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Journal of Petrology Volume 42 Number 11 Pages 2145-2167 2001
© Oxford University Press 2001
High Field Strength Element Fractionation in the Upper Mantle: Evidence from Amphibole-Rich Composite Mantle Xenoliths from the Kerguelen Islands (Indian Ocean)
1DÉPARTEMENT DE GÉOLOGIE–UMR 6524, UNIVERSITÉ J. MONNET, 42023 SAINT-ETIENNE, FRANCE
2GEMOC ARC NATIONAL KEY CENTRE, DEPARTMENT OF EARTH AND PLANETARY SCIENCES, MACQUARIE UNIVERSITY, 2109 N.S.W., AUSTRALIA
3DÉPARTEMENT DE SCIENCES DE LA TERRE–UMR 5570, ENS-LYON, 69364 LYON, FRANCE
Received June 19, 2000; Revised typescript accepted May 16, 2001
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONGEOLOGICAL SETTING AND XENOLITH...ANALYTICAL METHODSPETROGRAPHYMAJOR AND TRACE ELEMENT...DISCUSSIONCONCLUSIONSREFERENCES |
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A basanite dyke in the Kerguelen Archipelago contains abundant composite mantle xenoliths consisting of spinel-bearing dunites cross-cut by amphibole-rich veins. Two types of veins (thick and thin) have been distinguished: the thick veins represent almost complete crystallization products of highly alkaline melts similar to the host basanites, whereas thin veins are precipitates from fractionates of the parental melts to the thick veins. These fractionated fluids are enriched in H2O relative to the parental melts. The amphiboles in the thin veins are lower in Ti and higher in Nb, Ta, Zr and Hf than amphiboles in the thick veins. This fractionation of high field strength elements (HFSE) is consistent with a combination of the changing composition of the fractionated fluids and the change in intrinsic amphibole–fluid partition coefficients for HFSE in fluids with higher aH2O and lower aTiO2. The trace element content of amphiboles disseminated in dunitic wall-rocks is closely related to the composition of adjacent veins and thus these amphiboles are precipitates from fluids percolating into the dunite from the veins. Disseminated amphibole reflects the composition of the percolating melt, which is similar to that of the associated veins.
KEY WORDS: mantle amphibole; Kerguelen; HFSE fractionation; mantle HFSE; mantle xenoliths
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL SETTING AND XENOLITH...ANALYTICAL METHODSPETROGRAPHYMAJOR AND TRACE ELEMENT...DISCUSSIONCONCLUSIONSREFERENCES |
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Composite ultramafic xenoliths in volcanic rocks provide unique information about the extraction, interaction and migration processes of melts in the upper mantle (e.g. Irving, 1980
; Nielson & Wilshire, 1993; Witt-Eickschen et al., 1998). Studies of such xenoliths have improved our understanding of trace-element fractionation processes involved in the geochemical evolution of the upper mantle. In addition, amphibole, ilmenite and phlogopite are metasomatic phases that play a crucial role in the dispersion and residence of trace elements in the upper mantle because they incorporate (sometimes exclusively) some of the important trace elements such as high field strength elements (HFSE; Ti, Zr, Hf, Nb, Ta) or large ion lithophile elements (LILE; Ba, Sr) that are incompatible in olivine–orthopyroxene–clinopyroxene–spinel peridotites (e.g. Griffin et al., 1988; O’Reilly & Griffin, 1988; O’Reilly et al., 1991; Ionov et al., 1997). We report in this paper results of a major- and trace-element study of constituent minerals in composite mantle xenoliths of spinel-bearing dunites cross-cut by amphibole-rich (± ilmenite ± phlogopite) veins. Our data provide new insights into the distribution and fractionation of trace elements between these veins and their mantle wall-rocks at a centimetre scale and on the element partitioning between minerals within the veins (amphibole, ilmenite and phlogopite). This knowledge is crucial to understanding the chemical budget of modal metasomatic processes.
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GEOLOGICAL SETTING AND XENOLITH OCCURRENCE |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL SETTING AND XENOLITH...ANALYTICAL METHODSPETROGRAPHYMAJOR AND TRACE ELEMENT...DISCUSSIONCONCLUSIONSREFERENCES |
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The Kerguelen Islands are situated at the northern end of the Kerguelen oceanic plateau (25 km x 106 km). They have evolved by basaltic activity from a location near the SEIR (South East Indian Ridge, Iceland-type setting) to a present-day intraplate setting above the Kerguelen plume (Hawaiian-type setting); magmatic activity has extended over 45 m.y. (Giret, 1993
; Giret et al., 1997).
Dykes that form part of the youngest alkaline volcanic activity of the islands (Weis et al., 1993) crop out in the southeastern province of the Kerguelen Archipelago (Jeanne d’Arc and Ronarc’h Peninsulas; Fig. 1). A basanitic dyke related to this young magmatic activity from the Phonolite Valley (Ronarc’h Peninsula) contains the composite mantle xenoliths (dunites with amphibole-rich veins). The xenoliths have sub-rounded shapes and range in size from a few centimetres to 10 cm. Three major xenolith types are very abundant and have been described by Grégoire et al. (1997, 1998, 2000a,2000b) and Moine et al. (1998, 2000). They comprise: (1) amphibole ± phlogopite-bearing dunites and harzburgites that represent mantle wall-rock; (2) lherzolites, websterites and metagabbros that are Al-augite series or Type 2 xenoliths (Wilshire & Shervais, 1975; Frey & Prinz, 1978) representing high-pressure crystallization of basaltic melts of tholeiitic–transitional affinity; (3) alkaline clinopyroxenites that correspond to deep segregates of alkali basaltic affinity.
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The composite xenoliths that are the focus of this study are dunites with amphibole-rich veins from the first group and are described in detail below.
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ANALYTICAL METHODS |
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Only one vein (sample MG91-144) was large enough to be analysed for its bulk-rock composition. Its central part, without any olivine grains from the wall-rock peridotite, was ground in an agate mill. Major and minor elements (Cr, Ni, Sc, V, Co, Cu and Zn) in the bulk-rock were analysed by X-ray fluorescence spectrometry (XRF) at the ‘École des Mines’ at St Etienne (France) using the method of Gruffat (1992)
. The concentration of trace elements in the bulk-rock sample MG91-144 [rare earth elements (REE), Li, Be, Ga, Mo, Cs, Ba, Cs, Rb, Th, U, Nb, Ta, Pb, Sr, Zr, Ti, Y] were analysed by solution inductively coupled plasma mass spectrometry (ICP-MS) at the University of Grenoble (France) using the method of Barrat et al. (1996). The modal composition of sample MG91-144 was calculated by mass balance (based on the major element contents of bulk-rock and constituent minerals) (see Table 1).
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Major-element compositions of the constituent minerals of the xenoliths were determined by a Cameca Camebax SX 50 microprobe at Macquarie University (Australia) using wavelength-dispersive spectrometry (WDS), with 15 kV accelerating voltage, sample current of 20 nA, a beam diameter of 2–3 µm, and natural and synthetic minerals as standards. Matrix corrections were made by PAP procedures (Pouchou & Pichoir, 1984). Counting times were 20–40 s and no values are reported below detection limits (0·01–0·04 wt %).
Concentrations of 29 trace elements (REE, Ba, Rb, Th, U, Nb, Ta, Pb, Sr, Zr, Hf, Ti, Y, Sc, V, Co and Ni) in minerals were determined in polished sections (100–200 µm thick) using a Perkin Elmer Elan 6000 ICP-MS system coupled with a Continuum Surelite I-20 Q-switched Nd:YAG laser ablation system (LA-ICP-MS) at Macquarie University. Crater size is 15–50 µm. Detection limits range from 10 ppb for U and Th, to 2 ppm for Ni. The NIST 610 glass standard was used for calibration of relative element sensitivities and each analysis was normalized using Al2O3 (spinel), MgO (olivine, phlogopite, amphibole) and TiO2 (ilmenite) values determined by electron microprobe as an internal standard. A more detailed description of laser operating conditions, calibration values for the NIST 610 glass standard and error analysis has been given by Norman et al. (1998).
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PETROGRAPHY |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL SETTING AND XENOLITH...ANALYTICAL METHODSPETROGRAPHYMAJOR AND TRACE ELEMENT...DISCUSSIONCONCLUSIONSREFERENCES |
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The composite mantle xenoliths are spinel-bearing dunites with cross-cutting amphibole-rich veins. These dunites differ from most Kerguelen dunites by the lack of clinopyroxene (Grégoire et al., 2000b
). Three samples (MG91-144, Arc83-01 and MG91-143; Figs 2 and 3) display phlogopite-poor and ilmenite-bearing thick amphibole-rich veins (5–12 mm across) commonly associated with thin phlogopite-free and ilmenite-free amphibole veins (<5 mm across; MG91-144 and MG91-143). The thin veins are commonly connected to the thicker ones and the width of the thick vein may decrease near the contact with the thin veins (see Fig. 2a). The thin connected veins decrease in width away from the thick veins. All veins have sharp, straight contacts with the wall-rocks indicating fluid movement (and vein formation) by crack propagation (e.g. Wilshire, 1987; O’Reilly, 1989). Thick veins do not display evidence of zoning. The thick veins have a porphyroblastic microstructure characterized by large porphyroblasts (original phenocryst) of cloudy amphibole (up to 8 mm across) surrounded by small, clear neoblasts of amphibole (<500 µm across). Amphibole porphyroblasts have light brown cores showing ilmenite exsolution and contain abundant phlogopite inclusions; they have a brown rim free of inclusions and exsolution. Ilmenite and rare phlogopite grains also occur interstitially in the neoblastic part of the thick veins. The thin hornblendite veins have a granoblastic microstructure and contain only light brown amphibole (500 mm across). Light brown amphibole occurs in the dunitic wall-rocks adjacent to both the thick and thin veins as interstitial grains surrounding spinel. The abundance and size of these amphibole grains decreases away from the veins. The dunite wall-rocks display a fine- to coarse-grained granoblastic microstructure with olivine ranging from 0·1 to 1 mm across. Spinel occurs as small interstitial rounded or subhedral grains (500 µm) and is corroded when surrounded by amphibole.
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As this paper focuses on the chemical variation in amphibole from the thick and thin veins and in the dunitic wall-rock, each sample is described in detail below.
Sample MG91-144 contains two thin veins (0·1–1 mm) connected to a thick vein (8–12 mm). One section shows a sharp contact zone between the thick vein and the dunite wall-rock (MG91-144.1, Fig. 2a). Another shows the intersection of the two thin veins with the thick vein, which thins near the connection point (MG91-144.2, Fig. 3a). The thick vein contains abundant ilmenite concentrated in the porphyroblastic zone.
Sample ARC83-01 shows a thick vein of variable thickness (5–10 mm) containing numerous olivine xenocrysts inferred to have been dislodged from the dunite wall-rock (Fig. 2d).
Sample MG91-143 displays a thin vein (1–3 mm) connected to a thick vein (8–10 mm). One section shows the two veins separated by 3 cm (MG91-143.2, Fig. 4a) whereas another section (cut 6 cm from the thick vein) contains only the thin vein (MG91-143.1, Fig. 5a).
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In summary, thick veins are characterized by the presence of zoned amphibole porphyroblasts with pale, cloudy cores (containing ilmenite and phlogopite inclusions) and dark rims; rare phlogopite and ilmenite may occur interstitially. Thin veins contain only granoblastic pale amphibole with no ilmenite or phlogopite inclusions or interstitial grains. Textural relationship in the thick vein and between thick and thin vein excludes successive injection in the vein.
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MAJOR AND TRACE ELEMENT COMPOSITIONS OF MINERALS IN THE COMPOSITE XENOLITHS |
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Dunite wall-rocks
Olivines in the dunite wall-rocks of the composite xenoliths have very homogeneous Fo contents, which are lower (87–88) than those of olivines in the Kerguelen mantle dunites that do not contain amphibole-rich veins (Fo 88–91, Grégoire et al., 1997
). They display low Cr contents (40–85 ppm) and their Ca (620–785 ppm) and Al (30–50 ppm) contents are within the range of olivines from other Kerguelen mantle dunites and harzburgites (Grégoire et al., 2000b).
Spinels are Mg–Al chromites [mg-number 32–42, where mg-number = 100 x Mg/(Mg + Fetot); cr-number 45–57, where cr-number = 100 x Cr/(Cr + Al)]. Their MgO and Al2O3 contents decrease (MgO from 12 to 9·8 wt % and Al2O3 from 24 to 14 wt %) and their FeO and TiO2 contents increase (FeO from 29·9 to 38 wt % and TiO2 from 0·5 to 4 wt %) in the vicinity of the amphibole-rich veins (Fig. 6). Spinels are commonly high in V (783–1820 ppm), Co (358–528 ppm) and Ni (1820–2790 ppm), and systematically low in Sc (0·4–6·6 ppm). These characteristics are similar to those of spinels of Kerguelen mantle peridotites (Grégoire et al., 2000b).
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Amphibole veins and adjacent dunite wall-rocks
Summary
Ilmenites in the thick veins are MgO rich (11·5–12·6 wt % with 39–48 mol % geikielite). Ilmenite from sample MG91-144 is high in Nb (471 ppm), Ta (40 ppm), Zr (245 ppm) and Hf (4·3 ppm), and its trace element pattern (Fig. 7) is therefore characterized by large positive Nb, Ta, Zr and Hf anomalies. It also contains significant amounts of Sc (26·5 ppm), V (610 ppm), Cr (4785 ppm), Co (235 ppm) and Ni (1675 ppm).
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Phlogopite inclusions in the pale cloudy cores of the amphibole in the thick veins have mg-numbers of 87–87·5. Phlogopites are high in TiO2 (
4·4 wt %) and low in Cr2O3 (<0·5 wt %). They are characterized by very low REE contents (La 0·1 ppm, Yb 0·06 ppm) and significant LILE (Rb 575–600 ppm, Ba 3665–3925 ppm, Sr 150–160 ppm, Pb 4·75–5·35 ppm) and HFSE (Nb 24·5–40 ppm, Ta 1·05–2·90 ppm, Zr 5·5–11 ppm, Hf 0·25–0·35 ppm) contents. Phlogopite trace element patterns (Fig. 7) are therefore characterized by large positive Rb, Ba, Nb, Ta, Pb, Sr and Ti anomalies, and slightly positive Zr and Hf anomalies. Phlogopite also displays significant Ga (67–188 ppm) and Ni (1496–1567 ppm) contents.
Amphibole shows wide variations in trace element composition between thick veins, thin veins and dunitic wall-rock. Its composition (Fig. 8) ranges from Cr-poor and Ti-rich pargasite (Cr2O3 0·4 wt %, TiO2 3·7 wt %, mg-number 81) in the thick veins to Cr-rich and Ti-poor pargasite (Cr2O3 1·65 wt %, TiO2 0·9 wt %, mg-number 84) in the adjacent dunite wall-rock and finally to Cr-poor and Ti-poor pargasite (Cr2O3 0·34 wt %, TiO2 0·84 wt %, mg-number 84·4) in thin veins. Optical zoning in amphibole porphyroblasts represents a very small core–rim variation in the titanium content (TiO2 3·5–3·7 wt %).
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Detailed compositional variations and traverses
Amphibole shows significant and consistent differences in composition depending on its microstructural location in thick veins, thin veins or in dunite adjacent to veins.
Amphiboles in the dunitic wall-rocks near the thick veins of phlogopite- and ilmenite-bearing hornblendite (e.g. MG91-144) show similar trace element patterns but lower incompatible trace element abundances compared with amphibole in the thick veins (Fig. 3b and c).
The trace element content of amphibole in sample MG91-144.2 (Table 2, Figs 3 and 9) decreases from the core of the thick vein to the dunitic wall-rock on a centimetre scale (Nb 65–40 ppm, Ta 3·5–1·15 ppm, Zr 147–78·5 ppm, Hf 3·65–1·70 ppm, La 62–30 ppm, Sm 9·70–6·05 ppm, Yb 2·50–1·75 ppm). The amphibole of the thin veins in sample MG91-144.1 (Fig. 2 and Table 3) is lower in TiO2 (2·20 wt %) and REE (La 36·4 ppm, Sm 7·40, Yb 2·25 ppm) and higher in HFSE (Nb 113 ppm, Ta 14 ppm, Zr 263 ppm, Hf 9·40 ppm) than the amphibole of the thick vein (TiO2 3·30 wt %, Nb 78 ppm, Ta 5·6 ppm, Zr 204 ppm, Hf 6·4 ppm).
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Amphibole in the thick vein of sample ARC83-01 shows similar trace-element contents (Table 4) to those of the thick veins of samples MG91-144.1 and -144.2 (Tables 2 and 3). The amphiboles in the thin vein in sample MG91-143.1 (Fig. 5 and Table 5) are high in HFSE (Nb 221 ppm, Ta 16·5 ppm, Zr 410 ppm, Hf 12 ppm). In contrast, the HFSE content of the amphibole in the adjacent dunitic wall-rock decreases over a 3 cm distance away from the thin vein (Nb 152–81 ppm, Ta 11·6–2·9 ppm, Zr 351–133 ppm, Hf 10·80–1·35 ppm) whereas its TiO2 content increases (0·95–1·78 wt %) and REE remain constant. The other section of sample MG91-143 (MG91-143.2) displays both a thick and thin vein (Fig. 4). This amphibole has the same compositional range as that in thick and thin veins in the other samples (Table 6). The amphibole of the dunitic wall-rock (located between the two types of veins in that sample) displays an intermediate composition (Figs 4 and 10).
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Amphiboles in thick veins and dunitic wall-rock within 1 cm of the vein show light REE (LREE) and middle REE (MREE) enriched [(La/Yb)N = 12, (La/Sm)N = 3] patterns with large negative U, Pb, Sr and Zr anomalies and slightly negative Hf anomalies (Fig. 3b and c). In contrast, amphibole in the thin veins displays slightly less enriched trace element patterns characterized by negative Ti anomalies and large positive Ta, Nb and Hf anomalies (Figs 4d and 5b).
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL SETTING AND XENOLITH...ANALYTICAL METHODSPETROGRAPHYMAJOR AND TRACE ELEMENT...DISCUSSIONCONCLUSIONSREFERENCES |
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Previous studies of composite mantle xenoliths have interpreted veins cross-cutting mantle peridotites as representing crystallization products of melts that have undergone varying degrees of fractionation in the mantle (e.g. Irving, 1980
; Nielson & Wilshire, 1993; Witt-Eickschen et al., 1998). Compositional variation between veins is a predicted consequence of this differentiation process (Witt-Eickschen et al., 1998).
Composition of the percolating melt
There are two possible end-member models for the formation of amphibole-rich veins: either dynamic crystallization with precipitation of amphibole, ilmenite and phlogopite from a melt percolating through the veins or in situ crystallization (freezing) of a melt.
If dynamic crystallization prevailed, and if the minerals were in equilibrium with the melt, the melt composition can be calculated using published mineral–melt partition coefficients (Brenan et al., 1995; Chazot et al., 1995; La Tourrette et al., 1995; Ionov et al., 1997; Grégoire et al., 2000b). Such calculations result in melts with high incompatible trace element abundances and negative HFSE and Ti anomalies (Fig. 11) comparable only with carbonatitic melts (Rock, 1987; Hamilton et al., 1989; Hornig-Kjarsgaard, 1998).
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On the other hand, the whole-rock major element composition of the veins is similar to that of volatile-rich basanitic magmas (e.g. Conquéré, 1971; Wass & Rogers, 1980; Rock, 1987; Fabriès & Lorand, 2000). Moreover, the measured and calculated (using modal compositions and trace element contents of minerals) bulk-rock trace element contents of the thick veins (sample MG91-144) are very similar to those of the mica- or amphibole-bearing ultramafic highly alkaline lavas (basanites, nephelinites or leucitites) from Kerguelen [Fig. 12 and Table 1; Moine (2000) and Moine et al. (2000) for samples AGL91-29, BOB93-640n, OVZK680, MG91-66, MG91-60, MG91-68, MG91-145, GM92-enc2, GM92-enc3 and GM92-enc4].
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These observations are consistent with the interpretation that amphibole-rich thick veins in Kerguelen composite xenoliths represent almost complete crystallization of a melt showing compositional affinities with the ultramafic and highly alkaline lavas from Kerguelen. Witt-Eickschen et al. (1998) proposed a similar origin for one of the amphibole-rich veins in a composite mantle xenolith from the Eifel (Germany). The slightly higher heavy REE (HREE) content of the thick veins in comparison with those of the highly alkaline lavas may be related either to a small degree of amphibole fractionation as amphibole may have a partition coefficient for HREE and silicate melt >1 (e.g. Witt-Eickschen & Harte, 1994; Brenan et al. 1995), or to an exchange of HREE from dunitic wall-rock that reacted with infiltrated melt. Thus, in summary, the thick veins are inferred to represent near-total cumulates from a volatile-bearing highly alkaline undersaturated silicate melt with minor local modification by mineral segregation and wall-rock interaction processes.
Origin of compositional differences in amphibole of thick and thin veins
Petrographic evidence indicates that thin veins have formed by injection of melt as a result of crack propagation from the thick veins into the dunitic wall-rock. Amphiboles that constitute the thin veins display trace-element compositions contrasting with those of amphiboles in the thick veins, especially for HFSE (Ti, Nb, Ta, Zr and Hf) and also, to a much lesser extent, for LREE and Rb, Ba and Sr (e.g. Figs 4, 5 and 10). The melt that formed thin veins is inferred to be fractionated from the melt parental to the thick veins by crystallization of the amphibole, ilmenite and phlogopite in the thick veins. This process and the injection of evolved melt to form the thin vein may explain the high and low Ti contents of amphibole in the thick and thin veins respectively, as titanium behaves as an compatible element in ilmenite, amphibole and phlogopite (e.g. Brenan et al., 1995; La Tourrette et al., 1995; Ionov et al., 1997; Grégoire et al., 2000a).
Furthermore, crystal and chemical studies of Ti-amphibole (e.g. Dyar et al., 1993; Popp et al., 1995; Zanetti et al., 1996; Tiepolo et al., 1999a, 1999b, 2000) suggest that amphibole containing higher Ti is likely to be lower in H2O and vice versa. This low H2O content can be explained by a process of substitution–dehydrogenation related to the octahedral crystallographic sites in amphibole and involving Fe3+ and Ti (Zanetti et al., 1996; Tiepolo et al., 1999a, 1999b, 2000). The relevant substitutions include
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Indeed, low-Ti amphibole in the thin veins has a higher H2O content than high-Ti amphibole in the thick veins (Moine et al., 2000; see samples MG91-144 and GM92-163). These observations suggest that the fractionated secondary melt that precipitated amphibole in the thin veins could have a higher volatile content (H2O) than the original melt parental to the thick veins. But how can there be enrichment in H2O in a melt that has fractionated by crystallizing 95% amphibole (plus small amounts of phlogopite)? To address this potential paradox, we have measured the H2O content of the amphiboles in the thick vein using the technique of Bigeleisen et al. (1952), involving the measurement of hydrogen gas yields during isotopic analysis, and we obtained a relatively low content (H2O+ =1·48 wt %; Moine et al., 2000). Finally if the initial H2O content of the melt was >1·5 wt %, H2O behaves as an incompatible element for the high-Ti amphibole of the thick veins and therefore H2O content increases in the fractionated melt (Damph/water 0·2; La Tourette et al., 1995).
Petrographic evidence and variations of Ti, LREE, Rb, Ba, Sr and H2O contents confirm that thin veins have formed by injection of fractionated fluid by crack propagation from the thick veins into the dunitic wall-rock. The compositional evolution of this secondary fluid is tracked by the changing trace element contents of the different amphiboles. If the residual melt propagated through thin cracks when significant amounts of amphibole had already formed in the thick veins, then the composition of the amphibole in the thin veins is controlled by this crystallization of amphibole in the thick veins that partitions significant Ti, but not HFSE and LREE. However, no increase in LREE content is observed to parallel the increase in HFSE, nor any significant variation of [Zr/Hf]N in thin vein amphibole as would be expected with significant crystallization of amphibole, along with some early ilmenite and phlogopite. These observations therefore do not support a simple crystallization process.
The significant concentrations of Nb, Ta, Zr and Hf in the exsolved ilmenite may explain the lower contents of these elements in the coexisting amphibole in the thick veins compared with those of the amphibole in the thin ilmenite-free veins. Indeed, Dilm/amph or Dilm/phl values are >1 for Nb, Ta and Zr (Table 7). The effect of coexisting ilmenite is very obvious in MG91-144, which is characterized by a heterogeneous distribution of the ilmenite in the thick vein. Section MG91-144.1 contains much less modal ilmenite than section MG91-144.2 and the amphibole has higher Nb, Ta, Zr and Hf abundances (Tables 2 and 3, Figs 2b and 3b).
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In addition to simple fractionation of the parental fluid, the variation in the trace element content observed in the amphibole of the thick and thin veins could also reflect variation in mineral–melt partition coefficients relevant to fluids of different compositions resulting from this fractionation. Consideration of changes in aTiO2 and aH2O of the precipitating fluid is important (Tiepolo et al., 1999a, 2000).
Amphibole–melt partition coefficients for the HFSE may vary as a function of the aTiO2 in the melt as well as the Ti content in the amphibole. Melt cation concentration, speciation and melt–crystal site constraints at given physical conditions may be also important factors as shown for clinopyroxene–melt partitioning (e.g. Green et al., 2000; Hill et al., 2000). Indeed, Tiepolo et al. (2000) argued that amphiboles crystallizing from a Ti-poor melt may incorporate more Nb and Ta than amphiboles crystallizing from a Ti-rich melt. Nb and Ta, like Ti, go into the M1 octahedral site and the lower the aTiO2 is in the melt, the more Nb and Ta will enter the M1 site to balance dehydrogenation. However, this process may not be the major factor controlling the enrichment in Nb and Ta in the amphiboles in the Kerguelen thin veins because it cannot explain the parallel enrichment in Zr and Hf observed in our samples (sometimes resulting in positive Zr and Hf anomalies; Fig. 5).
Moreover, the experimental study of Bureau & Keppler (1999) has shown that there is miscibility between silicate melts and hydrous fluids for the P–T conditions of the upper mantle. Differences in partition coefficient values may be correlated with an increase in the hydrous fluid component–silicate melt component ratio (i.e. higher aH2O) in the fractionated melt crystallizing in the thin veins. Experimental studies (e.g. Tatsumi et al., 1986; Brenan & Watson, 1991; Keppler, 1996) show that clinopyroxene–hydrous fluid partition coefficients for HFSE (Nb, Ta, Zr and Hf) are significantly >1 whereas the clinopyroxene–hydrous fluid partition coefficients for other trace elements are <1 (LILE) or 
1 (REE); that is, the HFSE behave as compatible elements and partition strongly into the clinopyroxene relative to the hydrous fluid. It must be emphasized that these experimental partition coefficients were determined for clinopyroxene and hydrous fluid, and clinopyroxene is absent in the veins we have studied. Therefore we are extrapolating that amphibole may behave like clinopyroxene in its partitioning behaviour for HFSE, REE and hydrous fluid at upper-mantle conditions. In support of this extrapolation is the observation that the partition coefficients for HFSE between amphibole and clinopyroxene are >>1, and are close to unity for REE (e.g. Witt-Eickschen & Harte, 1994; Chazot et al., 1995; Vannucci et al., 1995; Ionov et al., 1997; Grégoire et al., 2000a) and so it is a reasonable assumption that the HFSE partition coefficients for amphibole–fluid are probably also significantly >1.
Compositional variations of amphibole in veins and wall-rocks
As detailed above, amphiboles in the dunitic wall-rocks adjacent to the thick veins of phlogopite- and ilmenite-bearing hornblendite (e.g. MG91-144) show similar trace-element patterns but lower incompatible trace element abundances than amphiboles in the thick veins. Amphiboles at the contacts of the thick veins have compositions intermediate between the amphibole from the veins and those from the dunite (Fig. 9). The amphibole in the dunite wall-rock is inferred to have crystallized from the secondary fractionated melt from the thick veins percolating on a centimetre scale into the mantle wall-rock and reacting with the dunite. As described by Woodland et al. (1996), the amphibole-bearing veins were mostly responsible for pervasive metasomatism at a centimetre scale. Such crystal–melt reaction is observed petrographically as the dunite spinel is commonly surrounded by the amphibole and corroded. This type of reaction can explain the lower trace-element abundances of the dunite amphiboles and also their higher Cr content. Abundances of incompatible elements for amphibole decrease in the peridotite wall-rock with distance from the vein. Elements that are more compatible in amphibole than in other minerals of wall-rock, such as Ti, Rb, Ba and Sr, have large gradients and indicate that amphibole crystallization was synchronous with melt infiltration and acted as buffer for such elements. Titanium behaves as a compatible element for amphibole (as discussed above) and is therefore fractionated during the percolation of the melt, and Ti content decreases progressively outwards from the thick veins into the wall-rock (Fig. 9). Fractionation also is observed for the HFSE in the wall-rock of thin veins but the decrease of content is accompanied by a decoupling between Nb and Ta, and between Zr and Hf. The behaviour of HFSE is characterized by an inversion of (Nb/Ta)N and (Zr/Hf)N (<1 in the thin vein and up to 1·6 in the wall-rock) (Fig. 5b), possibly suggesting more efficient removal of Ta and Hf from infiltrating melt by the newly formed phases. If amphibole alone can account for a more efficient Hf removal (DHfamp/melt values are almost twice those for Zr), the increase of Nb relative to Ta requires some ilmenite crystallization, or other Ti-oxides such as rutile or armalcolite, given the similar D values for Nb and Ta in amphibole. These accessory minerals have been observed in other metasomatized mantle peridotites (Bodinier et al., 1996; Grégoire et al., 2000b).
We have also studied compositional variation in amphiboles in the wall-rock located in an analytical traverse between the thick vein and the thin vein of sample MG91-143 (Figs 4 and 10) and compared the successive compositions with those occurring near the thick vein in sample MG91-144 (Figs 3 and 9) and far from any thin vein, to assess the possibility of a mixing process between the thick and thin vein melts on a centimetre scale.
Evidence of a mixing process between the two melts percolating from each vein type is obvious from the variation in Nb, Ta, Zr and Hf contents of the dunitic amphibole with distance from the thick vein in sample MG91-143.2 (Fig. 10). The HFSE show a sharp increase rather than a systematic decrease away from the thick vein as they do away from the thick vein of sample MG91-144.2 (Fig. 9). This increase is towards the thin vein and is inferred to track the influence of evolved fluid percolating from this thin vein. The evolved fluid penetrating into the wall-rock from the thin vein is higher in volatiles (including the hydrous component) and crystallizes late-stage amphiboles with high HFSE and low Ti. Conversely, the influence of the melt from the thick veins (crystallizing the most Ti-rich amphiboles) is evidenced by the increase in the Ti content of the dunitic amphiboles away from the thin vein and towards the thick vein (Fig. 10).
The mixing process is not recorded by the REE and LILE contents of the amphiboles (Fig. 10) probably because the difference in concentration between the two melts was not sufficiently high. Indeed, the amphiboles do not record any significant variation in REE ratios. This lack of fractionation of the REE in the dunitic amphiboles is consistent with the suggestion of Bodinier et al. (1990) and Zanetti et al. (1996) of a ‘diffusion-controlled metasomatism’, a process where the composition of the infiltrating melt crystallizing the amphiboles is influenced by chemical exchange with the veins through diffusion processes. Elements with different partition coefficients but having similar diffusivity coefficients may have similar concentration gradients, and the development of chromatographic fronts in the wall-rocks of the veins is thus prevented. However, it must be noted that the scale of this process in the Kerguelen xenoliths (centimetre scale) is different from that for the Ronda or Lherz lherzolite massif (decimetre to metre scale; Bodinier et al., 1990; Vannucci et al., 1995; Woodland et al., 1996; Zanetti et al., 1996).
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CONCLUSIONS |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL SETTING AND XENOLITH...ANALYTICAL METHODSPETROGRAPHYMAJOR AND TRACE ELEMENT...DISCUSSIONCONCLUSIONSREFERENCES |
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- (1) The thick veins are compositionally very similar to highly alkaline, undersaturated basanitic dykes cropping out at the surface and are inferred to be almost total crystallization products of similar melts in the mantle.
- (2) Amphiboles comprising over 95% of the mode in thick and thin veins in Kerguelen dunites (mantle wall-rocks) record striking differences in Ti, Ta, Nb, Hf and Zr that provide evidence for fractionation of these elements within the upper mantle. Petrographic observations indicate that thick and thin veins are genetically related. Thick veins formed by crystallization of an original silicate melt with high aTiO2 and crystallized dominant Ti-rich amphibole low in H2O, Ta, Nb, Hf and Zr, and displaying inclusions of subordinate ilmenite ± phlogopite. Rare interstitial ilmenite and phlogopite also occur. The crystallization of these mineral phases resulted in the enrichment of the residual fluid in volatiles (H2O) and Nb, Ta, Zr and Hf, and its depletion in Ti. Fractionated fluid is released by crack propagation from the thick veins to form the thin veins that exclusively contain amphibole low in Ti and high in Nb, Ta, Zr and Hf.
- (3) Differences in composition of the amphiboles in the thick and thin veins are inferred to reflect differences in aTiO2 and particularly in aH2O of their respective parent melts. The observed partitioning of HFSE may reflect change in melt concentrations, crystal–melt site occupancy constraints that are still unclear at this stage and evolution of the crystal–melt partitioning of trace elements as a function of aH2O.
- (4) The trace-element content of amphiboles disseminated in the dunitic wall-rocks is closely related to that in the nearest (thick or thin) vein(s) and thus reflects centimetre-scale variation of the composition of the percolating melt, which may therefore be similar to that of thick or thin veins.
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ACKNOWLEDGEMENTS |
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This work has been made possible by the generous assistance and technical expertise of N. J. Pearson, A. Sharma and C. Lawson (GEMOC Geochemical Analysis Unit). We thank W. L. Griffin for helpful comments. This work was carried out under a ‘Co-tutelle’ arrangement between the University of Jean-Monnet, St Etienne, and Macquarie University, Sydney, and supported by the French CNRS UMR 6524 and 5570, and Australian Research Council Large and Small Grants (S. Y. O’Reilly) Scheme. B.N.M. acknowledges support from the Région Rhône–Alpes (Programme ‘EMERGENCE’) and the French Minister of Education and Research. We also thank for their support the French Polar Research and Technology Institute (IFRTP, Brest, France). This paper is Publication 235 in the ARC National Key Centre for Geochemical Evolution and Metallogeny of Continents (GEMOC). We wish to thank H. Downes, D. Smith and an anonymous reviewer for their constructive comments on this paper.
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FOOTNOTES |
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*Present address: Université P. & M. Curie–IPGP, Laboratoire de Géochimie des Systèmes Volcaniques, ESA 7046 CNRS, case courrier 109, 4 place Jussieu, 75005 Paris, France. Fax: 33 (0)1-44-27-39-11. E-mail: moine{at}ipgp.jussieu.fr
Present address: Department of Geological Sciences, University of Cape Town, Rondebosch 7700, South Africa.
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