Journal of Petrology | Volume 40 | Number 11 | Pages 1721-1744 | 1999
© Oxford University Press 1999
Evolution of Heterogeneous Lithospheric Mantle in a Plume Environment Beneath the Kerguelen Archipelago
1 DéPartement Des Sciences De La Terre Et De L'environnement, Université Libre De Bruxelles CP 160/02, AV. F. D. Roosevelt 50, B-1050 Brussels, Belgium
2 Geology and Geophysics, Woods Hole Oceanographic Institution Woods Hole, MA 02543, USA
3 Laboratoire de GÉOLOGIE–CNRS UA10, Université Jean Monnet CNRS–UMR 6524, F- 42023 Saint-Etienne, France
Received October 1, 1998; Revised typescript accepted May 21, 1999
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ABSTRACT |
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 TOP ABSTRACT IntroductionGeological SettingPetrographyGeochemistryDiscussionTemporal Evolution of Kerguelen...Appendix: Analytical ProceduresReferences |
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A combined petrographic, geochemical and Sr–Nd–Pb isotopic investigation of peridotite xenoliths from the Kerguelen Archipelago (southern Indian Ocean) provides new insights into melt migration mechanisms and the sources of heterogeneities in the mantle associated with the long-lived (
115 my) Kerguelen mantle plume. Large variations of trace element concentrations in clinopyroxenes and their isotopic compositions reflect the strong imprint of complex, multi-stage metasomatic episodes during evolution of the lithospheric mantle under the Kerguelen Archipelago. Two metasomatic agents have been identified that have interacted with the mantle peridotite matrix: (1) a basaltic melt, and (2) a carbonatitic melt that produced extremely high and variable incompatible element abundances in clinopyroxenes, which are attributed to chromatographic effects associated with metasomatic melt transport by porous flow through the mantle. Isotopic compositions of 12 peridotite xenoliths indicate that both types of metasomatic melts are related to the alkaline magmatism produced by the Kerguelen plume. In contrast, isotopic data from a single dunite xenolith indicate the strong influence of a continental lithospheric component, probably derived from Gondwanaland, that either forms part of the Kerguelen Plateau or was incorporated into the mantle beneath Kerguelen and mixed with plume-derivedmaterial. Our geochemical study of Kerguelen xenoliths testifiesto the importance of plumes as mechanisms for producing metasomatic melts with highly variable compositions and for entraining different components that may act as contaminants for eruptedlavas. KEY WORDS: geochemistry; isotopes; Kerguelen plume; mantle chromatography; metasomatized xenoliths
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Introduction |
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TOPABSTRACTIntroductionGeological SettingPetrographyGeochemistryDiscussionTemporal Evolution of Kerguelen...Appendix: Analytical ProceduresReferences |
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It is now well accepted that the Earth's mantle behaves as a viscous fluid over geologic time-scales. The variation and amplitude of mantle geochemical heterogeneities reflect the efficiency of heat and mass transfer processes as well as the composition of entrained components (e.g. Hauri et al., 1994
). The heterogeneous nature of the convecting mantle has been delineated through the geochemical study of (1) mid-ocean ridge basalts (MORB) and oceanic island basalts (OIB) (e.g. Zindler & Hart, 1986; Hofmann, 1997), and (2) mantle xenoliths, typically characterized by a much greater geochemical variability than their host and related basalts (e.g. Hauri et al., 1993). Mantle plumes represent the main mechanism for entrainment in the mantle (Hart et al., 1992; Hauri et al., 1994).
The study of mantle xenoliths associated with ocean island hotspot volcanism can provide important information on melt extraction processes and sources of heterogeneities in the mantle. The main purpose ofthis paper is to address the formation of compositional heterogeneities in the mantle associated with the Kerguelen plume from a study of peridotite xenoliths from the Kerguelen Archipelago in the southern Indian Ocean. The Kerguelen plume is remarkable among mantle plumes for the following reasons (e.g. Weis & Frey, 1996):
- it is one of the longest lived, resulting in at least 115 my of volcanic activity in the Indian Ocean since the break-up of Gondwanaland, and yielding the large Kerguelen Plateau and its conjugate Broken Ridge (110–85 Ma) (Whitechurch et al., 1992; Mahoney et al., 1995), the Ninetyeast Ridge hotspot track (82–38 Ma) (Weis & Frey, 1991; Weis et al., 1991), and the Kerguelen and Heard oceanic islands (39–0 Ma) (Gautier et al., 1990; Weis et al., 1993, 1998; Barling et al., 1994; Yang et al., 1998) (Fig. 1a). The Kerguelen plume volcanic activity generated both a large igneous province and a hotspot track, and has evolved in tectonic environment from initially occupying a Southeast Indian Ridge (SEIR) centred setting to a current intraplate setting on the Antarctic plate through ridge-jumps.
- It has produced both volcanic and plutonic rocks (Dosso & Murthy, 1980; Gautier et al., 1990; Weis & Giret, 1994) on the Kerguelen Archipelago, with isotopic compositions at one end of the ocean island array and distinct from other hotspot oceanic islands such as Iceland or Hawaii (Zindler & Hart, 1986).
- It is inferred to be the main source of geochemical and isotopic anomalies present in the Indian Ocean seafloor basalts (Barling et al., 1994; Weis & Frey, 1996).
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Mineralogical and geochemical studies on the Kerguelen basic and ultrabasic xenoliths have shown the presence of diverse petrologic types—dunite, lherzolite, harzburgite, websterite, pyroxenite, metagabbro (Grégoire, 1994
; Mattielli, 1996; Grégoire et al., 1997). Most of these studies have focused on the basic xenoliths that have geochemical and isotopic characteristics consistent with being re-equilibrated cumulates (mostly in the granulite facies) from magmas formed through interaction between a depleted SEIR-type component and the Kerguelen plume (Grégoire et al., 1994; Mattielli, 1996; Mattielli et al., 1996; Valbracht et al., 1996).
In this paper, we present a petrographic, geochemical and Sr–Nd–Pb isotopic study of 12 Kerguelen peridotite xenoliths. Our discussion focuses on the origin of heterogeneities in the mantle neighbouring the Kerguelen plume, with implications for (1) the composition and the provenance of metasomatic melts in the mantle, and the degree and timing of the metasomatic reactions; (2) the influence of different source components such as the Kerguelen plume, oceanic lithosphere, asthenosphere and continental lithosphere; (3) the relationships between the deep lithospheric levels and the erupted lavas on the Kerguelen Archipelago itself. Our study demonstrates a complex and multi-stage metasomatic evolution of the lithospheric mantle and provides additional evidence for the occurrence of carbonate-rich metasomatism as a common process in the mantle (e.g. Schiano et al., 1994).
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Geological Setting |
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TOPABSTRACTIntroductionGeological SettingPetrographyGeochemistryDiscussionTemporal Evolution of Kerguelen...Appendix: Analytical ProceduresReferences |
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The Kerguelen Archipelago is located on the Antarctic Plate in the southern Indian Ocean (Fig. 1a). With a surface area of 6500 km2 above sea level, the Kerguelen Archipelago lies on the northern submarine Kerguelen Plateau. Crustal thicknesses for the combined plateau and the overlying archipelago are estimated to be in the range of 20–25 km (Charvis et al., 1993
). The archipelago represents the last 39 my of the long-lived Kerguelen plume volcanic activity (115 my). The exposed rocks on the Kerguelen Archipelago consist predominantly (80%) of subhorizontal traps of transitional to highly alkaline basalts (Gautier et al., 1990; Yang et al., 1998), locally intruded by differentiated volcanic and plutonic complexes (Giret & Lameyre, 1983; Weis et al., 1993, 1998; Weis & Giret, 1994) (Fig. 1b).
Systematic sampling of xenoliths in basaltic lavas of the Kerguelen Archipelago was conducted during the archipelago field mapping campaigns (1988–1998). The most numerous and diverse outcrops occur in the Southeast Province (Fig. 1c), where approximately one ton of xenoliths was collected (Grégoire, 1994; Mattielli, 1996). Additional outcrops have subsequently been discovered in the north and northeast of the archipelago (e.g. in the Courbet Peninsula; Hassler & Shimizu, 1994).
The Southeast Province is composed of basaltic lavas with a regional dip towards the north (Leyrit et al., 1990) (Fig. 1c). The topography of the region is controlled by extrusions (plugs, vents, domes, cupolas and lava flows) of differentiated lavas that represent about 4% of the surface area (Weis et al., 1993). Several plutonic complexes have been identified in the Southeast Province (Leyrit et al., 1990); amongst these complexes, the Val Gabbro yields the oldest K–Ar age (39 ± 3 Ma) on the archipelago (Giret & Lameyre, 1983). The lavas in the Southeast Province correspond to two main volcanic phases (Leyrit et al., 1990; Weis et al., 1993): a 22–20 Ma lower Miocene mildly alkaline series consisting of basalts and trachytes, and a 10.2–6.6 Ma upper Miocene highly alkaline series with basanites, tephri-phonolites and phonolites.
Mafic and ultramafic xenoliths are found exclusively in outcrops of the youngest and most alkaline volcanic rocks (Fig. 1b, fig.1c). Most of the xenolith-bearing outcrops are basanitic or lamprophyric dykes (Dôme Rouge, Val Phonolite, Pointe de l'Espérance, Le Trièdre, Tour de Pise, Mont Thompson, Val du Levant) or lava flows (Le Pouce and Rivières aux Macaronis) and more rarely breccia pipes (Mont Tizard), or limburgitic lava flows (Pointe Suzanne). The Dôme Rouge and the Mont Tizard localities are the main xenolith outcrops on the archipelago because of the abundance (75% of the total nodule collection), size (up to 50 cm in diameter for Mont Tizard nodules) and diversity of their xenoliths (Fig. 2a).
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Petrography |
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TOPABSTRACTIntroductionGeological SettingPetrographyGeochemistryDiscussionTemporal Evolution of Kerguelen...Appendix: Analytical ProceduresReferences |
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Modal proportions and textures, including pyroxene habit, are the most distinctive petrographic criteria for discriminating between the different types of Kerguelen peridotite xenoliths. The occurrence of a secondary wehrlitic assemblage, and melt and fluid inclusions, indicate that the xenoliths record a relatively recent modification of the mantle peridotite matrix by infiltrating melts.
The peridotite xenoliths represent 30% of the total xenolith collection. We have subdivided these peridotite xenoliths into four groups based on modal proportions and textures: lherzolite (12%), dunite (31%), protogranular harzburgite (40%) and poikilitic harzburgite (17%). All peridotite xenoliths are coarse- to medium-grained, with a mean diameter of 15 cm, sub-rounded or ovoid, without foliation or lineation. They display protogranular or poikilitic textures, with olivine and orthopyroxene crystals ranging in size from 2 to 10 mm, but locally grade into porphyroclastic textures, with olivine and pyroxene neoblasts (
1 mm) surrounding stretched and weakly oriented porphyroclasts (2–5 mm). The nodules are fresh. They may locally show altered rims, with intergranular thin basaltic glass, minor serpentinization or iddingsitization of olivine, and magnetite coronae around spinel.
Lherzolite xenoliths constitute the least abundant group. Only one sample representative of this group is described in this study: lherzolite 92–372, which consists of colourless olivine, pale green orthopyroxene and clinopyroxene, and olive-green spinel (Fig. 2b). The texture is more equigranular than in the other peridotites. Olivine grains contain no kink-bands and are more strongly polygonized. Pyroxene grains are rarely exsolved and may occur interstitially. Spinel crystals are mainly interstitial or included in olivine.
Dunite nodules consist of olivine (92–98 vol. %), clinopyroxene (1–5 vol. %) and spinel (0.5–3 vol. %) [for modal compositions estimated by point-counting and a least-squares method from whole rocks and mineral compositions, see Mattielli, (1996) and Grégoire et al., (1997)]. Olivine grains occur as either large kinked or strained porphyroclasts or smaller strain-free polygonized crystals. Undeformed, pale green clinopyroxene is typically interstitial, but can be included in olivine or contain olivine and spinel inclusions. Clinopyroxene contains rare thin orthopyroxene exsolution lamellae. Pale brown to black spinel grains are interstitial, but also occur as subhedral inclusions in olivine or clinopyroxene. Compared with other dunites, sample 91–114 is distinguished by its large size (diameter of 15 cm), the form of olivine porphyroclasts (more strongly polygonized and elongated in a preferential direction) and its slightly more altered and fissured general appearance (Fig. 2c).
Harzburgites are by far the predominant xenolith type in the collection. They contain olivine (67–77 vol. %), orthopyroxene (17–28 vol. %), pale green clinopyroxene (1–5 vol. %) and dark red spinel (0.5–4 vol. %). On the basis of pyroxene habit, the harzburgite xenoliths were divided into two general groups (Grégoire et al., 1997): (1) protogranular harzburgites (samples 91–38, 91–42) that consist mostly of anhedral porphyroclasts of olivine and orthopyroxene; spinels typically form a symplectitic texture (or cluster; up to 1 cm in diameter) in association with orthopyroxene and clinopyroxene (Fig. 2d); and (2) poikilitic harzburgites characterized by the occurrence of a secondary ‘wehrlitic’ assemblage—mainly clinopyroxene plus olivine and spinel. In poikilitic harzburgites, the secondary clinopyroxenes are undeformed, show primary twinning and occur either as large, poikilitic grains with orthopyroxene ± olivine or spinel inclusions (Fig. 2e), or as fine-grained neoblasts ( 1 mm) with olivine and spinel in patches between primary orthopyroxene and olivine (Fig. 2f).
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Geochemistry |
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TOPABSTRACTIntroductionGeological SettingPetrographyGeochemistryDiscussionTemporal Evolution of Kerguelen...Appendix: Analytical ProceduresReferences |
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Because the Kerguelen peridotite xenoliths have evolved in a hotspot environment, where degrees of melting and melt circulation are expected to be important (e.g. Johnson et al., 1990
), these xenoliths may be particularly favourable for observing major and trace element fractionation (coupled or decoupled) in the lithospheric mantle.
Major element compositions
Major element compositions of the Kerguelen peridotite xenoliths still reflect to a certain degree compositional control by partial melting; however, they mainly indicate subsequent modifications following metasomatic processes. Spinel and clinopyroxene major element compositions are especially sensitive indicators; they show significant variation and correlate with rock type (dunite, harzburgite and lherzolite) and with texture (protogranular vs poikilitic harzburgites). None of the minerals display significant major element variations between the rims and cores, however, and recrystallization and deformation do not result in chemical modification as neither porphyroclasts nor neoblasts show significant differences in major element contents (Grégoire, 1994).
Differences in mineral compositions do not necessarily imply significant variations in bulk-rock compositions in the Kerguelen xenoliths. Spinel and clinopyroxene, which contain the most variable and distinctive major element contents, represent the lowest modal proportions (up to 3.5 and 5 vol. %, respectively) of the xenoliths, whereas olivine with relatively homogeneous compositions (Fo87–92) is the most abundant phase.
The lherzolite 92–372, like all other Kerguelen lherzolites, clearly differs from harzburgite and dunite xenoliths by high Al2O3 (6.33 wt %), CaO (3.43 wt %) and Na2O (0.22 wt %), and low MgO (39.47 wt %) contents. Kerguelen dunite and harzburgite nodules have refractory bulk compositions, showing strong depletions in ‘basaltic components', especially in CaO (0.29–1.08 wt %) and Al2O3 (0.15–1.15 wt %), and enrichments in MgO (43.7–47.7 wt %) relative to primitive mantle compositions (Hart & Zindler, 1986; McDonough & Sun, 1995) (Fig. 3). With the lowest mg-number (87), and the highest Fe2O3t, the dunite 91–114 shows the least refractory bulk composition of all the investigated nodules.
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), dunite (
), and protogranular (
) and poikilitic (
) harzburgite xenoliths] [data from Grégoire, (1994)In dunite and lherzolite nodules, clinopyroxenes have diopsidic compositions characterized, respectively, by lower MgO (15.1–17.5 wt %) and higher Al2O3 (6.0 wt %) contents relative to harzburgite nodules (Grégoire et al., 1997
). Clinopyroxenes in dunite 91–114 stand out by virtue of their high TiO2 contents (0.71–1.06 wt %). Clinopyroxene compositions in harzburgites vary from diopside (En49–51Fs3–4Wo45–47) in the protogranular harzburgites to Mg-augite (En51–53Fs4–7Wo41–44) in the poikilitic harzburgites. Clinopyroxenes in poikilitic harzburgites differ from those in protogranular harzburgites by significantly higher Al2O3, Cr2O3 and Na2O contents (up to 5.35, 2.44 and 3.05 wt %, respectively) (Grégoire et al., 1997). Within poikilitic harzburgites, major element contents do not vary as a function of clinopyroxene habit, i.e. no compositional difference is observed between isolated and poikilitic clinopyroxene grains [compare the results of 53 clinopyroxene analyses in sample 92–509 (M. Grégoire, unpublished data, 1995)].
Spinels (Mg–Al chromites) show a large range of cr-number values for nearly constant mg-number, especially in the harzburgites (mg-number: 60–77; cr-number: 33–68) (Grégoire, 1994). The cr-number values are highest (up to 68) in the poikilitic harzburgites, and decrease progressively from protogranular harzburgites, dunites and lherzolites, with the lowest value (5) in lherzolite 92–372 (Fig. 4). Spinels from dunite 91–114 have low mg-number (65) and the highest TiO2 (0.99 wt %) and Fe2O3 (8.02 wt %) contents. Fe2O3 and TiO2 contents decrease progressively from dunite to poikilitic harzburgite (Fe2O3: 2.68–4.52 wt %; TiO2: 0.05–0.24 wt %), protogranular harzburgites (Fe2O3: 1.86–2.01 wt %; TiO2: 0.05–0.12 wt %) and finally, lherzolite 92–372 (Fe2O3: 1.30–1.48 wt %; TiO2: 0.04–0.10 wt %) (Grégoire, 1994).
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The relatively strong refractory compositions of bulk rocks and mineral phases, especially in protogranular harzburgites, could be typical of upper-mantle material that has undergone substantial partial melting. Olivine and clinopyroxene compositions in the protogranular harzburgites follow the theoretical compositional evolution [e.g. illustrated by variations of mg-number in clinopyroxene vs modal olivine contents; see discussion by Grégoire et al., (1997)
] calculated for peridotitic residues undergoing increasing degrees of partial melting. However, the chemical characteristics of Kerguelen peridotite xenoliths mainly reflect subsequent chemical modifications caused by metasomatic processes, as illustrated by the following observations:- the Kerguelen lherzolite 92–372 clearly differs from abyssal peridotites (Hamlyn & Bonatti, 1980; Johnson et al., 1990) by higher Al2O3 and FeO contents and modal orthopyroxene and spinel contents.
- The mineral composition trends in dunite, poikilitic harzburgite, and lherzolite xenoliths do not follow the typical evolution observed in models of mantle partial melting [see discussion by Grégoire et al., (1997)].
- Relative to abyssal dunites (Dick & Bullen, 1984; Bonatti & Michael, 1989), mineral compositions in Kerguelen dunites show lower mg-number values and higher Al2O3 contents (Fig. 4). Unlike chemical variations in abyssal peridotites (Niu, 1997), Kerguelen dunites do not show correlations with modal olivine contents, forsterite contents or bulk mg-number. In contrast, dunites with high modal olivine (up to 98%) define positive correlations between MgO and SiO2 contents.
- Clinopyroxenes in poikilitic harzburgites show strong enrichments in Na2O and Cr2O3, relative to abyssal peridotites. It is noteworthy that Kerguelen harzburgites show some analogies (similar modes, similar high forsterite contents and enrichments in MgO in clinopyroxenes) with abyssal peridotites dredged close to hotspots, especially near Bouvet (Johnson et al., 1990).
- The composition of spinels in abyssal peridotites has been considered by Dick & Bullen, (1984) as a sensitive indicator of degrees of partial melting in the mantle. Spinels in Kerguelen harzburgites (Grégoire, 1994) display high cr-number (Fig. 4), a strong increase in Cr abundances associated with decrease in Al and Mg abundances, and, for poikilitic harzburgites, extremely high cr-numbers (up to 70) that are not observed for abyssal harzburgites. High cr-number values may reflect high degrees of partial melting in the mantle (Dick & Bullen, 1984). However, Kelemen & Dick, (1995) have suggested that high Cr abundances may indicate interaction between basaltic melt and mantle matrix, producing the dissolution of clinopyroxene ± orthopyroxene ± spinel with low Cr–Al ratios and the formation of secondary spinels depleted in Al and enriched in Cr. Kelemen & Dick, (1995) found that variations in cr-number relative to Ti abundances in spinels constitute reliable indicators that distinguish interaction products, characterized by enrichments in both Cr and Ti, from melting products, characterized by increasing Cr and decreasing Ti. Without a clear cr-number–Ti trend, most of the spinels in the Kerguelen harzburgites indicate a parallel increase of both cr-number and Ti (Mattielli, 1996) which, together with the large variations in cr-number and Ti concentrations for nearly constant mg-number, suggests a reaction of the mantle lithosphere with ascending melt.
Incompatible element compositions
In situ trace element concentrations of clinopyroxene in the Kerguelen peridotite xenoliths (Table 1) were obtained using secondary ion mass spectrometry (SIMS) (see Appendix for full details on the analytical procedure). Particular attention was paid to avoiding any melt or fluid inclusions during clinopyroxene analyses. Clinopyroxene controls the rare earth elements (REE) and Sr, and to a lesser extent Zr and Ti abundances in four-phase ‘dry’ peridotites (e.g. Zindler & Jagoutz, 1988; Blusztajn & Shimizu, 1994). Compositional heterogeneities in clinopyroxenes within or among Kerguelen xenoliths indicate incomplete melt–rock reactions, yielding information on reaction progress and recording a relative temporal signal of metasomatism. On the basis of the chondrite-normalized REE abundance patterns of clinopyroxenes, three groups of Kerguelen peridotite xenoliths are identified: the weakly light REE (LREE)-enriched lherzolite 92–372, the LREE-enriched dunites and harzburgites, and the LREE-depleted dunite 91–114 (Fig. 5).
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Weakly LREE-enriched lherzolite 92–372
Clinopyroxenes from sample 92–372 show concave-upward REE patterns [(Ce/Yb)n = 1.8–3.5; (Sm/Yb)n = 0.9–1.4] with an abrupt slope change at Nd (Fig. 5a), and systematically positive Eu anomalies. Consistent with Grégoire, (1994)
, we propose that the Eu anomalies result from a subsolidus reaction between early-formed plagioclase and olivine forming the orthopyroxene–clinopyroxene–spinel assemblage. The high Sr and Pb abundances in 92–372 (Table 1) are also consistent with the early occurrence of plagioclase in this sample.
LREE-enriched dunites and harzburgites
Clinopyroxenes of dunite and harzburgite xenoliths show considerable variations of trace element abundances coupled with textural changes (Fig. 5b–e), as well as major element composition variations (see Grégoire et al., 1997). Whereas heavy REE (HREE) abundances are relatively constant, LREE abundances show large variations with significant enrichments relative to chondrite abundances. For dunite xenoliths, the clinopyroxenes show (Ce/Yb)n that ranges from 5.6 to 9.4 (Cen = 29–93), negative Zr and Ti anomalies, and weakly negative or no Sr anomalies relative to REE (Zr = 7.6–82 ppm; Ti = 1705–2042 ppm; Sr 334 ppm) (Fig. 5b). Clinopyroxenes in the poikilitic harzburgites are characterized by the highest REE abundances and LREE enrichments of all the Kerguelen xenoliths (Fig. 5c Fig.5d). (Ce/Yb)n varies from 6.2 to 33 and REE patterns are characterized by steep negative slopes. Clinopyroxenes of the protogranular harzburgite show average REE abundances 10 times lower than those in the poikilitic harzburgites (Fig. 5e); they have concave-upward REE patterns with (Ce/Yb)n varying from 2.6 to 3.1. It is noteworthy that Mattielli et al., (1992) and Hassler & Shimizu, (1998) have reported harzburgite nodules from the Southeast Province and the Courbet Peninsula with LREE-depleted clinopyroxenes [(Ce/Yb)n down to 0.1; (La/Sm)n < 1] thus confirming the extreme variability in trace element abundances in the harzburgite xenoliths.
Clinopyroxenes in both harzburgite groups are depleted in Ti and Zr relative to REE (Fig. 5c, Fig. 5e), with Ti/Eu (74–972) lower than the chondritic value (Ti/Eu in C1 chondrites is 7815; McDonough & Sun, 1995). The negative anomalies in Ti, Zr and Sr are even more pronounced in the poikilitic harzburgites, especially in the samples 91–8 (Ti/Eu = 108–526; Ti/Zr = 0.4–86) and 133A1–1 (Ti/Eu = 107; Ti/Zr = 28; Srn = 16), which presents at once the lowest Ti, Zr and Sr concentrations and the highest REE abundances for the entire population analysed in this study. Clinopyroxenes in the xenolith 91–8 exhibit substantial trace element heterogeneity (Fig. 5d) [(Ce/Yb)n = 6.8–15; Zrn = 0.5–75] not correlated with grain morphology—there is no systematic difference in chemistry between discrete or symplectitic clinopyroxene grains.
LREE-depleted dunite 91–114
In our peridotite suite, this is the only xenolith with LREE-depleted clinopyroxene [(Ce/Yb)n < 0.6] (Fig. 5f). In addition, unlike other dunites, the clinopyroxenes in 91–114 show no anomaly in Ti and a positive anomaly in Zr. The whole-rock chemistry of the dunite 91–114 shows a weak LREE enrichment [(Ce/Yb)n = 7.8], still present although weaker when the whole rock is leached [(Ce/Yb)n = 1.9] (Fig. 5f).
Melt and fluid inclusions
In an attempt to characterize the metasomatic agents in the lithospheric mantle beneath Kerguelen, Schiano et al., (1994) undertook a study of melt and fluid inclusions trapped in Kerguelen peridotite xenoliths. Three types of secondary cogenetic inclusions are hosted by silicate minerals in dunite and harzburgite xenoliths: silicate melt inclusions, carbonate-rich inclusions and CO2 fluid inclusions. These inclusions form trails along fracture planes. Carbonate-rich melt inclusions are physically connected with the silicate melt inclusions, indicating the former existence of a homogeneous melt that later unmixed into two separate melts by immiscibility. Carbonate-rich melt inclusions contain aggregates of calcite crystals, whereas silicate melt inclusions include kaersutite, diopside, rutile, ilmenite and magnesite (Schiano et al., 1994). The silicate melt inclusions are characterized by normative quartz and feldspar compositions with SiO2 60 wt %, Al2O3 20 wt %, Na2O and K2O 4–5 wt % each, FeO and MgO < 3 wt %, Cl > 1000 ppm, H2O 
1.2% and oversaturation with CO2. The trace element signature is characterized by LREE enrichments and high field strength element (HFSE) depletions [e.g. (Ce/Yb)n = 16.2 and Ti/Zr = 17 (much lower than typical OIB; >60)] (Schiano et al., 1994). The silicate–carbonate melt inclusions trapped in dunite and harzburgite xenoliths cannot result from melting of anhydrous peridotite assemblage; they must be produced by migration of an exotic metasomatic melt through the lithospheric mantle (Schiano et al., 1994).
Isotopic results
The Sr–Nd–Pb isotopic compositions for the Kerguelen metasomatized xenoliths display a large range of variations. The data allow us (1) to constrain the provenance of the metasomatic melts and their relationships with the Kerguelen plume, (2) to establish genetic relationships between the lithospheric mantle and the lavas erupted on the archipelago and those produced by the earlier plume activity, and (3) to characterize the enriched mantle reservoir, as the Kerguelen lavas have been classified as EM I OIB type on the basis of trace element compositions (Weaver, 1991), but have been considered for their isotopic characteristics by Weis et al., (1993) to be intermediate between EM I and EM II [defined by Hart & Zindler, (1986)].
The complete isotopic data set discussed in this paper, including data reported by Mattielli et al., (1996), is given in Table 2 for leached clinopyroxene separates and whole rocks. Complete analytical details are given in the Appendix. The variations in isotopic ratios cover a large range of values: 87Sr/86Sr varies from 0.70487 to 0.70721; 143Nd/144Nd from 0.51264 to 0.51214; and 206Pb/204Pb from 17.71 to 18.60 (Fig. 6– 8). In comparison, 87Sr/86Sr in the Hawaiian peridotite xenoliths varies from 0.7025 to 0.7043 (Frey & Roden, 1987; Okano & Tatsumoto, 1996).
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), protogranular (small 
)], with isotopic ratios (labelled fields) for lavas from the Kerguelen Archipelago [including Upper Miocene Series (SE UMS) from the Southeast Province], the Kerguelen Plateau (Sites 747, 748, 749, 750, 738), the Ninetyeast Ridge (90ER), and the Southeast Indian Ridge (SEIR) (Salters et al., 1992
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The isotopic variations are correlated with the chemical variations of Kerguelen peridotite xenoliths. Whereas the isotopic ratios are inversely correlated in Sr and Nd for all the Kerguelen peridotites (Fig. 6), they define no coherent linear array in Pb–Pb diagrams (Fig. 8). Pb isotopic data clearly form two distinct isotopic groups and distinguish dunite with low 206Pb/204Pb (<18.270) from harzburgite xenoliths with high 206Pb/204Pb (>18.390) (Fig. 7 and 8). The dunite and harzburgite groups described above share similar 87Sr/86Sr, but the 143Nd/144Nd values of the dunites (<0.51257) are lower than those of the harzburgites (<0.51263) (Fig. 6). Despite significant geochemical differences between the protogranular and poikilitic harzburgites, both types are isotopically similar. The lherzolite 92–372, which is chemically distinct from the other peridotite xenoliths, has the most depleted isotopic signature of all the investigated peridotite xenoliths, with low 87Sr/86Sr (0.70487) and high 143Nd/144Nd (0.51264), 206Pb/204Pb (18.464) and 207Pb/204Pb (15.600). As expected from its distinct geochemical characteristics, the one sample with a distinctive isotopic composition is the dunite 91–114, with extremely low 206Pb/204Pb (17.713) and 143Nd/144Nd (0.51214), and high 87Sr/86Sr (0.70721) (Fig. 6– 8). Sample 91–114 significantly extends the isotopic range documented for the Kerguelen hotspot system. Except for a few lower values of 143Nd/144Nd, isotopic ratios of the other peridotite xenoliths are included in the field defined by the Kerguelen Archipelago lavas (Gautier et al., 1990
; Weis et al., 1993, 1998; Yang et al., 1998) (Fig. 6– 8). It is noteworthy that the dunite sample 92–491, which has the lowest Pb concentration of all the peridotite xenoliths, shows especially high 207Pb/204Pb (15.628).
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Discussion |
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TOPABSTRACTIntroductionGeological SettingPetrographyGeochemistryDiscussionTemporal Evolution of Kerguelen...Appendix: Analytical ProceduresReferences |
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Host basalt–xenolith interactions do not explain the geochemical signatures of the Kerguelen xenoliths and therefore different multi-stage metasomatic processes are inferred. We propose two main metasomatic agents: a basaltic melt and a carbonatitic melt. We were aware of host basalt–xenolith interaction risks and we therefore minimized potential effects of this interaction by selecting all analysed clinopyroxenes as far as possible from basaltic veins. No petrographic relationship exists between basaltic veins and specific clinopyroxene habits. Furthermore, geochemical and isotopic studies have provided no evidence of basalt–xenolith interaction effects:
- the isotopic compositions for individual xenoliths are clearly distinct from those of the host basalts; the xenoliths are enriched in Sr–Nd radiogenic isotopes and depleted in 206Pb compared with those of the host basalts [Fig. 5 and 8a of Mattielli et al., (1996)].
- Minerals in peridotite xenoliths are compositionally different from those in the host basalts. For example, clinopyroxenes in the xenoliths are more magnesian and deficient in FeO and MnO when compared with the host basalts from Dôme Rouge and Mont Tizard (Leyrit et al., 1990), and clearly present higher LREE abundances and HFSE depletions.
- Trace element abundances and major element contents of the silicate–carbonate melt inclusions (Schiano et al., 1994) imply a melt composition distinct from the basaltic host magma.
Interactions with a basaltic metasomatic melt
The lherzolite sample 92–372 represents the least abundant ultrabasic xenolith group found in the Kerguelen Archipelago. This sample is clearly distinguishable from the dunite and harzburgite xenoliths by extremely low MgO, and high Al2O3 and CaO contents. However, its bulk Al2O3 and FeO contents are too high and cr-number in the spinels too low to be considered as a fertile abyssal lherzolite. This sample may be part of the basic–ultrabasic xenolith subtype IIa (two pyroxenes + spinel-bearing xenoliths) of Grégoire et al., (1994). These xenoliths are inferred to have crystallized from tholeiitic–transitional basaltic magmas and undergone subsolidus reactions involving destabilization of early olivine and plagioclase. This is consistent with the positive anomalies in Eu and Sr relative to REE in the clinopyroxenes of 92–372 (Fig. 5a).
The LREE enrichments in the clinopyroxenes of 92–372 reflect a cryptic metasomatism. The composition of the metasomatic melt has been estimated by calculating the trace element composition of a hypothetical melt that would be in equilibrium with the clinopyroxenes. As with the studies of Kelemen et al., (1992) and Blusztajn & Shimizu, (1994), we have used the clinopyroxene–basaltic melt partition coefficients from Hart & Dunn, (1993) (Fig. 9a). The resultant chondrite-normalized trace element profiles lie within the field of trace element abundances of the Kerguelen Archipelago lavas, specifically the alkaline basalts (Fig. 9a) (Davies et al., 1989; Gautier et al., 1990; Weis et al., 1993; Yang et al., 1998).
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). The cpx–basalt partition coefficients from Hart & Dunn, (1993)Relative to other Kerguelen xenoliths, this lherzolite possesses the most ‘depleted’ isotopic composition. The isotopic ratios in Sr (0.70487), Nd (0.51264) and Pb (206Pb/204Pb = 18.464) are included in the field of isotopic compositions for the alkaline lavas from the Kerguelen Archipelago (Table 2; Fig. 6– 8). The isotopic characteristics of these lavas have been demonstrated as reflecting the Kerguelen plume signature (Weis et al., 1993
, 1998; Yang et al., 1998). This indicates that the cryptic metasomatism in the mantle lithosphere, as shown in the lherzolite 92–372, results from an alkaline basaltic melt originating from the Kerguelen plume.
Interactions with a carbonatitic metasomatic melt
Petrographic and chemical evidence for the presence of carbonatitic metasomatic melt
The distribution of melt or fluid inclusions along fracture planes in dunite and harzburgite xenoliths indicates that melt injection was before, or synchronous with, deformation of the xenoliths, and thus allowed exotic melt to invade the mantle peridotite matrix. Physical continuity between silicate melt inclusions and carbonate-rich melt inclusions indicates that the two melts were initially a single melt and separated subsequently by immiscibility (Schiano et al., 1994). On the basis of their major element and incompatible element compositions and high volatile contents, the silicate–carbonate melt inclusions are interpreted as a metasomatic melt phase, with carbonatitic composition, distinct from the host basaltic magma composition. We are aware that direct comparisons of mantle carbonate-rich metasomatic liquids with natural carbonatites may be misleading (Ionov et al., 1993), as most natural carbonatites are plutonic rocks, strongly differentiated, with numerous accessory minerals that can significantly influence trace element distribution. For suitable comparisons, it is necessary to use compositional data from carbonatite lavas (e.g. Keller, 1981; Deans & Roberts, 1984; Church & Jones, 1995; Keller & Spettel, 1995). We will refer to the metasomatic melt as ‘carbonatitic’ in the sense that it represents mantle-derived carbonate-rich material with trace element signatures typical of carbonatitic volcanism: strong enrichment in LREE over HREE, and strong depletion in Zr, Hf and Ti. The petrogenesis of primary carbonatite liquids and carbonate–silicate immiscibility processes is still poorly constrained. Despite convincing evidence from mantle peridotite xenoliths for carbonate–silicate immiscibility (Ionov et al., 1993; Kogarko et al., 1995), the complexity probably results from the large range of primary carbonatite compositions and P–T conditions established for immiscibility processes between carbonatite and silicate magmas (e.g. Green & Wallace, 1988; Kjarsgaard & Hamilton, 1989; Wyllie, 1989; Canil, 1990; Hamilton & Kjarsgaard, 1993). Furthermore, the presence of primary carbonates in mantle peridotite xenoliths has only rarely been reported, because of carbonate destabilization at high temperature.
The secondary wehrlitic mineral assemblage (clinopyroxene, olivine, spinel) found in the poikilitic Kerguelen harzburgites (Fig. 2e, Fig. 2f) is similar to that occurring in other mantle nodules that are inferred to have been metasomatized by ‘carbonatite melts’ or ‘carbonate-rich melts’ (Green & Wallace, 1988; Dautria et al., 1992; Kogarko et al., 1995; Ionov et al., 1996). Second generation clinopyroxene in the Kerguelen xenoliths shows strong similarities with clinopyroxene of secondary wehrlitic assemblages (olivine, clinopyroxene, spinel, and carbonate–silicate glass patches) reported by Kogarko et al., (1995) for the Monta Clara harzburgite nodules; these clinopyroxenes have low TiO2 and very high Na2O contents, similar to those in Kerguelen harzburgite xenoliths (Na2O: up to 2.44 wt % in poikilitic harzburgites). To explain the formation of secondary ‘wehrlitic’ assemblages, Green & Wallace, (1988) and Yaxley & Green, (1996) proposed a metasomatic reaction between sodic dolomitic melt and lithospheric wall-rock phases (at P 20 kbar), modelled as carbonate melt + orthopyroxene = clinopyroxene + olivine + CO2-rich fluid. The net effects of the postulated decarbonation reaction are to decrease or eliminate modal orthopyroxene, with an accompanying increase in modal clinopyroxene and olivine. Consistent with these effects, the Kerguelen poikilitic harzburgites show undeformed clinopyroxene crystals enclosing resorbed orthopyroxene (Fig. 2e). Moreover, the Kerguelen harzburgites show evidence for variable degrees of this metasomatic reaction: the poikilitic harzburgites contain less modal orthopyroxene (21 vol. %) and more modal olivine (74 vol. %) than protogranular harzburgites [respectively, 25 and 70 vol. %; mean values from Grégoire, (1994)], and the clinopyroxenes in poikilitic harzburgites have higher Al2O3, Cr2O3 and Na2O contents relative to protogranular harzburgites.
Chromatographic fractionation associated with porous flow percolation of carbonatitic metasomatic melt?
Although the variability of trace element profiles in clinopyroxene within and among dunite and harzburgite xenoliths can be used to constrain the process of metasomatic melt percolation, the contrasted interrelationships of HFSE and REE abundances in these clinopyroxenes can also be used to determine the composition of the metasomatic melts. Patterns with extremely variable LREE enrichments and almost constant HREE have been attributed to chromatographic effects associated with metasomatic melt transport by porous flow through the mantle (e.g. Navon & Stolper, 1987; Bodinier et al., 1990). A characteristic feature of this process is that concentration fronts of trace elements develop as elements move through the porous peridotite matrix at rates that are inversely proportional to their solid–melt partition coefficients. Given factors such as melt velocity, percolation distance in the matrix, matrix porosity, span of time and extent of interaction, complete equilibrium with the percolating melt may not be achieved and large variations in the incompatible element distribution can be created; in contrast, under specific conditions, REE in a mantle peridotite can be re-equilibrated with infiltrated melt (e.g. Navon & Stolper, 1987; Hunter & McKenzie, 1989; Hauri & Hart, 1994; Bedini et al., 1997).
We assume that trace-element-rich melt inclusions have not contributed significantly to the heterogeneous clinopyroxene signatures in the Kerguelen peridotite xenoliths, on the basis of our careful in situ ion microprobe analyses of these clinopyroxenes. Trace element abundance variability in clinopyroxenes from the Kerguelen harzburgite and dunite xenoliths can be interpreted as a chromatographic fractionation effect during percolation of a carbonatitic metasomatic melt by porous flow through a mantle peridotite. This is based on:
- the unusual wide range of variations for LREE and HFSE abundances in clinopyroxenes of dunite and harzburgite xenoliths for relatively constant HREE abundances (the dunite and poikilitic harzburgite clinopyroxenes have Cen concentration ranges respectively 13 times and 37 times larger relative to Ybn) (Fig. 5b, Fig. 5c);
- the concave-upward REE patterns of clinopyroxenes in protogranular harzburgites that are similar to profiles occurring in other mantle nodules, which are also inferred to have resulted from incomplete equilibrium between the mantle matrix and a percolating melt, or from addition of small amounts of ‘trapped’ melt to residual peridotites, associated with a chromatographic effect (e.g. Navon & Stolper, 1987; Takazawa et al., 1992; Kelemen & Dick, 1995) (Fig. 5e);
- the presence of pronounced variations of trace element abundances (especially in LREE and HSFE) for clinopyroxene grains in the poikilitic harzburgite 91–8 (Fig. 5d). However, localized heterogeneities among nearby crystals may require very localized differences in melt migration pathways.
Assuming that Kerguelen harzburgite and dunite xenoliths were metasomatized by a single melt type, the composition of the metasomatic melt has been estimated by calculating the trace element concentration of a hypothetical melt that would be in equilibrium with the clinopyroxenes showing the most pronounced melt–rock interaction effects. If multiple metasomatic pulses with variable melt compositions occurred, then the calculated equilibrium melt would be a hypothetical ‘integrated’ melt. With the highest REE abundances, and the most pronounced LREE enrichments and HFSE anomalies, clinopyroxenes from the poikilitic harzburgite 133A1–1 (Fig. 5c) may have recorded the most significant interaction with a metasomatic melt, strongly enriched in LREE, but depleted in HFSE, and represent the closest approach to the composition produced through equilibration with the migrating melt. This calculation method does not account for the effects of melt composition, pressure and temperature on the trace element exchange equilibrium. Our goal is to determine the relative element fractionation and not the absolute concentration value. We have used the clinopyroxene–carbonatitic melt partition coefficients from Klemme et al., (1995). As expected, the putative equilibrium melt is characterized by high concentrations of LREE and depletions in Zr and Ti (Fig. 9b). Its trace element pattern fits extremely well with the composition field of carbonatites (Keller, 1981; Nelson et al., 1988; Woolley & Kempe, 1989; Church & Jones, 1995), characterized by low Zr and Ti abundances and highly LREE-enriched profiles with a steep slope (Fig. 9b). In contrast, the calculated trace element pattern for the metasomatic melt is clearly distinct from those of basalts, especially the Kerguelen Plateau or Kerguelen Archipelago basalts (Davies et al., 1989; Gautier et al., 1990; Alibert, 1991; Weis et al., 1993, 1998; Yang et al., 1998) [even using the clinopyroxene–basaltic melt partition coefficients from Hart & Dunn, (1993)] (Fig. 9a, Fig. 9b).
Carbonatitic melt is an effective agent of mantle metasomatism because of its extremely high large-ion lithophile element (LILE) abundances. Experimental studies (e.g. Sweeney, 1994) have demonstrated the low solubility of Zr and Ti contrasting with the high solubility of LREE in carbonatitic melts. These melts should be extremely mobile in the mantle because of their low viscosities and low dihedral angle values (Hunter & McKenzie, 1989). Consequently, carbonatitic melts should be able to separate from their residue at melt fractions as low as 0.02 vol. %, much lower than for silicate melts. Such low melt fractions should produce strong partitioning of incompatible elements into melts. The carbonatitic nature of the metasomatic melt may thus significantly amplify the effects of chromatographic melt migration. Accordingly, the originally depleted mantle peridotites beneath the Kerguelen Archipelago were considerably enriched in REE (especially in LREE), but not in Ti and Zr, through interactions with a metasomatic melt of carbonatitic composition. This explains the high (Ce/Yb)n and (La/Sm)n and low Ti/Eu and Ti/Zr values (lower than chondritic ratios) of Kerguelen peridotite xenoliths, clearly different from those of Kerguelen Archipelago lavas, including the most evolved alkaline lavas from Southeast Province [(Ce/Yb)n 21.7, (La/Sm)n < = 8, Ti/Eu < = 8077; Weis et al., 1993]. This carbonatitic metasomatism also implies important modifications of mineral assemblages (secondary ‘wehrlitic’ assemblages) and major element compositions of mineral phases (pyroxenes and spinel).
Provenance of carbonatitic metasomatic melt
The isotopic compositions of the Kerguelen harzburgite and dunite xenoliths can be used to characterize the source of the carbonatitic metasomatic melts. Mass balance calculations show that >70% of the bulk Sr and Nd abundances of all the dunite and harzburgite samples is contained in the clinopyroxene grains. Our discussion on the trace element patterns has demonstrated that the clinopyroxenes clearly show the effects of interaction with carbonatitic metasomatic melts. These melts have high concentrations of Sr, Nd and Pb and will therefore dominate the isotopic characteristics of peridotite xeno- liths. Consequently, isotopic ratios in the acid-leached whole rock and clinopyroxenes for a single sample are not distinguishable within experimental error (Table 2). Incomplete equilibration between peridotite matrix and metasomatic melt does not alter the isotopic signature imposed by the metasomatic melt. For example, a factor of up to 10 difference in incompatible element abundances between protogranular and poikilitic harzburgites does not lead to significantly different isotopic ratios. Given that the poikilitic harzburgites have recorded the most significant interactions with the metasomatic melt and that one of them presents geochemical characteristics closest to the equilibrium composition of the migrating melt, their isotopic compositions are the most suitable candidates for representing those of the carbonatitic metasomatic melt.
The age of the Kerguelen peridotite xenoliths or the metasomatic melt is not yet known. We cannot calculate crystallization ages because the xenoliths have not been closed systems and there have been significant modifications of their initial LREE abundances. Only time since metasomatism, probably a relatively recent event, can be estimated (Hassler & Shimizu, 1994). However, the evolution of isotopic ratios with time in Kerguelen peridotite xenoliths can be roughly estimated with calculations of age-corrections of their isotopic ratios for in situ decay using: (1) the age of the main volcanic activity on the archipelago [from 30 to 22 Ma (Nicolaysen et al., 1996); an average of 26 Ma is considered for the age-correction calculations] and age of the oldest rocks on the Kerguelen Archipelago (39 Ma; Cantagrel et al., 1990) and on the Plateau (115 Ma; Leclaire et al., 1987), and (2) parent–daughter abundance ratios measured by isotope dilution (ID) on acid-leached samples (Table 2-see the Appendix for leaching details). The age corrections for 208Pb/204Pb are even more complex because Th data were obtained by inductively coupled plasma mass spectrometry (ICP-MS) on unleached whole rocks. The age correction for 207Pb/204Pb is insignificant because of the low 235U concentration. The relatively low Rb/Sr, Sm/Nd, U/Pb and Th/Pb abundance ratios in the Kerguelen xenoliths generate small corrections for age-adjusted Sr, Nd and Pb isotope ratios: 0.02% for 87Sr/86Sr, 0.006% for 143Nd/144Nd and 0.2% for 206Pb/204Pb over 26 my, and 0.03%, 0.009% and 0.3%, respectively, over 39 my. For 115 my, the age corrrections are larger but still small when compared with the large range of isotopic variations for the Kerguelen xenoliths (0.3%, 0.1% and 4% in 87Sr/86Sr, 143Nd/144Nd and 206Pb/204Pb, respectively). The age corrrections do not modify the isotopic data distribution nor the interpretation of results.
As previously noted, the isotopic compositions of harzburgite xenoliths show significant correlations with those of Kerguelen Archipelago lavas, especially the flood basalts from the Southeast Charbon and Crozier Sections (Gautier et al., 1990; Weis et al., 1993, 1997; Damasceno et al., 1997; Yang et al., 1998) (Fig. 6– 8). Weis et al., (1993, 1998) and Yang et al., (1998) showed the dominance of an enriched initial isotopic signature in archipelago lavas during 30 my, supporting the interpretation of Weis et al., (1998) that the Kerguelen plume has 87Sr/86Sr = 0.7051–0.7058, 143Nd/144Nd = 0.51263–0.51249 and 206Pb/204Pb = 18.02–18.27. With age range from 24.5 to 24.9 Ma (Nicolaysen et al., 1996), lavas from the Southeast Charbon and Crozier Sections show isotopic compositions that represent exclusively that of the Kerguelen plume. Consequently, we infer that the isotopic characteristics of metasomatized Kerguelen harzburgites result from the Kerguelen plume. This implies that carbonatitic melts are associated with the alkaline magmatism produced by the Kerguelen plume. Peridotite xenoliths from other localities, for example, southeast Australia (Yaxley et al., 1991), Tanzania (Rudnick et al., 1993) and Tubuai (Hauri et al., 1993), also interpreted as having interacted with carbonatitic melts, exhibit isotopic compositions similar to young alkaline volcanic activity in their respective areas. Thus, there is growing evidence based on xenolith studies that carbonatitic metasomatism may be genetically related to relatively recent alkaline volcanism in continental and oceanic environments.
Compared with the harzburgite xenoliths, the dunites, which show an offset to low 143Nd/144Nd (Fig. 6) and distinct 206Pb/204Pb for a given 207Pb/204Pb (Fig. 8), require a more complex interpretation. Several hypotheses can be explored, as follows.
- The metasomatic melt that dominated the isotopic composition of dunite xenoliths reflects the isotopic signature of Kerguelen plume. Relative to the Kerguelen Archipelago lavas, the dunite xenoliths have Sr and Pb isotopic ratios (excluding the Pb data of 92–491) that lie within the field defined by the alkaline to highly alkaline lavas younger than 10 Ma (Fig. 6 and 7). Nevertheless, the dunite xenoliths have Nd isotopic ratios out of the range reported for all lavas and plutons exposed on the Kerguelen Archipelago (Fig. 6), and their Pb isotope ratios tend towards lower values of 206Pb/204Pb, i.e. pointing towards the field of the Kerguelen Plateau (Fig. 7 and 8). We could infer that the difference in Nd isotope ratios between the Kerguelen dunites and Kerguelen Archipelago lavas results from their difference in age. The dunite xenoliths could display less radiogenic Nd isotopic ratios because they are older relative to the archipelago lavas. Moreover, the xenoliths may present differences in age amongst themselves, also implying differences in Nd isotope ratios between xenoliths. Until we obtain age constraints for the Kerguelen xenoliths, this issue unfortunately cannot be resolved.
- The isotopic signatures of the Kerguelen dunite xenoliths show the influence of a lithospheric source component derived from the submarine Kerguelen Plateau. Relative to fields of archipelago lavas, the Kerguelen Plateau lavas (Sites 747, 748, 749 and 750) define separate but parallel 207Pb/204Pb vs 206Pb/204Pb and 143Nd/144Nd vs 87Sr/86Sr trends, essentially characterized by an offset to lower 206Pb/204Pb and 143Nd/144Nd (Fig. 6– 8). Sr–Nd isotopic compositions of the dunites partly overlap those of the Site 747 and 748 lavas, which is not the case for Pb isotopic data. The difference in Pb isotopes would be enhanced if the data for the Kerguelen Plateau and xenoliths were age-corrected for 110 Ma [age for ODP (Ocean Drilling Program) Site 747 basalts, Whitechurch et al., (1992)], because of higher U/Pb ratios in the Kerguelen Plateau basalts relative to the xenoliths. Compared with the other Kerguelen peridotite xenoliths, the dunites display the lowest values in 206Pb/204Pb, close to those of the Kerguelen Plateau basalts. For the dunite 92–491, with low 206Pb/204Pb and distinctly higher 207Pb/204Pb and 208Pb/204Pb ratios falling within the range of those for the Kerguelen Plateau lavas (Fig. 8), the influence of a component derived from the Kerguelen Plateau is much more obvious (see the next hypothesis for more discussion of this sample). We are aware that the xenoliths were collected on the Kerguelen Archipelago, which lies on the northern Kerguelen Plateau, a Cenozoic feature (Coffin & Eldholm, 1994; Weis et al., 1999), in contrast to the Mesozoic southern Kerguelen Plateau. Recent isotopic data from three dredged Kerimis sites (Weis et al., 1999) on the northern Kerguelen Plateau show relatively high 87Sr/86Sr, and low 143Nd/144Nd and 206Pb/204Pb values that are matched remarkably well by the isotopic ratios of the dunite xenoliths.
- The specific Pb isotopic composition (high 207Pb/204Pb and 208Pb/204Pb for a given low 206Pb/204Pb) of the dunite 92–491 may reflect the imprint of a component derived from continental lithosphere. Despite the strong impact by a metasomatic melt evidenced by LREE enrichments (Fig. 5b), sample 92–491 shows the lowest Pb concentration of all the investigated xenoliths (Table 2), which could account for its distinct isotopic composition. As the ratio of Pb concentrations between the upper continental crust and mantle is much higher than those for Sr and Nd (Rudnick & Goldstein, 1990), Pb concentrations and isotope ratios are much more sensitive to continental contamination than either Sr or Nd. High 207Pb/204Pb is one characteristic typical of ancient crustal rocks (e.g. Mahoney et al., 1995) and the Pb isotopic ratios of 92–491 may indicate a continental affinity. Moreover, the Pb isotopic data of 92–491 fall within the range of values reported for the Rajmahal traps (eastern India) and Bunbury lavas (Western Australia), which reflect a continental lithosphere imprint (Mahoney et al., 1995; Frey et al., 1996) (Fig. 8), i.e. they have relatively high 207Pb/204Pb and 208Pb/204Pb for a given lower 206Pb/204Pb.
When isotopic data from the northern Kerguelen Plateau itself (ODP Leg 183) are reported, we will be able to distinguish between the hypotheses for interaction with the Kerguelen Plateau or for the input of a continental component to account for the differences in 206Pb/204Pb between the dunites and the harzburgites. The continental lithosphere influence, more obvious for sample 92–491, will be discussed in more detail in light of the isotopic data from 91–114.
Continental lithospheric component signature
Dunite 91–114 represents a ‘fertile’ piece of the lithosphere beneath the Kerguelen Archipelago. With the lowest mg-number (87), the dunite 91–114 shows the least refractory bulk composition of all the investigated nodules (Grégoire et al., 1997). The leached dunite 91–114 has very low 206Pb/204Pb (17.713) and 143Nd/144Nd (0.51214), and high 87Sr/86Sr (0.70721). The leached 91–114 clinopyroxene, however, shows lower 87Sr/86Sr (0.70561) and higher 143Nd/144Nd (0.51246) (Table 2 and Fig. 6– 8). Mass balance calculations show that <20% of the bulk Sr, Nd and Pb abundances resides in clinopyroxene, depleted in LREE, and therefore this phase does not dominate the isotopic composition of the whole rock. As 87Sr/86Sr is higher in the bulk rock than in the clinopyroxenes, the more radiogenic Sr component may reside in an intergranular contaminant phase. The leaching procedure was, however, specifically designed to remove any alteration or intergranular phase.
The Sr and Nd isotopic composition of 91–114 is the most enriched yet measured for any lava, pluton or anhydrous xenolith from the Kerguelen Archipelago (Vance et al., 1989; Weis et al., 1993, 1998; Yang et al., 1998) (Fig. 6). Hassler & Shimizu, (1994) reported one hydrous xenolith (a phlogopite-bearing clinopyroxenite from the Courbet Peninsula) with another extreme isotopic composition (87Sr/86Sr = 0.7087 and 143Nd/144Nd = 0.51200 for clinopyroxene separates). The isotopic composition of 91–114 points towards the isotopic field of Site 738 in the southern Kerguelen Plateau, which clearly shows a continental signature (Mahoney et al., 1995). We infer that the isotopic and geochemical characteristics of 91–114 reflect the influence of a component derived from the continental lithosphere. During Gondwanaland break-up, some continental lithosphere material may have been incorporated into the newly forming Indian Ocean lithosphere or deeper into the mantle and entrained by plume materials (Storey et al., 1989, 1992; Mahoney et al., 1995; Hassler & Shimizu, 1998). Depletions in Nb and Ta relative to La and Th are particularly diagnostic indicators of continental crust signature in volcanic rocks (Mahoney et al., 1995), and high 207Pb/204Pb and 87Sr/86Sr coupled with low 143Nd/144Nd are typical characteristics of ancient continental lithosphere. The very high (La/Ta)p (7.3) coupled with the high 207Pb/204Pb (16.29) and 87Sr/86Sr of the dunite 91–114 indicate a continental lithosphere affinity.
None of the volcanic or plutonic rocks on the Ninetyeast Ridge and the Kerguelen Archipelago have geochemical signatures indicating a continental influence (Weis et al., 1993, 1998; Weis & Giret, 1994; Frey & Weis, 1996; Yang et al., 1998). There is, however, evidence of assimilation of a continental component in young alkaline lavas from Heard Island (Barling et al., 1994) and in the lavas at ODP Site 738 on the southernmost part of the Kerguelen Plateau, nearest the Antarctic continental margin (Alibert, 1991; Mahoney et al., 1995). These plateau lavas have significant relative depletions in Nb and Ta [with (La/Ta)p and (La/Nb)p 2], and high 207Pb/204Pb, 87Sr/86Sr coupled with low 143Nd/144Nd and 206Pb/204Pb (Fig. 6– 8). The southern tip of the Kerguelen Plateau (Site 738) together with other Indian Ocean volcanic features associated with the early volcanic activity of the Kerguelen plume, such as the Rajmahal traps in India, the Naturaliste Plateau and Bunbury lavas in western Australia and Broken Ridge, were emplaced in new ocean basins formed by continental break-up, i.e. on continental margins or in proximity to rifted continental margins. As a result, some of these lavas present geochemical signatures reflecting a continental imprint (Mahoney et al., 1983, 1995; Frey et al., 1996). The isotopic ratios of the dunite 91–114 are comparable with those of the most contaminated lavas of these Indian Ocean volcanic features (such as eastern Broken Ridge and Naturaliste Plateau) (Fig. 8).
Our Sr–Nd–Pb isotopic results for the dunite 91–114 support the Re–Os evidence (Hassler & Shimizu, 1998) for a continental lithosphere component beneath the northern Kerguelen Plateau. Indeed, our results are consistent with the interpretation based on unradiogenic Os and ancient Re-depletion ages (to 1.36 Ga) obtained on peridotites from the Courbet Peninsula (Hassler & Shimizu, 1998). We propose therefore that a component derived from continental lithosphere was incorporated into the mantle below Kerguelen, probably during the break-up of Gondwanaland. Alternatively, unexplored parts of the Kerguelen Plateau may include Gondwanaland continental lithosphere pieces. The recent ODP Leg 183, which drilled six sites on the plateau, should document this alternative. As none of the Ninetyeast Ridge and Kerguelen Archipelago lavas show a clear geochemical signature of a continental lithosphere component, this component may be spatially restricted to the lower lithosphere beneath the Kerguelen Plateau or to specific parts of the plateau (southern tip and unexplored parts).
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Temporal Evolution of Kerguelen Peridotite Xenoliths |
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TOPABSTRACTIntroductionGeological SettingPetrographyGeochemistryDiscussionTemporal Evolution of Kerguelen...Appendix: Analytical ProceduresReferences |
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Trace element abundances of Kerguelen peridotite xenoliths show a large variability both on a small scale (sample size) and a larger scale (single locality and the whole province), indicating the complex and multi-stage petrogenetic evolution of these xenoliths. An early formation stage of the Kerguelen peridotite xenoliths by mantle partial melting is reflected by relatively strong depletions in basaltic major element components, particularly in protogranular harzburgites. However, evidence of a partial melting event is overprinted by subsequent chemical modifications. All petrographic, geochemical and isotopic features converge to indicate that the residual Kerguelen peridotite xenoliths interacted with at least two types of metasomatic melts: (1) a basaltic melt, as evidenced in the lherzolite 92–372; (2) a carbonatitic melt that generated the crystallization of new clinopyroxene grains and produced a secondary wehrlitic assemblage in poikilitic harzburgites and dunites.
Extremely variable and high incompatible element abundances in the clinopyroxenes from the poikilitic harzburgites and dunites probably resulted from chromatographic fractionation effects associated with porous flow percolation of carbonatitic melt through peridotite mantle matrix. Trace element heterogeneities observed on centimetre-scales in harzburgites (e.g. sample 91–8) could not have survived for a long period of time at mantle temperatures, suggesting that the carbonatitic melt invasion occurred relatively shortly before the eruption of the host lava with their xenoliths. This is supported by a metasomatic event age estimated at 20 Ma by Hassler & Shimizu, (1994) from a ‘two-point Rb–Sr isochron’ measured on a phlogopite-bearing clinopyroxenite xenolith from the Courbet Peninsula. Assuming that the melt inclusions do not influence geochemical compositions analysed in the clinopyroxenes from Kerguelen dunite and harzburgite xenoliths, the similarity of trace element patterns obtained for clinopyroxenes and silicate–carbonate melt inclusions suggests that clinopyroxenes and inclusions may be produced from a common carbonatitic metasomatism occurring at successive evolution stages of the Kerguelen xenoliths. Isotopic signatures of the Kerguelen harzburgite and lherzolite xenoliths indicate that both the basaltic metasomatic melt and carbonatitic metasomatic melt are related to the alkaline magmatism produced by the Kerguelen plume.
To summarize the main elements of the history for Kerguelen peridotite xenoliths, a simplified model is illustrated in Fig. 10. The upwelling of plume material induced deformation of the peridotite mantle matrix. The permeability and porosity were modified and fracture planes were created, which facilitated the transport of metasomatic melts through the mantle matrix. Schiano et al., (1994) showed that the entrapment of the melt or fluid inclusions occurred at a minimum trapping pressure of 12.5 kbar at 1250°C, before or synchronous with peridotite deformation. Reactions between peridotites and infiltrated melts occurred, especially at decreasing melt mass, producing small fractions of metasomatic melts enriched in volatiles and incompatible elements. Because of their low viscosity, these melts migrated upwards and produced drastic changes in composition.
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Our isotopic study has allowed us to trace the source of heterogeneities in the Kerguelen peridotite xenoliths. The results testify to the importance of the Kerguelen plume for producing variable mantle metasomatic episodes in addition to being a major mechanism for entraining diverse source components. Sr, Nd and Pb isotopic data from sample 91–114 show the strong influence of a continental lithophere component that may also have affected, but to a lesser extent, the dunite 92–491. Therefore, some of the lithospheric materials beneath the archipelago carry signatures of continental contamination and may act as contaminants for Kerguelen lavas. However, as is consistent with previous studies on plutonic and volcanic rocks from the Kerguelen Archipelago (Weis & Giret, 1994
; Weis et al., 1998; Yang et al., 1998), our results show that this continental component is spatially restricted. The ODP Leg 183 should constrain this component in space and in composition. |
Appendix: Analytical Procedures |
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TOPABSTRACTIntroductionGeological SettingPetrographyGeochemistryDiscussionTemporal Evolution of Kerguelen...Appendix: Analytical ProceduresReferences |
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Trace element and REE concentrations in whole rocks were analysed by inductively coupled plasma-mass spectrometry (ICP-MS) using a VG PlasmaQuad PQ2 (Musée Royal de l'Afrique Centrale, Tervuren) (André & Ashepkov, 1996
). The samples were prepared with alkaline and acid dissolution, under ultraclean conditions. Five international standards (PCC1, DST1, W1, ANG, BHVO1) were calibrated externally and accuracy was checked by analysis of other international standards (Nim-N, Nim-D, UB-N). Because of the limited database on the low REE abundances in the ultramafic standards, we checked the reproducibility of the analyses using sample duplicates. For REE abundances >1 ppm, 1–0.1 ppm, <100 ppb and <10 ppb, the reproducibility of measurements corresponds respectively to 5%, 10–15%, 50–100% and >100% of the measured value. Clinopyroxenes were analysed in situ for Sr, Zr, Ti and REE (La, Ce, Nd, Sm, Eu, Dy, Er and Yb) by secondary ion mass spectrometry (SIMS), using a Cameca IMS-3f ion microprobe at the Woods Hole Oceanographic Institution (Shimizu & Hart, 1982). The accuracy of the results is within ±5–7% for Ti, Sr, Y and Zr, and ±10% for REE.
A subset of fresh xenoliths was carefully selected for isotopic analysis to encompass the numerous petrographic types and the large range of major and trace element compositions. To avoid contamination effects produced during interaction with the host basalts, we used only the central parts of the samples. Reduced to fragments (2–3 cm in diameter) with a hammer that was covered by thick plastic sheets, the samples were not sawn, to avoid metal splinters. We crushed about 200 g of each sample in an agate jar and/or mortar, which were demonstrated (Frey et al., 1991) to be devoid of detectable contamination effects, especially for Pb isotopes. After homogenization of the coarse-grained powder, one fraction reserved for whole-rock analyses was crushed in an agate jar until the mean grain size reached 100–250 µm; the remaining coarse fraction was stored for later mineral separation. All sample powders were rinsed with methanol. Mineral phases were concentrated with a Franz magnetic separator. Clinopyroxene grains were selected by hand-picking in methanol with a binocular microscope (with further crushing to finer grain sizes) to remove all heterogeneous crystals with cracks or cloudy surfaces.
Trace element analyses were performed on unleached powders. For isotopic analysis, the samples were leached to remove secondary alteration phases. For whole-rock powders, we used a ‘cold’ acid leaching procedure comparable with that of Mahoney (1987) and Weis & Frey (1991), i.e. elimination of fine-grained material by repeated removal of the 6 N HCl immediately after being placed for 30 min in an ultrasonic bath. This leaching procedure was repeated until the yellow (Fe) colour of the solution was gone. We controlled the strength (time and acid quantities) of the leaching, to remove alteration phases without destroying the crystal structures (especially for olivine). Weight losses were typically about 25%, although for the dunites they reached 50%. The different leachate solutions were stored.
A different leaching procedure was performed on the separated minerals. Adapted from the literature (e.g. Polvé, 1983; Zindler & Jagoutz, 1988), it consisted of four major steps: (1) crystal surface cleaning and elimination of fines by removal of the ‘hot’ acid solutions (2.5 N HCl and 6 N HCl) immediately after 30 min on a hot plate (130°C); (2) grain cleaning with cold 5% HF; (3) rinse with 2.5 N HCl; (4) after each step, storage of the leachate solution and careful grain rinse with quartz-distilled water. Weight losses were typically about 20%.
Two different dissolution procedures were chosen to minimize the contamination risks for Pb and to dissolve resistant minerals, such as spinel and garnet. A mixture of HF and HNO3 (in a proportion of 4:1) was added to the sample powder in a Savilex beaker placed on a hot plate (180°C) for 10 days. After drying, 6 N HCl was added several times and slowly evaporated. For powders including resistant minerals, HClO4 was added to the mixture of HF + HNO3. After 10 days, undissolved residues were transferred to a Teflon bomb with HF + HNO3. The bomb was placed into a stainless steel closed cylinder and then in an oven (180°C) for a week. Finally, after evaporation, 6 N HCl acid was added to the residue in the bomb, which was stored again in the oven for 3 days. The reagents were purified twice in a sub-boiling still.
The final attack solutions were in 6 N HCl, and were centrifuged and then split into two aliquots. The type and amount of spike to be added were determined to minimize the error magnification factors (see De Bièvre & Debus, 1965; J. Barling, personal communication, 1994) and the mass discrimination error. We used mixed spikes for 204Pb-235U and 148Nd-150Sm, and single spikes for 84Sr and 87Rb. The samples were processed following standard chemical separation procedures on anion exchange columns (Weis & Frey, 1991). For Pb and U, the entire chemical procedure was performed in a clean, over-pressurized (>3 mmHg) laboratory. Total blank values for Pb for the entire chemical procedure were 0.5 ng when the ‘normal’ dissolution procedure was applied and 0.7 ng when dissolution required HClO4 acid. Total blank values for Sr and Nd were typically below 1 ng. Blank values for the column separations were 10 times less than the total blank values, which are essentially controlled by the dissolution procedure.
Elemental concentrations (Rb, Sr, Sm, Nd, U and Pb) were determined by ID on a Finnigan Mat 260 mass spectrometer at the Université Libre de Bruxelles. Pb and U concentrations were measured in the temperature range of 1050–1150°C. Standard deviations for Rb, Sr, Sm, Nd, U and Pb concentrations were <1% (in some analytical Rb and U runs, they were 4%). Fractionation corrections for Rb, Sr, Sm and Nd were calculated on the basis of the Boelrijk (1968) equations. For Pb, the measured ratios were corrected by a factor of 0.13% ± 0.05% per a.m.u. on the basis of repeated analyses of the NBS981 Pb standard [see Weis & Frey (1991)]. Sr and Nd isotopic compositions were measured in the dynamic mode on a VG Sector 54 multicollector mass spectrometer at the Université Libre de Bruxelles. We loaded Sr onto single Ta filaments with 1 M H3PO4 and Nd on triple Ta–Re filaments with 0.1 M H3PO4. For each run, the 146–145–144–143 and 88–87–86–84 isotopes were normalized to 146Nd/144Nd = 0.7219 and 86Sr/88Sr = 0.1194, respectively. Repeated measurements of standards yielded 143Nd/144Nd = 0.511732 ± 10 and 145Nd/144Nd = 0.348409 ± 4 (2
m on 32 measurements) for the Merck Nd standard and 87Sr/86Sr = 0.710274 ± 6 (2m on 32 measurements) for the NBS987 Sr standard. Pb isotopic compositions were measured in static mode on the same VG Sector 54 multicollector mass spectrometer. Pb was loaded onto single Re filaments using the H3PO4–silica gel technique (e.g. Cameron et al., 1969). All the Pb isotopic analyses were performed in the temperature range of 1080–1160°C after 50 min of warm-up and a 10 min pause at 1050°C. All the results were corrected for mass fractionation by repeated analyses of the NBS981 Pb standard [0.12 ± 0.006% per a.m.u. (2m, n = 18)]. Between-run precision was better than 0.1% for 206Pb/204Pb and 207Pb/204Pb, and better than 0.15% for 208Pb/204Pb.
The leaching, dissolution and chemical separation procedure were duplicated for nearly all the samples and in some cases triplicated. This provided Sr, Nd and Pb isotopic compositions as well as ID results that are consistently reproducible and identical within experimental error (Table 2). The isotopic composition differences for duplicates are as small as 7 x 10–6 for 87Sr/86Sr, 2 x 10–6 for 143Nd/144Nd and 12 x 10–3 for 206Pb/204Pb.
It may be possible to characterize the composition of the intergranular contaminant phase that was present in the natural unleached xenoliths by analysing trace element abundances and isotopic compositions in leachate solutions, unleached and leached splits for a single sample. Comparison of ICP-MS data on unleached whole rocks and ID data on leached whole rocks shows no significant difference for Nd and Sm concentrations within experimental error. In contrast, for Sr and Rb, unleached whole rocks have respective abundances up to 11 and 62 times higher than those of leached whole rocks. For U and Pb, we excluded any ICP-MS ID comparisons because of the low concentrations in these elements close to the detection limit of ICP-MS technique. ID data on leachate solutions reveal lower abundances in Sm, Nd and Sr (not in Rb) than those of leached splits. Rb concentrations are nevertheless lower in the leachate solutions relative to unleached splits. Leachate solutions and unleached splits actually show isotopic ratios systematically higher in Sr (from 1 x 10–4 to 1.2 x 10–3) and lower in Nd than those of leached splits for a single sample. We attribute the discrepancies in concentration and isotopic composition between acid-leached and unleached splits to the presence of a contaminant phase, rich in Rb-Sr and poor in Sm-Nd, that apparently derives from interaction of the nodules with local groundwater and/or the host magma.
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Acknowledgements |
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We thank F. A. Frey for his careful and constructive comments on an earlier version of this manuscript, D. H. Lindsley for his judicious remarks on the current manuscript, and E. A. Dunworth, A. Le Roex and an anonymous reviewer for their detailed and constructive journal reviews. We would like to thank L. André for ICP-MS REE analyses at the ‘Musée Royal de l'Afrique Centrale’. This research programme was supported by the Belgian FNRS (Fonds National de la Recherche Scientifique) (FNRS 1.5.030.91 [EC] F and 1.5.019.95 [EC] ), and we thank the ‘Mission de Recherche’ of the IFRTP (Institut Français pour la Recherche et Technologie Polaires) for critical support in the field.
* Corresponding author. Telephone: +32-2-650-22-69. Fax: +32-2-650-22-26. e-mail: nmattiel{at}ulb.ac.be
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  D. Weis, S. Ingle, D. Damasceno, F. A. Frey, K. Nicolaysen, J. Barling, and Leg 183 Shipboard Scientific Party Origin of continental components in Indian Ocean basalts: Evidence from Elan Bank (Kerguelen Plateau, ODP Leg 183, Site 1137) Geology, February 1, 2001; 29(2): 147 - 150. [Abstract] [Full Text] [PDF]
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