evolved (e.g. picrite Pd/Ir = 25, andesite Pd/Ir = 216,
Fig. 4f ). This is because the M-series picrites contain
higher abundances of Ir and Os than the more evolved
rocks. The M–C picrite contains more Os, Ir, Pt and Re
than the other Grenada picrites (see Table 2). The
M–C picrite is depleted in Ru relative to Os and Ir. A
particularly unusual feature of the M–C group is that
the M–C basalt (Gd5) contains more Os than the M–C
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SIDEROPHILE ELEMENTS AND SUBDUCTION
picrite (i.e. 540 ppt vs 300 ppt; Table 2). In all other
Grenada rocks analysed, basaltic rocks contain significantly less Os than the picrites.
The C-series basalts (Fig. 4d) are depleted in Os relative
to Ir (i.e. Os/Ir in Gd18 is 0·25). They contain lower
concentrations of Ir, Os and Ru than the M-series picrites,
slightly less Pt than the high-La/Y M-series picrites, but
equivalent Pt to the low-La/Y M-series picrites. The Cseries basalts contain higher concentrations of Pd than
the andesites, cumulates and low-La/Y picrites, and
overlap with the range of Pd concentrations found in the
high-La/Y M-series picrites and M–C transitional series
(Table 2). The C-series PGE patterns have a steep
positive slope between Pt and Pd (Fig. 4d). Re concentrations within the C-series basalts are higher than in
the picrites, comparable with andesite Gd25 and lower
than in andesite Gd17 (Table 2).
Gd25, the more evolved of the two andesites analysed
(with 64% SiO2, compared with 59% for Gd17), contains
lower concentrations of all PGE except Ru than Gd17.
Both andesites are extremely depleted in Ir (<5 ppt), Os
(<20 ppt) and Ru compared with the picritic rocks, but
they contain higher concentrations of both Os and Ru
than the C-series basalts (Table 2; Fig. 4f ). It should be
noted, however, that the Ru data for each of the andesites
are limited to one analysis (see Table 2) and hence it is
impossible to assess the reproducibility of these results.
Pt concentrations within the andesites are generally lower
than in the M-series picrites and comparable with those
in the C-series basalts. Pd concentrations within the
andesites are lower than within any other group of
Grenada rocks and hence positive slopes are seen between
Pd and Re in the andesite PGE patterns (Fig. 4e). Re
concentration within andesite Gd17 is higher than in
any other Grenada sample (Table 2).
Cumulates. The PGE patterns obtained for the different
cumulates are highly variable (Fig. 4g and h). Four
separate aliquots of Gd1 were analysed to assess the extent
to which a nugget effect may influence the measured PGE
abundances. Despite the fact that in terms of overall
abundances, variations of >100% (2 × SD of the mean)
are observed, the shape of the patterns for Gd1 replicate
reasonably well (Fig. 4g). PGE concentrations are higher
(particularly for Ir, Ru and Pd) within the hornblenderich cumulate Gd1 than in the plagioclase–hornblende
cumulates Gd2 and Gd3 (Table 2). The cumulates contain similar Os and Ir concentrations to the M-series
picrites, with the exception of Gd1, which contains significantly higher Ir concentrations (260 ppt). Gd1 and
Gd2 both have strongly positive slopes to their PGE
patterns between Os and Ir (Fig. 4g and h), which
contrast dramatically with the negative slopes of the PGE
pattern between Os and Ir for the andesites (Fig. 4e).
The Ru concentration in Gd1 is comparable with that
of the M-series picrites and higher than for other Grenada
samples. Ru concentrations in Gd2 and Gd3, by contrast,
are lower than in any other Grenada samples. All of the
cumulates contain high concentrations of Pt (>4·2 ppb;
see Table 2) compared with the other Grenada samples
(except the M–C picrite). Concentrations of Pd and Re
are low in Gd1 and Gd2, and these cumulates have
negative sloping patterns from Pt to Re. Gd3 contains
more Re than the other cumulates (Fig. 4h) and has a
positive slope between Pd and Re.
Comparison of Grenada PGE with Izu–Bonin and
sediment PGE
The overall shapes of the PGE patterns for the primitive
rocks of both Grenada and Izu–Bonin are very similar
(Fig. 5a and b). The Grenada low-La/Y picrites most
resemble the high-Ca boninites (Fig. 5a) whereas the
high-La/Y picrites most resemble the intermediate-Ca
boninites (Fig. 5b). The greatest difference between the
primitive rocks of Grenada and Izu–Bonin is in their Re
concentrations (Table 2), as the Grenada M-series picrites
are all significantly depleted in Re compared with the
Izu–Bonin boninites. The exception is the anomalous
M–C transitional picrite AMG6157 (Fig. 5b) whose Re
concentration is similar to the Re concentration of the
boninites (Table 2). The M–C picrite is enriched in Os
and Ir compared with the boninites.
Of the more evolved rocks from the two arcs, the PGE
patterns of the Izu–Bonin andesite (IB1; Fig. 5c) and the
Grenada C-series basalt (Fig. 5c) show most similarity.
The Izu–Bonin andesite is considerably more evolved
(SiO2 of 61·7%) than the Grenada C-series basalt (SiO2
of 47·8%), but it contains slightly higher concentrations
of all PGE, except Ir (Fig. 5c). The Grenada andesites
(Gd17 and Gd25) have a marked depletion in Ir and Pd
compared with the Izu–Bonin rhyolite (IB21) and andesite
(IB1; Fig. 5c and d). The Grenada high-silica andesite
(Gd25) also contains less Re but higher concentrations
of Os, Ru and Pt than the Izu–Bonin rhyolite (IB21; Fig.
5d). Overall, the PGE patterns for the evolved rocks of
Izu–Bonin are much ‘smoother’ than those for Grenada
(Fig. 5d); they do not have significant depletion of one
PGE relative to another as recorded by the Grenada
rocks.
Comparison of PGE abundances in sediments likely
to be subducted below Grenada with PGE abundances
in a typical Grenada high-La/Y M-series picrite (Gd16;
Fig. 5e) shows that the terrigenous sediment (average of
RC13-175 and V24-260) contains lower concentrations
of all PGE, but not Re, than the M-series picrite. In
contrast, the Mn-rich sediment is enriched in IPGE and
Re relative to the M-series picrite, but contains similar
Pt and Pd concentrations.
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Relationships between arc lava major and trace element
compositions and PGE systematics
Fig. 5. (a)–(d) Comparison of chondrite-normalized ( Jochum, 1996;
Naldrett, 1997) PGE patterns for Grenada and Izu–Bonin volcanic
rocks. Gd C-series basalt is an average of Gd21 and Gd18. IB67 is an
average of two replicate analyses. IC, intermediate-Ca boninite; HC,
high-Ca boninite, and is an average of IB5 and IB40; ICaBrz, intermediate-Ca bronzite andesite. Re depletion in picrites compared with
boninites should be noted (a, b). C-series basalts have very similar
patterns to Izu–Bonin andesites (c). (e) Comparison between a Grenada
M-series picrite and typical sediments subducted below Grenada, where
Mn Sed (av) is an average of the replicate analyses Mn-1 and Mn-2,
and Lithic Sed (av) is the average of RC13-175 and V24-260.
Correlations between the PGE and most lithophile and
even chalcophile elements within the arc rocks studied
are remarkably scarce, particularly within the M-series
picrite group. Some general trends in relative concentrations within the fractionation suites were recognized and are illustrated in Fig. 6.
Ir concentrations (and IPGE in general) correlate positively with MgO in both the Izu–Bonin and Grenada
suites (Fig. 6a). This overall relationship is much less
pronounced when considering PPGE vs MgO plots (Fig.
6b), although high Pd and high MgO tend to correlate
within the individual Grenada andesite, cumulate and
C-series groups. The same relationship is true between
the PGE and Ni.
Ir also correlates positively with TiO2 in the C-series,
the andesites and particularly within the cumulates of
Grenada. Gd1, the hornblende-rich cumulate, is significantly enriched in both Ir and TiO2 compared with
the Grenada lavas (Fig. 6c). Such good correlation is not
observed between TiO2 and Pd or Re, and both are
depleted in the cumulates relative to the C-series basalts
and M-series picrites. The Izu–Bonin samples have much
lower and more constant Ti abundances over a range of
SiO2 than the Grenada rocks, but again there is a
positive correlation between TiO2 and Ir, if the rhyolite
is excluded, which contains much less Ir (Fig. 6c). Some
correlation occurs between Ir and V through the Grenada
fractionation suite (Fig. 6d), with the concentration of
both elements decreasing from the cumulates, through
the C-series basalts, to the andesites. Cumulate Gd1 is
enriched in both V and Ir. The M-series picrites form a
cluster in the Ir–V plot and do not define any trend. The
Izu–Bonin samples have a positive correlation between V
and Ir (Fig. 6d). Likewise, Ir and Cr both decrease in
concentration from the primitive to the more evolved
rocks in the Grenada and Izu–Bonin samples (Fig. 6e).
High Cr correlates with high Ir content in Gd1 and
a positive correlation is observed between Ir and Cr
concentration in the other cumulate samples. There are
no clear relationships between Cr and the PPGE, or Cr
and Re.
Within the arc lava samples, systematic relationships
between the PGE and Cu are also lacking, despite the
fact that these elements are chalcophile and should
behave in a similar manner. Cu concentrations are low
in the cumulates and the M–C picrite of Grenada, even
though the rocks are enriched in Ir. The M-series picrites
and C-series basalts have similar Cu contents despite the
fact that the C-series contain lower concentrations of Ir
(and Os) than the picrites. The M-series picrites do,
however, tend to have both higher Pd and Cu contents
than the more evolved Grenada samples, particularly the
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SIDEROPHILE ELEMENTS AND SUBDUCTION
Fig. 6. Selected major and trace element vs PGE plots for the Grenada fractionation suite. Log scales are used for the PGE and Cr data.
Correlations between the PGE and both major and trace elements are rare in the arc rocks studied. However, a noteworthy feature is the good
correlation between Ir and MgO (a), but little correlation between Pd and MgO (b) in Grenada samples, suggesting IPGE fractionation occurs
simultaneously with olivine, but PPGE fractionation does not. Correlation of Ir with Ti (c) and V (d) within Grenada cumulates and andesites,
and between Ir and Cr within the Grenada cumulates and Izu–Bonin samples (e), indicates that Ir distribution may be linked to fractionation
of such phases as magnetite and spinel. Re, the most chalcophile element studied, shows little correlation with Cu (f; an indicator of S saturation
and sulphide segregation), hence Re distribution may be controlled by phases other than sulphide in an arc setting. Re and Pd are highest in
high-La/Y M-series picrites from Grenada (g and h) indicating that minor enrichment in these elements may have occurred as a result of
increased fluid input in the Grenadian sub-arc mantle wedge.
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andesites, which contain low concentrations of both Cu
and Pd (see Fig. 9 below). The behaviour of Re and Cu
is also largely decoupled in the Grenada samples. The
picrites and C-series basalts have similar Cu contents,
but the picrites contain less Re (Fig. 6f ). The andesites
with high Re concentrations by contrast have low Cu
concentrations (Fig. 6f ). The Izu–Bonin samples have
near-invariant Re concentrations over a range of Cu
concentrations and possibly record a negative correlation
between Pd and Cu (Figs 6f and 9).
La/Y can be used as an indication of both fluid addition
and degree of partial melting (high La/Y indicates lowdegree partial melt and/or high fluid input), and hence
may illustrate the influence of these factors on PGE
distribution. Within the more primitive arc samples, there
is a weak inverse correlation between La/Y and Re,
which extends through the Izu–Bonin samples, the Grenada M–C series, the C-series and to the high-La/Y Mseries picrites (Fig. 6g). A similar correlation does not
occur between Ir and La/Y. Within the M-series picrites
of Grenada, there is a correlation of higher Pd, Pt
and Re contents within the high-La/Y M-series picrites
compared with the low-La/Y M-series picrites (Fig. 6h).
Grenada Os and Sr isotope data
All of the Grenada samples analysed have 187Os/188Os
isotope ratios that are more radiogenic than either depleted mantle ( 187Os/188Os = 0·124) or fertile mantle
( 187Os/188Os = 0·13) (Table 3). AMG6103, a high-La/
Y picrite, has the most radiogenic signature ( 187Os/
188
Os = 0·1644). The M–C picrite (AMG6157) has
the highest Os concentration (300 ppt), but the least
radiogenic Os signature ( 187Os/188Os of 0·1337; Table
3). There is a general inverse correlation of Os isotope
signature with Os concentration in the samples analysed
(Fig. 7b). The five picrites analysed show positive correlations between MgO and Os isotope ratio (Fig. 7a).
Although the dataset is small there also appears to be a
correlation between magma series type and Os systematics, i.e. the high-La/Y picrites have higher 187Os/
188
Os ratios and lower Os concentrations than the lowLa/Y picrites (Fig. 7c). As such, there is a positive
correlation between 187Os/188Os and 1/Os within the
primitive rocks of Grenada. There is also a positive
correlation between Os and Sr isotope ratios within the
Grenada lavas, with the high-La/Y picrites having both
the most radiogenic Os and Sr signatures (see Fig. 11,
below). The M–C picrite (AMG6157) conversely has the
least radiogenic 87Sr/86Sr and 187Os/188Os ratios (Table
3). The cumulate Gd1 has the highest 87Sr/86Sr ratio but
contains less radiogenic Os than the high-La/Y picrites
(Table 3).
NUMBER 1
JANUARY 2002
PETROGENETIC MODELS FOR
GRENADA AND IZU–BONIN ARC
MAGMAS
Overviews of the current models for lava petrogenesis in
both Grenada and Izu–Bonin are now presented to
provide a framework within which to consider the significance of the PGE data.
Petrogenesis of Grenada lavas
The trace element signature of LILE and LREE enrichment relative to HFSE and to MORB of the Mseries picrites (Fig. 2a) indicates that they were generated
by hydrous melting of MORB-source mantle variably
enriched in fluid and sediment components from the
subducting slab (Thirlwall et al., 1996). The parental
magmas of the Grenada lavas are thought to have been
derived from low-degree (><10%) partial melting in the
sub-arc mantle wedge at >100 km depth (Arculus, 1973;
Thirlwall et al., 1996). This is well within the garnet
lherzolite stability field and hence explains the pattern
of HREE depletion relative to MORB seen within the
picrites (Fig. 2a).
All lavas on Grenada are thought to be ultimately
derived from fractional crystallization of picrites with
>15% MgO (Thirlwall et al., 1996). Such primary
magmas erupted essentially unaffected by fractionation,
as the M-series picrites, although they experienced minor
late-stage crustal contamination. A schematic diagram
to illustrate the way in which each of the Grenada
samples can be generated during high-level fractionation
processes is shown in Fig. 8. Amphibole-dominated fractionation of M-series picrites (Fig. 8) produces the more
evolved M-series basalts and andesites (such as Gd17;
Thirlwall & Graham, 1984; Thirlwall et al., 1996), with
their characteristic Ti and compatible element depletion
(Fig. 2b). Hornblende-rich cumulates significantly enriched in Ti, V and certain PGE, such as Gd1, are
probably the by-product of this fractionation (see Fig. 8).
The mineralogy and chemistry of the most primitive
C-series basalts (>7·5% MgO, e.g. Gd21) are consistent
with an origin via fractionation of >16% olivine from
a picritic parental magma, probably at the base of the
arc crust (Thirlwall et al., 1996; Fig. 8). This explains the
observed Ni and Cr depletion in C-series basalts relative
to M-series picrites (Fig. 2b). More evolved C-series
basalts (e.g. Gd18) are derived via further fractionation of
augite and plagioclase with minor olivine and magnetite.
High-SiO2 andesites (e.g. Gd25) are the end-product of
C-series differentiation via amphibole-dominated fractionation and assimilation–fractional crystallization
(AFC; Arculus, 1978). Cumulates rich in plagioclase and
amphibole, such as Gd2 and Gd3, are likely to have
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SIDEROPHILE ELEMENTS AND SUBDUCTION
Table 3: Os isotopic signatures of Grenada lavas (measured by N-TIMS)
Sample name
187
Gd1, cumulate
Gd8, low-La/Y picrite
Os/188Os (±2 × RSD)
Os (ppb)
87
0·14618±0·000790
0·074
0·14148±0·000532
0·087
Gd10, low-La/Y picrite
0·14263±0·000259
AMG6078, high-La/Y picrite
Sr/86Sr
Sr (ppm)
La/Y
0·70507
236·4
0·12
0·70481
255·7
0·30
0·085
0·70481
254·7
0·30
0·14738±0·000282
0·075
0·70493
529·2
1·07
AMG6103, high-La/Y picrite
0·16441±0·000489
0·045
0·70498
679·0
1·03
AMG6157, M–C picrite
0·13366±0·000211
0·302
0·70455
576·0
0·57
Modelling parameters
187
Os (ppb)
87
Sr (ppm)
Os/188Os
Sr/86Sr
M-series mantle source
0·129
3·0
0·70445
Subducted sediment 1
8·0
0·05
0·717
35
Subducted sediment 2
8·0
0·98
0·717
125
Arc crust
1·0
0·02
0·709
1200
Low La/Y M-picrite
0·129
0·35
0·70455
216
Low-Os picrite
0·145
0·02
0·70493
529
125
Sr isotope data for AMG samples from Thirlwall et al. (1996). Sr isotope data for Gd samples are from this study. The
correlation between high Sr and Os ratios and high-La/Y picrites should be noted. End-members’ compositions used to
model effects of mixing of sediment–arc crust with primitive Grenada lavas (i.e. to generate Sr contents) and isotope data
taken from Thirlwall et al. (1996).
Fig. 7. (a) Relationship between Os isotopic ratio and MgO content—the most primitive samples (i.e. highest MgO) have the most radiogenic
Os; this signature cannot be explained by simple late-stage AFC processes. (b) Os concentration vs Os isotope composition of picritic Grenada
lavas. (c) Os isotope composition vs La/Y (note correlation of radiogenic Os with the high-La/Y group of the M-series picrites). Low-La/Y
picrites (i.e. least slab-fluid input) have least radiogenic Os.
formed in high-level magma chambers during these late
stages of differentiation (see Fig. 8). On the basis of Pb
isotope evidence, the M–C transitional series is thought
to be derived from mantle more enriched in a subducted
sediment component and less enriched in a slab-fluid
component than the C-series source [see Thirlwall et al.
(1996) for a full description of isotope constraints on
magma genesis]. In all other respects they have a similar
petrogenetic history to the C-series basalts (Thirlwall et
al., 1996). The trace element pattern for the M–C basalt
(Gd5) analysed in this study (Fig. 2b) suggests that it has
suffered less olivine fractionation than the C-series basalts
because its Ni concentration is more equivalent to the
M-series picrites.
Petrogenesis of Izu–Bonin lavas
The overall melting regime in which the boninites were
generated has been constrained to shallow depths
(>30 km) and high temperatures (1250°C; Murton et al.,
1992; Pearce et al., 1992b). The boninite trace element
signature of REE and HFSE depletion relative to both
MORB (Fig. 2c) and the Grenada suite can be attributed
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Fig. 8. Schematic illustration of the magma-plumbing system below Grenada and the stages of fractionation involved in generating the range
of lava compositions found on Grenada [based on the models of Thirlwall et al. (1996)]. It is proposed that Gd1 originates from fractionation
of an M-series picrite whereas Gd2 and Gd3 are produced during high-level fractionation of C-series basalts.
to remelting of recently depleted MORB-source mantle
above a subduction zone (Murton et al., 1992; Pearce et
al., 1992b). Enrichment of the most fluid-mobile LILE
within the boninite relative to MORB (see Fig. 2c),
however, suggests that some refertilization of the mantle
wedge from slab-derived fluxing has occurred. It is estimated that the boninite source had lost 10–15 wt % of
melt at a ridge, before undergoing a further 5–10 wt %
melting at the onset of subduction beneath the Izu–Bonin
arc (Pearce et al., 1992a).
The dykes in which the high- and intermediate-Ca
boninites are found were probably generated in individual
batch melting episodes and did not experience magma
chamber processes (Murton et al., 1992). Murton et al.
(1992) proposed that the intermediate-Ca boninite came
from a more depleted source than the high-Ca boninite.
The intermediate-Ca bronzite andesite and rhyolite are
products of fractionation of variable amounts of olivine,
pyroxene, plagioclase and spinel, and pyroxene, plagioclase, quartz and magnetite, respectively, from more
primitive boninitic magmas (Murton et al., 1992; Pearce
et al., 1992b). Evidence of fractionation can clearly be
seen in the trace element patterns (Fig. 2c) as the rhyolite
is enriched in incompatible LILE and REE and depleted
in Ti, V (amphibole), Ni (olivine) and Cr (spinel).
DISCUSSION
PGE behaviour in relation to melting,
fractionation and S saturation in Grenada
and Izu–Bonin
Because Ir and Os concentrations decrease as samples
become more evolved in both Grenada (with the exception of the M–C transitional series) and Izu–Bonin,
there is an obvious link between fractionation and removal of the highest temperature, most compatible PGE
from the magma. Correlation of Ir and Os with both
MgO and Ni indicates that the IPGE and olivine tend
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SIDEROPHILE ELEMENTS AND SUBDUCTION
Fig. 9. Pd vs Cu as a potential measure of S saturation (Vogel & Keays, 1997). It should be noted that the boninite field from Hamlyn et al.
(1985) has significantly higher Pd contents than the boninites analysed during this study. Rocks from both Izu–Bonin and Grenada become
more S saturated as fractionation proceeds. It should be noted that the S saturation line (dashed) is the empirically derived line of Brooks et al.
(1999) and is thus not proof of S saturation.
to be removed from the melt simultaneously. Compatibility of Ir and Os within the olivine crystal lattice
cannot be inferred, however, as it is highly likely that
the IPGE are either removed in a phase co-crystallizing
with olivine, such as sulphide, or are included as alloys
within such cumulus phases (Alard et al., 2000).
It is sometimes assumed that Os and Ir behave in an
identical manner during mantle melting and fractionation, because of the near-uniform Os/Ir ratios of
>1 that have been observed within both mantle residues
and high-degree partial melts (Barnes et al., 1985; Brügmann et al., 1987). Os/Ir ratios within samples studied
in this project, however, vary according to sample composition. For example, in Grenada picrites (e.g. Gd10)
Os/Ir is close to unity, whereas in the cumulate Gd1
Os/Ir is 0·3. In the Izu–Bonin suite, Os/Ir is 0·3 in the
boninites (e.g. IB67) and 0·7 in the andesite (IB1). Thus,
either phases crystallize or processes occur that are capable of fractionating Os and Ir. Caution should therefore
be exercised in estimating Ir abundances in evolved rocks
based on Os abundances and vice versa.
Candidates for phases capable of fractionating Os from
Ir are uncertain, as relationships between the PGE and
most other elements are poor; however, within the Grenada cumulate Gd1 (probably fractionated directly from
a primitive M-series magma; see Fig. 8), there is a
correlation between high TiO2 and high Ir. The high
TiO2 undoubtedly arises as a result of the amphibolerich nature of this sample, but coupled high V contents
in Gd1 (Fig. 6e) also suggest that magnetite is an important phase in this rock. This sample is thus unique in
that magnetite accumulation and Ir enrichment coincide.
The Grenada plagioclase–hornblende cumulates (Gd2
and Gd3) contain much lower concentrations of all PGE
than Gd1. We presume this is because the C-series
magmas had already undergone several stages of magma
pooling, fractionation and PGE segregation before this
final high-level fractionation stage in which these cumulates were produced.
Ru behaviour within the suites from both Grenada
and Izu–Bonin is difficult to explain. Initially, Ru appears
to behave compatibly (e.g. Ru content is lower in the Cseries basalts than in the primitive M-series picrites on
Grenada). Ru behaviour then seems to reverse and
concentrations become higher again within the andesites
(Gd17 and IB1) of both Grenada and Izu–Bonin, before
finally decreasing within the most evolved rocks (Gd25
and IB 21; see Table 2). The multivalent nature of Ru
certainly dictates that its compatibility with other mineral
phases will be governed by its oxidation state. Hence the
distribution of Ru may, more than that of the other
PGE, be controlled by changes in fO2 as well as fS2, which
occur within high-level magma chambers.
Pt and Pd distribution in the M-series picrites appears
to be related to their La/Y signatures. The Grenada
high-La/Y M-series picrites generally contain more Pt
and Pd (the most fluid-mobile of the PGE) than the lowLa/Y M-series (see Table 2) even though these series
have similar MgO content and hence fractionation histories. This enrichment is most easily explained by preferential Pt and Pd mobilization in subduction-derived
fluids or volatiles. If partial melting was solely responsible
for La/Y we would expect to see Pt and Pd enrichment
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in the higher-degree partial melts (i.e. the low-La/Y
picrites) and not vice versa.
The intermediate-Ca boninite (IB67) contains much
higher concentrations of Pt than the high-Ca boninites.
The major difference in their genesis is that the intermediate-Ca boninites were derived from a more depleted
mantle source than the high-Ca boninite (Murton et al.,
1992). This implies that Pt was concentrated in a mantle
phase such as residual sulphide and that this phase was
accessed during the second episode of melting beneath
Izu–Bonin. Pd does not show similar enrichment in the
intermediate-Ca boninite. Thus, either residual phases
are capable of fractionating Pt from Pd during partial
melting in a subduction regime, or subduction-derived
fluids can fractionate these two elements.
Pd appears to behave semi-incompatibly during crystal
fractionation, as it is more enriched within the Izu–Bonin
bronzite andesite (7·22 ppb) than in the Izu–Bonin boninites (average 2·96 ppb). The same is true in Grenada,
where the C-series basalts and M–C basalt contain more
Pd than the low-La/Y M-series picrites (Table 2). Pd
concentrations are then much lower in the evolved rocks
in both Grenada and Izu–Bonin. Thus, a phase that
readily scavenges Pd, or with which Pd is compatible,
must have segregated from the magmas late in both
systems. One possibility is that S saturation occurred
within high-level magma chambers during silicate fractionation. Removal of sulphide following S saturation
would rapidly deplete a melt of its PGE budget. In the
Grenada system this may have occurred during evolution
of the C-series basalts and M-series picrites towards
andesitic compositions. The cumulates analysed in this
study, however, do not contain the ‘missing’ Pd and
sulphide, as both their Pd and Cu concentrations are
low (Fig. 9).
The relationship between Pd and Cu, where Cu is
used as a proxy for sulphur, has been used by some
workers to assess extent of sulphur saturation in magmatic
rocks (Vogel & Keays, 1997; Brooks et al., 1999). Pd is
plotted in preference to the other PGE because it is the
most chalcophile of the PGE and so its abundance
should be most intimately linked to S saturation and S
precipitation from a melt. On the basis of such a plot
(Fig. 9), the Izu–Bonin andesite and Grenada basalt
(Gd21), which contain high concentrations of Pd, would
appear to be undersaturated with respect to S whereas
the more evolved rhyolite of Izu–Bonin (IB21) and the
andesites of Grenada appear to be S saturated. Hence,
we conclude that S saturation appears to play a key role
in controlling Pd abundances in the evolved arc rocks.
The Izu–Bonin boninites (IB40 and IB67) are more S
undersaturated than the Grenada picrites. This is consistent with derivation of the boninites from a more
residual mantle source already depleted in sulphide. It
should be noted that the boninites analysed during this
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study have much lower Pd contents and hence appear
less S undersaturated than boninites previously analysed
(Hamlyn et al., 1985).
If S saturation controls Pd abundances within the more
evolved arc rocks, the same does not appear to be true
for Re. In the Izu–Bonin system, the sample that contains
the least Cu (IB40) contains one of the highest Re
concentrations of the group (see Table 2). Similarly,
Grenada andesites have high Re but low Cu concentrations (Fig. 6g). Thus, Re appears to remain within
the melt after removal of Cu (and hence sulphide). Re
shows larger variations, within the primitive arc rocks
studied here, than do the PGE. The Izu–Bonin boninites
contain significantly more Re than the Grenada picrites
(Fig. 5a). One possible reason for this difference is that
the first-stage, low-degree melting events at Izu–Bonin
caused concentration of Re within residual mantle sulphides (Keays, 1995). These sulphides may then have been
consumed during the subsequent melting episode that
generated the boninites (Hamlyn et al., 1985). This model
is problematical, as there is no strong evidence for enrichment of any of the PGE in the boninites relative to
the Grenada picrites. An alternative hypothesis is that
Re may have been retained by a phase stable within the
mantle wedge of Grenada, but not in Izu–Bonin. The
most likely candidate, given that the Grenada picrites
are generated at much greater depths than the boninites
(100 km vs 30 km), is garnet and there is good experimental evidence that garnet can accommodate Re
within its crystal structure (Righter & Hauri, 1998).
Hence, residual garnet in the Grenada source retains Re
during magma genesis, whereas the shallow-level melting
of the boninites is not conducive to Re retention in the
source and the primitive magmas are enriched in Re.
The PGE systematics of the Grenada M–C series are
poorly understood. The simplest explanation of their
PGE signatures would be that this group suffered less
fractionation; however, in terms of MgO and Ni content
(Table 1), the M–C picrite appears to be more fractionated than the M-series picrites. The fact that M–C
transitional compositions occur at only one geographical
location does suggest a peculiarity in their genesis. One
way in which elevated PGE signatures in the M–C series
could be obtained is by remelting an isolated block of
residual mantle (as in boninite genesis) in which PGE
were concentrated in a residual phase. That such blocks
of depleted material may be present in the Grenada subarc mantle wedge has been suggested by the discovery
of unradiogenic Os within a peridotite xenolith in an Mseries picrite, implying an anomalously old age for this
piece of arc mantle (Parkinson et al., 1998b; see Fig. 10).
The lithophile element signatures of the M–C series,
however, do not support any great differences in their
source or genesis compared with other Grenada picrites
or basalts. In this sense, the PGE may reveal information
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SIDEROPHILE ELEMENTS AND SUBDUCTION
Fig. 10. Range of Os isotopic signatures for Grenada samples vs rocks from other tectonic settings. Picrites fall predominantly within 187Os/
188
Os range defined by OIB. The star symbols represent previous Grenada Os isotope measurements, G.X. being a Grenada peridotite xenolith
(Parkinson et al., 1998b), and its respective host lava (I. J. Parkinson, personal communication, 1999). Stippled field, chondrite ( Jochum, 1996);
MW, Cascades and Ichinomegata mantle-wedge harzburgites (Brandon et al., 1996); IB Harz, Izu–Bonin seamount harzburgites (Parkinson et
al., 1998a); CAL, Cascades Arc lavas (Borg et al., 2000); MORB data are from Ravizza & Pyle (1997); OIB data are from Fryer & Greenough
(1992) and Widom & Shirey (1996). It should be noted that the M–C picrite (AMG6157) with least radiogenic Os signature is still equally as
radiogenic as the most enriched sub-arc mantle-wedge peridotites of Brandon et al. (1996). The cumulate (Gd1), which should have experienced
greatest contamination by AFC, does not have the most radiogenic Os signature.
Fig. 11. Sr–Os isotope relationships of Grenada picrites plus one cumulate xenolith (see key for symbols). Mixing curves are represented to
depict simple bulk mixing. Curve A, mantle wedge source plus ‘typical ocean-floor sediment’ (subducted sediment 1); curve B, primitive, highOs picrite plus arc crust; curve C, mantle wedge source plus high-Os sediment (subducted sediment 2); curve D, low-Os picrite magma containing
radiogenic Os, plus arc-crust. End-members used are given in Table 3 [Sr contents and isotope data taken from Thirlwall et al. (1996)]. All
mixing calculations were performed using calculated atomic weights and atomic proportions for all end-members.
about the source of the M–C series that the lithophile
elements do not, as lithophile element signatures in arc
lavas would be rapidly dominated by recent subduction
input and crustal contamination.
With the exception of the M–C magma series, overall
concentrations of PGE, particularly the IPGE, are significantly lower within arc rocks than in other high-MgO
rocks from different tectonic settings such as komatiites
(Brügmann et al., 1987), kimberlites (McDonald et al.,
1995) and plume-related picrites (Brooks et al., 1999). For
example, picrites from West Greenland (average MgO
>18%) have Ir concentrations of 0·8–1·94 ppb and Pd
concentrations of 4·34–11·54 ppb (Woodland, 2000). As
we are assuming that the Grenada picrites have undergone minimal fractionation and PGE loss in this way,
the only other explanation for their low PGE concentrations must be related to extent of partial mantle
melting. The Grenada picrites were generated by >10%
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partial mantle melting (Thirlwall et al., 1996), whereas
plume-related picritic magmas are the products of much
higher-degree partial melting of >25% (Brooks et al.,
1999). Thus, it seems that, even in the presence of
oxidizing fluids in a subduction setting, PGE-bearing
phases are not broken down efficiently at low degrees of
partial mantle melting, and higher degrees of melting
are required to elevate PGE abundances in resultant
lavas.
Implications for PGE behaviour in
subduction systems
Despite their complex petrogenetic histories the primitive
lavas of both Grenada and Izu–Bonin have similar PGE
patterns. This is surprising considering their contrasting
alkalic and boninitic compositions, the greater degree of
depletion in the Izu–Bonin source region and the different
fractionation histories of the two magma series. Thus,
on the basis of these two arc systems, it seems that major
differences in subduction zone conditions that affect other
trace elements, such as variation in sediment input and
fertility of the mantle source, have not greatly influenced
the arc-lava PGE signatures.
PGE abundances within the sediments analysed in this
study, particularly the terrigenous sediments that are
probably the greatest constituent of sediments subducted
below Grenada, are very low (Table 2). This dictates
that bulk mixing of very large quantities of such sediment
would be required within the mantle to significantly
affect PGE concentrations of the Grenada sub-arc mantle
wedge. A large contribution from subducted sediment to
these magmas has been effectively ruled out by Thirlwall
et al. (1996), who estimated that mixing of 2% or less of
sediment into the Grenada source adequately accounts
for the Sr, Nd and Pb isotopic signatures of the arc lavas.
Brandon et al. (1996) have suggested that Os can be
stripped from the mantle wedge above a subduction zone
by slab-derived fluids. The low Os contents of the picritic
and boninitic arc lavas analysed here do not indicate
significant Os enrichment via slab fluids. This may either
be a consequence of Os depletion from the mantle melts
as a result of fractionation processes, or because Os has
been retained within mantle phases that are actually
stabilized by fluxing with slab fluids (Borg et al., 2000).
Analysis of harzburgitic nodules exhumed from the Grenadian mantle wedge may present answers as to the
location of this ‘missing’ Os and should be a target for
future work.
PPGE/IPGE ratios within the primitive lavas analysed
in this study are higher than corresponding ratios in
supra-subduction harzburgites [i.e. harzburgite Pd/Ir =
1·28 (Rehkämper et al., 1997), low-La/Y M-series picrite
(Gd11) Pd/Ir =33·42]. In addition to the presence of
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Pt–Pd enrichment in the high-La/Y picrites, this provides
some evidence that the PPGE have greater transport
efficiency than the IPGE in fluids released from the
subducting slab. This was also a conclusion reached by
Rehkämper et al. (1997). We still exercise a certain amount
of caution in making this interpretation for the rocks
studied here, as IPGE abundances (and hence PPGE/
IPGE ratio) are so strongly controlled by fractionation
of early cumulus phases. PGE signatures even within
primitive picritic lavas, such as those found on Grenada,
are predominantly governed by the degree of mantle
partial melting and fractionation processes, which would
readily disguise primary sediment or fluid signatures. We
suggest therefore that PGE signatures in rocks of basaltic
composition cannot reliably be used to indicate sourceregion characteristics.
Becker (2000) interpreted low Re/Os ratios within
eclogites, blueschists and mafic granulites to indicate that
Re is selectively mobilized relative to Os during slab
dehydration. Alternatively, it should be noted that in
certain subduction environments, Re may be retained in
the slab, as some eclogites have high Re/Os ratios and
Re contents (Pearson et al., 1995; Ruiz et al., 1998). As
with Os, Re concentrations in the Grenada picrites are
anomalously low. Thus, if Re was preferentially stripped
from the slab during subduction beneath Grenada, this
‘excess’ Re signature did not survive melt or fluid transport through the mantle wedge to be recognized within
the erupted arc lavas. Because Re is more incompatible
than the PGE, its signature in the picrites is more likely
to reflect source characteristics rather than fractionation
processes (assuming that the erupted primitive rocks
remained S undersaturated as indicated in Fig. 9). Thus,
retention of Re within garnet in the mantle wedge beneath
Grenada is proposed as the most likely explanation for
the contrasting signature of Re depletion in Grenada
relative to Izu–Bonin.
This study did not provide any means to directly assess
the transport efficiency of the PGE in subduction-derived
fluids. The low overall PGE concentrations in the Grenada picrites suggest that the PGE are not highly mobile
in slab-derived fluids, although the possibility remains that
the PPGE are more mobile than the IPGE. Remelting of
mantle modified by addition of slab-derived fluids (with
or without PGE) will generate melts intermediate in
composition between depleted mantle and a subduction
component. As the transport efficiency of PGE between
slab and mantle wedge appears to be poor, and as there
is even debate regarding PGE composition of depleted
mantle, it seems unrealistic to try to assess sediment
recycling processes within subduction zones by use of
PGE patterns, even in primitive lavas. In the absence of
clear evidence from PGE abundances to indicate input
of slab-derived material to the subduction systems studied,
Os isotope tracer studies may prove more informative.
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Implications of Os isotope data for
Grenada picrites
The young age of the Grenada picrites (<2 Ma), coupled
with their low Re/Os ratios, results in negligible age
correction of Os isotope ratios. Thus, in this section we
plot and discuss the measured isotope ratios. The Grenada picrites have Os isotope compositions (Table 3)
that are more radiogenic than estimates of primitive
mantle or chondritic meteorites (Meisel et al., 1996; Shirey
& Walker, 1998). The Grenada M-series picrites and
cumulate xenolith are more enriched in radiogenic Os
than mantle-wedge peridotite xenoliths from the Cascades and Ichinomegata sub-arc (Brandon et al., 1996),
the Izu–Bonin mantle-wedge harzburgites (Parkinson et
al., 1998a) and the one peridotite xenolith reported from
Grenada (Parkinson et al., 1998b; Fig. 10). The Grenadian
M–C picrite has a 187Os/188Os ratio comparable with the
most radiogenic sub-arc peridotites thought by Brandon
et al. (1996) to contain crustal Os (Fig. 10). The Grenada
samples also show a similar range in Os isotope compositions to those of the primitive Cascades Arc lavas
analysed by Borg et al. (2000; Fig. 10), especially those
that have similar Os concentrations.
In addition to being radiogenic, the Os isotope compositions of the Grenadian lavas show a general inverse
correlation with their Os concentrations (Fig. 7). Such a
relationship can be characteristic of two-component mixing between a radiogenic, low-Os component and a
relatively unradiogenic, higher-Os component. In a normal subduction environment there are at least two possible explanations for such a signature: (1) mixing of a
subduction component, either slab-derived aqueous fluids
or sediment, with mantle in the arc wedge; (2) mixing of
radiogenic Os acquired from late-stage crustal contamination, as the magmas assimilate arc crust during
ascent to the surface, with mantle-derived melts.
Inverse correlations between Os isotope ratios and
concentrations have been observed in both ocean island
basalts (Widom & Shirey, 1996) and flood basalts (Chesley
et al., 1998), and have been interpreted as resulting from
contamination of mantle-derived magmas with crust at
high levels via AFC processes. Similarly, Lassiter & Luhr
(1999) concluded that the inverse correlation between
Os concentration and Os isotope ratio in Mexican arc
lavas was a result of late-stage contamination by crustal
Os. Lassiter & Luhr (1999) extended this conclusion to
other suites (Alves et al., 1999) of evolved arc rocks where
such relationships are observed. In contrast, Os isotopic
and elemental systematics in primitive Cascades lavas
cannot be easily modelled by AFC-type processes and
have been interpreted to represent the addition of
radiogenic Os to the source of the lavas from a slab
component (Borg et al., 2000).
Despite their primitive nature, the relatively low Os
concentrations in some of the Grenada picrites do render
them sensitive to late-stage contamination from the arc
crust, so perhaps the simplest explanation for the inverse
relationship between Os isotope composition and Os
concentration in Grenada picrites is that it reflects latestage, high-level interaction of arc magmas with the arc
crust suprastructure. However, this hypothesis needs to
be examined in more detail before invoking it to explain
all the Os isotope variation in these lavas. The primitive,
magnesium-rich nature of the Grenada picrites indicates
that they are unlikely to have been extensively affected
by AFC processes in the arc crust. As this crust is Tertiary
in age (Arculus, 1973), it should not have developed an
extremely radiogenic Os isotope composition, and this
factor will moderate the effects of such contamination
on the lava isotopic compositions. General correlations
between MgO content and Sr, Nd and Pb isotope compositions of the Grenada magma series have been identified and related to the effects of AFC processes (Thirlwall
et al., 1996). These data have been used to infer that
Grenada picrites have experienced a maximum of >2%
assimilation of arc crust.
The five picrites analysed here show a positive correlation between MgO content and Os isotope ratio (Fig.
7a), i.e. the opposite of that expected if their Os isotope
compositions were dominated by AFC-type processes.
For this reason we have not extensively modelled AFC
processes in discussing the Os isotope characteristics of
the Grenada picrites. Furthermore, likely trajectories for
contamination of the picrite parent magma with arc crust
show much steeper trends on a combined Sr–Os isotope
plot (Fig. 11) than the array defined by the picrites. It is
also noteworthy that the cumulate xenolith, Gd1, which
is more likely to reflect AFC-type processes, is displaced
above the picrite array in Fig. 11. This sample could
have been generated by between 1 and 2% arc-crust
contamination of a magma with a composition similar
to picrite Gd8 or Gd10 (Fig. 11). The inverse correlation
between 187Os/188Os and Os concentration (Fig. 7b) could
simply represent late-stage crustal contamination, with
the lowest Os magmas being most susceptible to contamination. However, the positive correlation of Os isotopes with MgO argues against this and the correlation
in Fig. 7a could be a function of magmatic or source
processes in the different Grenada magma series. In this
regard, we note that the samples with high 187Os/188Os
are also the high-La/Y M-series picrites (Fig. 7c) and so
Os isotope composition does correlate with features of
source geochemistry, which in turn will influence how
much Os a primitive melt can contain.
From Fig. 11 it appears difficult to generate the Sr–Os
isotope systematics of the Grenada picrites by any type of
interaction with arc-crust using reasonable end-member
compositions and simple mixing processes (Table 3; curve
B, Fig. 11). First, very large amounts of crust would be
required to shift the Os isotopic composition to the
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radiogenic values observed in the picrites. Second, likely
compositions of crustal contaminants do not have the
requisite Sr/Os ratios to create the slope of the trajectory
in Fig. 11. From this reasoning and the arguments
presented above, we suggest that the Os isotope compositions of the Grenada picrites we have analysed are
not dominated by high-level crustal interaction, or AFC
processes. There will undoubtedly be some contribution
of radiogenic Os to the picrite magmas from late-stage
crustal interaction. The slope generated by such processes
(curve D, Fig. 11) means that subtle effects would be
very difficult to detect when superimposed on the main
picrite trend, but we suspect that AFC is not the main
process affecting the Os isotope composition of the picrites. This contrasts with the strong influence of small
amounts of late-stage crustal interaction on Sr–Nd–Pb
isotopes (Thirlwall et al., 1996).
If late-stage crustal influences on the Os isotope compositions of the Grenada picrites are small it seems likely
that their radiogenic character is a reflection of their
source compositions. The radiogenic nature of the picrites
indicates that radiogenic Os, probably from the subducting slab–sediment mix, was incorporated into the
picrite source region. One possible interpretation of the
positive correlation between MgO and 187Os/188Os (Fig.
7a) is that it represents the increasing effects of slabderived fluids, producing larger degrees of melting and
introducing more radiogenic Os into the source region.
This is consistent with increasing radiogenic Sr isotope
signatures (Fig. 11) and increasing La/Y (Fig. 7c) in the
picrites, both of which could be produced via slab-derived
fluids. In such a model the high La/Y signature of some
picrites would be generated mainly by the greater addition
of slab fluids rather than by lower degrees of mantle
melting as suggested by Thirlwall et al. (1996). The
hypothesis of radiogenic Os accompanying high-La/Y
slab-derived fluids clearly requires more stringent testing
with more detailed Os isotope studies.
A model invoking the influx of radiogenic Os from
slab-derived fluid is not without problems. The slope of
the correlation between Os and Sr isotopes for the picrites
(Fig. 11) is very shallow and cannot be generated by
simple bulk mixing between the mantle wedge and even
sediment with highly radiogenic Os, using normal sediment Os concentrations (Table 2; curve A, Fig. 11). The
only way of replicating the slope of the picrite Sr–Os
isotope array is by invoking mixing with Os-rich sediment,
with very high Os/Sr (curve C, Fig. 11), similar to the
Mn-rich ocean-floor sediment analysed in Table 2. Using
such a composition the slope of the Sr–Os array defined
by four of the five picrites can be approached and their
compositions generated by mixing of between <0·5%
and <2% sediment-derived Os with a mantle-wedge
composition with 187Os/188Os of 0·129 (Meisel et al.,
1996). This level of sediment interaction is consistent
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with that estimated from Sr–Nd–Pb isotope constraints
(Thirlwall et al., 1996). Using less radiogenic Os isotope
estimates for the mantle-wedge compositions increases
the amount of sediment-derived Os required in the mix.
Using the very unradiogenic Os isotope values of the
Grenada peridotite xenoliths analysed by Parkinson et al.
(1998b) obviously requires much greater sediment-derived
Os. The sample with the most radiogenic 187Os/188Os
(AMG6103) plots to the right of the other four picrites
in the Os–Sr isotope diagram and is very difficult to
model by sediment mixing without invoking extreme
Os/Sr ratios and low 87Sr/86Sr in the would-be sediment
end-member.
Although Mn-rich clays (such as the one analysed in
this study) are relatively abundant on the ocean floor, it
is unlikely that they formed the sole flux of sedimentderived PGE into the source of the Grenada magmas.
Hence, more realistic mixing curves would lie between
curves A and C in Fig. 11 (i.e. considerably steeper than
the array defined by the picrites). A possible solution to
this problem is if the transport behaviour of sedimentderived Os and Sr from subducted sediment, into the
mantle wedge, was very different and occurred mostly
via fluids rather than the simplified mixing scenarios
depicted in Fig. 11. In this model, Os and other PGE
may be transported in oxidized halogen-rich slab- or
sediment-derived fluids that have a high Os/Sr ratio.
Such a model has been previously suggested by Brandon
et al. (1996) to explain Os isotope systematics and abundances in arc-derived peridotite xenoliths. This type of
model would have to assume that the transport efficiency
of Sr in fluids carrying Os was low. In all these models
we do not allow for the potential complexity introduced
by the dissolution of mantle peridotites with relatively
unradiogenic Os isotope compositions that are contained
within some Grenada picrites. This introduces an extra
level of complexity that is very difficult to quantify.
Whatever the ultimate origin of the Os–Sr isotope
systematics in the Grenada picrites it would seem likely
that some subducted crustal-derived Os was incorporated
into their source that is visible above the effects of latestage crustal contamination that dominate some arc
magmas (e.g. Lassiter & Luhr, 1999). The mechanism
for this incorporation is unclear, but it is likely that the
relative amount of sediment-derived Os compared with
mantle-wedge Os was low; of the order of a few percent.
This agrees with estimates for sediment contribution
based on Sr, Nd and Pb isotopes (Thirlwall et al., 1996).
Such small amounts of sediment-derived Os are unlikely
to noticeably affect PGE abundances and ratios in rocks
such as the Grenada picrites and explain why the mean
PGE abundances of such rocks do not differ greatly from
non-subduction-related magmas.
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compositions of the Grenada picrites solely by AFC
processes. Thus, we infer that small amounts of radiogenic
Os, probably from the subducting slab–sediment mix,
were incorporated into the picrite source region, possibly
by Os-rich fluids. The detailed mechanism of Os transport
and incorporation into the melting regime is not well
understood and should be a target for future work.
CONCLUSIONS
The picritic rocks of Grenada provide an ideal opportunity to study the process of PGE recycling within
a subduction system, as they have undergone less fractionation and crustal contamination than most other
subduction-related lavas. Despite this, PGE concentrations (particularly for the IPGE: Ir, Os, Ru) and
Re concentrations are lower than in other lavas of
comparable MgO content from non-subduction settings.
Overall PGE distributions in the rocks of Grenada and
Izu–Bonin are not strongly correlated with other lithophile element abundances. There are relationships between certain IPGE and elements such as Ni and Ti that
suggest removal of IPGE from the magma with specific
cumulus phases. Within the Grenada suite, Pt and Pd
show minor enrichment in lavas with greater slab-fluid
input (as shown by correlation with high La/Y). Thus,
the PPGE may have slightly greater transport efficiency
in slab-derived fluids than do the IPGE. Melt genesis in
a subduction regime does not, however, cause substantial
enrichment of any one PGE relative to another.
Considering their complex petrogenetic histories and
their derivation from sources of differing fertility with
differing slab-sediment contributions, the Izu–Bonin boninites and Grenada picrites have very similar PGE patterns and concentrations. Further work may confirm that
all subduction systems have a common PGE signature.
Re, however, is markedly depleted in the most primitive
samples from Grenada compared with Izu–Bonin. This
is most easily explained by retention of Re within garnet
in the Grenadian mantle wedge, as partial melting occurs
at much greater depths beneath Grenada than at Izu–
Bonin.
Oceanic sediments (unless highly metalliferous) contain
very low concentrations of PGE and therefore bulk
mixing of large quantities into a mantle source would be
required to significantly affect PGE concentrations within
the mantle wedge. Furthermore, even if PGE are quantitatively transferred from the subducting slab to the
mantle wedge via fluids, their signatures may not be
recognized within resultant arc lavas, as a result of the
complex melt transport and fractionation histories within
an arc environment. As such, study of PGE concentrations within arc lavas is probably not a sensitive
gauge of sediment recycling into the magma source
region. IPGE signatures even in primitive picritic rocks
appear to be modified by early crystal fractionation and
separation.
Os isotope studies provide a potentially more sensitive
monitor of the role of sediment recycling during arc
magma genesis, but only if the extent to which late-stage
crustal contamination has affected Os isotopic signatures
can be tightly constrained. Combined Sr–Os isotopic
studies suggest that it is difficult to generate the radiogenic
ACKNOWLEDGEMENTS
We thank Rick Carlson for generous access to the NTIMS facility at DTM, Carnegie Institution of Washington, and Gordon Irvine for running three of the Os
samples. Professor Julian Pearce provided the Izu–Bonin
samples and is thanked for useful discussion on aspects
of subduction-zone geochemistry. The Ocean Drilling
Program Lamont core store provided sediment samples.
Dr Chris Ottley and Ron Hardy (Durham University) are
thanked for assistance with ICP-MS and XRF analyses.
Support for this project was provided (S.J.W.) by NERC
Grant GT 4/95/71 E. D.G.P. acknowledges funding
from NERC Grant JR99 DUPEEQ during the completion of this project. This manuscript was greatly improved following reviews by Mark Rehkamper, Thijs
Van Soest and Ian Parkinson, and they are thanked very
much for their time and help.
197
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