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Journal of Petrology | Volume 43 | Number 7 | Pages 1327-1339 | 2002
© Oxford University Press 2002
Hf Isotope Evidence for a Miocene Change in the Kerguelen Mantle Plume Composition
1DÉPARTEMENT DES SCIENCES DE LA TERRE ET DE L’ENVIRONNEMENT, UNIVERSITÉ LIBRE DE BRUXELLES, CP160/02, AVENUE F. D. ROOSEVELT, 50, B-1050 BRUSSELS, BELGIUM
2ÉCOLE NATIONALE SUPÉRIEURE DE LYON, UMR CNRS 5570, 69364 LYON, FRANCE
Received June 13, 2001; Revised typescript accepted February 20, 2002
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONSAMPLE SELECTIONANALYTICAL PROCEDURERESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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We report high-precision multiple collector inductively coupled plasma mass spectrometry (MC-ICP-MS) analyses of Hf isotopic compositions for 39 Kerguelen Archipelago volcanic rocks that add a new perspective to the temporal evolution of the Kerguelen mantle plume. Samples cover the entire range of chemical and Sr, Nd and Pb isotopic variations reported for the archipelago, and vary in age from 29 to 0·1 Ma. Their Hf isotopic compositions show the largest variation for an individual oceanic island and
Hf correlates negatively with alkalinity. The Hf–Nd isotopic compositions of the pre-Miocene flood basalt group (>25 Ma) are distinct from those of the <10 Ma group of Upper Miocene and Quaternary Mt. Ross volcanic rocks and plot along separate en-echelon Hf–Nd arrays. The Lower Miocene Series mildly alkaline basalts from the Southeast Province are transitional between the two groups and indicate a major replacement of plume source components over a relatively short period of time. The geochemical systematics of the >25 Ma group of flood basalts require interaction between a shallow reservoir of asthenospheric mantle or oceanic lithosphere and the Kerguelen plume core, best represented by the Mt. Crozier basalts on the archipelago. This scenario is consistent with the relative proximity of the Kerguelen Archipelago to the Southeast Indian Ridge (
300–400 km) when the >25 Ma flood basalts erupted. The younger alkaline volcanic rocks were generated by low-degree melting in the plume envelope, an area of enriched mantle material, different from the plume core, and presumably formed by different combinations of old recycled oceanic crust and plateaux. The enriched geochemical character of the Kerguelen plume core supports the hypothesis that the EM I-type plume source includes recycled oceanic plateaux. KEY WORDS: Hf isotopic geochemistry; Kerguelen plume; mantle dynamics; plume–ridge interaction
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONSAMPLE SELECTIONANALYTICAL PROCEDURERESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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Spatial and temporal isotope variations in basalts can provide important constraints on the origin of the mantle source of hotspots, plume dynamics and the interaction of plumes with the oceanic lithosphere. The relationships between ocean island basalts (OIB) and mantle geochemistry have been investigated in recent publications on both ridge-axis hotspots (Iceland; Hanan & Schilling, 1997
; Chauvel & Hémond, 2000; Kempton et al., 2000), and within-plate hotspots (Hawaii; e.g. Lassiter et al., 1996; Lassiter & Hauri, 1998; Blichert-Toft et al., 1999). The Kerguelen plume is an important case to study because (1) basalts related to the Kerguelen hotspot show isotopic signatures falling between the two enriched mantle components, EM I and EM II (Zindler & Hart, 1986), and are therefore critical for delimiting the range of enriched isotopic variations for OIB, and (2) during formation of the Kerguelen Archipelago the environment of the hotspot changed from ridge-centred at 40 Ma to the present intra-plate position. The archipelago has been centred on or near the hotspot because the Antarctic plate has remained essentially stationary with respect to the hotspot reference frame since the Eocene (Duncan & Richards, 1991). The recent extensive and systematic sampling of volcanic stratigraphic sections from the Kerguelen Archipelago provides a new framework within which to study the evolution of the Kerguelen plume.
The Kerguelen Archipelago formed on the northernmost part of the Kerguelen Plateau on the Antarctic plate, far from continental margins. In contrast to the northern part of the Kerguelen Plateau, the southern and central parts began to grow within the newly formed Indian Ocean basin near the rifted continental margins between 119 and 90 Ma (Pringle et al., 1994). The discovery of Proterozoic continental crustal rocks within the basaltic basement of Elan Bank (the western salient of the Southern Kerguelen Plateau) is consistent with contamination of Cretaceous Kerguelen plume magmas by partial melting of isolated continental crust fragments (e.g. Frey et al., 2000b; Weis et al., 2001). To date, evidence for contamination by continental crustal components has not been found in the Cenozoic Kerguelen Archipelago volcanic rocks (Weis et al., 1998a; Yang et al., 1998), despite some evidence for a minor continental component in the genesis of a few ultramafic xenoliths sampled by young (6–10 Ma) basanitic plugs and dykes on the archipelago (Hassler & Shimizu, 1998; Mattielli et al., 1999).
Here we present a detailed study on Hf isotopes in Kerguelen Archipelago volcanic rocks, which provides important constraints on the composition and evolution of the Kerguelen plume source. We have focused on the Lu–Hf isotopic system because it is a sensitive monitor of source components and mineralogy, as the different chemical behaviour of the parent and daughter elements results in larger variations in 176Hf/177Hf relative to 143Nd/144Nd. Until this study, the Hf isotope record for the Kerguelen volcanic rocks was extremely limited and produced by less Hf-sensitive thermal ionization mass spectrometry (TIMS) and hot-SIMS (secondary ionization mass spectrometry) techniques (Patchett, 1983; Salters & Hart, 1991; Salters & White, 1998). The new dataset presented here contains Hf isotope compositions for 39 samples of Kerguelen Archipelago volcanic rocks, mostly basalts dated from 29 to 0 Ma (Weis et al., 1993, 1998b; Nicolaysen et al., 2000).
The purpose of our Hf isotopic study on Kerguelen Archipelago lavas is threefold: (1) to establish the Hf isotopic signature of the Kerguelen plume; (2) to estimate the temporal Hf isotope variations in the Kerguelen Archipelago volcanic rocks; (3) to add another isotopic dimension to the evolution of the Kerguelen plume composition and determine the nature of the components assimilated by the plume during its ascent.
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SAMPLE SELECTION |
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TOPABSTRACTINTRODUCTIONSAMPLE SELECTIONANALYTICAL PROCEDURERESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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Thirty-nine samples were selected to represent the widest possible range in major and trace element and isotopic compositions of volcanic rocks from the Kerguelen Archipelago. The time- and space-dependent compositional evolution of Kerguelen Archipelago basaltic volcanism has recently been described in several studies (e.g. Weis et al., 1998a
; Yang et al., 1998; Frey et al., 2000c). Volcanism on the archipelago occurred from 29 Ma to the present day, although the majority of the flood basalts were erupted between 29 and 24 Ma (Nicolaysen et al., 2000). With decreasing eruption age, the flood basalts become more alkalic (Weis et al., 1993, 1998b; Yang et al., 1998).
We analysed 28 flood basalt samples from basaltic stratigraphic sections from throughout the archipelago, which include, from NW to SE: Mt. Bureau (29·3 Ma), Mt. Rabouillère (29 Ma), Mt. Tourmente (26 Ma) and Mt. Crozier (24·8 Ma) (Fig. 1). We also analysed four samples from the Lower Miocene Series (LMS, 24·8 Ma) of the Southeast Province (three basalts and one trachyte). The flood basalts form >85% of the exposed surface of the archipelago and range in composition from transitional (Na2O + K2O = 2·4) for the oldest sections (29–26 Ma) to alkaline (Na2O + K2O = 7·4) for the younger sections (25 Ma). Another seven samples are from the youngest archipelago flows of the Upper Miocene Series (UMS) of the Southeast Province (basanite and phonolite, <10 Ma) and Mt. Ross (basaltic trachyandesite and trachyte, <2 Ma; except one gabbro dated at 21 Ma) (Fig. 1). All of the latter samples are characterized by their petrographic freshness. In contrast, the older sections, and especially Mt. Bureau and Mt. Rabouillère samples, contain secondary minerals, dominantly zeolites (Yang et al., 1998). The ages of the selected samples cover the entire period of volcanic activity on the archipelago from 29 Ma (Mt. Bureau) to 0·1 Ma (Mt. Ross) (Weis et al., 1998b; Nicolaysen et al., 2000) (Fig. 1). The lavas show a large variation in MgO contents ranging from 13·3 wt % for the less evolved basalts to 0·05 wt % for the trachytes. The majority of samples, however, typically contain 2·3–8 wt % MgO. The provenance, rock type and age of the samples are summarized in Table 1, together with references for detailed petrographic and chemical descriptions and petrogenetic interpretations.
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ANALYTICAL PROCEDURE |
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TOPABSTRACTINTRODUCTIONSAMPLE SELECTIONANALYTICAL PROCEDURERESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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Hf was chemically purified in the clean laboratory at the Université Libre de Bruxelles and its isotopic composition was analysed by multiple collector inductively coupled plasma mass spectrometry (MC-ICP-MS) on the VG Plasma 54 instrument at the École Normale Supérieure de Lyon following the procedure described by Blichert-Toft et al. (1997)
. About 250 mg of rock powder was dissolved with a 3:1 mixture of concentrated HF (40%) and HNO3 (65%) in Savillex vials at 130°C for 2 days. After evaporation, concentrated HF was added to the residue and the closed Savillex beaker was left on a hot plate for another 2 days, bringing the maximum amount of Hf into solution while the precipitation of fluoride salts entrained the rare earth elements (REE). Subsequent Hf purification required a two-stage elution procedure involving anion and cation exchange resins [compare the chemical separation procedure of Blichert-Toft et al. (1997)]. Total procedural Hf blanks were <23 pg, which is at least a factor of 105 lower than the amount of processed Hf.
The Hf isotopic compositions were analysed in static mode with cup efficiency factors assigned to the Faraday cups to correct for collector biases (<150 ppm). All measured Hf isotopic ratios were corrected for W and Ta, and Lu and Yb interferences on masses 180 and 176, respectively, by monitoring the isotopes 182W and 181Ta, and 175Lu and 173Yb. Mass fractionation was normalized to 179Hf/177Hf of 0·7325 using an exponential law. Internal precision on Hf isotope compositions was in the range of 10–30 ppm, and the external reproducibility measured on pure Hf standard solutions was better than 40 ppm. From two measurement sessions over a 1 year period, 18 analyses of the JMC-475 Hf standard gave an unweighted mean for 176Hf/177Hf of 0·282162 ± 24 (2
m). Good reproducibility was also demonstrated by duplicate analyses of three samples, for which values agree within 0·2–0·4 Hf units (Table 1).
Two samples were leached following the procedure described by Weis & Frey (1991). The isotopic differences between the leachate and the residue for both samples were within the reproducibility obtained on duplicate measurements (Table 1). As a result, Hf isotopic compositions were measured on unleached powders, whereas the isotopic compositions of Sr, Nd and Pb were analysed on leached powders (Weis et al., 1998b; Yang et al., 1998; Frey et al., 2002a), except for the first survey study in the Southeast Province (Weis et al., 1993).
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RESULTS |
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TOPABSTRACTINTRODUCTIONSAMPLE SELECTIONANALYTICAL PROCEDURERESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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The Hf isotopic compositions for the Kerguelen Archipelago volcanic rocks show the largest observed variation so far for an individual oceanic island for all OIB samples constituting the mantle array (Fig. 2). 176Hf/177Hf varies from 0·28267 to 0·28307, corresponding to 14 units of
Hf. [All the initial Hf were calculated with values of (176Hf/177Hf)CHUR(0) = 0·282772 and (176Lu/177Hf)CHUR(0) = 0·0332 (Blichert-Toft & Albarède, 1997). For calculations of initial Nd we used (143Nd/144Nd)CHUR(0) = 0·512638 and (147Sm/144Nd)CHUR(0) = 0·1966 (Jacobsen & Wasserburg, 1980).] Our analyses are presented in Table 1, together with Nd isotopic compositions reported in the literature. In comparison, the Hawaii array (Blichert-Toft et al., 1999) and the OIB array (Salters & White, 1998; Vervoort et al., 1999) cover nine and 21 Hf units, respectively. The age corrections for the Kerguelen samples do not modify the overall Hf isotopic data distribution and do not significantly affect the Hf isotopic ratios discussed in this paper [the maximum correction is 1·2 x 10-5, i.e. 0·5 Hf unit, on the 176Hf/177Hf value of a 29 Ma Mt. Bureau basalt; it is typically around (5–6) x 10-6 for most of the flood basalts].
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) analysed in this study and reported in Table 
), Kerguelen Archipelago (hatched circles): Salters & Hart (1991)
The Kerguelen Archipelago Hf–Nd isotopic array as a whole is linear with an intercept I of +2·87 and a slope S of 1·55, i.e. slightly steeper than slopes of the global mantle array (1·33) and the terrestrial array (1·36) (Vervoort et al., 1999) (Fig. 2). Individual oceanic island arrays (Salters & White, 1998; Blichert-Toft et al., 1999; Kempton et al., 2000) are commonly characterized by shallower slopes (e.g. Hawaiian basalts, S 1·00; Samoa, 0·72; Comores, 1·25). The Pitcairn Island array also has a slope at 1·0 (Eisele et al., 2002), whereas arrays for Walvis Ridge, another EM I-type locality, and Iceland (Kempton et al., 2000), have steeper slopes (S > 1·5). A strong isotopic contrast is observed in Hf between the pre-Miocene flood basalts and the group of Upper Miocene and Quaternary Mt. Ross volcanic rocks (Fig. 3), similar to that described for Pb isotopes (Weis et al., 1998a, 2002). The pre-Miocene basalts include the Mt. Bureau, Mt. Rabouillère, Mt. Tourmente and Mt. Crozier volcanic sections and show variations in 176Hf/177Hf from 0·28281 to 0·28307. The Upper Miocene Series and Mt. Ross volcanic rocks have less radiogenic Hf isotopic compositions and range in 176Hf/177Hf from 0·28267 to 0·28276. In a plot of Hf vs Nd, each of these groups forms an elongated field with a slope of 1·1, whereas the Lower Miocene Series (0·28280–0·28283) appears to represent a transition between the two groups at almost constant Hf isotopic composition. The Hf isotopic differences between these groups are correlated with the alkalinity index: high Hf basalts are tholeiitic and transitional, whereas low Hf volcanic rocks are dominantly alkalic from the Upper Miocene Series and Mt. Ross (Fig. 4).
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Previously published Hf isotopic data for basalt related to the Kerguelen plume, all determined by TIMS and hot-SIMS, are scarce. There are data for six samples from the Kerguelen Archipelago (Salters & Hart, 1991; Salters & White, 1998) and five samples from throughout the Kerguelen Plateau (Salters & Hart, 1991). In a Nd–Hf plot these results overlap our dataset, and they also show a strong distinction in isotopic signatures between the Kerguelen Plateau and Archipelago basalts, except for the one sample of Site 749 (Fig. 3). Patchett (1983) reported five Hf isotope analyses for archipelago lavas. In a Hf–Nd isotopic diagram, these data points fall outside the field of Kerguelen basalts (as defined by all the other data, including our new data). Their Nd isotopic compositions (Dosso & Murthy, 1980) are significantly lower (by 0·00011 on average with a 2m error on 143Nd/144Nd of 28 x 10-6) than any of the more recent Nd isotope data obtained on Kerguelen lavas (over 160 samples). These samples are not reported in the figures of this study. In contrast, results reported by Salters & Hart (1991) and Salters & White (1998) overlap our dataset and emphasize the strong distinction in isotopic signatures between the Kerguelen Plateau and Archipelago basalts. Discussion of new Hf isotope results for the Cretaceous Kerguelen Plateau basalts (Mattielli et al., 2000) will be published elsewhere.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONSAMPLE SELECTIONANALYTICAL PROCEDURERESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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The new hafnium isotopic results demonstrate the important differences between the younger (<10 Ma) volcanic rocks and the older (pre-Miocene) basalts (Fig. 3). The en-echelon arrays in the
Hf vs Nd plot correspond to two different periods of magmatic activity, with the Lower Miocene Series from the Southeast Province as a transitional series between the two. The Hf isotopic contrast between the younger volcanic rocks and the Mt. Crozier basalts from the pre-Miocene period is also consistent with the contrast between their 206Pb/204Pb values and their 
7/4 and 8/4 values [ values as defined by Hart (1984)]. The younger volcanic rocks have distinctly lower 206Pb/204Pb, whereas the Mt. Crozier basalts have the most radiogenic Pb isotopic compositions that we consider as the best representation of the Kerguelen plume (Weis et al., 1998a, 2002). In contrast, the older transitional flood basalts, including those of Mt. Bureau, Mt. Rabouillère (Yang et al., 1998), Mt. des Ruches and Mt. Fontaine (Doucet et al., 2002), which erupted on the NW part of the archipelago [i.e. the closest to the Southeast Indian Ridge (SEIR)], have more variable 206Pb/204Pb, together with lower 87Sr/86Sr and higher 143Nd/144Nd, reflecting the presence of a depleted component in their genesis.
To establish the intrinsic compositional characteristics of the Kerguelen plume, it is necessary to distinguish the chemical signatures generated by the plume itself from those arising from the plume’s interaction with shallow material on its way up to the surface. Several studies have suggested that much of the isotopic and chemical heterogeneity observed within individual OIB suites reflects different degrees of interaction between plume melts and the oceanic crust and the underlying lithospheric mantle (Chen & Frey, 1985; Storey et al., 1988; Halliday et al., 1995). An important tool in deciphering the interaction of the plume magmas with the lithospheric mantle and oceanic crust is the relationship between the uncontaminated isotopic ratios of the source magmas and their major element chemistry. Alkalinity, MgO and other major element contents of basaltic magmas are acquired during melting and modified during differentiation and interaction of the magmas with the overlying lithospheric mantle. Typically, variation in MgO contents results from shallow-level olivine fractionation, whereas variation in alkalinity results from changes in degree of melting in the lithosphere (Chen & Frey, 1985), depth of melting, or clinopyroxene fractionation in the deeper part of the suboceanic lithosphere. Correlation between isotopic ratios and major elements, therefore, indicates interaction of plume magmas with the lithosphere.
Depleted mantle component in pre-Miocene Kerguelen Archipelago basalts
Storey et al. (1988) and Gautier et al. (1990) identified a temporal evolution for Kerguelen Archipelago volcanism, going from older tholeiitic to transitional basalts with more depleted Sr and Nd isotopic characteristics, to more alkaline volcanic rocks with more enriched Sr and Nd isotopic characteristics. They correlated this evolution with the migration of the hotspot away from the SEIR. This interpretation was evaluated by Yang et al. (1998), who pointed out that the plume component has been present throughout the entire history of the archipelago as reflected by the presence of enriched Nd and Sr isotopic ratios in basaltic rocks ranging in age over several tens of million years. Nevertheless, a new study of some of the older basalts present in the northern part of the archipelago (Doucet et al., 2002) also documents interaction between depleted SEIR-type material and the Kerguelen plume. There are broad correlations between isotopic ratios and MgO in the northern archipelago sections: in Mt. Bureau and Mt. Rabouillère, a series of basalts [defined as the D-group by Yang et al. (1998)] are characterized by higher MgO contents and a more depleted isotopic signature; similarly, some of the older flood basalts of Mt. des Ruches and Mt. Fontaine show a positive correlation between isotopic ratios and MgO (Doucet et al., 2002).
In the present study, where the samples have a much broader age and compositional range, we also document correlation between MgO, alkalinity and isotopic compositions. This correlation is most notable between alkalinity and Hf (Fig. 4). The young, low Hf volcanic rocks are highly alkaline (4 < AI < 10, where AI is alkalinity index), whereas the oldest (28–29 Ma), high Hf flood basalts have tholeiitic–transitional compositions (-2·4 < AI < 0) (Yang et al., 1998; Doucet et al., 2002; Frey et al., 2002a). The 25 Ma flood basalts are alkaline in composition (Weis et al., 1993; Frey et al., 2000c; Damasceno et al., 2002). Albarède (1992) noted that in the older tholeiitic Kerguelen basalts Al behaves compatibly during differentiation. This also occurs in Iceland and Galapagos basalts but does not occur in intra-plate hotspots. This contrasting behaviour presumably reflects the disappearance of plagioclase from the liquidus with deeper levels of fractionation, as the lithosphere becomes progressively thicker when the plume moves away from the ridge.
It is important to consider, when discussing the origin of the Hf–Nd isotopic correlation as well as the correlation with MgO and alkalinity for the older group of pre-Miocene basalts, that the Kerguelen hotspot was located relatively close to the SEIR when these basalts were erupted. Site 1140 basalts in the Northern Kerguelen Plateau were formed at 34 Ma, at 50 km from the SEIR, and their geochemical and isotopic trends can be explained by binary mixing between SEIR-MORB and Kerguelen plume magmas (Weis & Frey, 2002). At the time of formation of the oldest Kerguelen Archipelago flood basalts (28–29 Ma), the SEIR was 350 km away (Royer & Sandwell, 1989). Abundant plagioclase phenocrysts and evidence for plagioclase fractionation from Sr depletion (Yang et al., 1998) require the existence of shallow magmatic reservoirs for the Mt. Bureau and Mt. Rabouillère sections. The radiogenic Hf and Nd and unradiogenic Pb end-member of the pre-Miocene basalt group may be represented by asthenospheric mantle or oceanic lithosphere. Isotopic data for the oldest archipelago basalts trend systematically towards the composition of the SEIR basalts in Hf–Nd, Sr–Nd and Hf–Pb isotope space (Figs 3 and 5). Hf isotopic data for Site 1140 basalts (Northern Kerguelen Plateau) extend this trend because they systematically plot between fields of the oldest Kerguelen Archipelago basalts and the SEIR basalts (Mattielli et al., 2000; Weis & Frey, 2002). The unradiogenic Hf and Nd and radiogenic Pb end-member on the pre-Miocene Hf–Nd trend, here best represented by Mt. Crozier basalts, is interpreted as essentially unmodified plume material (Weis et al., 1998a, 2002). The lack of linear Hf–Pb isotopic relationships (Fig. 5) requires the presence of a small amount of an additional component, probably similar to the one that dominates in the younger lavas.
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Major change in the source components at 20 Ma
The Hf–Nd isotopic trend for the younger lavas (<10 Ma) is parallel to that of the older flood basalts, but with distinctly lower Hf. A remarkable observation is that the transition to the younger group indicated by the variable 143Nd/144Nd at constant 176Hf/177Hf of the Lower Miocene Series requires a relatively rapid change in the composition of the source. The 20 Ma Kerimis basalts dredged on seamounts between Kerguelen Archipelago and Heard Island and interpreted as resulting from the activity of the Kerguelen plume plot within the lower trend for the younger (<10 Ma) volcanic rocks (Weis et al., 2002) (Figs 3 and 5). Hf and MgO for the pre-Miocene lavas form a general positive trend, but MgO is not correlated with Hf in the <10 Ma lavas, which have distinctly lower Hf than the pre-Miocene basalts (Fig. 4). In the 176Hf/177Hf vs 143Nd/144Nd and 206Pb/204Pb diagrams (Figs 3 and 5), the <10 Ma volcanic rocks trend towards the compositions of basalts from Site 747 on the Cretaceous Kerguelen Plateau, although the young archipelago lavas do not exhibit such low 206Pb/204Pb.
Three hypotheses to account for the isotopic systematics of the younger, <10 Ma, volcanic rocks are as follows:
- the presence of an ancient pelagic component, as invoked by Blichert-Toft et al. (1999) to explain the shallow Hf vs Nd slope of Hawaiian basalts, could account for the unradiogenic lead present in the younger Kerguelen lavas, but is contradicted by the lack of a Nb deficit in Mt. Ross lavas (Weis et al., 1998a). The absence of a Nb anomaly and the nearly chondritic Hf/Sm ratios (0·7 ± 0·1) (Weis et al., 1993, 1998b; Yang et al., 1998; Frey et al., 2000c, 2002a; Doucet et al., 2002) also exclude a significant contribution of continental crust material in the source of all Kerguelen Archipelago basalts.
- The enriched isotopic signature of the young archipelago lavas is even stronger in and dominates the Cretaceous basalts from the Southern Kerguelen Plateau and Elan Bank (Mattielli et al., 2000). For these Kerguelen Plateau basalts, this enriched signature may reflect incorporation of partial melts of continental crust fragments in the early stages of the Kerguelen plume activity (Frey et al., 2002b). The low 176Hf/177Hf, 143Nd/144Nd and 206Pb/204Pb are also present in the 20 Ma Kerimis basalts sampled in seamounts between Heard and Kerguelen Islands (Weis et al., 2002) (Figs 3 and 5). The 20 Ma basaltic seamounts and <10 Ma lavas may have assimilated Cretaceous Kerguelen Plateau lithosphere.
- Relative to the source of the pre-Miocene, plume-related basalts from Mt. Crozier, the source for these younger (20 and <10 Ma) lavas is located in a different zone of the Kerguelen plume.
To discriminate between the last two hypotheses, it is necessary to address the source characteristics of the Kerguelen plume at a broader scale.
Kerguelen plume source characteristics
The origin of the source characteristics of the Kerguelen plume is a subject of considerable debate (Class et al., 1993; Barling et al., 1994; Weis et al., 1998b, 2002; Yang et al., 1998). Following Weis et al. (1998a, 2002), and because of their widespread distribution and relatively homogeneous character, we suggest that the isotopic compositions of the basalts from Mt. Crozier are likely to reflect the mantle source composition of the main volume of the Kerguelen plume. The geochemical characteristics of these basalts, including their 187Os/188Os at 0·1395 ± 21 (Weis et al., 2000) are the least, if at all, affected by shallow components (oceanic lithosphere, Kerguelen plateau lithosphere or continental crust). We interpret these features as representing the core of the Kerguelen plume; they belong to the Dupal anomaly (Hart, 1984), which is characteristic of the southern hemisphere mantle.
In general, Kerguelen Archipelago basalts show geochemical characteristics intermediate between those of the EM I and EM II components, especially in Sr–Nd plots (Zindler & Hart, 1986). The presence of an EM I component in some older Kerguelen Plateau lavas (Sites 747) has been discussed by Frey et al. (2002b). In the case of these early products of the Kerguelen plume, the EM I feature can be related to contamination by lower continental crust material in the early stages of the break-up of Gondwana.
The EM sources are long-standing puzzles, especially EM I, because its geochemistry has been proposed to be consistent with contributions from recycled, ancient pelagic and/or metalliferous sediments (White, 1985; Weaver, 1991), recycled oceanic plateaux (Gasperini et al., 2000), or ancient subcontinental lithospheric mantle (see Hofmann, 1997, for review). The EM I reservoir is typically inferred to be derived from recycled oceanic crust containing a few percent of continent-derived sediments (White & Hofmann, 1982; Chauvel et al., 1992). The moderately enriched character of the Kerguelen plume end-member (Crozier) precludes ancient recycled MORB as the only source component (Hofmann & White, 1982). Although it may be argued that contamination by granulitic basement may account for the EM I ‘flavour’ at some parts of the Cretaceous Kerguelen Plateau where continental remnants have been identified (Frey et al., 2002b), the presence of EM I is equally the hallmark of some small volcanic oceanic islands (e.g. Pitcairn) that can hardly be suspected to harbour continental fragments (Eisele et al., 2002). The lack of a Nb anomaly and chondritic Hf/Sm ratios in the Crozier basalts further argue against the presence of a substantial amount of pelagic sediments and other continental detritus in the source of the Kerguelen plume. In contrast, oceanic lithosphere enriched by the emplacement of oceanic plateaux in the past (Gasperini et al., 2000), i.e. more enriched in incompatible elements, would, upon subduction and radiogenic ingrowth leading to higher 87Sr/86Sr and more radiogenic Pb ratios, become an appropriate mantle source of enriched Kerguelen plume material, as represented by the Mt. Crozier basalts.
Assuming the 25 Ma Mt. Crozier lavas reflect the main part of the plume, what is the origin of the low Hf–Nd component in the <10 Ma lavas? If contamination by Kerguelen Plateau material is invoked to explain the geochemical characteristics of some of the young Kerguelen alkaline magmas [hypothesis (2)], it does not explain how this component formed in the first place. The large size of the plateau, as well as its isotopic and age heterogeneity, indicates that a unique plateau component does not exist. The enriched end-member observed in the recent archipelago lavas and in the seamounts between Heard and Kerguelen may represent magmas emplaced at the periphery of the Kerguelen plume [hypothesis (3)]. An analogue is the North Arch magmas that are known to fringe the Hawaiian plume hundreds of kilometres from the main eruptive centres (Frey et al., 2000a) and have been suggested to represent melting of the mantle far in the wake of the Hawaiian hotspot (Ribe & Christensen, 1999). We favour the suggestion that the source of the younger lavas represents enriched mantle material with an age, and therefore isotopic compositions, different from those of the Kerguelen plume end-member reflected in Mt. Crozier basalts. This additional end-member presumably reflects a different combination of old recycled oceanic crust and plateaux in the plume source.
We propose a possible general scenario to account for the distinctive isotopic compositions of the Kerguelen Archipelago basaltic lavas. At 35 Ma, the Kerguelen hotspot was still close to the SEIR and magmas from the upwelling Kerguelen plume interacted with those from the nearby ridge (Weis & Frey, 2002). The plume-derived magmas, best represented by the 25 Ma Crozier basalts but also erupted throughout the archipelago, do not contain a depleted component. As inferred from trace element evidence, magma supply declined at 20 Ma (Frey et al., 2000c). This was probably the result of both the SEIR migrating away from the hotspot and a change in the nature of the mantle material constituting the source of magmatism. With the SEIR migrating away, the thicker oceanic or possibly Kerguelen Plateau lithosphere offered more efficient thermal blanketing and magma supply from the plume declined rapidly. The eruption centre moved into an area invaded by magmatic products emitted earlier, at the periphery of the hotspot, and the magmatism became more diffuse. The relatively static mantle of the Antarctic plate choked the hotspot or at least drastically reduced the magma supply to the surface. Large-degree melts of the hotspot core were rapidly replaced by small-degree melts of its envelope. As a result, the Kerguelen hotspot is now in a waning stage under a stagnant lithospheric plate.
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CONCLUSIONS |
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TOPABSTRACTINTRODUCTIONSAMPLE SELECTIONANALYTICAL PROCEDURERESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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Our Hf isotopic study of a selected subset of Kerguelen Archipelago basaltic rocks indicates the presence of two distinct groups in
Hf: the pre-Miocene flood basalts and the <10 Ma volcanic rocks with isotopic characteristics trending towards those of some older Kerguelen Plateau basalts. We interpret these differences as resulting from a change in the source: the Mt. Crozier basalts are formed by partial melting of the Kerguelen plume core whereas the younger volcanic rocks come from an outer zone of the plume, with different isotopic characteristics. We infer that both plume end-members result from the incorporation of various proportions of ancient recycled oceanic plateaux in the deep mantle source. The oldest tholeiitic–transitional flood basalts of the archipelago were formed at 450 km from the SEIR and have geochemical characteristics indicative of the presence of a depleted component. The tectonic environment for the eruption of Kerguelen plume magmas evolved from a ridge-centred position at 40 Ma to an intraplate position. This evolution generates various combinations of time, depth of melting and access to different level components that provide interesting multi-dimensional perspectives on the deep Kerguelen plume source.
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ACKNOWLEDGEMENTS |
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We are grateful to P. Télouk for his expertise in maintaining the P54 in top running condition in Lyon, and C. Maerschalk for the Sector 54 in Brussels. J. S. Scoates is thanked for comments and suggestions, and C. Chauvel, F. A. Frey, P. D. Kempton and J. D. Vervoort for their constructive reviews that helped improve the manuscript. Funding by the Communauté Française through the programme of ‘Actions de Recherches Concertées’ ARC No. 98/03-233 and the Tournesol Programme 1998 of the CGRI is gratefully acknowledged.
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FOOTNOTES |
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*Corresponding author. Present address: Earth and Ocean Sciences, University of British Columbia, Vancouver, B.C. V6T-1Z4, Canada. Telephone: 1-604-822.1697. Fax: 1-604-822.6088. E-mail: dweis{at}eos.ubc.ca
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