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Journal of Petrology | Volume 43 | Number 7 | Pages 1341-1366 | 2002
© Oxford University Press 2002
The Depleted Mantle Component in Kerguelen Archipelago Basalts: Petrogenesis of Tholeiitic–Transitional Basalts From the Loranchet Peninsula
1DÉPARTEMENT DES SCIENCES DE LA TERRE ET DE L’ENVIRONNEMENT, UNIVERSITÉ LIBRE DE BRUXELLES, B-1050 BRUSSELS, BELGIUM
2DEPARTMENT OF EARTH, ATMOSPHERIC AND PLANETARY SCIENCES, MASSACHUSETTS INSTITUTE OF TECHNOLOGY, CAMBRIDGE, MA 02139, USA
3LABORATOIRE DE GÉOLOGIE–PÉTROLOGIE, CNRS–UMR 6524, UNIVERSITÉ JEAN MONNET, 42023 SAINT-ETIENNE, FRANCE
Received June 20, 2001; Revised typescript accepted February 19, 2002
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONGEOLOGY OF THE KERGUELEN...PETROGRAPHY AND MINERAL...ANALYTICAL TECHNIQUES40Ar/39Ar CHRONOLOGYRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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A geochemical study of 28 Ma tholeiitic to transitional basalts from the Kerguelen Archipelago (Mont des Ruches and Mont Fontaine) indicates that three distinct magma types erupted within
1 Myr. Low-MgO basalts (4–6 wt %) in both sections are overlain by high-MgO basalts (7–13 wt %), mostly present in Mont Fontaine. Both high- and low-MgO basalts have nearly identical low 87Sr/86Sr and high 143Nd/144Nd and formed from similar parental magmas that represent mixtures between a depleted mantle component and the Kerguelen plume. The third magma type, predominant in Mont des Ruches, is represented by high-MgO basalts that are isotopically heterogeneous with isotopic ratios that are intermediate between those of the stratigraphically lower basalts and the Kerguelen plume compositions; this third magma type may have formed by mixing of similar material, but with a higher contribution from the Kerguelen plume. The depleted component involved in all three magma types is similar to the source for Southeast Indian Ridge basalts and is present in Kerguelen Archipelago basalts older than 26 Ma, which erupted when the ridge axis was <500 km away from the Kerguelen hotspot. Depleted heterogeneities intrinsic to the plume and entrainment of depleted mantle during plume ascent do not explain the marked cut-off in the presence of a depleted component in the archipelago basalts within a time interval of 1 Myr after 26 Ma. Mixing of depleted asthenosphere with the plume in sublithospheric channels during migration of the Southeast Indian Ridge axis away from the Kerguelen hotspot is proposed as a suitable explanation to account for the temporal distribution of the depleted component in basalts from the Northern Kerguelen Plateau and the >26 Ma Kerguelen Archipelago flood basalts; cessation of plume–ridge interactions may explain the absence of depleted basalts in the youngest sections that erupted further away from the ridge axis. KEY WORDS: Kerguelen Archipelago; basalt; geochemistry; depleted component; Kerguelen plume; mixing
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONGEOLOGY OF THE KERGUELEN...PETROGRAPHY AND MINERAL...ANALYTICAL TECHNIQUES40Ar/39Ar CHRONOLOGYRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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The generation of the
119 Ma to present Kerguelen Large Igneous Province (Duncan, 2002
) is related to activity of the Kerguelen mantle plume (e.g. Frey et al., 2000a). The Kerguelen Archipelago and part of the submarine Northern Kerguelen Plateau (Weis & Frey, 2002) represent the last 35 Myr of plume activity. Within this 35 Myr time interval, there is evidence for a decreasing degree of melting with decreasing eruption age (Weis et al., 1998b; Frey et al., 2000b). In an effort to evaluate the source components and the petrogenetic processes responsible for the various isotopic signatures observed in basalts from the Kerguelen Archipelago, systematic studies of the spatial and temporal geochemical variations in archipelago basalt sections have been undertaken (Weis et al., 1998b; Yang et al., 1998; Frey et al., 2000b, 2002a). These studies complement earlier survey studies of basalts from diverse locations on the Kerguelen Archipelago (Gautier et al., 1990; Weis et al., 1993).
The proportion of depleted mantle contributing to Kerguelen Archipelago flood basalts has been a matter of debate (Storey et al., 1988, 1989; Gautier et al., 1990; Weis et al., 1993; Yang et al., 1998). The relatively low (87Sr/86Sr)i (down to 0·7039, where the subscript ‘i’ indicates the age-corrected isotopic ratio for each individual section), (206Pb/204Pb)i (down to 18·2) and high (143Nd/144Nd)i (up to 0·5129) in 15% of the exposed tholeiitic to transitional basalts from the north–central 29·5 Ma Mont Bureau and 29 Ma Mont Rabouillère sections have been interpreted as reflecting assimilation of gabbroic oceanic crust (Yang et al., 1998). Mixing between Southeast Indian Ridge (SEIR) and Kerguelen plume magmas is documented at Ocean Drilling Program (ODP) Site 1140 (34 Ma; Duncan, 2002) in the Northern Kerguelen Plateau, 300 km north of the archipelago (Weis & Frey, 2002). Finally, the nearly isotopically homogeneous basalts from the 26 Ma Mont Tourmente section in the central Kerguelen Archipelago have lower 87Sr/86Sr and higher 143Nd/144Nd compared with estimates for the plume (Weis et al., 1998a, 2002); Frey et al. (2002a) have suggested that these basalts may reflect depleted heterogeneities intrinsic to the Kerguelen plume.
We have studied the Mont des Ruches and Mont Fontaine basaltic sections, which are located in the northwestern part of the Kerguelen Archipelago on the Loranchet Peninsula (Fig. 1). These lavas are considered to be amongst the oldest sections exposed on the archipelago as a result of a regional 2–5° SE dip of basaltic flows, which correlates with a general decrease in eruption ages from NW to SE in the archipelago (Nicolaysen et al., 2000). In this paper we use the age and geochemical characteristics of basalts in both sections to (1) constrain the composition and origin of the depleted component, (2) constrain interactions between the plume and the depleted component reservoirs, and (3) evaluate the contribution, if any, of contamination by continental material in the petrogenesis of basaltic lavas from the NW Kerguelen Archipelago.
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GEOLOGY OF THE KERGUELEN ARCHIPELAGO AND THE LORANCHET PENINSULA |
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TOPABSTRACTINTRODUCTIONGEOLOGY OF THE KERGUELEN...PETROGRAPHY AND MINERAL...ANALYTICAL TECHNIQUES40Ar/39Ar CHRONOLOGYRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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The Kerguelen Archipelago (6500 km2) is the emergent part of the Northern Kerguelen Plateau (Fig. 1). Spreading rates on the SEIR (Royer & Sandwell, 1989
) indicate that the archipelago location was along the ridge axis at 40·1 Ma and subsequently evolved toward its present intraplate location, 1400 km to the SW, as the ridge moved to the NE. Flood basalts (29–24 Ma; Nicolaysen et al., 2000) cover 85% of the surface of the archipelago (Fig. 1a). During emplacement of the flood basalts, the distance between the archipelago and the SEIR increased from 350 to 550 km. The topography of the nearly horizontal flood basalts is locally perturbed by younger volcanic–plutonic complexes (Giret & Lameyre, 1983; Weis & Giret, 1994). Basaltic sections up to 1000 m in height (Fig. 1a) have been exposed by glacial erosion and were systematically sampled during the last 10 years of the Kerguelen Archipelago mapping programme (CartoKer programme, Université Jean Monnet).
Flood basalts cover the entirety of the Loranchet Peninsula (Fig. 1) and are exposed in sections of 300–800 m height. NW–SE and NE–SW oriented fjords and near-vertical dykes cut the basalts and follow the two main fracture orientations reported on the peninsula (Nougier, 1970). Dykes and sills of trachyte and rhyolite, and minor gabbroic intrusions also occur on the peninsula. Plagioclase-phyric massive basalt flows, preferentially located in the lower parts of the volcanic sections, and olivine-phyric flows are common. The Mont des Ruches and Mont Fontaine sections are composed of numerous massive basaltic flows of 2–18 m thickness, which are separated by weathered or oxidized horizons (Fig. 1b). Mont des Ruches is 497 m high and is cut by a dyke of 15 m thickness. A sample of this dyke was not available for this study but it is described as a zoned dyke with a gabbroic core and a rhyolitic border. Additional field descriptions (P. Camps & M. Perrin, personal communication) indicate the presence of numerous sills in the area. Mont Fontaine is a succession of subhorizontal basaltic flows forming a section of 325 m height, located 15 km to the east of Mont des Ruches. There are no field relationships that allow us to place relative age constraints between the two stratigraphic sections.
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PETROGRAPHY AND MINERAL CHEMISTRY |
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TOPABSTRACTINTRODUCTIONGEOLOGY OF THE KERGUELEN...PETROGRAPHY AND MINERAL...ANALYTICAL TECHNIQUES40Ar/39Ar CHRONOLOGYRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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More than 80% of the examined samples from Mont des Ruches and Mont Fontaine are either plagioclase-phyric (four of 46 samples) or olivine-phyric (34 of 46 samples). Phenocryst contents and sizes range from 3 to 15 vol. % and
0·3 to 5 mm, respectively (Table 1). The groundmass is doleritic or subophitic and consists of fine-grained or poikilitic augite, plagioclase laths, oxides (<1 vol. %), olivine, and locally devitrified and/or altered brown glass. Alteration is present in 40% of the Mont des Ruches and Mont Fontaine samples (Table 1), and ranges from brown altered and devitrified groundmass, to serpentinized and iddingsitized olivine, to secondary zeolites (<1 vol. % when present) and rare calcite (<1%, when present, e.g. sample BY96-31). Alteration is also reflected in variable whole-rock loss on ignition (LOI) values from 1·2 to 6·7 wt %.
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Olivine and plagioclase compositions were analysed with a Cameca SX100 electon microprobe at the Université Blaise Pascal, Clermont Ferrand, France, using an accelerating voltage of 15 keV, a beam current of 15 nA, a beam size of 1 µm, and natural and synthetic standards. Compositions were determined on cores and rims of multiple grains from selected thin sections. Both the plagioclase and olivine analyses are consistent with mineral stoichiometry. Representative compositions are given in Table 2.
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Plagioclase-phyric (low-MgO) basalts
Plagioclase phenocrysts have a nearly constant size of 5 mm. They are typically characterized by a core of An70–80, a strongly resorbed border and a narrow fringe (up to 0·1 mm) of An40–50 (Table 2). They are commonly isolated in a fine-grained groundmass of augite, plagioclase and altered devitrified glass. Zoning can locally be complex: a single phenocryst in BY96-44 exhibits oscillatory zoning between An35 and An80 around a homogeneous core of An80.
Olivine-phyric (high-MgO) basalts
Olivine is abundant in the high-MgO basalts from the Mont des Ruches and Mont Fontaine sections (Table 1) and is of variable size (up to 3 mm diameter), abundance and habit. Skeletal or euhedral crystals may occur as isolated phenocrysts in a fine- or medium-grained doleritic or subophitic groundmass. Rare rounded olivine grains are observed. Olivine grains have core and rim compositions of Fo85–80 and Fo75–50, respectively (Table 2). One sample (BY96-37) contains an inclusion of fine-grained gabbro.
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ANALYTICAL TECHNIQUES |
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A total of 43 samples (200–300 cm3) were analysed for major and trace element compositions. Surface alteration was removed by a diamond-embedded saw, and then the cut surface was abraded using sandpaper to remove any saw traces and remaining alteration features. The samples were coarsely crushed in a hydraulic piston crusher (percussion method) before being reduced to powder in an agate shatterbox.
Two samples from the Mont Fontaine stratigraphic section and one from Mont des Ruches were selected for dating by the 40Ar/39Ar method. The results are summarized in Table 3. The relative paucity of plagioclase phenocrysts within these basalts required the analysis of whole-rock groundmass. Each sample was crushed to 
1 mm in diameter and sieved to a 0·8–0·3 mm size fraction. Using a Frantz magnetic separator, the olivine and clinopyroxene were removed to minimize excess magmatic argon. The samples were acid-leached to remove zeolite alteration phases, irradiated, and then analysed in the CLAIR facility at MIT (Nicolaysen et al., 2000).
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Major element oxides and some trace element concentrations were determined by X-ray fluorescence at the University of Massachusetts following the analytical procedure described by Rhodes (1996). Major element compositions are the mean of duplicate analyses (Table 4) and LOI is the weight loss on ignition after 30 min at 1020°C. Trace elements, including rare earth element concentrations, were determined by inductively coupled plasma mass spectrometry (ICP-MS) on an HP4500 system at the Katholieke Universiteit Leuven, Belgium. Samples were digested in sub-boiled 22M HF and 14M HNO3 then dissolved in 10 ml of sub-boiled 14M HNO3. After digestion, In (6 ppm), Tl (3·25 ppm), Re (8·6 ppm) and Rh (9·8 ppm) were added as internal standards. Li, Y, Ce and Tl were used for external HP4500 calibration. One blank was measured for each set of five samples and was used to correct the results for contamination during the dissolution. Except for U, Th, Pb and Ta, whose blank contribution can exceed 20% of the total amount in the sample, all blank contributions are = 10%, with the majority of blank contributions ranging between 0·01 and 2% of the amount in the sample. Repeated measurement of the BE-N (basalt) standard (Geostandards Newsletter, 1995) was used to calculate trace element concentrations in the samples. The precision of the measurements was controlled by replicate measurements (eight times, on a separately dissolved sample) of a basalt from ODP Leg 183, Site 1140 (31R1-57–61 cm; Weis & Frey, 2002; Table 5).
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Samples for Pb, Sr and Nd isotopic analyses were selected to encompass the entire range of compositions of the least altered samples. The chemical procedure used is that described by Weis & Frey (1991). Samples were acid-leached in HCl 6N (6–8 steps) to remove alteration phases. The weight lost by leaching was between 28 and 58%. Two complete duplicates (leached from separate powders) were also analysed and their values are within error (Table 6). Total blank values were = 1 ng for each isotopic system considered, which is negligible with respect to the abundance of the elements in the dissolved samples (i.e. >80 000, 400 and 5000 ng in average for Sr, Pb and Nd, respectively). Measurements were performed at Université Libre de Bruxelles on a multicollector thermal ionization mass spectrometer (Micromass VG Elemental Sector 54). Sr and Nd compositions were measured in the dynamic mode on a single Ta and triple Re–Ta filament, respectively. Sr and Nd isotopic ratios were normalized to 86Sr/88Sr = 0·1194 and 146Nd/144Nd = 0·7219, respectively. The average 87Sr/86Sr of the NBS 987 and 143Nd/144Nd of the Rennes Nd standards (Chauvel & Blichert-Toft, 2001) during the period of our analyses are 0·710279 ± 7 (2
m on 12 samples) and 0·511967 ± 10 (2m on 27 samples), respectively. Pb isotopic compositions were measured in the static mode, at temperatures between 1090 and 1150°C, on a single Re filament using the H3PO4 silica-gel technique. All Pb isotopic ratios were corrected for mass fractionation by 0·12 ± 0·04% per a.m.u., on the basis of 72 analyses of the NBS 981 Pb standard.
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40Ar/39Ar CHRONOLOGY |
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TOPABSTRACTINTRODUCTIONGEOLOGY OF THE KERGUELEN...PETROGRAPHY AND MINERAL...ANALYTICAL TECHNIQUES40Ar/39Ar CHRONOLOGYRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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For all three samples dated by the 40Ar/39Ar technique, >90% of the released argon defined plateaux (Fleck et al., 1977
), which yield ages of 28 Ma (Fig. 2, Table 3). Isochronal and plateau ages agree within error (Table 3). The ages for the top and basal samples from Mont Fontaine overlap within error, suggesting that this 325 m section of basalt erupted rapidly (<1 Myr). These new data fall within the range of ages previously reported for Kerguelen flood basalts (Nicolaysen et al., 2000) and are comparable with the 29 Ma Mont Bureau and Mont Rabouillère tholeiitic to transitional flood basalts from the north–central archipelago (Yang et al., 1998; Fig. 1a). This suggests that the emplacement of tholeiitic to alkalic flood basalts exposed on the archipelago occurred within 5 Myr, from 29 Ma in the north–central part of the archipelago to 24–25 Ma for mildly alkalic basalts from the Southeast Province (Weis et al., 1993; Frey et al., 2000b).
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RESULTS |
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TOPABSTRACTINTRODUCTIONGEOLOGY OF THE KERGUELEN...PETROGRAPHY AND MINERAL...ANALYTICAL TECHNIQUES40Ar/39Ar CHRONOLOGYRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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Major element chemistry
Basalts from Mont des Ruches and Mont Fontaine are tholeiitic to transitional (Fig. 3) and overlap in a plot of total alkalis vs silica with the
29 Ma Mont Bureau and Mont Rabouillère basalts. Two groups of 29 Ma basalts were defined by Yang et al. (1998): Group D (‘depleted’) represents basalts with high MgO contents (6·3–13·4 wt %), relatively low incompatible element abundances, and relatively low 87Sr/86Sr; and Group P (‘plume’) represents basalts with low MgO contents (3·4–5·9 wt %), relatively high incompatible element abundances, and relatively high 87Sr/86Sr. In Fig. 3, two groups are clearly shown in the Mont des Ruches and Mont Fontaine samples. As with the 29 Ma basalts, they are also distinguished on the basis of their MgO contents (Figs 4 and 6). The low-MgO basalts (<6 wt %) of Mont des Ruches and Mont Fontaine overlap with the 29 Ma Group P incompatible element-enriched basalts from Mont Bureau and Mont Rabouillère. The high-MgO basalts (>6 wt %) partly overlap with the 29 Ma Group D basalts of Yang et al. (1998), but have more tholeiitic compositions on average. In Mont des Ruches and Mont Fontaine, the low-MgO basalts systematically form the lower flows (Fig. 4), except for one sample in Mont des Ruches (BY96-31). The most alkalic sample (BY96-34) is most probably a sill; it is intersected by a dyke in the section (Fig. 1b). Despite the presence of low-temperature alteration products (Table 1), the alkali abundances do not seem to have been significantly disturbed, as there is an overall positive correlation between the alkalinity index (AI) and ratios of incompatible elements such as Nb/Zr (Fig. 4).
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The Mont des Ruches and Mont Fontaine sections are dominated by relatively high-MgO basalts (6·6–13·3 wt %) (Fig. 5), in contrast to the 3–5 wt % MgO basalts that cover most of the archipelago (Gautier et al., 1990; Weis et al., 1993, 1998b; Frey et al., 2000b). For many of the Mont des Ruches and Mont Fontaine high-MgO basalts, the whole-rock mg-number is too high relative to the olivine-basaltic liquid equilibrium curve (Roeder & Emslie, 1970) to be in equilibrium with the forsterite content of the olivine cores (Fig. 5). The equilibrium liquid compositions (see arrows in Fig. 6) were calculated by systematically removing olivine (5–10%) from the whole-rock composition. Relative to the whole-rock composition, these equilibrium liquids are 2 wt % lower in MgO, i.e. they change from 9–11 to 7–9 wt % MgO. This decrease clearly does not bring these samples back to the 3–5 wt % MgO range of most archipelago basalts.
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Trace element chemistry
With a few exceptions, such as BY96-31, -46, -84 and -101, the abundance of elements sensitive to secondary alteration, such as Ba, Sr and Rb, is correlated with Nb concentration (Fig. 7). This suggests that the Mont des Ruches and Mont Fontaine samples have trace element concentrations that have not been significantly disturbed by post-magmatic alteration.
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The Mont des Ruches and Mont Fontaine basalts show positive correlations between MgO and Ni and Cr (Fig. 8) that are consistent with both olivine (± Cr-spinel) fractionation to form the low-MgO basalts and olivine accumulation in high-MgO basalts.
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Zr and Nb concentrations in the studied basalts are well correlated and show three distinctive groups, which we call Groups 1, 2 and 3 (Fig. 9):
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- Group 1: the low-MgO basalts, one high-MgO sample (BY96-39) from Mont des Ruches and most of the basalts from Mont Fontaine (samples BY96-80, -85 and -86 are the exceptions) have Nb/Zr ratios (Nb/Zr 0·07–0·10) similar to the 29–26 Ma tholeiitic to transitional basalts from the archipelago (Fig. 9b; Yang et al., 1998; Frey et al., 2002a).
- Group 2: the relatively high-MgO basalts from Mont des Ruches (BY96-24 to -38, except BY96-30 and the low-MgO sample BY96-31), plus BY96-80 from Mont Fontaine, have higher Nb/Zr (
0·13) that overlaps with the mildly alkalic basaltic lavas from the southern archipelago (Fig. 9; Weis et al., 1993; Frey et al., 2000b).
- Group 3: two high-MgO basalts from Mont Fontaine (BY96-85 and -86) and three high-MgO basalts from Mont des Ruches (BY96-30, -33 and -34), have distinctly higher Nb/Zr (
0·17–0·19).
- Group 2: the relatively high-MgO basalts from Mont des Ruches (BY96-24 to -38, except BY96-30 and the low-MgO sample BY96-31), plus BY96-80 from Mont Fontaine, have higher Nb/Zr (
Isotope geochemistry
(87Sr/86Sr)i values correlate well with (143Nd/144Nd)i and indicate that Sr isotopic ratios of the acid-leached samples were not affected by alteration (Fig. 10). Consistent with their relatively depleted incompatible element abundance (low Nb/Zr), the Group 1 basalts (BY96-31 is the exception) have relatively low (87Sr/86Sr)i and high (143Nd/144Nd)i (<0·7049 and >0·5126, respectively; Fig. 10). Group 2 basalts have higher (87Sr/86Sr)i and lower (143Nd/144Nd)i (0·7048–0·7051 and 0·5126–0·5127, respectively) and basalts from Group 3 have much higher (87Sr/86Sr)i and lower (143Nd/144Nd)i values (0·7050–0·7056 and 0·5125–0·5126, respectively). There is an evolution upwards in the Mont des Ruches section from (87Sr/86Sr)i 0·7045 in the lower part (Fig. 4) to relatively high ratios of 0·7056 at 340 m. The uppermost three flows have a nearly constant and lower ratio of 0·7050, which is closer to the average value of the basalts for the section (0·7052). The Mont Fontaine Sr isotopic compositions are much more homogeneous with average initial ratios of 0·7046, except for three of the seven upper flows, which range up to 0·7049.
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In a Nd vs Sr isotopic ratio plot (Fig. 10), the Group 1 basalts, i.e. all of the Mont Fontaine basalts (except BY96-80, -85 and -86) and the low-MgO basalts (except BY96-31 from Mont des Ruches upper section), overlap with the 29 Ma tholeiitic to transitional Group D basalts (Yang et al., 1998). Group 2 basalts, i.e. mostly the Mont des Ruches high-MgO basalts, show limited variation and overlap the field of the enriched 24–25 Ma mildly alkalic basalts ratios field (i.e. 87Sr/86Sr0·7052; 143Nd/144Nd0·5126), representative of the Kerguelen plume signature (Weis et al., 1993, 2002; Frey et al., 2000b). Excluding BY96-33 and -34 from Group 3, which have extremely high (87Sr/86Sr)i (0·7056) and low (143Nd/144Nd)i (0·5125), the Mont des Ruches and Mont Fontaine basalts form a field that ranges from that for metagabbro xenoliths found in alkalic dykes and flows on the archipelago (Mattielli et al., 1996) to the field for the 24–25 Ma mildly alkalic basalts.
The studied basalts form fairly well-defined trends in 208Pb/204Pb and 207Pb/204Pb vs 206Pb/204Pb plots that overlap with the fields for other Kerguelen Archipelago basalts (Fig. 11). All Group 2 samples, together with the low-MgO sample located higher up in the Mont des Ruches section (BY96-31), have distinctly higher 208Pb/204Pb and 207Pb/204Pb for a given 206Pb/204Pb than the Group 1 samples (i.e. low-MgO basalts and the relatively incompatible element-depleted high-MgO basalts). Also, the studied basalts define trends in 208Pb/204Pb vs 206Pb/204Pb that lie above the trend for SEIR mid-ocean ridge basalt (MORB) (Mahoney et al., 2002).
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONGEOLOGY OF THE KERGUELEN...PETROGRAPHY AND MINERAL...ANALYTICAL TECHNIQUES40Ar/39Ar CHRONOLOGYRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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Stratigraphic geochemical evolution in the Mont des Ruches and Mont Fontaine tholeiitic–transitional basalts
Three groups of compositionally distinct basalts are present in the Mont des Ruches and Mont Fontaine sections. They do not show simple correlation between radiogenic isotopic ratios and MgO content. They were erupted sequentially in the following order:
- Group 1. Low-MgO plagioclase-rich tholeiitic to transitional basalts are characterized by relatively low (87Sr/86Sr)i (0·7044–0·7048) and Nb/Zr0·10, except the low-MgO sample BY96-31 located higher in Mont des Ruches section (0·7054 and 0·11, respectively). Tholeiitic to transitional olivine-rich high-MgO basalts, overlying the low-MgO basalts, have (87Sr/86Sr)i 0·7043–0·7049 and Nb/Zr0·07–0·10, and are relatively less enriched in incompatible elements. Only one basalt of this type is present in the Mont des Ruches section (BY96-39), whereas 18 of the 21 high-MgO basalts in Mont Fontaine section belong to this group.
- Groups 2 and 3. Transitional to slightly alkalic, olivine-rich, high-MgO basalts are slightly more enriched in incompatible elements than high-MgO basalts from Group 1 (Fig. 9). Their isotopic composition is variable [e.g. (87Sr/86Sr)i 0·7048–0·7056 and Nb/Zr0·13–0·19]. These groups contain 13 of the 14 high-MgO samples from Mont des Ruches but only three of the 21 high-MgO samples from Mont Fontaine. Together with the low-MgO sample from Mont des Ruches located higher in the section (Fig. 4), these groups have more radiogenic 207Pb/204Pb and 208Pb/204Pb for a given 206Pb/204Pb than Group 1 basalts. The lead isotopic stratigraphic variations (Fig. 4) are consistent with Sr and Nd isotopic variations and show enrichment upwards in the Mont des Ruches and Mont Fontaine sections. The trace element and isotopic compositions of Group 1 basalts suggest that both low-MgO and high-MgO basalts from this group had similar parental magmas. The Group 2 and 3 basalts are slightly more enriched in alkalis (Figs 3 and 4) and may have formed by a lower extent of melting as supported by their higher La/Yb (5–18 compared with 3–10, respectively) on average.
Temporal evolution in tholeiitic to transitional basalts from the Kerguelen Archipelago
A general temporal decrease in the proportion of a depleted component in the source of the Kerguelen Archipelago basalts was interpreted by Gautier et al. (1990) and Weis et al. (1993) as reflecting the decreasing contribution of a MORB source-related asthenospheric component with time, as the ridge moved away from the archipelago. In contrast, the coeval eruption at 29 Ma of depleted (Group D) and enriched (Group P) basalts, the latter with isotopic ratios similar to those proposed for the Kerguelen plume, led Yang et al. (1998) to argue that there was no systematic temporal geochemical trend in the archipelago. On the basis of high Ba/Th and Sr/Nd, reflecting a plagioclase-rich component despite the absence of plagioclase phenocrysts, Yang et al. (1998) inferred that Group D basalts had interacted with a gabbroic SEIR crust component. The positive correlation of 143Nd/144Nd with MgO content in high-MgO basalts from Mont Fontaine Group 1 suggests interaction of the ascending magmas with the lithosphere. In contrast, the Pb and Sr isotopic composition enrichment upwards in 28 Ma Mont des Ruches basalts is independent of MgO content and is not consistent with combined assimilation and fractional crystallization.
A MORB source-like component was present only during the formation of the oldest 29–28 Ma flood basalts and Yang et al. (1998) noted that a plume component was present throughout the growth of the Kerguelen Archipelago. The Sr and Nd isotopic data for the 28 Ma Mont des Ruches and Mont Fontaine basalts are consistent with these observations. A plot of 
8/4 vs (87Sr/86Sr)i (Fig. 12) shows the important variations of Pb isotopic compositions in these tholeiitic to transitional sections. In such a diagram, where 8/4 represents the deviation of 208Pb/204Pb for a given 206Pb/204Pb from the Northern Hemisphere Reference Line (Hart, 1984), Mont des Ruches and Mont Fontaine basalts define a mixing trend between the inferred composition of the Kerguelen plume (Weis et al., 1993, 1998b, 2002) and the SEIR N-MORB field (Mahoney et al., 2002). We suggest that the compositions observed in Mont des Ruches and Mont Fontaine basalts are consistent with simple binary mixing between an SEIR-like component and the Kerguelen plume component, and infer that the absence of a depleted SEIR-like component in the <26 Ma basalts in the archipelago reflects decreasing contribution of an SEIR-like source component.
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Nature of the depleted component in tholeiitic to transitional basalts from the Kerguelen Archipelago
A Nb/Y vs Zr/Y diagram has been useful in discriminating between source components contributing to Icelandic plume-related magmatism (Fitton et al., 1997). In such a diagram, Group 1 basalts from both Mont des Ruches and Mont Fontaine are distributed along a mixing trend (Fig. 13), located along the lower boundary of the Iceland Neovolcanic Zone basalt field (Fitton et al., 1997), between the Kerguelen plume inferred composition (Weis et al., 1993, 2002; Frey et al., 2000b) and SEIR N-MORB. The high-MgO basalts in Group 1 have lower Nb/Y and Zr/Y than the low-MgO basalts and may represent higher extents of partial melting of a similar source. Group 2 and 3 basalts plot mostly within the Iceland plume array (Fig. 13).
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The temporal geochemical trend observed from 29 to 24 Ma in the archipelago flood basalts shows a decreasing role for a depleted component as the SEIR moved from 350 to 550 km away from the archipelago. There are several possible origins for such a component, such as entrained asthenosphere at the periphery of the plume, migration of MORB material from the SEIR axis, and a depleted component within the plume itself. Below we address each of these possibilities using isotopic and geochemical systematics of the Kerguelen Archipelago flood basalts and we use Fig. 14 to illustrate a possible scenario.
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Is a depleted component intrinsic to the Kerguelen mantle plume?
There has been much discussion about the origin of a depleted component in the petrogenesis of Icelandic plume-related basalts (e.g. Fitton et al., 1997
; Hanan et al., 2000; Kempton et al., 2000) and the need for heterogeneities intrinsic to the Icelandic plume has become widely accepted. The heterogeneity of the Galápagos plume has been related to both shallow mixing with local asthenosphere and intrinsic heterogeneity within the plume (White et al., 1993; Kurz & Geist, 1999; Harpp & White, 2001). In general, the Kerguelen Archipelago flood basalts, especially the younger alkalic flood basalts, have nearly uniform Sr and Nd isotopic ratios and these have been attributed to the Kerguelen plume (Weis et al., 1998b, 2002; Frey et al., 2000b) without any contribution from a depleted component. It has been suggested that the 28–29 Ma Kerguelen Archipelago flood basalts with lower 87Sr/86Sr and higher 143Nd/144Nd contain a proportion of an SEIR-related component (Yang et al., 1998; and this study). A MORB-source depleted mantle component appears to be required from the Hf and Nd isotopic systematics of the Kerguelen Archipelago basalts (Mattielli et al., 2002). Frey et al. (2002a) have noted that the 26 Ma basalts from Mont Tourmente are nearly homogeneous in Sr and Nd isotopic compositions, but that they have ratios distinct from those proposed for the Kerguelen plume (Figs 10 and 12). They considered the possibility that these different isotopic ratios reflect plume heterogeneity. As the Mont Tourmente basalts straddle the isotopic gap that exists between the Mont Fontaine samples (Figs 10–12), an alternative explanation is that the Mont Tourmente basalts reflect a homogeneous source formed by efficient mixing of plume and MORB-source components. If this interpretation is valid, an explanation is required for the absence of such efficient mixing in the Mont Bureau, Rabouillère, Ruches and Fontaine sections. We have demonstrated a stratigraphic evolution in the 28 Ma Mont des Ruches and Mont Fontaine basaltic compositions with enrichment upwards in the sections (Fig. 4). This is also true for the 29 Ma Mont Bureau section, where most enriched basalts (Group P) are preferentially located at the top of the section (Yang et al., 1998). These stratigraphic changes in the 29–28 Ma basaltic sections may reflect replenishment in the magma plumbing system, with replenishing magmas resulting from a higher contribution of the Kerguelen plume component and providing more heterogeneous compositions than those observed in the 26 Ma Mont Tourmente basalts. In summary, we propose that depleted heterogeneities intrinsic to the plume are not necessary to account for the abrupt decrease in the role of a depleted component at 25 Ma (see Weis & Frey, 2002, fig. 15); the cessation of sampling of depleted heterogeneities within 1 Myr seems unlikely as the increase in lithosphere thickness within this short time interval would not be important enough to abruptly limit the extent of partial melting.
Is the depleted component entrained, depleted upper mantle?
An entrained, depleted upper mantle component should be mostly present in the Kerguelen plateau lithosphere, which formed by melting of the head of the Kerguelen plume, where most of the entrained depleted mantle may have mingled with the upwelling plume (e.g. Campbell & Griffiths, 1990). Only one ODP Leg 120 site shows the important involvement of a depleted component (Site 749; Frey et al., 2002b). Alternatively, melting of plume and entrained depleted mantle may occur in ponded plume material areas, which can reach lateral extensions of thousands of kilometres (Nataf, 2000), and where mingling is more efficient. On the basis of the geochemical data we are unable to determine if the depleted component was depleted mantle entrained during plume ascent or depleted material associated with the SEIR.
Is the depleted component the upwelling asthenospheric source for SEIR basalts?
Rare earth element patterns of lower-crustal metagabbro xenoliths found in the alkalic dykes and flows on the archipelago are similar to those of gabbroic dykes related to MORB and to plagioclase-rich cumulates sampled at Indian Ocean ridges (Grégoire et al., 1998). Although the isotopic compositions of the Kerguelen metagabbro xenoliths, with 143Nd/144Nd 0·5127–0·5129 and 87Sr/86Sr 0·70425–0·70475 (Mattielli et al., 1996), do not overlap with SEIR MORB, they overlap with the 29–28 Ma depleted basalts from the Kerguelen Archipelago [e.g. low (87Sr/86Sr)i of 0·7040–0·7048; Fig. 10]. Hence an SEIR component may be present in these xenoliths.
Plume–ridge interactions involving asymmetric spreading along the south Mid-Atlantic Ridge and subsequent sublithospheric channelling of plume material to the ridge axis have been proposed, based on geochemical observations of short-wavelength variations in the ratios of incompatible elements, such as La/Sm and Nb/Zr (Schilling et al., 1985). Locally, the isotopic compositions of lavas from the Easter seamount chain have been used to argue for bidirectional flow between the spreading ridge axis and the Easter plume (Haase, 1996). Geochemical evidence of mixing between plume- and ridge-derived material is observed as far as 400 km away from the ridge axis in the Foundation chain, where the Pacific–Antarctic Ridge moves towards the Foundation hotspot (Maia et al., 2000). In the opposite case, when a ridge moves away from a hotspot location, as is the case for the SEIR relative to the Kerguelen hotspot, numerical experiments (e.g. Ito et al., 1997) show that hotspot–ridge interaction will be enhanced and active at larger distances than when the ridge moves towards the hotspot. Numerical experiments from Yale & Phipps Morgan (1998) show the important effect of asymmetric spreading on flow of plume-related material toward a ridge. Also, seismicity along the 81–85°E fracture zone on the Antarctic plate between the Kerguelen Plateau and the Amsterdam–Saint Paul Plateau (Fig. 1) has been interpreted as thermal and bending stresses in the lithosphere overlying a thermal anomaly resulting from channelled flow between the Kerguelen hotspot and the SEIR axis (Bergman et al., 1984).
The 34 Ma basalts from the Northern Kerguelen Plateau at Site 1140, which was 200 km away from the SEIR at its formation (Fig. 14), show evidence of plume– and upwelling depleted mantle–magma mixing (Weis & Frey, 2002). The Kerguelen hotspot was 350–450 km away from the ridge axis when the compositionally heterogeneous 29–28 Ma flood basalts erupted (Fig. 14); these 29–28 Ma basalt compositions also reflect mixing of plume and SEIR material, but with a significantly higher contribution of the Kerguelen plume component than for depleted basalts from Site 1140 (Figs 10–13). These differences may indicate that mixtures of plume and upwelling asthenosphere material formed during asymmetric spreading of the SEIR (oval labelled 1+4 in Fig. 14). We propose that plume and ridge source material mingled below the lithosphere and that these heterogeneous mixtures of upwelling plume and depleted mantle source for SEIR basalts have subsequently melted in the plume stem (Fig. 14) to form the heterogeneous coeval depleted and enriched compositions observed in the 29–28 Ma flood basalts from the northwestern Kerguelen Archipelago. This scenario best resolves the temporal decreasing contribution of SEIR mantle source in the Northern Kerguelen Plateau and the Kerguelen Archipelago, as the ridge moved from 200 km to 560 km away from the Kerguelen hotspot. This interpretation could also explain the presence of enigmatic topographic highs observed on the ocean floor between the Northern Kerguelen Plateau and the SEIR axis [see global topographic map of Smith & Sandwell (1997)].
Did continental crust contaminate Mont des Ruches and Mont Fontaine basalts?
Contrary to the Cretaceous Kerguelen Plateau, which records significant continental contamination in the early products of Kerguelen plume activity (Mahoney et al., 1995; Weis et al., 2001; Frey et al., 2002b; Ingle et al., 2002), the Mont des Ruches and Mont Fontaine basalts have isotopic compositions that do not reflect contamination by lower or upper continental crust (e.g. Fig. 12). The positive correlation of their Sr isotopic ratios with Nb/Zr (Fig. 4) is inconsistent with the potential involvement of continental crust, which has high 87Sr/86Sr and low Nb/Zr. The relative enrichment in Nb compared with Kerguelen Plateau basalts argues against continental contamination because continental crust is depleted in Nb (e.g. Sun & McDonough, 1989). On a larger scale, there is also no continental contamination documented at ODP Site 1140, 300 km north of the archipelago, in the Northern Kerguelen Plateau (Weis & Frey, 2002).
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CONCLUSIONS |
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TOPABSTRACTINTRODUCTIONGEOLOGY OF THE KERGUELEN...PETROGRAPHY AND MINERAL...ANALYTICAL TECHNIQUES40Ar/39Ar CHRONOLOGYRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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Strong geochemical and isotopic heterogeneities in the 28 Ma Kerguelen Archipelago basalts [e.g. (87Sr/86Sr)i
0·7043–0·7056], compared with the relative homogeneity and enriched composition of 24–25 Ma mildly alkaline basalts on the archipelago (e.g. 87Sr/86Sr 0·7052, Weis et al., 1993; Frey et al., 2000b) are consistent with variable amounts of mixing between a upwelling depleted mantle asthenospheric component and the Kerguelen plume source. Evidence for mixing between plume and ridge-related material from 34 Ma in the Northern Kerguelen Plateau (ODP Site 1140) to at least 28 Ma, i.e. from 200 km to 400 km away from the ridge axis, suggest that plume–ridge interactions occurred as the SEIR migrated away from the Kerguelen hotspot (as far as 450 km away from the SEIR axis). This is consistent with previous interpretation of seismicity on the oceanic floor between the Northern Kerguelen Plateau and the SEIR that could reflect the existence of a sublithospheric channel flow between the Northern Kerguelen Plateau and the SEIR. We propose that during asymmetric spreading along the SEIR, a two-way channel flow permitted an efficient mingling of the upwelling depleted mantle with the Kerguelen plume. Mixtures of the enriched and depleted sources, which are still present in the top of the plume stem, were subsequently melted and the resulting magmas ascended through the Kerguelen Plateau lithosphere. Entrained depleted mantle during plume ascent is an unlikely explanation by itself for the temporal geochemical trends observed in the Kerguelen Archipelago; depleted heterogeneities intrinsic to the plume source also seem unlikely to explain the marked cut-off in the presence of depleted basalts on the Kerguelen Archipelago after 26 Ma.
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ACKNOWLEDGEMENTS |
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We are grateful to D. Christie for providing fields for Nb–Zr–Y compositions for the SEIR MORB shown in Figs 9 and 13. We thank G. Michon, B. Quemeneur and Y. Julliot for their field descriptions of the Loranchet Peninsula geology. We also thank the captain and crew of the Marion Dufresne II, the IFRTP and the TAAF for logistical support. We are especially grateful to J. Hertogen and J. Mareels for their help in trace element ICP-MS data acquisition. M. Veschambre is thanked for technical assistance during the microprobe analyses, and C. Maerschalk for technical assistance for mass spectrometry. This paper benefited from very constructive reviews from H. J. Yang, A. Pietruszka and J. F. Allan. This work was supported by an ARC grant (Actions de Recherches Concertées) from the Communauté Française de Belgique, ARC 98/03-233), and by a grant from the Région Rhône-Alpes (Eurodoc). X-ray fluorescence major and trace element acquisition was financed by US NSF EAR Grant 9814313 (F.A.F.).
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FOOTNOTES |
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*Corresponding author. Present address: Département des Sciences de la Terre et de l’Environnement, Université Libre de Bruxelles, B-1050 Brussels, Belgium. Telephone: +32-2-650-22-46. Fax: + 32-2-650-37-48. E-mail: sdoucet{at}ulb.ac.be
Present address: Department of Geology, Kansas State University, Manhattan, KS 66506, USA
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REFERENCES |
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