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Journal of Petrology | Volume 43 | Number 7 | Pages 1207-1239 | 2002
© Oxford University Press 2002
Involvement of Continental Crust in the Formation of the Cretaceous Kerguelen Plateau: New Perspectives from ODP Leg 120 Sites
154-1226, DEPARTMENT OF EARTH, ATMOSPHERIC AND PLANETARY SCIENCES, MASSACHUSETTS INSTITUTE OF TECHNOLOGY, CAMBRIDGE, MA 02139, USA
2DEPARTMENT OF EARTH AND ENVIRONMENTAL SCIENCES, CP 160/02, UNIVERSITÉ LIBRE DE BRUXELLES, AV. F. D. ROOSEVELT, 50, B-1050 BRUSSELS, BELGIUM
3VERNADSKY INSTITUTE OF GEOCHEMISTRY AND ANALYTICAL CHEMISTRY, KOSYGIN ST. 19, 117975, MOSCOW, RUSSIA
Received May 7, 2001; Revised typescript accepted December 17, 2001
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONSAMPLING AND PETROGRAPHYSAMPLE PREPARATION AND...GEOCHEMICAL RESULTSDISCUSSIONREFERENCES |
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The basaltic basement of the large igneous province formed by the Kerguelen Plateau and Broken Ridge in the southeastern Indian Ocean has been sampled by three Ocean Drilling Program cruises (Legs 119, 120 and 183). Although the Cretaceous parts of this plateau formed in the embryonic Indian Ocean basin, presumably by melting associated with the Kerguelen plume, trace element abundances and isotopic ratios of Sr, Nd and Pb of Cretaceous basalt from several drill sites indicate that continental lithosphere was involved in their petrogenesis. On the basis of relative depletions in Nb, Ta and Th, and isotopic characteristics similar to those of EMI ocean island basalt, lavas from Leg 120 Site 747 in the Central Kerguelen Plateau contain a component derived from lower continental crust. On the basis of relative abundances of Sr and Eu and EMI-like Pb isotopic ratios, the source of basalt from Leg 120 Site 750 in the northeastern part of the Southern Kerguelen Plateau also contained a component derived from lower continental crust; in this case, the crustal component formed as a plagioclase-rich, clinopyroxene-bearing cumulate. Basalts from Leg 120 Site 749 define two distinct isotopic (Sr, Nd and Pb) groups which differ from the isotopic fields for Site 747 and 750 basalts. Among Site 749 lavas, there is subtle evidence for a continental component, broadly similar (i.e. moderate 206Pb/204Pb
18·0) to that expressed more obviously in basalt from Leg 119, Site 738 on the southern edge of the Southern Kerguelen Plateau and Leg 183 Site 1137 on Elan Bank. The continental components in the Kerguelen Plateau basalts may have resided in a heterogeneous mantle plume that was formed, in part, by deep recycling of crust. It is more likely, however, that slivers of Gondwana lithosphere reside within the lithosphere and asthenosphere of the Indian Ocean mantle where they contaminate both plume-derived and mid-ocean ridge basaltic magmas. KEY WORDS: Ocean Drilling Program Leg 120; large igneous provinces; Kerguelen Plateau; basalt geochemistry
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONSAMPLING AND PETROGRAPHYSAMPLE PREPARATION AND...GEOCHEMICAL RESULTSDISCUSSIONREFERENCES |
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The submarine Kerguelen Plateau is a dominantly Cretaceous large igneous province in the southern Indian Ocean that stands 2–4 km above the surrounding deep ocean basins. The plateau has been divided into distinct domains: Southern Kerguelen Plateau (SKP), Central Kerguelen Plateau (CKP), Northern Kerguelen Plateau (NKP), Elan Bank and Labuan Basin (Fig. 1). Together with the formerly contiguous Broken Ridge, this large igneous province covers an area of 2 x 106 km2 (Coffin & Eldholm, 1994
). It has been interpreted as an igneous structure formed from the initial volcanism associated with the Kerguelen mantle plume (e.g. Morgan, 1971; Duncan & Storey, 1992; Coffin et al., 2002). Abundant geophysical data are consistent with a dominantly basaltic to gabbroic, 20 km-thick crust [see references given by Shipboard Scientific Party (2000)]. Volcanic features attributed to the Kerguelen plume include the 119–110 Ma SKP, the 103–95 Ma CKP and Broken Ridge (Coffin et al., 2002; Duncan, 2002), and the 82–38 Ma Ninetyeast Ridge, which is interpreted as the Kerguelen plume hotspot track (e.g. Weis et al., 1992). Cenozoic to recent volcanism attributed to the Kerguelen plume includes part of the NKP, the Kerguelen Archipelago and Heard and McDonald Islands (Fig. 1 and Shipboard Scientific Party, 2000). It is likely that the Kerguelen plume is now beneath the Cretaceous Kerguelen Plateau (Weis et al., 2002); hence, the spatially diffuse recent island volcanism (Fig. 1) may reflect the difficulty of establishing magma ascent paths through the thick plateau lithosphere (Frey et al., 2000b; Weis et al., 2002).
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Despite formation within the Indian Ocean basin, there has been a long-term discussion (e.g. Dietz & Holden, 1970) regarding the role of continental lithosphere in forming the Kerguelen Plateau. In some areas, such as the SKP and Elan Bank, a western salient of the plateau, geophysical data provide evidence for continental fragments within the crust of the plateau (Operto & Charvis, 1996; Charvis et al., 1997). Moreover, at two locations, ODP Site 738 on the SKP and ODP Site 1137 on Elan Bank, tholeiitic basalts contain a component derived from continental crust (Mahoney et al., 1995; Weis et al., 2001). Determining the role of continental lithosphere in the petrogenesis of Kerguelen Plateau basalt as a function of eruption age and location is essential to understanding the breakup of eastern Gondwana and subsequent evolution of the Indian Ocean basin.
Dredges from the Labuan Basin, 77°E Graben, and Skiff Bank (Fig. 1) have recovered metamorphic and granitic rocks, but these have been interpreted as ice-rafted debris (Ramsay et al., 1986; Bassias et al., 1987; Montigny et al., 1993; Weis et al., 2002). The first indisputable samples of the Cretaceous igneous crust forming the Kerguelen Plateau were dredged from the 77°E Graben in the SKP (Bassias et al., 1987; Leclaire et al., 1987; Davies et al., 1989) (Fig. 1). These dredge samples are dominantly altered, tholeiitic basalt. Leclaire et al. (1987) reported a K/Ar age of 114 Ma for plagioclase separated from a basalt. Isotopic ratios of Sr and Nd in the basalts from the 77°E Graben largely overlap with the broad field defined by lavas from the Cenozoic Kerguelen Archipelago (Weis et al., 1989, 1993). In contrast, the 206Pb/204Pb ratios of these Cretaceous dredge basalts are lower than those of Kerguelen Archipelago lavas. Davies et al. (1989) found that these dredged basalts have higher Th/Ta and La/Nb than typical for ocean island basalts (OIB), and Storey et al. (1989) proposed that these high ratios may indicate a component derived from continental lithosphere. A compelling conclusion could not be made from these few data for dredged basalts, but subsequent drilling by ODP Leg 119 at Site 738 in the SKP (Fig. 1) recovered tholeiitic basalt that clearly contains a continental crust component; these basalts have 
Nd(T) = -13 to -7, (87Sr/86Sr)T = 0·7070–0·7130, very high 207Pb/204Pb relative to 206Pb/204Pb and relative depletion in Nb and Ta (Alibert, 1991; Mahoney et al., 1995). Most recently, Weis et al. (2001) and Ingle et al. (2002a) showed that tholeiitic basalts from ODP Leg 183 Site 1137 on Elan Bank (Fig. 1) also contain a continental crust component. Moreover, at this site clasts of continental crust occur in a conglomerate intercalated with the basalts (Frey et al., 2000a; Nicolaysen et al., 2001; Ingle et al., 2002b). Although they are separated from the surrounding continental margins by deep ocean basins, basalts at Sites 738 and 1137 erupted on the periphery of the Kerguelen Plateau closest to the rifted continental margins (Fig. 1).
The objective of this paper is to evaluate the role of continental crust during basalt petrogenesis at other drill sites within the CKP and SKP; specifically ODP Leg 120, Site 747 in the SE CKP, Site 749 in the interior of the northern part of the SKP, and Site 750 at the NE edge of the SKP (Fig. 1). Tholeiitic basalt was recovered at these sites (Storey et al., 1992). Alkalic basalt was recovered at the base of Hole 748C, but this basalt is not interpreted as part of the igneous basement (Shipboard Scientific Party, 1989a). Isotopic (Sr, Nd, Pb and Hf) data for the tholeiitic basalt from Sites 747, 749 and 750 showed that each drill site is isotopically distinct, but these isotopic characteristics were initially interpreted as being consistent with an oceanic origin (Salters et al., 1992). Storey et al. (1992), however, noted that basalts from Sites 747 and 750 are offset to low 206Pb/204Pb from the field of Kerguelen Archipelago lavas and that some have a relative Nb depletion. They suggested that the source of these Kerguelen Plateau basalts was contaminated by components derived from the Gondwana continental lithosphere. These data are not sufficient for a rigorous assessment of the role for a continental crustal component in these lavas because there are too few isotopic and trace element abundance data for the same samples. We obtained a comprehensive dataset for major and trace element abundances and isotopic ratios for basalt from Sites 747, 749 and 750 to use these data to evaluate the role of a continental crustal component in the interior parts of the Kerguelen Plateau.
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SAMPLING AND PETROGRAPHY |
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TOPABSTRACTINTRODUCTIONSAMPLING AND PETROGRAPHYSAMPLE PREPARATION AND...GEOCHEMICAL RESULTSDISCUSSIONREFERENCES |
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Site 747
This site is in the Central Kerguelen Plateau (Fig. 1) where 53·9 m of basement basalt was penetrated (20·5 m recovered) below 295·1 m of sediment. Although the basalt–sediment contact was not recovered, the oldest sediment is early Santonian in age (
87 Ma) (Shipboard Scientific Party, 1989b). The basalts are highly altered and difficult to date using K–Ar or 40Ar/39Ar techniques (Whitechurch et al., 1992). An 40Ar/39Ar age of 85 Ma was reported by Pringle et al. (1994), but this age is now considered unreliable (Duncan, 2002). The basement core is interpreted to consist of up to 12 subaerially erupted basaltic lava flows separated by zones of basaltic breccia (Shipboard Scientific Party, 1989b). Site 747 lavas range from aphyric to moderately phyric (<10% phenocrysts) to lavas with 10–15% phenocrysts forming two assemblages: olivine–plagioclase and olivine–plagioclase–clinopyroxene (Table 1). The vesicularity of the basalts, up to 10%, indicates subaerial or shallow-water eruption. To variable extents all samples are altered and contain veins. Vesicles are filled with clay minerals and veins are filled with zeolites. Olivine phenocrysts are euhedral (0·7–2 mm) but replaced by clay minerals, serpentine and iddingsite. Plagioclase phenocrysts (An70–80) occur as unaltered euhedral tabular crystals (0·5–2 mm) and as pseudomorphs replaced by zeolite and smectite. Unaltered augite phenocrysts (0·5–2 mm) are subhedral to anhedral. The texture is microlitic to subophitic with the matrix formed of plagioclase laths, augite and interstitial Fe–Ti oxides.
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Site 749
This site is in the northwestern part of the Southern Kerguelen Plateau (Fig. 1) where 47·5 m of basement basalt was penetrated (23·3 m recovered) below 202 m of sediments. The age of the oldest sediment is 54–55 Ma, but the sediment–basalt contact was not recovered (Shipboard Scientific Party, 1989c). K/Ar and 40Ar/39Ar ages of 110 Ma have been determined for the basalt (Whitechurch et al., 1992; Pringle et al., 1994; Storey et al., 1996). The recovered basaltic basement consists of five subaerial lava flows intruded by a dike.
Site 749 lavas (Table 1) range from aphyric (Units 1, 3 and 4) to moderately plagioclase-phyric (5–10% of 1–2 mm phenocrysts in Unit 5) to highly plagioclase-phyric (20–30% of An62 to An84 laths ranging from 0·5 to 20 mm in Unit 6). These lavas are vesicular and variably altered; some plagioclase phenocrysts are replaced by clay minerals. The microlitic matrix consists of plagioclase (An40–50), augite and titanomagnetite.
Site 750
This site is in the northeastern part of the Southern Kerguelen Plateau (Fig. 1) where 34 m of basement basalt was penetrated (22·9 m recovered) below 671·7 m of sediment. The oldest sediments are Albian terrigenous deposits containing abundant fossil wood and other land-derived organic matter (Shipboard Scientific Party, 1989d; Francis & Coffin, 1992; Mohr & Gee, 1992). Whitechurch et al. (1992) reported ages of 101 Ma (K–Ar) and 118 Ma (40Ar/39Ar) for a Site 750 basalt; Coffin et al. (2002) reported a 112·4 ± 0·4 Ma age (40Ar/39Ar) for plagioclase from a Site 750 basalt. The basement basalts are divided into five units consisting of at least four different lava flows; Units 3 and 4 may belong to a single flow.
Site 750 lavas are moderately phyric basalt (Table 1). The Unit 3 lava is highly altered with clays replacing plagioclase and glass and filling cavities. As discussed below, its chemical composition has been significantly changed by post-magmatic processes (e.g. K2O/Na2O = 6·4, Table 2).
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Units 5 and 6 are massive, moderately plagioclase- and clinopyroxene-phyric basalts. In Unit 5, 11·5 m thick, the plagioclase phenocrysts (An60–67 and 0·5–2 mm in length) are euhedral tabular crystals forming a variolitic texture. Unit 5 lavas have two different textures: medium- to coarse-grained, centimeter-scale subophitic bands or clots separated by fine-grained basalt (see Shipboard Scientific Party, 1989d, fig. 34). The bands are phenocryst rich and show grading in size distribution. In the subophitic bands euhedral clinopyroxene (up to 2 mm) poikilitically encloses plagioclase. These bands have been interpreted as dislodged cumulates, or mechanical segregations produced during flow or as products of magma mixing (Shipboard Scientific Party, 1989d ). In Unit 6 lavas, tabular plagioclase crystals (0·7–1 mm in length) are associated with subhedral clinopyroxene phenocrysts. Unit 6 lavas are more altered than Unit 5 lavas; secondary minerals include clay minerals, zeolites and calcite filling vesicles and veins.
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SAMPLE PREPARATION AND ANALYTICAL PROCEDURES |
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TOPABSTRACTINTRODUCTIONSAMPLING AND PETROGRAPHYSAMPLE PREPARATION AND...GEOCHEMICAL RESULTSDISCUSSIONREFERENCES |
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To analyze the least altered samples, small rock chips from the interior of the core were selected to avoid large filled vesicles, veins and regions of intense alteration. These chips were powdered in an agate shatterbox. As in our previous studies of lavas related to the Kerguelen plume (e.g. Frey et al., 1991
, 2000b) abundances of major oxides and some trace elements were determined by X-ray fluorescence (XRF) spectrometry (Table 2) at the University of Massachusetts (Rhodes, 1996). Other trace elements were determined by instrumental neutron activation analysis (INAA) (Table 2) at the Massachusetts Institute of Technology (MIT) (Ila & Frey, 1984, 2000). In addition, we report trace element analyses determined at MIT by inductively coupled plasma mass spectrometry (ICP-MS) (Table 3) using a Fisons VG Plasmaquad 2+S with both internal and external drift monitors. In general, compared with data determined by XRF and INAA, the ICP-MS data agree within ±10% (La, Ce, Nd, Sm, Eu, Rb, Sr, Ba, Nb, Ta, Zr and Hf); however, at Th <0·5 ppm and Rb <1 ppm, the discrepancies are higher because the INAA (Th) and XRF (Rb) data are not precise at these low abundances. Abundances of Y (15%) and Yb and Lu (8%) determined by ICP-MS are higher than those determined by XRF and INAA, respectively.
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Samples for radiogenic isotope analyses (Table 4) were selected on the basis of their relative freshness and to encompass the largest range of chemical composition. The chemical procedure used is similar to that described by Weis et al. (1987) with improvements discussed by Weis & Frey (1991). To remove secondary or alteration phases all samples were acid-leached repeatedly with warm 6N HCl until a clear solution remained. Total blank values were <1 ng for all three isotopic systems considered. Such values are negligible in view of the elemental concentrations in the samples (Table 3). Sr and Nd isotopic ratios were measured on single Ta filaments and triple Re–Ta filaments, respectively, in the dynamic mode, on a VG Sector 54 multicollector mass spectrometer with an internal precision better than 1 x 10-5 unless specified in Table 4. Sr isotopic ratios were normalized to 86Sr/88Sr = 0·1194 and Nd isotopic ratios (147, 146, 145, 144, 143 and 142), measured as metal to 146Nd/144Nd = 0·7219. The average 87Sr/86Sr value of the NBS 987 Sr standard was 0·710270 ± 14 (2
on 62 samples) and analyses of Rennes Nd standard (Chauvel & Blichert-Toft, 2001) yielded 143Nd/144Nd = 0·511968 ± 12 (2 on 61 samples). The precision for Sr and Nd isotopic ratios can be evaluated by comparing duplicate analyses, which agree within the 2 uncertainties for six of seven samples (Table 4). Pb isotopic ratios were measured on single Re filaments using the H3PO4–silica gel technique. All Pb isotopic ratios were corrected for mass fractionation (0·12 ± 0·024% per a.m.u.) on the basis of 72 analyses of the NBS 981 Pb standard for a temperature range between 1090°C and 1150°C. Between-run precisions were better than 0·1% for 206Pb/204Pb and 207Pb/204Pb and better than 0·15% for 208Pb/204Pb.
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GEOCHEMICAL RESULTS |
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TOPABSTRACTINTRODUCTIONSAMPLING AND PETROGRAPHYSAMPLE PREPARATION AND...GEOCHEMICAL RESULTSDISCUSSIONREFERENCES |
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Major element compositions
It is apparent from their petrography, weight loss on ignition (LOI; typically 1–5% but some up to 10%, Table 2), and weight loss after the acid leaching before isotopic analyses, that the samples have been strongly affected by post-magmatic alteration. In addition, zeolite-facies minerals are abundant in amygdules (Sevigny et al., 1992
). Despite the mineral and compositional changes caused by alteration and metamorphism, these lavas are undoubtedly tholeiitic basalt. Firstly, their petrographic characteristics and inferred phenocryst crystallization sequence, olivine and plagioclase followed by clinopyroxene, are distinct from the alkalic analcite-bearing basalt overlying basement at Site 748 (Shipboard Scientific Party, 1989a). Secondly, although Na, K and to a lesser extent Si are mobile during post-magmatic alteration, the tholeiitic nature of basalts from Sites 747, 749 and 750 in a Na2O + K2O vs SiO2 classification plot (Fig. 2) is consistent with the relative abundances of immobile incompatible elements. For example, at a given MgO content, lavas from these sites vary systematically in TiO2 content, with TiO2 increasing in the order 750 to 747 and 749, to 748 (Fig. 3). This sequence is the same as that for increasing alkalis at a given SiO2 content (Fig. 2). A dramatic example of alkali mobility is shown by the two analyses from Unit 3 at Site 750. These samples have a weight loss on ignition of 7 and 10%, high K2O (4·8–5·3%) and atypically low Na2O (0·4–0·8%) contents; in addition, they have relatively low SiO2, high total iron, and they have lost most of their CaO (2·2–0·3%) (Table 2, Fig. 2). In general, there is a trend for SiO2/Al2O3 to decrease with increasing LOI. In contrast, TiO2 was apparently relatively immobile, as the TiO2 contents of Unit 3 lavas are similar to those of other Site 750 basalt (Table 2). Also distinctive is the plagioclase-rich basalt from Unit 6 at Site 749 (Table 1). These lavas have high Al2O3 (20%) and CaO (11·5–12·5%) coupled with low total iron and TiO2 abundances (Table 2, Fig. 3). These compositional features, which reflect the abundance of plagioclase (20–30%, Table 1), indicate that this basalt contains cumulate plagioclase.
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Trace element abundances
Abundances of the highly incompatible elements Th, La, Ce, Nb and Ta in Leg 120 lavas from Sites 747, 749 and 750 are strongly correlated; e.g. in trace element vs Th plots the correlation coefficients exceed 0·96 (Fig. 4). Abundances of U, Ba, Pb, Zr and Hf are also positively correlated with Th content, although there are a few aberrant points, which probably reflect the effects of post-magmatic processes [e.g. the anomalously high Ba (Fig. 4) and K (Table 2) in the clay-rich sample 750-14R1 from Unit 3]. Abundances of the mobile alkali metals K and Rb are not correlated with Th content; K/Rb ranges from 425 to 1900. This wide range reflects the effects of post-magmatic processes; hence we do not use K and Rb for petrogenetic interpretations. Abundance variations of Yb and Sr with Th are also poorly correlated. As discussed below, the scatter in Sr reflects the well-known compatibility of Sr in feldspar; whereas lavas from each site define different Yb–Th trends.
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Primitive mantle-normalized plots show several important features of the relative abundances of incompatible elements in these Kerguelen Plateau basalts (Fig. 5). Most of the lavas have (Ba/Rb)PM much greater than unity (PM denotes normalized to primitive mantle ratio of 11) and (Ba/Rb)PM varies widely (0·7–8·5). As with K/Rb, we infer that such anomalous ratios reflect the effects of subaerial and submarine alteration processes that have affected the Rb contents of these basalts. For Site 747 lavas, the slope in Fig. 5a is negative from La to Yb, i.e. relative enrichment in the more incompatible elements. However, the highly incompatible elements Th, Nb and Ta are relatively depleted compared with the adjacent elements Ba and La; sample 14R2 has a particularly high La/Nb (Fig. 5a). In addition, Sr is relatively depleted. Compared with Site 747 basalts, Site 749 basalts define a shallower negative slope in a primitive mantle-normalized plot (Fig. 5b). However, the two plagioclase-rich samples from Unit 6 have very marked enrichments in Sr and Eu; also the aphyric samples from Unit 1 (Table 1) have relative enrichments in Sr and Eu (Fig. 5b). Among Leg 120 lavas, those from Site 750 have the lowest abundances of incompatible elements, and they also have significant relative enrichments in Sr and Eu and a depletion in Ti, Zr and Hf (Fig. 5c). Surprisingly, these lavas do not contain a large abundance of plagioclase phenocrysts (e.g. lavas from Units 5 and 6 at Site 750 typically contain 6–7·5% plagioclase, Table 1, Fig. 6b). Relative to the other flow units at Site 750, Unit 6 basalts have higher abundances of rare earth elements (REE) and several other incompatible elements (Fig. 5c).
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In summary, the relative abundances of Sr and Eu provide constraints on the petrogenesis of several basaltic units from each site. Except for the highly altered Site 750 sample 14R1, the relative enrichments of Sr and Eu are highly correlated (Fig. 6a). In lavas from Site 747, (Sr/Nd)PM decreases from 1 to 0·5 with increasing abundances of incompatible elements, such as La (Fig. 6c). This correlation is expected from plagioclase fractionation; that is, La and Nd are incompatible and Sr is compatible in plagioclase. In contrast, for the Site 750 lavas (Sr/Nd)PM increases from 1 to 2·5 with only a slight decrease in La content (Fig. 6c). This correlation is consistent with accumulation of plagioclase.
Isotopic ratios of Sr, Nd and Pb
Site 747
Isotopic data are available for 14 samples [nine in Table 4 and five from Salters et al. (1992; personal communication, 1992)]. The initial Sr and Nd isotopic ratios form a tightly grouped cluster that overlaps in 87Sr/86Sr but is offset to lower 143Nd/144Nd than the <10 Ma lavas in the Kerguelen Archipelago (Fig. 7a). The Pb isotopic ratios of Site 747 lavas differ substantially from those of Kerguelen Archipelago lavas (Fig. 8). In detail, the Pb isotopic data for nine Site 747 lavas form two clusters, with four samples from cores 15 and 16 at the bottom of the hole having slightly higher Pb isotopic ratios than the overlying basalt (Fig. 8).
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In general, the isotopic data of Salters et al. (1992) agree with our new data. An important exception are Pb isotopic data reported by Salters et al. (1992) for three samples powdered on the ship (samples 747-16R-2, 85–87 cm; 749-15R-2, 35–37 cm; and 750-16R3, 134–136 cm). These samples yielded aberrant highly radiogenic Pb ratios, which we did not find in our Pb isotopic analyses of 21 acid-leached samples powdered in agate (see insets in Fig. 8). We infer that these three aberrant samples were contaminated with Pb during shipboard sample preparation; therefore we exclude the Salters et al. (1992) Pb isotopic data for these three samples from our subsequent discussion.
Site 749
Isotopic data are available for 12 samples [seven in Table 4 and five from Salters et al. (1992)]. There is general agreement between the two datasets for Sr and Nd. The measured Sr and Nd data for Site 749 basalts are at the depleted end (i.e. relatively high 147Nd/144Nd and low 87Sr/86Sr) of the array defined by lavas attributed to the Kerguelen plume; the age-corrected ratios, however, are displaced to lower 143Nd/144Nd at a given 87Sr/86Sr (Fig. 7a). In detail, Sr and Nd isotopic data for Site 749 lavas define two groups, with basalts from Units 3 and 4 having higher 87Sr/86Sr and lower 143Nd/144Nd than basalts from Units 1, 5 and 6 (Table 4 and Fig. 7a). These two groups do not have different Pb isotopic ratios (Table 4 and Fig. 8). Compared with Kerguelen Archipelago lavas, Site 749 basalts have 206Pb/204Pb and 207Pb/204Pb most similar to the field for <10 Ma lavas from the Kerguelen Archipelago (Fig. 8), but they are not similar to these Kerguelen Archipelago lavas in 208Pb/204Pb or in Sr and Nd isotopic ratios (Figs 7a and 8). In Pb–Pb isotopic plots Site 749 lavas form vertical arrays that lie along a downwards extension of the vertical trends defined by basalts from Leg 183 Site 1137 on Elan Bank (Fig. 8). This vertical trend defined by Site 1137 lavas has been interpreted as reflecting contamination by continental crust (Weis et al., 2001; Ingle et al., 2002a).
Site 750
Isotopic data are available for 10 samples [six in Table 4 and four from Salters et al. (1992; personal communication, 1992)]. For Sr and Nd there is general agreement between the two datasets, with Unit 6 having distinctly lower 143Nd/144Nd and higher 87Sr/86Sr than the other units (Fig. 7a). Unit 6 lavas are also relatively enriched in incompatible elements (Fig. 5). A surprising result is that basalts from Sites 749 and 750 have similar 143Nd/144Nd, but all Site 750 basalts have distinctly higher 87Sr/86Sr (Fig. 7). Although 87Sr/86Sr age corrections are uncertain because of Rb mobility, the sample from Unit 6 at Site 750 with normal Ba/Rb (11·2, Fig. 5c) is also offset to high 87Sr/86Sr (Fig. 7a). The Unit 6 sample that is distinctive in Sr and Nd has slightly higher 208Pb/204Pb and 207Pb/204Pb than other Site 750 basalts (Table 4). Despite their differences in Sr and Nd isotopic ratios (Fig. 7a), the Pb isotopic fields for Site 750 and 747 basalts overlap, with both sites having low 206Pb/204Pb (17·5) compared with younger lavas associated with the Kerguelen plume (Fig. 8).
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONSAMPLING AND PETROGRAPHYSAMPLE PREPARATION AND...GEOCHEMICAL RESULTSDISCUSSIONREFERENCES |
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Mantle source for basalt forming the Kerguelen Plateau and Broken Ridge
A working hypothesis has been that much of the Kerguelen Plateau and Broken Ridge formed when the Kerguelen plume head impinged upon recently formed Indian Ocean lithosphere at
115 Ma (e.g. Shipboard Scientific Party, 2000). The uppermost igneous surface of this large igneous province, however, did not form over a brief time period (Duncan, 2002). In addition, a plot of 87Sr/86Sr vs 206Pb/204Pb (Fig. 7b) shows that: (1) drill sites on the Cretaceous Kerguelen Plateau (738, 747, 749, 750 and 1137) have distinctive isotopic signatures; (2) Cretaceous lavas associated with the Kerguelen plume are offset to low 206Pb/204Pb compared with Cenozoic lavas erupted in the Kerguelen Archipelago and at Heard Island and the Late Cretaceous to Cenozoic lavas erupted on the Ninetyeast Ridge. In subsequent discussion we evaluate the sources of this isotopic heterogeneity.
Role of a continental component: Site 747
Basalts from the Southern Kerguelen Plateau at ODP Site 738 have isotopic ratios of Sr, Nd and Pb (Figs 7 and 8) and relative depletions of Nb and Ta that clearly indicate contamination by continental lithosphere, probably crust (Alibert, 1991; Mahoney et al., 1995). Although not diagnostic, a common feature of basalt contaminated by continental crust is anomalously low Ti/Zr (e.g. Frey et al., 1996; Kent et al., 1997). For example, Ti/Zr is low in basalts from Site 738; Site 747 basalts from the Central Kerguelen Plateau also have low Ti/Zr (Fig. 9). In contrast, basalts with relative enrichment in Sr and Eu, i.e. all Site 750 samples and basalts from Units 1 and 6 at Site 749 (Fig. 6), have anomalously high Ti/Zr relative to the primitive mantle estimate of 115 (Fig. 9).
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A more diagnostic signature of continental crust is relative depletion in Nb and Ta (e.g. Thompson et al., 1984). As seen in Fig. 5a, basalts from Site 747 are relatively depleted in Th, Nb and Ta. To evaluate this depletion, we use a (Th/Nb)PM vs (La/Nb)PM plot (Fig. 10a and b), which not only shows the effects of continental crust assimilation, but in some cases may distinguish between upper and lower continental crust (Fitton et al., 1998a, 1998b; Shipboard Scientific Party, 2000). In this plot Site 747 data extend outside the field for mid-ocean ridge basalt (MORB) and OIB to (La/Nb)PM >1, but unlike basalts from ODP Site 738, (Th/Nb)PM is <1·2 for Site 747 basalts, except for two samples in adjacent cores, 14R1 and 14R2, which define a trend projecting toward the field of Site 738 basalts (Fig. 10a and b). The trend of increasing (La/Nb)PM at constant (Th/Nb)PM defined by most Site 747 basalts (Fig. 10b) is similar to that for North Atlantic MORB from the lower flow units at ODP Leg 152, Site 917, near eastern Greenland (Fig. 10c), which are contaminated by the Archean crust of eastern Greenland, specifically, lower-crustal granulite-facies gneiss (Fitton et al., 1998a, 1998b, 2000). The effects of continental contamination are very evident in Hole 917A basalt because these MORB-like lavas have much lower abundances of incompatible elements than most plume-related lavas. It should be noted that the combination of anomalously high La/Nb and normal Th/Nb in most Site 747 samples is unlike recent estimates for average lower continental crust (Fig. 10b) but such ratios are typical of some Archean granulites (e.g. the Lewisian, see W&T in Fig. 10c).
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The distinctive isotopic characteristics of Site 747 lavas are important in evaluating the source components that contributed to these lavas. In all figures involving Sr, Nd and Pb isotopic ratios, Site 747 lavas plot near the enriched OIB component EMI (Figs 7 and 8). The origin of this component has been discussed extensively and a possible scenario, largely based on Pb isotopic ratios, is that the EMI OIB contain a component derived from ancient pelagic sediments recycled into the mantle by subduction (e.g. Ben Othman et al., 1989; Weaver, 1991; Eisele et al., 2002). An alternative proposal is that continental lithosphere is detached during continental breakup and dispersed into the asthenosphere where it is subsequently sampled by MORB- or plume-related volcanism (e.g. Mahoney et al., 1992; Saunders et al., 1992). In particular, Archean granulitic lower crust typically has low Rb/Sr, and is relatively depleted in U and in some cases Th (e.g. Rudnick et al., 1985; Rudnick & Presper, 1990). Therefore, such rocks have relatively low 87Sr/86Sr for a given 143Nd/144Nd and retarded Pb isotopic ratios (e.g. Rudnick & Goldstein, 1990; Halliday et al., 1993); the Lewisian granulites are an extreme example (Fig. 11). Although ancient recycled slabs containing pelagic sediments can also explain the isotopic characteristics of EMI OIB (Ben Othman et al., 1989; Weaver, 1991; Eisele et al., 2002), there is no evidence that modern marine sediments have the relatively low (Th/Nb)PM and high (La/Nb)PM characteristic of most Site 747 lavas (Fig. 10b). For example, only four of 40 marine sediments analyzed by Plank & Langmuir (1998) have (Th/Nb)PM <2 and the average global subducting sediment has (Th/Nb)PM = 6·5. If these modern sediments are representative of ancient recycled sediments, a scenario using such sediments to explain the isotopic characteristics of Site 747 lavas will not explain the trend to high (La/Nb)PM at relatively low and constant (Th/Nb)PM (Fig. 10b). Therefore we conclude that the geochemical characteristics of most Site 747 basalt, i.e. the elevated La/Nb without high Th/Nb, relatively low 143Nd/144Nd at a given 87Sr/86Sr, and low 206Pb/204Pb and 207Pb/204Pb (Figs 7, 8 and 10), indicate contamination of plume-derived basalt by lower continental crust.
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) for average lower continental crust (Rudnick & Goldstein, 1990
Role of a continental component: Site 749
Compared with Site 747 basalt, a role for continental crust is not as obvious in basalt from Site 749 on the northwestern part of the Southern Kerguelen Plateau. Nevertheless, the displacement of basalt from Units 3 and 4 at Site 749 to lower 143Nd/144Nd and higher 87Sr/86Sr, with (La/Nb)PM and (Th/Nb)PM >1 (Figs 7 and 10b) coupled with a vertical array in 207Pb/204Pb vs 206Pb/204Pb (Fig. 8) are consistent with a role for upper continental crust in the petrogenesis of these lavas. This crustal component is geochemically similar to that found in basalt from Site 1137 basalt (Ingle et al., 2002b).
Role of plagioclase in the petrogenesis of Site 747, 749 and 750 basalt
The petrogenetic role of plagioclase for basalt from Sites 747 and 749 appears to be simple. Site 747 basalts are relatively depleted in Sr (Fig. 5a), and the extent of depletion, as measured by (Sr/Nd)PM, increases with increasing incompatible element abundances (Fig. 6c). This trend is expected in residual melts after fractionation of plagioclase, which characteristically has high Sr/Nd and low abundances of incompatible elements such as REE (e.g. Bindeman et al., 1998). Basalts from Units 3, 4 and 5 at Site 749 define a similar trend (Fig. 6c). The two Unit 6 samples from Site 749 are plagioclase rich (Table 1), have high (Sr/Nd)PM and Eu/Eu*, and low abundances of incompatible elements (Figs 3, 5b and 6). These geochemical characteristics are consistent with accumulate plagioclase. Surprisingly, the aphyric Site 749 Unit 1 lavas also have high (Sr/Nd)PM 1·6 and (Eu/Eu*) 1·07 (Fig. 6a). This observation hints at a complexity that is even more apparent in data for Site 750 lavas.
Deciphering the role of plagioclase in the petrogenesis of Site 750 basalt is much more complex. Each of the seven samples from Units 4 and 5 is strongly enriched in Sr and Eu [(Sr/Nd)PM 1·8–2·4, Eu/Eu* 1·06–1·14)]. In these respects they are similar to the plagioclase cumulates forming Unit 6 at Site 749 (Table 1, Fig. 6a). However, the modal proportion of phenocryst plagioclase in these Site 750 lavas ranges from 6 to 10% (Table 1, Fig. 6b); hence, the geochemical signature of plagioclase [(Sr/Nd)PM and (Eu/Eu*) >1] does not arise from large amounts of cumulate plagioclase. An alternative explanation is that the source for Site 750 basalt had a plagioclase geochemical signature.
The Pb isotopic ratios of Site 750 basalt are typical of the EMI type of OIB (Fig. 8). The origin of EMI geochemical characteristics was recently discussed by Gasperini et al. (2000) and Lustrino et al. (2000), who studied lavas from Sardinia that share many important geochemical features with Site 750 basalt (Figs 11 and 12). In particular, the relative enrichments of Sr, Ba, Eu and Pb, i.e. (Sr/Nd)PM, (Ba/Th)PM and Eu/Eu* >1 and Ce/Pb ratios (15) less than that of typical OIB (25), in the Site 750 basalt and Sardinia lavas (Fig. 12) are geochemical characteristics of plagioclase. Gasperini et al. (2000) argued that lavas from the Logudoro volcanic field of Sardinia are a particular sub-class of the EMI type, whose mantle source contained ancient recycled gabbros with abundant cumulate plagioclase when they formed. We conclude that a component formed as a plagioclase-rich cumulate was in the source of Site 750 basalt. Although this plagioclase may have been eliminated by subsequent recrystallization, in a closed system the geochemical characteristics of cumulate plagioclase are retained.
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One important difference between Logudoro and Site 750 lavas is that the latter do not have the anomalously low CaO/Al2O3 that is expected of plagioclase-rich rocks or lavas derived from sources that formed as plagioclase-rich cumulates (Fig. 13). The CaO/Al2O3 ratio of a low-pressure cumulate is, however, very sensitive to the modal proportions of clinopyroxene and plagioclase (Fig. 13). The high Sc abundances (42–44 ppm, Table 2), an element compatible in clinopyroxene, and high CaO/Al2O3 of Site 750 basalt relative to that of the Logudoro lavas is consistent with a source component that had cumulate clinopyroxene in addition to plagioclase. Less than 5% clinopyroxene is required to increase CaO/Al2O3 from the 0·4 of Logudoro lavas to the higher values of Site 750 lavas (Fig. 13). The atypically high Ti/Zr of Site 750 lavas (Fig. 9) is also consistent with a source containing cumulate clinopyroxene, as (Ti/Zr)cpx/(Ti/Zr)melt is 3 (Hart & Dunn, 1993; Skulski et al., 1994; Johnson, 1998). In contrast, the very low CaO/Al2O3 of the Logudoro lavas requires a very high proportion of sodic plagioclase (An56 in Fig. 13), and the absence of cumulate clinopyroxene.
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The origin of the plagioclase and clinopyroxene cumulate component and processes involved in creating the source of Site 750 basalt
Although the geochemical data for Site 750 basalt require a source component that originated as a cumulate of plagioclase and clinopyroxene, such data do not constrain the setting where this component formed or the processes that created Site 750 basalt. There are several alternative scenarios.
(1) The recycling via subduction of plagioclase-rich cumulates into the deep mantle where they become a plume component, perhaps as mafic layers in a peridotite matrix (Fig. 14a)
Specifically, for Sardinia basalt Gasperini et al. (2000) suggested deep recycling of plagioclase-rich gabbros associated with oceanic plateaux. Observations in favor of this hypothesis are the occurrences in massive peridotites of aluminous mafic layers with marked relative enrichments in Sr and Eu; these mafic layers are interpreted to have formed within the crust as plagioclase-rich cumulates that were recycled to at least the depths required for garnet stability (Takazawa et al., 1999; Morishita et al., 2002).
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(2) The assimilation of plagioclase-rich cumulates in the oceanic crust by plume-derived magmas (Fig. 14b)
Yang et al. (1998) proposed that some plume-derived magmas in the Kerguelen Archipelago assimilated in situ plagioclase-rich gabbros formed at an oceanic spreading center. Certainly plagioclase-rich cumulates are abundant in the oceanic lower crust (e.g. Benoit et al., 1996), and there is evidence for assimilation of plagioclase-rich cumulates formed in the oceanic crust (Bédard et al., 2000; Danyushevsky et al., 2001). As some lower-crustal gabbros in the Indian Ocean have non-radiogenic Pb isotopic ratios (206Pb/204Pb 17·4, 207Pb/204Pb 15·35, 208Pb/204Pb 37·0, Hart et al., 1999), such gabbros could explain the relatively non-radiogenic Pb in Site 750 basalt.
Granulites, formerly plagioclase-rich cumulates, exist in the lithosphere beneath the Kerguelen Archipelago (Grégoire et al., 1994). On the basis of isotopic data (Sr, Nd, Pb and Os) these granulites are related to Kerguelen Archipelago lavas, and they do not have the relatively non-radiogenic Pb isotope ratios found in Site 750 basalt (Mattielli et al., 1996; Hassler, 1999).
(3) Incorporation of plagioclase-rich lower continental crust into the upper mantle (Fig. 14c)
A small amount of lower continental crust mixed with upper-mantle peridotite could be a suitable source for Site 750 basalt because many lower continental crustal xenoliths are interpreted to have formed as plagioclase-rich cumulate rocks (e.g. Rudnick et al., 1986; Halliday et al., 1993). Also, Archean lower crust (especially granulites) is characterized by unradiogenic Pb isotopic ratios; hence such rocks could explain the trend to low Pb isotopic ratios defined by Site 750 basalts (Figs 8 and 11). Such a model has been proposed for Plio-Pleistocene lavas from Sardinia, where incorporation of lower continental crust into the mantle has been associated with continent–continent collision (Lustrino et al., 2000). A more appropriate process for the Kerguelen Plateau is that continental lithosphere was detached and incorporated into the asthenosphere or newly formed oceanic lithosphere during Gondwana breakup. This continental lithosphere was subsequently melted during the extension and plume-related volcanism that formed the Indian Ocean basin and Kerguelen Plateau, respectively.
Average lower continental crust is relatively depleted in Nb (Fig. 10a and c); therefore, an argument against a role for continental crust is the absence of Nb depletion in Site 750 basalt (Fig. 10b). There are, however, examples of mafic granulite xenoliths that lack a relative Nb depletion [e.g. table 6 of Rudnick & Fountain (1995), and samples from Fidra, Scotland, of Halliday et al. (1993)]. Moreover, appealing to a component derived from lower oceanic crust does not solve this problem because oceanic gabbros in the Oman ophiolite and from Hole 735B in the Southwest Indian Ridge have (La/Nb)PM >1·5 (Benoit et al., 1996; Hart et al., 1999).
In summary, each of these models can explain important geochemical characteristics of Site 750 basalt. However, on the basis of the evidence for a component derived from continental crust in Kerguelen Plateau basalt from ODP Sites 738, 747 and 1137, we suggest that the most plausible process for the petrogenesis of Site 750 basalt involves a contribution from lower continental crust that was introduced into the Indian Ocean asthenosphere during Gondwana breakup.
CONCLUSIONS
Tholeiitic basalts from the edges of the Kerguelen Plateau, ODP Site 738 in the Southern Kerguelen Plateau and ODP Site 1137 on Elan Bank (Fig. 1) are plume-derived magmas whose geochemical characteristics reflect a component derived from the continental crust (Mahoney et al., 1995; Ingle et al., 2002a; Weis et al., 2002). These crustal components are characterized by relative depletion in Nb and Ta and relatively high 207Pb/204Pb and 208Pb/204Pb at a moderate 206Pb/204Pb ratio (17·8–18). Lavas from the northwestern part of the Southern Kerguelen Plateau (Site 749) are not as obviously contaminated by continental crust, but they may contain lesser amounts of the continental crustal contaminant found in Site 1137 basalt.
In the interior of the Central (Site 747) and northeastern part of the Southern Kerguelen Plateau (Site 750), the igneous basement is also formed of tholeiitic basalt that was contaminated by continental crust. The nature of this contaminant, however, differs at the two sites and they are distinct from the continental crust components present in basalt from Sites 738 and 1137. At Site 747, a role for lower continental crust is indicated by relative depletion in the Th, Nb and Ta abundances, very low 206Pb/204Pb, a trend to high 207Pb/204Pb at a given 206Pb/204Pb and offset to low 143Nd/144Nd at a given 87Sr/86Sr. In contrast, Site 750 basalts are not depleted in Th, Nb and Ta, but they have Pb isotopic characteristics similar to Site 747 lavas. In addition, the source of Site 750 lavas contained a component that formed as a plagioclase-rich cumulate containing clinopyroxene. Such rocks occur in the lower oceanic and continental crust; given the evidence for diverse components derived from continental crust at other locations in the Kerguelen Plateau, we suggest that a continental component also contributed to Site 750 basalt.
Of course, a critical question is: do these different geochemical signatures reflect (1) a very heterogeneous plume composed in part of deeply recycled crust or (2) contamination of plume material by fragments of continental lithosphere dispersed within the asthenosphere and lithosphere of the Indian Ocean? We favor the latter hypothesis because small clasts of continental crust have been found intercalated within basalts at Site 1137 on Elan Bank (Nicolaysen et al., 2001; Weis et al., 2001; Ingle et al., 2002b), and seismic data are interpreted to indicate continental crust within the lithosphere at Elan Bank (Charvis et al., 1997) and the northern part of the southern Kerguelen Plateau (Operto & Charvis, 1996). Not surprisingly, given the heterogeneity of continental lithosphere, geochemically distinctive continental components are required by basalts from Sites 738, 747, 750 and 1137.
In addition, there is increasing recognition that other Indian Ocean basalts contain a component derived from continental lithosphere. For example, the 80 Ma olivine tholeiitic basalts forming the Aphanasey Nikitin Rise in the central basin of the Indian Ocean have (La/Nb)PM–(Th/Nb)PM systematics very similar to Site 747 lavas, (La/Nb)PM ranges from 1·5 to 2·3 but (Th/Nb)PM is low, ranging from 0·65 to 1·2. These Aphanasey Nikitin basalts have isotopic characteristics that are more extreme than those for EMI (Fig. 11). Borisova et al. (2001) proposed that these geochemical data are best explained by mantle-derived tholeiitic magmas assimilating ancient (>109 a) lower Gondwanan crust residing in the Indian Ocean lithosphere. Also, the active Southwest Indian Ridge from 39° to 41°E is characterized by unusual isotopic ratios for MORB. These young lavas define a trend to low 143Nd/144Nd at a given 87Sr/86Sr, and they have Pb isotopic ratios even lower than those for EMI (Mahoney et al., 1992; Fig. 11). Mahoney et al. (1992) concluded that the source of these MORB contained stranded continental lithosphere, thermally eroded from the India–Madagascar continent by the Marion plume. Moreover, a role for continental lithosphere in the petrogenesis of oceanic basalt is not limited to the Indian Ocean; for example, Douglass et al. (1999) found evidence for a subcontinental lithospheric mantle component contributing to South Atlantic MORB.
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ACKNOWLEDGEMENTS |
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This research used samples provided by the Ocean Drilling Program (ODP). The ODP is sponsored by the US National Science Foundation (NSF) and participating countries under management of Joint Oceanographic Institutions Inc. Funding for this research was provided by the US Science Support Program, NSF Grant EAR-9814313, the Belgian Communauté Française de Belgique ARC Grant 98/03-233 and the Fonds National de la Recherche Scientifique (FNRS) Grant 2.4579.99. The FNRS funds Belgium membership in the ODP program. We thank C. Maerschalk for operation of the chemistry laboratory associated with the mass spectrometer, P. Ila for supervision of MIT Neutron Activation Analysis Facility, B. Grant for ICP-MS expertise and J. M. Rhodes for access to the University of Massachusetts X-ray Fluorescence Facility. The review comments of M. Coffin, G. Fitton, R. Kent, J. Mahoney, R. Rudnick and V. Salters are very much appreciated. We thank M. Coffin for suggesting and assisting in preparation of Fig. 14.
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FOOTNOTES |
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*Corresponding author. Fax: 617-253-7102. E-mail: fafrey{at}mit.edu
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TOPABSTRACTINTRODUCTIONSAMPLING AND PETROGRAPHYSAMPLE PREPARATION AND...GEOCHEMICAL RESULTSDISCUSSIONREFERENCES |
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