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Journal of Petrology | Volume 43 | Number 7 | Pages 1367-1387 | 2002
© Oxford University Press 2002
Flood Basalt from Mont Tourmente in the Central Kerguelen Archipelago: the Change from Transitional to Alkalic Basalt at 
25 Ma
1DEPARTMENT OF EARTH, ATMOSPHERIC AND PLANETARY SCIENCES, MASSACHUSETTS INSTITUTE OF TECHNOLOGY, CAMBRIDGE, MA 02139, USA
2DEPARTMENT OF GEOSCIENCES, UNIVERSITY OF MASSACHUSETTS, AMHERST, MA 01003, USA
3DEPARTMENT OF EARTH AND ENVIRONMENTAL SCIENCES, CP 160/02, UNIVERSITÉ LIBRE DE BRUXELLES, AV. F. D. ROOSEVELT, 50, B-1050, BRUSSELS, BELGIUM
4LABORATOIRE DE GÉOLOGIE, UNIVERSITÉ JEAN MONNET, CNRS–UMR 6524, 23 RUE DU DOCTEUR PAUL MICHELON, 42023, SAINT-ÉTIENNE, CEDEX 2, FRANCE
Received May 7, 2001; Revised typescript accepted February 14, 2002
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONGEOLOGYSAMPLE DESCRIPTIONANALYTICAL TECHNIQUESRESULTSDISCUSSIONSUMMARYREFERENCES |
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The surface of the Cenozoic Kerguelen Archipelago, constructed on the Kerguelen Plateau in the southern Indian Ocean, is dominantly flood basalt. With the objective of understanding the Cenozoic history of the Kerguelen mantle plume, the age and geochemical characteristics of this flood basalt province are being determined by studying stratigraphic sections of basalt flows at several locations. Sections from the NW, north–central, east and SE parts of the archipelago have been studied. Here we report results for a 597 m succession of lavas from Mont Tourmente from the Plateau Central, a region of the archipelago that has not been studied in detail. Mont Tourmente lavas range from
26 Ma dominantly transitional basalts in the lower 80% of the section to 25·3 Ma dominantly alkalic basalts in the upper part of the section. The timing of this change from transitional to alkalic volcanism within the Mont Tourmente section is consistent with that defined by the older 28–29 Ma transitional basalts in the north and the 25 Ma alkalic lavas erupted in the east. This change in basalt composition may be related to migration of the archipelago away from the plume or to increasing lithosphere thickness over the 5 Myr of flood basalt volcanism. The alkalic and transitional Tourmente lavas are nearly homogeneous in isotopic ratios of Sr, Nd and Pb. They have lower 87Sr/86Sr and higher 143Nd/144Nd compared with lavas from the eastern sections, which have been proposed to be characteristic of the Kerguelen plume. Consequently, the Mont Tourmente isotopic data may reflect heterogeneity within the plume or a constant proportion of a depleted component mixed with the high 87Sr/86Sr and low 143Nd/144Nd plume. In contrast to many of the Cretaceous Kerguelen Plateau lavas, there is no evidence in trace element abundances or Sr and Nd isotopic ratios that the Cenozoic Kerguelen Archipelago lavas were influenced by continental lithosphere. KEY WORDS: Kerguelen Archipelago; Kerguelen plume; flood basalt; igneous geochemistry
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONGEOLOGYSAMPLE DESCRIPTIONANALYTICAL TECHNIQUESRESULTSDISCUSSIONSUMMARYREFERENCES |
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To understand the Cenozoic volcanism attributed to the Kerguelen plume we are studying stratigraphic sections of the flood basalt lavas that cover most of the Kerguelen Archipelago (Fig. 1). Our objective is to define and understand the temporal and spatial variations in the geochemical characteristics of these lavas. Studies of sections from the SE (Frey et al., 2000
), NE (Damasceno et al., 2002), north–central (Yang et al., 1998) and NW (Doucet et al., 2002) show that there are important regional geochemical differences in the flood basalts. These differences include: (1) a temporal change from older tholeiitic–transitional to younger alkalic lavas and (2) isotopic (Sr, Nd, Pb) heterogeneity in the oldest, 28–29 Ma basalts (Yang et al., 1998; Doucet et al., 2002), which contrasts with the isotopic homogeneity of younger, 25 Ma, sections (Weis et al., 1998; Frey et al., 2000).
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In this paper we focus on a section from Mont Tourmente, which is located in the central part of the archipelago in a region known as Plateau Central (Fig. 1). In a survey study of the flood basalts Gautier et al. (1990) found that the Sr and Nd isotopic ratios of the transitional basalts from this region (four samples) are intermediate between those of the alkalic basalts in the SE and NE and the tholeiitic to transitional basalts in the NW. A goal in this paper is to determine the generality of the survey results by Gautier et al. (1990) and to answer the question—are lavas erupted in the Plateau Central geochemically distinct from basalts erupted in other parts of the archipelago? Previously, Frey et al. (2000) showed that the trend to more alkalic lavas with decreasing age in the Kerguelen Archipelago reflects (1) a decreasing supply of magma from the plume, (2) a temporal decrease in extent of melting and (3) a temporal increase in depth of melt segregation. If radiogenic isotopic ratios of Plateau Central lavas differ from those of younger alkalic flood basalts, these data bear on important issues such as the heterogeneity of the Kerguelen plume and changes in mixing ratios between source components derived from plume, asthenosphere and oceanic lithosphere.
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GEOLOGY |
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TOPABSTRACTINTRODUCTIONGEOLOGYSAMPLE DESCRIPTIONANALYTICAL TECHNIQUESRESULTSDISCUSSIONSUMMARYREFERENCES |
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Mont Tourmente is located in the northern part of the Plateau Central in the Kerguelen Archipelago (Fig. 1). This region is characterized by basaltic flows that extend for very long distances and are only cut locally by small streams; glaciation, however, has exposed vertical sections, up to 800 m high. The total volume of magmatism represented by the Plateau Central is a significant portion of the flood basalts. Like the other volcanoes in the area, Mont Tourmente has a general oval shape with a clear east–west orientation.
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SAMPLE DESCRIPTION |
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TOPABSTRACTINTRODUCTIONGEOLOGYSAMPLE DESCRIPTIONANALYTICAL TECHNIQUESRESULTSDISCUSSIONSUMMARYREFERENCES |
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Sixty-four samples (93-351 to 93-414) were collected from a 597 m section (Fig. 2). On the basis of 40Ar/39Ar dating, Nicolaysen et al. (2000)
reported eruption ages of 26·0 ± 1·0 Ma for sample 93-414 at the bottom of the section and 25·3 ± 0·7 Ma for sample 93-354 from near the top of the section (Fig. 2); therefore this section formed within 1 Myr. Except for two samples, all of the samples contain <5% phenocrysts and microphenocrysts, dominantly plagioclase with lesser amounts of clinopyroxene. In contrast, samples 93-401 and 93-404 contain 8–10% plagioclase and 3–4% clinopyroxene phenocrysts and microphenocrysts. The scarcity of phenocrysts in most of the Mont Tourmente lavas contrasts with the abundant phenocrysts, especially plagioclase, in the younger, 24–25 Ma, alkalic flood basalts erupted to the east at Mont Crozier (Damsaceno et al., 2002).
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Abundances of major and several trace elements (Rb, Sr, Ba, V, Cr, Ni, Zn, Ga, Y, Zr, Nb and Ce) in 64 samples were determined by X-ray fluorescence (XRF), and in a subset of 19 samples abundances of Sc, REE, Hf and Th were determined by instrumental neutron activation analysis (INAA) (Table 1). Sixteen samples were analyzed for Sr and Nd isotopes and 13 for Pb isotopes (Table 2).
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RESULTS |
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Major elements
In a total alkalis vs SiO2 classification plot, Mont Tourmente lavas straddle the alkalic–tholeiitic boundary (Fig. 3). Most of the Mont Tourmente lavas are less alkalic than the slightly younger (
25 Ma) alkalic lavas forming the flood basalts in the NE (Mont Crozier section) and SE (Ravin Jaune and du Charbon sections), and they overlap with the transitional flood basalts forming the Mont Bureau and Mont Rabouillère sections in the north–central part of the Kerguelen Archipelago (Fig. 3). Within the Mont Tourmente section there is, however, a temporal change from transitional to alkalic basalt. The uppermost 13 lavas, 93-351 to 93-363 from 597 to 485 m, are characterized by relatively low SiO2 contents, and 11 of the 13 are alkalic or very close to the tholeiitic–alkalic boundary (Figs 3 and 4). We conclude that these 11 lavas form a distinctive geochemical group, which we designate as the ‘Upper Alkalic Group’. Two other alkalic lavas (93-394, 93-398) occur lower in the section; they differ from the uppermost samples in having relatively high K2O (1·3–1·4%, Table 1) and SiO2 (>49%) contents (Fig. 4b).
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) indicating samples within the alkalic field. It should be noted that nine of the uppermost 13 lavas are alkalic and all 13 samples have relatively low SiO2 contents. Sample numbers are indicated for the two alkalic lavas lower in the section and transitional sample 380, which has a low SiO2 content.
The 64 Mont Tourmente lavas have low MgO contents (Table 1), ranging from 4·05% (sample 93-407) to 6·38% (sample 93-404). At a given MgO content, relative to the transitional lavas the upper alkalic lavas have higher abundances of P2O5 and TiO2 and lower SiO2 contents (Fig. 5). In contrast, all samples define a similar negative trend for Na2O–MgO and a positive trend for CaO–MgO that levels off at 11% CaO when MgO exceeds 5·5% (Fig. 5). The latter trend is consistent with control of lava compositions by the onset of clinopyroxene fractionation as MgO decreased below 5·5%. Abundance of Al2O3 also decreases with decreasing MgO content, but there is considerable scatter with no distinction between alkalic and transitional lavas (Fig. 5).
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Trace elements
Like TiO2 and P2O5, the abundances of incompatible trace elements, such as Nb, define a broad inverse trend with MgO content with the upper alkalic lavas generally having the highest abundances of incompatible elements at a given MgO content (Figs 5 and 6). Consistent with their relatively low MgO contents, the Mont Tourmente lavas have low Ni contents (<100 ppm). The positive correlations of Ni and Cr with MgO are consistent with fractionation of mafic phases, such as olivine and pyroxene (Fig. 6); however, olivine is not present in these lavas. Abundance of Sc ranges from 30 to 34 ppm for MgO >5·5%, but MgO and Sc contents are positively correlated at lower MgO contents (Fig. 6). Like CaO, Sc is compatible in clinopyroxene; therefore both the CaO–MgO and Sc–MgO trends are consistent with clinopyroxene as a major fractionating phase for magmas with <5·5% MgO.
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Plagioclase is the major phenocryst phase in these lavas. Abundance of Sr, an element compatible in plagioclase, is not positively correlated with abundances of an incompatible element, such as Nb; in fact, there is a poorly defined negative trend (Fig. 7). An important role for plagioclase fractionation is indicated by Sr/Ce ratios, which are close to primitive mantle ratios in the highest MgO lavas but decrease to (Sr/Ce)PM 0·4 with decreasing MgO (Fig. 8). This relative depletion of Sr is apparent in primitive mantle (PM) normalized plots where Sr abundances are nearly constant at 15–17 times the PM abundance (Fig. 9); the mean Sr content is 335 ± 11 ppm for 64 lavas from 597 m of section. The limited range in Sr and Sc contents (13% and 30%, respectively) relative to the range of Nb abundances, a factor of 1·8, implies that the bulk solid–melt partition coefficients for Sr and Sc were near unity. This inference requires a fractionating assemblage dominated by the observed phenocryst phases, qualitatively 50% feldspar and 30% clinopyroxene (assuming that the mineral–melt partition coefficient for Sr is 2 for plagioclase and 3 for clinopyroxene); these proportions are typical for low-pressure evolution of basaltic magma (Toplis & Carroll, 1995).
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Other important features in the primitive mantle-normalized plots are the negative slopes from Nb to Yb (Fig. 9); that is, unlike some basalts from the Cretaceous Kerguelen Plateau (e.g. Mahoney et al., 1995; Frey et al., 2002), these Cenozoic Kerguelen Archipelago lavas are not relatively depleted in Nb (or Ta). In fact, none of the flood basalt sections in the Kerguelen Archipelago include lavas that are relatively depleted in Nb (e.g. Yang et al., 1998; Frey et al., 2000; Doucet et al., 2002). There are strong positive correlations between abundances of incompatible elements, such as Nb, Ce, Zr and Y, which are relatively immobile during post-magmatic alteration; as with TiO2 and P2O5 abundances, the Upper Alkalic Group samples tend to have the highest abundances of these incompatible elements (Fig. 7).
The aphyric sample 93-406 is anomalously enriched in Y and Ce (Fig. 7) and several other incompatible elements (Fig. 9). Part of this enrichment reflects its relatively low MgO content (4·2%) and the effects of fractional crystallization [low Sc and (Sr/Ce)PM in Figs 6 and 8, respectively]. However, the exceptionally high La and Ce contents (Fig. 9) and deviation from the trend defined by other Mont Tourmente lavas (Nb–Ce panel in Fig. 7) may reflect post-magmatic alteration. In both hand specimen and thin section this sample is highly weathered. Anomalous enrichments in Y and rare earth elements (REE) have been observed in many localities, including Hawaiian tholeiitic shields (Fodor et al., 1992; Frey et al., 1994), alkalic lavas in French Polynesia (Joron et al., 1991; Cotten et al., 1995) and tholeiitic lavas in southeastern Australia (Price et al., 1991). The enrichment process can occur during the earliest stages of weathering and involves localized mobilization of Y and REE with their deposition in groundmass phosphate. High rainfall, which is characteristic of the windward sides of Hawaiian shields and the Kerguelen Archipelago, may promote this process.
Another major feature of the primitive mantle-normalized plots are the strong depletions in Rb and very high ratios of Ba/Rb (Fig. 9), a ratio that is usually fairly constant in unaltered oceanic basalts (Hofmann & White, 1983). Also, abundances of Rb and Nb are not correlated (Fig. 7), and the range in Rb content, from 0·6 to 42·5 ppm, is much greater than that for the immobile incompatible element Nb (factor of 1·8). We infer that Rb was mobile during post-magmatic alteration. The wide range in K/Rb, 210–4000, is also consistent with this interpretation. In particular, the trend to high K/Rb at low K2O/P2O5 (Fig. 10) is the same alteration trend as shown by Hawaiian shield lavas (e.g. Feigenson et al., 1983; Roden et al., 1994); i.e. Rb is more mobile than K2O. Despite the obvious differences in climate, in these areas of high rainfall the alkali metals, K and especially Rb, are leached from the lavas, leading to anomalously high K/Rb and low K2O/P2O5. A similar result was found for Kerguelen Archipelago flood basalts from Monts Bureau and Rabouillère (see Yang et al., 1998, fig. 4b). For Mont Tourmente lavas loss of K could affect the tholeiitic–alkalic classification plot by converting alkalic lavas to tholeiitic lavas, but in general both types of lavas range widely in K2O/P2O5 (Fig. 10).
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Isotopes (Sr, Nd, Pb)
The Sr and Nd isotopic ratios of 16 Mont Tourmente lavas define an inverse 143Nd/144Nd–87Sr/86Sr trend centered at 0·5127 and 0·7047, respectively (Fig. 11a). Compared with the wide range of Sr and Nd isotopic ratios found in lavas from the Kerguelen Archipelago, Mont Tourmente lavas are relatively homogeneous in isotopic ratios of Sr, Nd and Pb with no systematic isotopic differences between the alkalic and transitional lavas (Fig. 11, Table 2). In contrast to the relative isotopic homogeneity of lavas at Mont Tourmente (25–26 Ma), isotopic heterogeneity is typical in slightly older sections of the Kerguelen Archipelago flood basalts; i.e. the Mont Bureau and Mont Rabouillère (29 Ma) sections and the Mont Fontaine and Mont des Ruches (28 Ma) sections (Fig. 11a). Mont Tourmente lavas have 87Sr/86Sr and 143Nd/144Nd intermediate to the extremes defined by lavas from these older sections.
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precision indicated by error bars, but there is no systematic difference between alkalic and transitional lavas (
Relative to the 25 Ma alkalic basalts erupted in the eastern part of the archipelago (Mont Crozier and Ravin du Charbon in Fig. 1) the Tourmente lavas have lower 87Sr/86Sr and higher 143Nd/144Nd (Fig. 11a). Despite these differences in Sr and Nd isotopic ratios, Pb isotopic ratios of Mont Tourmente lavas largely overlap with the younger alkalic flood basalts from Mont Crozier and Ravin du Charbon; however, some Mont Tourmente lavas extend to lower Pb isotopic ratios and overlap with the fields for the older 28–29 Ma basaltic sections from Mont Fontaine and Mont Bureau (Fig. 11b).
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONGEOLOGYSAMPLE DESCRIPTIONANALYTICAL TECHNIQUESRESULTSDISCUSSIONSUMMARYREFERENCES |
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Temporal geochemical evolution of the Kerguelen Archipelago flood basalts
Important geochemical characteristics of the flood basalts forming the Kerguelen Archipelago vary systematically with location and eruption age. Transitional basalts are exposed in north–central sections [the
29 Ma Mont Bureau and Mont Rabouillère sections (Yang et al., 1998)] and in NW sections [the 28 Ma Mont Fontaine and Mont des Ruches sections (Doucet et al., 2002)], whereas 25 Ma alkalic basalts are exposed in eastern sections [Ravin du Charbon and Ravin Jaune sections (Frey et al., 2000) and Mont Crozier (Damasceno et al., 2002)]. In the Mont Tourmente section from the Plateau Central (Fig. 1) a change from transitional to alkalic lavas occurs within the succession of lavas with primarily transitional basalt in the lower 80% of the section and dominantly alkalic basalt in the upper 112 m of this 597 m section. The timing of this change in lava composition at Mont Tourmente, 26·0 ± 1 Ma for a transitional basalt at the bottom of the section to 25·3 ± 0·7 Ma for an alkalic basalt near the top of the section (Fig. 2), is consistent with the change from transitional basalt in the northern sections (28–29 Ma) to alkalic basalt in the eastern sections (25 Ma). At a given MgO content, the alkalic lavas have higher abundances of incompatible elements than most of the transitional lavas (e.g. P, Ti and Nb in Figs 5 and 6). The inferred decrease in extent of melting with decreasing eruption age at Mont Tourmente is consistent with the temporal trend inferred from studies of basalt sections in the eastern part of the archipelago (Frey et al., 2000).
Like the flood basalts in the eastern sections, the alkalic basalts in the Mont Tourmente section are distinguished by their relatively low SiO2 content (Fig. 12a), a result consistent with melt segregation from a peridotite at relatively high pressure (e.g. Gaetani & Grove, 1998, table 8). For alkalic lavas from the SE Province and Mont Crozier, pressures of melt segregation within the garnet stability field are inferred on the basis of relatively low heavy rare earth element (HREE) and Y contents at a given abundance of a highly incompatible element, such as Nb or Th (Fig. 12b; Frey et al., 2000, fig. 6). There is, however, no evidence for residual garnet during the petrogenesis of the transitional or alkalic Mont Tourmente lavas, which define a Y–Nb trend that is distinct from that of basalts in the SE Province and Mont Crozier sections. The Y–Nb trend of Mont Tourmente lavas overlaps with the data for the transitional flood basalts from Mont Bureau and Mont Rabouillère (Fig. 12). At a given MgO content, there are also important differences between flood basalt sections in Ce/Y, La/Yb and Nb/Zr (Fig. 13). Such ratios increase as extent of melting decreases, especially when garnet is a residual phase. These ratios in Mont Tourmente lavas, both transitional and alkalic, from the Plateau Central are lower than in the alkalic flood basalts from the eastern sections (Crozier and SE Province), and they overlap with the P-type transitional lavas in the north–central Mont Bureau and Mont Rabouillère sections (Fig. 13). Apparently all of the Mont Tourmente magmas segregated within the stability field of spinel peridotite, but on the basis of their relatively low SiO2 content (Fig. 12a) the uppermost alkalic magmas segregated at pressures higher than the transitional lavas.
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In summary, studies of several basalt sections through the flood basalt forming the Kerguelen Archipelago show that at 25 Ma the extent of melting decreased and melt segregation occurred at higher pressures, reaching the garnet stability field beneath the eastern part of the archipelago. At 25 Ma there were, however, important differences between the magma sources. The Sr and Nd isotopic ratios of Mont Tourmente lavas are distinct from those of the alkalic lavas erupted in the east (Fig. 11a). In fact, these isotopic ratios in Mont Tourmente lavas are intermediate between the isotopic extremes of the older transitional basalt erupted in the northern sections at Mt. Bureau and Mt. des Ruches (Fig. 11a).
Heterogeneous Kerguelen plume or varying mixing ratios between plume, asthenosphere and lithosphere?
Isotopic data for ocean-island basalt suites attributed to mantle plumes, such as Hawaii, Iceland and Galapagos, indicate considerable isotopic heterogeneity within each suite. Does this heterogeneity dominantly reflect intrinsic differences within the plume or variable proportions of components derived from the plume, asthenosphere and lithosphere? This question has generated considerable debate (for Hawaii: Lassiter & Hauri, 1998; Keller et al. 2000; Regelous et al., 2002; for Iceland: Hanan et al., 2000; Kempton et al., 2000; for Galapagos: White et al., 1993; Blichert-Toft & White, 2001).
Similarly, the wide range of radiogenic isotopic ratios in lavas from the Kerguelen Archipelago (Fig. 11) may be explained by two endmember interpretations.
- Intrinsic geochemical heterogeneities embedded within the Kerguelen plume with the alkalic flood basalts in the Kerguelen Archipelago (Mont Crozier and Ravin du Charbon in Fig. 11) representing the low melting component in the plume. This component, with relatively high 87Sr/86Sr and low 143Nd/144Nd, also dominates in the P-type transitional flood basalts from Mont Bureau and Mont Rabouillère (Fig. 11). Other transitional flood basalts from the northern sections (Mont Fontaine, Mont des Ruches, D-type from Mont Bureau and Mont Rabouillère) range to lower 87Sr/86Sr and higher 143Nd/144Nd (Fig. 11). These transitional lavas may contain a relatively depleted component intrinsic to the Kerguelen plume that was sampled only at higher extents of melting.
Within the Kerguelen Archipelago the isotopic characteristics of Mont Tourmente lavas are intermediate between the enriched and depleted extremes. Although this intermediate character may represent mixing, an important result is that the alkalic and transitional lavas from Mont Tourmente have similar isotopic ratios (Table 2). Two-component melt mixing will result in a correlation between isotopic ratios and magma composition; therefore it is unlikely that Mont Tourmente lavas reflect mixing between enriched alkalic and depleted transitional to tholeiitic magmas. Mont Tourmente lavas may represent magmas derived from variable extents of melting from a nearly homogeneous part of the plume with intermediate isotopic ratios.
- Alternatively, the isotopic heterogeneity of Kerguelen Archipelago lavas may reflect varying mixing proportions between components derived from the plume, asthenosphere and lithosphere, either oceanic or continental (Doucet et al., 2002).
In regard to continental lithosphere, in several areas sampled by basement drilling Cretaceous plume-derived lavas forming the uppermost part of the Kerguelen Plateau were apparently contaminated by continental lithosphere (Mahoney et al., 1995; Weis et al., 2001; Frey et al., 2002; Ingle et al., 2002). Also, mantle xenoliths with a continental affinity have been found in Kerguelen Archipelago lavas (Hassler & Shimizu, 1998; Mattielli et al., 1999). There is, however, no compositional evidence that Kerguelen Archipelago basalts, including lavas from Mont Tourmente, contain a continental component (e.g. Fig. 9; Yang et al., 1998; Doucet et al., 2002; Frey et al., 2002, fig. 10).
In regard to the role of oceanic lithosphere,15% of the lava flows in the 29–30 Ma sections from Mont Bureau and Mont Rabouillère have relatively low 87Sr/86Sr and high 143Nd/144Nd (Group D in Fig. 11) coupled with the trace element characteristics of plagioclase-rich cumulates; Yang et al. (1998) concluded that these lavas represent plume-derived magmas that assimilated plagioclase-rich cumulate rocks in the lower oceanic crust. Lavas with these geochemical characteristics do not occur in the Mont Tourmente section.
A role for a mid-ocean ridge basalt (MORB)-related asthenosphere component in the source of Kerguelen Archipelago lavas was inferred by Storey et al. (1988) and Gautier et al. (1990), who argued that as the Kerguelen Archipelago evolved from an early ridge-centered stage at 40 Ma to its present intraplate setting, the proportion of depleted asthenosphere in the source decreased. This interpretation was based on an inferred increase in 87Sr/86Sr and decrease in 143Nd/144Nd with eruption age in the Kerguelen Archipelago. Today this model is less compelling because Yang et al. (1998) found that most of the lavas in the oldest studied sections of the flood basalt, the transitional basalt from Mont Bureau and Mont Rabouillère, have Sr and Nd isotopic ratios that overlap with those of the younger alkalic flood basalts at Mont Crozier and Ravin du Charbon (Fig. 11). Although there is no simple trend of Sr and Nd isotopic ratios varying with age, the archipelago lavas with relatively low 87Sr/86Sr and high 143Nd/144Nd occur only within the oldest, 28–30 Ma, sections of the flood basalt (Yang et al., 1998; Doucet et al., 2002). A larger proportion of a depleted component is inferred for some of these oldest lavas, but there is no evidence that the uppermost parts of the flood basalt formed at a time when the archipelago was near the Southeast Indian Ridge (SEIR); i.e. the oldest flood basalts in the archipelago erupted at 30 Ma (Nicolaysen et al., 2000; Doucet et al., 2002) much younger than the 40 Ma juxtaposition of the archipelago and SEIR. There are, however, intercalated 34 Ma MORB- and plume-related lavas at Ocean Drilling Program (ODP) Leg 183 Site 1140 on the northernmost Kerguelen Plateau (Weis & Frey, 2002).
In summary, the near isotopic homogeneity of Mont Tourmente lavas (Fig. 11) contrasts with the isotopic diversity of intercalated lava flows at ODP Site 1140 north of the Kerguelen Archipelago and at Monts Fontaine, des Ruches, Bureau and Rabouillère in the archipelago. In addition, the isotopic ratios of Mont Tourmente lavas differ from those proposed for the Kerguelen plume (Weis et al., 1998). If much of the Plateau Central region of the Kerguelen Archipelago proves to be isotopically similar to the Mont Tourmente lavas, this may be evidence for intrinsic isotopic heterogeneity of the Kerguelen plume that complements the evidence for plume heterogeneity provided by basalts from the Ninetyeast Ridge, which is a hotspot track related to the Kerguelen plume (Fig. 1 inset; Frey & Weis, 1995).
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SUMMARY |
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TOPABSTRACTINTRODUCTIONGEOLOGYSAMPLE DESCRIPTIONANALYTICAL TECHNIQUESRESULTSDISCUSSIONSUMMARYREFERENCES |
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Studies of flood basalt sections from the Kerguelen Archipelago show several important trends. With decreasing eruption age from
29 Ma in the north to 25 Ma in the east, the flood basalts of the Kerguelen Archipelago changed from transitional to alkalic basalt. Consistent with this result, the 26·0–25·3 Ma, 597 m lava section at Mont Tourmente in the Plateau Central region includes an abrupt change from transitional to alkalic flood basalts. By analogy with the growth stages of Hawaiian volcanoes (Clague & Dalrymple 1987), this temporal trend implies a decrease in extent of melting and may indicate that the archipelago was not centered above the plume at 25 Ma. Alternatively, magma flux from the mantle decreased at 25 Ma as the lithospheric thickness beneath the archipelago increased. Consistent with this inference, the role of residual garnet during partial melting was important for the eastern alkalic basalt sections (Frey et al., 2000), but not for the northern transitional basalt sections. This change in residual mineralogy is not observed in the Mont Tourmente Section from the Plateau Central. Another possibility is that at 25 Ma the intrinsic plume flux was diminishing, possibly becoming less focused (Frey et al., 2000). Evidence in favor of the latter alternative is that over the last 20 Myr alkalic lavas have erupted as post-flood basalts in the Kerguelen Archipelago (Weis et al., 1993), and have formed Heard and McDonald Islands, 400 km SE of the Kerguelen Archipelago (Barling et al., 1994), and several seamounts located between the Kerguelen Archipelago and Heard Island (Weis et al., 2002).
Samples of flood basalt from the oldest sections of the Kerguelen Archipelago are isotopically heterogeneous, whereas those from the younger sections are relatively homogeneous with relatively high 87Sr/86Sr, and low 143Nd/144Nd and radiogenic Pb isotope ratios. This enriched component is also present in the oldest 29 Ma, archipelago lavas. The depleted component, relatively low 87Sr/86Sr and high 143Nd/144Nd, may be intrinsic within the plume or related to the oceanic lithosphere (Yang et al., 1998) or asthenosphere (Doucet et al., 2002). The lavas in the Mont Tourmente section are nearly isotopically homogeneous, with Sr, Nd and Pb isotopic ratios intermediate between the enriched and depleted components. Also, there is no isotopic distinction between transitional and alkalic lavas. In contrast to Cretaceous Kerguelen Plateau basalts (e.g. Frey et al., 2002), there is no evidence that Mont Tourmente lavas contain components derived from continental lithosphere. It is possible that Mont Tourmente lavas dominantly reflect the Kerguelen plume.
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ACKNOWLEDGEMENTS |
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This research was supported by US NSF EAR Grant 9814313 (F.A.F.) and ARC GRANT 98/03-233 (D.W.), and FNRS Grant 1.5.186.98 (D.W.). We thank Dr J. Scoates for discussions regarding the geology of the Plateau Central, Dr P. Ila for supervision of the MIT Neutron Activation Analysis Facility, C. Maerschalk for chemical processing of samples before isotopic analyses, and G. Xu for assistance in preparing figures. Finally, we thank J. Barling, G. Fitton and R. Kent for their constructive reviews.
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FOOTNOTES |
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*Corresponding author. E-mail: fafrey{at}mit.edu
Present address: Geology Department, Kansas State University, Manhattan, KS 66506, USA.
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REFERENCES |
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TOPABSTRACTINTRODUCTIONGEOLOGYSAMPLE DESCRIPTIONANALYTICAL TECHNIQUESRESULTSDISCUSSIONSUMMARYREFERENCES |
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