JOURNAL OF PETROLOGY
VOLUME 40
NUMBER 11
PAGES 1721–1744
1999
Evolution of Heterogeneous Lithospheric Mantle in a Plume Environment Beneath the Kerguelen Archipelago N. MATTIELLI1,∗, D. WEIS1, J. S. SCOATES1, N. SHIMIZU2, J.-P. MENNESSIER1, M. GRE´GOIRE3, J.-Y. COTTIN3 AND A. GIRET3 DE´PARTEMENT DES SCIENCES DE LA TERRE ET DE L’ENVIRONNEMENT, UNIVERSITE´ LIBRE DE BRUXELLES,
1
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 LABORATOIRE DE GE´OLOGIE–CNRS UA10, UNIVERSITE´ JEAN MONNET, CNRS–UMR 6524, F- 42023 SAINT-ETIENNE,
3
FRANCE
RECEIVED OCTOBER 1, 1998; REVISED TYPESCRIPT ACCEPTED MAY 21, 1999
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, multistage 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-derived material. Our geochemical study of Kerguelen xenoliths testifies to 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 erupted lavas.
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 of this paper is to address the formation of compositional
∗Corresponding author. Telephone: +32-2-650-22-69. Fax: +32-2650-22-26. e-mail:
[email protected]
Oxford University Press 1999
geochemistry; isotopes; Kerguelen plume; mantle chromatography; metasomatized xenoliths KEY WORDS:
INTRODUCTION
JOURNAL OF PETROLOGY
VOLUME 40
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): (1) 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. (2) 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). (3) 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). 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 (Gre´goire, 1994; Mattielli, 1996; Gre´goire 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 (Gre´goire 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
NUMBER 11
NOVEMBER 1999
the occurrence of carbonate-rich metasomatism as a common process in the mantle (e.g. Schiano et al., 1994).
GEOLOGICAL SETTING 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 (Gre´goire, 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, c). Most of the xenolith-bearing outcrops are basanitic or lamprophyric dykes (Doˆme Rouge, Val Phonolite, Pointe de l’Espe´rance, Le Trie`dre, Tour de Pise, Mont Thompson, Val du Levant) or lava flows (Le Pouce and Rivie`res aux Macaronis) and more rarely breccia pipes (Mont Tizard), or limburgitic lava flows (Pointe Suzanne). The Doˆme Rouge and the Mont Tizard localities are the main xenolith outcrops on the
1722
MATTIELLI et al.
MANTLE HETEROGENEITY AND KERGUELEN PLUME
Fig. 1. (a) Location of the Kerguelen Large Igneous Province including the Ninetyeast Ridge, Broken Ridge, and the Kerguelen Archipelago and Heard Island on the Kerguelen Plateau. (b) Location of the Prince de Galles Peninsula and the Southeast Province, with the Jeanne d’Arc and Ronarc’h Peninsula, on the Kerguelen Archipelago (after Nougier, 1970; Gautier et al., 1990). (c) Location of the xenolith outcrops in the Prince de Galles Peninsula (Pointe Suzanne) and the Southeast Province (after Leyrit et al., 1990), with sample numbers reported in this study.
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).
PETROGRAPHY 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 mediumgrained, 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 (Ζ1 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
1723
JOURNAL OF PETROLOGY
VOLUME 40
NUMBER 11
NOVEMBER 1999
Fig. 2. (a) Photograph of an alkaline basanitic dyke from the Doˆme Rouge outcrop containing abundant ultramafic and mafic xenoliths (2 nodules/dm3) (length of hammer is 30 cm). (b) Photomicrograph of lherzolite 92-372 showing interstitial spinel crystals (pleonaste). (c) Photomicrograph of dunite 91-114 showing the polygonized and elongated porphyroclasts of olivine and the more altered and fissured general aspect compared with other dunites. (d) Photomicrograph of a protogranular harzburgite showing typical symplectitic texture consisting of diopside, enstatite and spinel (chromite). (e) Photomicrograph of poikilitic harzburgite 91-8, which is characterized by the occurrence of secondary and undeformed, poikilitic clinopyroxene including orthopyroxene and/or spinel and olivine. (f ) Photomicrograph of poikilitic harzburgite 91-8 showing a secondary ‘wehrlitic’ assemblage that consists of fine-grained neoblasts (Ζ1 mm) of clinopyroxene, spinel (chromite) and olivine in patches between primary orthopyroxene and olivine. Except for the dunite 91-114, all the photomicrographs have been made with polarized light (ol, olivine; cpx, clinopyroxene; opx, orthopyroxene; sp, spinel; gl, basaltic glass; serp, serpentine).
1724
MATTIELLI et al.
MANTLE HETEROGENEITY AND KERGUELEN PLUME
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 Gre´goire 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 (Gre´goire 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 (Ζ1 mm) with olivine and spinel in patches between primary orthopyroxene and olivine (Fig. 2f ).
GEOCHEMISTRY 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 (Gre´goire, 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. 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 (Gre´goire 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
1725
JOURNAL OF PETROLOGY
VOLUME 40
NUMBER 11
NOVEMBER 1999
Fig. 3. Plot of SiO2, Al2O3, Fe2O3t (FeO = 0·9 × Fe2O3t), CaO vs MgO contents (in wt %) showing major element compositions in whole rocks of the Kerguelen peridotite xenoliths [lherzolite (Η), dunite (Β), and protogranular (Μ) and poikilitic (Α) harzburgite xenoliths] [data from Gre´goire (1994)].
to 5·35, 2·44 and 3·05 wt %, respectively) (Gre´goire 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. Gre´goire, unpublished data, 1995)]. Spinels (Mg–Al chromites) show a large range of crnumber values for nearly constant mg-number, especially in the harzburgites (mg-number: 60–77; cr-number: 33– 68) (Gre´goire, 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 92372 (Fig. 4). Spinels from dunite 91-114 have low mgnumber (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 %) (Gre´goire, 1994).
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 Gre´goire 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: (1) 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. (2) 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 Gre´goire et al. (1997)].
1726
MATTIELLI et al.
MANTLE HETEROGENEITY AND KERGUELEN PLUME
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 crnumber 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 crnumber and Ti concentrations for nearly constant mgnumber, suggests a reaction of the mantle lithosphere with ascending melt.
Incompatible element compositions Fig. 4. Plot of cr-number [molar Cr × 100/(Cr + Al)] vs mg-number [molar Mg × 100/(Mg + Fe2+)] of spinel in Kerguelen peridotite xenoliths (symbols as in Fig. 3) determined by electron microprobe (Gre´goire, 1994), compared with the spinel compositions in abyssal peridotites (shaded field) (Dick & Bullen, 1984).
(3) 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. (4) 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). (5) 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 (Gre´goire, 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
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).
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 Gre´goire (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.
1727
JOURNAL OF PETROLOGY
VOLUME 40
NUMBER 11
NOVEMBER 1999
Table 1: Representative trace element compositions in clinopyroxene (ppm)
Sample:
Lherzolite
Dunite
92-372
91-114
cpx 1
cpx 2
cpx 3
92-286
cpx 1
cpx 2
cpx 3
92-491
cpx 1
cpx 2
cpx 1
cpx 2
La
4·65
3·66
4·76
0·98
0·47
0·79
20·0
31·5
23·8
27·2
Ce
3·93
3·61
5·16
3·41
1·62
2·09
17·8
35·8
51·5
56·7
Nd
1·30
1·23
1·31
3·50
2·02
1·37
3·23
4·86
19·12
20·96
Sm
0·48
0·44
0·41
1·05
0·94
0·55
0·95
1·29
4·72
3·50
Eu
0·36
0·28
0·22
0·51
0·34
0·41
0·49
0·47
1·41
1·27
Dy
0·48
0·48
0·58
2·81
2·08
1·55
1·07
1·24
3·13
3·41
Er
0·53
0·47
0·35
1·63
1·51
1·17
0·77
0·92
1·68
1·90
Yb
0·40
0·55
0·41
1·83
1·59
1·40
0·88
1·06
1·60
Ti
1067
1251
959
Sr
178
193
219
Y
2·69
Zr
8·18
(Ce/Yb)n (Ce/Sm)n
3·07
4544
39·7
4731
30·8
12·3
10·6
7·99
59·1
2·7
1·8
3·5
0·52
0·28
2·0
2·1
3·1
0·81
0·43
130
118
120
Ti/Eu
29790
4468
4359
Protogranular
Poikilitic
harzburgite
harzburgite
91-38
91-42
91-8
cpx 1
cpx 1
cpx 1
9·85
70·3
cpx 2
1835
169
291
317
334
6·28
6·11
16·0
14·9
8·40
7·62
78·4
81·9
0·41
5·6
9·4
8·9
9·3
0·95
4·7
6·9
2·7
4·0
26·0
22·4
91·8
13288
cpx 3
2042
51·5
80·0
8107
1830
7·67
56·8
11511
203
240
3480
3927
133A1-1
92-502
cpx 1
cpx 1
1451
1442
92-509 cpx 2
4·55
10·7
5·59
6·28
2·10
4·71
23·0
8·26
8·32
Nd
0·45
0·89
6·31
1·63
1·23
73·50
Sm
0·15
0·43
1·18
0·35
0·30
14·80
6·34
6·18
6·44
6·47
5·33
Eu
0·03
0·14
1·18
0·35
0·30
3·55
2·25
2·24
2·05
2·35
1·65
Dy
0·11
0·57
0·57
0·30
0·29
6·37
3·33
2·82
3·18
3·30
2·49
Er
0·18
0·41
0·39
0·20
0·40
3·32
1·02
0·78
0·99
1·09
0·95
Yb
0·19
0·50
0·43
0·34
0·33
1·61
1·06
0·92
1·11
1·28
Sr
14·0
681 21·8
Y
0·47
3·00
Zr
2·90
5·30
(Ce/Yb)n
3·1
2·6
(Ce/Sm)n
3·5
2·7
Ti/Zr
25·5
128
Ti/Eu
73·9
681
128 57·5 2·61 291 14·9
153 31·7
158 25·7
9·34
10·2
cpx 1
1·80
73·9
9·92
cpx 4
Ce
189
10·2
cpx 3
La
Ti
78·4
1·69
1705
16·2
3·02
Ti/Zr
Sample:
4159
16·4
27·8
27·5
27·0
28·4
47·6
19·2
19·4
19·6
20·3
25·3
0·98
379
1758
1769
1996
1659
1322
114
178
176
181
174
269
1·50
1·41
24·6
2·58
1·84
13·4
8·09 230
7·79 230
8·32 236
7·16 215
10·40 58·2
6·8
6·9
32·6
7·3
8·3
6·7
6·2
4·87
6·0
6·9
3·2
1·1
1·1
1·0
1·1
2·2
0·44
59·3
85·7
28·3
7·6
7·7
8·4
7·7
22·7
108
440
526
107
783
Values normalized to the C1 chondrite abundances from Sun & McDonough (1989).
1728
791
972
706
13·5
801
MATTIELLI et al.
MANTLE HETEROGENEITY AND KERGUELEN PLUME
(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 Ζ 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, d). (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 concaveupward 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, e), 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.
Fig. 5. Patterns of REE, Sr, Zr and Ti abundances normalized to the C1 chondrite abundances (Sun & McDonough, 1989) for clinopyroxenes and whole rocks (crosses) of the Kerguelen lherzolite (a), dunite (b, f ), poikilitic harzburgite (c, d), and protogranular harzburgite (e) xenoliths. Each pattern represents a single analysis, except in (c), where the patterns for clinopyroxenes represent the average of three grain analyses (identical within experimental error) (cpx, clinopyroxene; wr, whole rock).
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 Gre´goire 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
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
1729
JOURNAL OF PETROLOGY
VOLUME 40
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; 143 Nd/144Nd from 0·51264 to 0·51214; and 206Pb/204Pb from 17·71 to 18·60 (Figs 6–8). In comparison, 87Sr/86Sr in the Hawaiian peridotite xenoliths varies from 0·7025 to 0·7043 (Frey & Roden, 1987; Okano & Tatsumoto, 1996). 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
NUMBER 11
NOVEMBER 1999
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 (18·390) (Figs 7 and 8). The dunite and harzburgite groups described above share similar 87Sr/86Sr, but the 143 Nd/144Nd values of the dunites (
