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Trace of the Kerguelen mantle plume: Evidence from seamounts between the Kerguelen Archipelago and Heard.. Weis, Dominique 2011

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Trace of the Kerguelen mantle plume: Evidence from seamounts between the Kerguelen Archipelago and Heard Island, Indian Ocean Dominique Weis Département des Sciences de la Terre et de l’Environnement, Université Libre de Bruxelles CP160/02, Avenue F.D. Roosevelt, 50, B-1050 Brussels, Belgium (dweis@ulb.ac.be) Currently at Earth and Ocean Sciences, University of British Columbia, Vancouver, V6T-1Z4, Canada (dweis@eos.ubc.ca) Frederick A. Frey Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA (fafrey@mit.edu) Roland Schlich, Marc Schaming, and Raymond Montigny Ecole et Observatoire des Sciences de la Terre (ULP), Institut de Physique du Globe de Strasbourg (UMR CNRS 7516), 67084 Strasbourg, France (mschaming@eost.u-strasbg.fr; Raymond.montigny@eost.u-strasbg.fr) Dimitri Damasceno and Nadine Mattielli Département des Sciences de la Terre et de l’Environnement, Université Libre de Bruxelles CP160/02, Avenue F.D. Roosevelt, 50, B-1050 Brussels, Belgium (ddamasce@mac.com; nmattiel@ub.ac.be) Kirsten E. Nicolaysen Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA Currently at Département des Sciences de la Terre et de l’Environnement, Université Libre de Bruxelles CP160/02, Avenue F.D. Roosevelt, 50, B-1050 Brussels, Belgium (knicol@ksu.edu) James S. Scoates Département des Sciences de la Terre et de l’Environnement, Université Libre de Bruxelles CP160/02, Avenue F.D. Roosevelt, 50, B-1050 Brussels, Belgium (jscoates@ulb.ac.be) Currently at Earth and Ocean Sciences, University of British Columbia, Vancouver, V6T-1Z4, Canada (jscoates@eos.ubc.ca) [1] The gravity and bathymetric highs on the Kerguelen Plateau in the southern Indian Ocean between the Kerguelen Archipelago and Heard Island are seamounts formed of Miocene alkalic basalts similar to those found on the islands. Dredging during the Kerimis survey cruise recovered >1 ton of mostly basaltic rocks. One of the dredges (D6) yielded a large volume of in situ alkalic picritic pillow basalts, the first picritic lavas recovered that are related to the Kerguelen plume. K-Ar and 40Ar-39Ar ages are between 18 and 21 Ma for all but one sample, and these ages are only slightly younger than the main phase of volcanism on the archipelago. The dredged lavas form three distinct groups based on chemical and isotopic compositions. Incompatible element abundance ratios overlap with compositional groups defined by lavas from both the Kerguelen Archipelago and Heard Island indicating that alkalic volcanism in this region of the Kerguelen G3GeochemistryGeophysicsGeosystems Published by AGU and the Geochemical Society AN ELECTRONIC JOURNAL OF THE EARTH SCIENCES Article Volume 3, Number 6 20 June 2002 10.1029/2001GC000251 ISSN: 1525-2027 Copyright 2002 by the American Geophysical Union 1 of 27 Plateau has been spatially diverse. Olivine and picritic basalts have Sr and Nd isotopic characteristics similar to most of the lavas exposed on the archipelago and those proposed for the Kerguelen plume. However, compared to Kerguelen Archipelago lavas, the picritic basalts have relatively low 206Pb/204Pb which is a characteristic of Cretaceous basalt forming some parts of the Kerguelen Plateau. We propose that the apparent age trend of the lavas from 34 Ma in the Northern Kerguelen Plateau (ODP Leg 183, Site 1140) to 24–30 Ma on the Kerguelen Archipelago to 18–21 Ma on the dredged submarine volcanoes, and even possibly to recent volcanism on Heard and McDonald Islands, may correspond to the Tertiary hot spot track of the Kerguelen plume. Components: 12,583 words, 11 figures, 7 tables. Keywords: Kerguelen; trace elements; isotopes; mantle plume; hot spot track; picrites; geochemistry. Index Terms: 1025 Geochemistry: Composition of the mantle; 1040 Geochemistry: Isotopic composition/chemistry; 1065 Geochemistry: Trace elements (3670). Received 11 October 2001; Revised 19 February 2002; Accepted 19 February 2002; Published 20 June 2002. Weis, D., F. A. Frey, R. Schlich, M. Schaming, R. Montigny, D. Damasceno, N. Mattielli, K. E. Nicolaysen, and J. S. Scoates, Trace of the Kerguelen mantle plume: Evidence from seamounts between Kerguelen Archipelago and Heard Island, Indian Ocean, Geochem. Geophys. Geosyst., 3(6), 10.1029/2001GC000251, 2002. 1. Introduction [2] The Kerguelen mantle plume has produced 15.2 to 24.1  106 km3 of basaltic magmas (minimum and maximum volumes assuming off- and on-ridge emplacement, respectively [Coffin and Eldhom, 1994]) for at least 120 million years [Weis et al., 1989a; Barron et al., 1989; Schlich et al., 1989]. The Kerguelen plume played an important role in gen- erating major tectonic features in the Indian Ocean [e.g., Weis et al., 1991; Frey et al., 2000a] and subsequently in the genesis of geochemical anoma- lies in Indian Ocean floor basalts [Dosso et al., 1988; Mahoney et al., 1992; Weis and Frey, 1996; Graham et al., 1999]. The long volcanic record of the Kerguelen plume includes a large igneous prov- ince (the giant oceanic plateau Kerguelen Plateau- Broken Ridge), which may be associated with plume initiation, the >5000 km long 82–38 Ma Ninetyeast Ridge hot spot track, which is the longest linear feature on Earth that formed as the Indian Plate migrated rapidly northwards over the plume, and recently active oceanic islands (Kerguelen Archipelago, McDonald and Heard Islands) on the Antarctic Plate (Figure 1). [3] The surface expression of the Kerguelen plume during the late Cretaceous and early Tertiary resulted in the formation of the Southern and Central Kerguelen Plateau, Broken Ridge, and Ninetyeast Ridge. In particular, Ninetyeast Ridge provides compelling evidence for the trace of the Kerguelen hot spot as the Indian plate moved north- wards over the plume [Duncan and Richards, 1991]. The current location of the hot spot is uncertain; Eocene and younger lavas occur on both the Kerguelen Archipelago and Heard Island, which are separated by 440 km, and there is current volcanism at McDonald Island (Figure 1). At 34 Ma, tholeiitic basalts erupted in the location of ODP Site 1140 of Leg 183 in the northern part of the Northern Kerguelen Plateau [Frey et al., 2000a; Duncan and Pringle, 2000; Weis and Frey, 2000] (Figure 2). Over 250 km to the south (Figure 1), the transitional to alkalic flood basalts that cover >85% of the Kerguelen Archipelago formed between 30 and 24 Ma [Nicolaysen et al., 2000]. The younger, volumetrically minor basanites and differentiated lavas in the southeast and southern part of the Kerguelen Archipelago occur principally as dikes or plugs in the flood basalts and have ages between 10 and 6 Ma for the Upper Miocene series of the southeast Province [Weis et al., 1993] and between 2 Ma and 0.1 Ma at Mount Ross [Weis et al., 1998b]. The most recent volcanic activity on the Kerguelen Archipelago has been alkalic with abun- Geochemistry Geophysics Geosystems G3 weis et al.: trace of the kerguelen mantle plume 10.1029/2001GC000251 2 of 27 dant evolved lavas (trachytes and phonolites). To the south, Heard Island (Figure 2) also has a record of volcanism from middle Eocene to Recent; how- ever, Pleistocene-Holocene lavas are volumetrically dominant [Barling et al., 1994]. Both Heard Island and the nearby McDonald Island have had historic eruptions (1998). This brief geochronologic sum- mary illustrates why there is significant uncertainty in the location of the Kerguelen hot spot and there- fore in plate tectonic reconstructions of the Indian Ocean. Most plate reconstruction models locate the hot spot on the west of the Kerguelen Archipelago in order to fit Ninetyeast Ridge [Duncan and Storey, 1992; Steinberger, 2000], whereas the record of recent volcanic activity would put the hot spot beneath Heard Island. [4] Constraining the current location of the Kergue- len hot spot is of fundamental importance, not only for understanding the history of the Kerguelen Figure 1. Map of the southeastern Indian Ocean showing major physiographic features related to the activity of the Kerguelen plume [Coffin et al., 2000], as well as the drill sites of ODP Legs 119, 120 (circles) and 183 (stars) and dredge sites on the Kerguelen Plateau (squares). Black symbols indicate that basement was reached. Geochemistry Geophysics Geosystems G3 weis et al.: trace of the kerguelen mantle plume 10.1029/2001GC000251 3 of 27 plume, but also for reconciling plate tectonic recon- structions of the Indian Ocean basin. Understanding the tectonic history of the Indian Ocean will also potentially shed light on the breakup of eastern Gondwana [Nicolaysen et al., 2001] and the subse- quent complex mid-ocean ridge reorganizations, which may have included one or more ridge jumps. Importantly, theKerguelen plume is characterized by distinctive geochemical signatures, very different from those of Iceland or Hawaii [Zindler and Hart, 1986; Weis et al., 1989b], and as such can yield unique information about the origin of mantle het- erogeneities. This paper presents the first geochro- nologic and geochemical study of seamounts between Kerguelen Archipelago and Heard Islands. Notably the first picritic basalts related to the Ker- guelen plume were recovered from one of these seamounts. The ages of dredged seamount lavas and their geochemical affinity with other Kerguelen lavas provide compelling evidence for anOligocene- to-Recent hot spot track of the Kerguelen plume. 1.1. Kerguelen Plume and Kerguelen Plateau [5] The 2  106 km2 Kerguelen Plateau (Figure 1) has been divided into several distinct domains [Houtz et al., 1977; Schlich, 1982; Coffin et al., Figure 2. Satellite-derived gravity field for the Northern Kerguelen Plateau (scale in 1/10 mgals) showing the MD109-Kerimis track chart, dredge sites (pink), proposed drilling sites (cyan) and ODP Leg 183 site locations (grey). The two white lines represents the transect shown on Figure 11. Geochemistry Geophysics Geosystems G3 weis et al.: trace of the kerguelen mantle plume 10.1029/2001GC000251 4 of 27 1986;Munschy et al., 1994]: (1) northern Kerguelen Plateau (NKP) which has shallow water depths (<1000 m) and includes the Kerguelen Archipelago, (2) central Kerguelen Plateau (CKP) which is also relatively shallow, contains a large sedimentary basin (Kerguelen-Heard Basin), and includes the volcanically active Heard and McDonald islands, (3) southern Kerguelen Plateau (SKP) which is characterized by deeper water depths (1500– 2500 m), contains a major sedimentary basin (Rag- gatt Basin), and is tectonically more complex than the NKP and CKP, (4) Elan Bank, which extends westward from near the boundary between the CKP and the SKP and has water depths less than 2000 m, and (5) Labuan Basin, which adjoins the CKP and SKP to the east, and is a deep (>3500m), extensively faulted, thickly sedimented (>2000 m) basin whose structure is similar to that of the main Kerguelen Plateau. The dredged seamounts are located on the NKP and northern CKP (Figure 1). [6] Dredging and drilling of basement has yielded tholeiitic basalts with ages of 110–119 Ma for the Southern Kerguelen Plateau, 108–110 for Elan Bank, and 94–102 Ma for the central Kerguelen Plateau and Broken Ridge [Leclaire et al., 1987; Pringle and Duncan, 2000]. The geochemical char- acteristics of these basalts are consistent with der- ivation from the Kerguelen plume [Davies et al., 1989;Weis et al., 1989a; Salters et al., 1992; Storey et al., 1992; Coffin et al., 2000], but basalts from the southern tip of the plateau (Site 738) and Elan Bank (Figure 1) have geochemical signatures reflecting the presence of a continental lithospheric compo- nent [Mahoney et al., 1995; Weis et al., 2001]. Although the Kerguelen Archipelago, which is constructed on the northern Kerguelen Plateau, has been studied in detail [Gautier et al., 1990; Weis et al., 1993, 1998b; Yang et al., 1998; Frey et al., 2000b; Nicolaysen et al., 2000], the submar- ine basement of the northern Kerguelen Plateau had not been sampled prior to this study and the sub- sequent ODP drilling Leg 183 [Frey et al., 2000a]. 1.2. Kerimis Survey Cruise: Dredge Locations and Descriptions [7] New multichannel seismic reflection data were collected in March 1998 during the Kerimis (Ker- guelen, Imagerie Multifaisceau et Imagerie Sismi- que) survey cruise of the French research vessel Marion Dufresne 2 [Schlich et al., 1998]. Five transects, 2000 km in total, were shot over the northern Kerguelen Plateau and over several large seamounts located to the south and to the west of the Kerguelen Archipelago (Figure 2). The first south- west-northeast transect (MD109-05) was located to the northwest of the archipelago between ODP sites 1139 and 1140. It extends southward from the northern boundary of the NKP across the sub- marine platform of the archipelago and crosses Leclaire Rise (or Skiff Bank) at almost a right angle. This transect and a northwest-southeast transect (MD109-06) identify a prominent basement sur- face, which may indicate that throughout the NKP subaerial erosion occurred before subsequent sub- sidence and sedimentation. A north-south transect east of the archipelago (MD109-07) crosses the deep Cretaceous Kerguelen-Heard sedimentary basin [Schlich et al., 1971; Munschy and Schlich, 1987]. This transect and two additional transects (MD109-08 and MD109-09) cross several large seamounts with clearly defined dipping basement reflector wedges. These data coupled with results of the previous surveys (1970, 1972, and 1981) were used to identify locations for dredging during the Kerimis cruise and to survey the basement for drilling sites 1139 and 1140 of ODP Leg 183 [Coffin et al., 2000]. [8] More than 1 ton of basalt was recovered from four dredge sites (Figures 1 and 2). Dredge 1 (D1: 50210S-63480E) on the Northern Kerguelen Pla- teau, west of the Kerguelen Archipelago, was located on the flank of Skiff Bank and collected 200 kg of samples of diverse rock types including aphyric basaltic to trachybasaltic lavas, microgab- broic to alkali granitic intrusive rocks, and sedi- mentary rocks. With the exception of the rare fresh basalts (<5%), all the samples are small (10–20 cm), rounded and coated by Fe-Mn. These rocks will be discussed elsewhere in conjunction with studies of lavas recently recovered from Skiff Bank at ODP Leg 183 Hole 1139 [Frey et al., 2000a;Kieffer et al., 2000]. [9] Dredges 4 (D4: 51310S-71090E) and 5 (D5: 51310S-71080E) were on the northern part of the Geochemistry Geophysics Geosystems G3 weis et al.: trace of the kerguelen mantle plume 10.1029/2001GC000251 5 of 27 central Kerguelen Plateau on the most northerly of the seamounts that are distributed linearly between the Kerguelen Archipelago and Heard Island (Figure 2). D4 and D5 traversed the lower and upper southeast flank of this structure. These dredges recovered 700 kg of basalt, with fea- tures reflecting an in situ origin (fresh broken surfaces, columnar shapes, minimal Mn crusts). These basalts have abundant centimeter-size oli- vine/clinopyroxene phenocrysts that distinguish them from the basalts of the Kerguelen Archipe- lago where plagioclase phenocrysts are relatively common [Gautier et al., 1990]. [10] Dredge 6 (D6: 51010S-071040E) was on the northeast flank of the same structure and collected >1 ton of very fresh samples, mainly picritic pillow basalts with abundant olivine phenocrysts. The fresh broken surfaces of the pillows indicate that these are in situ samples. The convex surfaces of the pillows are Fe-Mn-coated glass. Vesicles are not abundant, but pipe vesicles 5–15 cm in length radiate from the pillow cores. 2. Analytical Techniques 2.1. Mineral Chemistry [11] Olivine and clinopyroxene mineral composi- tions (Tables A1 and A2) were determined using a CAMECA SX100 electronprobe (CNRS – Blaise Pascal University, Clermont-Ferrad, France) with the following settings: accelerating voltage, 15 kV; beam current, 15 nA; beam size, 3 mm; counting time, 15 s; and a ZAF correction factor. Calibration was made with natural and synthetic standards. 2.2. K-Ar and 40Ar-39Ar Dating [12] Details on analytical techniques for the K-Ar and 40Ar-39Ar dating is given by Montigny et al. [1988] and Henry et al. [1998] The set of con- stants recommended by Steiger and Jäger [1977] was used for age calculation. The 120–150 mg aliquots of standard biotite LP6 (129 Ma) were utilized as flux monitors. Uncertainties represent- ing estimates of analytical precision are quoted as 2s and were calculated using the procedure of Cox and Dalrymple [1967] for K-Ar conventional dates and using that of Albarède [1976] for 40Ar-39Ar dates. 2.3. Major and Trace Element Concentrations [13] Abundances of the major elements and the trace elements Rb, Sr, Ba, V, Ni, Zn, Ga, Y, Zr, Nb, La, and Ce were determined by X-ray fluo- rescence (XRF) at the University of Massachusetts, Amherst [Rhodes, 1996]. All reported XRF data are the mean values of duplicate analyses. Abundances of the trace elements Sc, Cr, Co, Hf, Ta, Th, and several rare earth elements were determined by instrumental neutron activation at MIT [Ila and Frey, 2000]. The accuracy and precision of these techniques were discussed by Rhodes [1996] and Ila and Frey [2000]. 2.4. Sr, Nd, and Pb Isotopic Compositions [14] Samples for Sr, Nd, and Pb isotope analyses were selected to encompass the entire range of chemical compositions. The chemical procedure used was similar to that described by Weis et al. [1987] with improvements as discussed by Weis and Frey [1991]. All of the samples were acid- leached to remove secondary or alteration phases. Total blank values were <1 ng for all three isotopic systems considered. Such values are negligible in view of the elemental concentrations in the sam- ples. Sr and Nd isotopic compositions were meas- ured on single Ta filaments and triple Re-Ta filaments, respectively, in the dynamic mode on a VG Sector 54 multicollector mass spectrometer with an internal precision better than 1  105 unless specified in Table 4. Sr isotopic ratios were normalized to 86Sr/88Sr = 0.1194 and Nd isotopic ratios (for isotopes 147, 146, 145, 144, 143, and 142, measured as metal), to 146Nd/144Nd = 0.7219. The average 87Sr/86Sr value of the NBS 987 Sr standard was 0.71027 ± 2 (2sm on 48 samples) and analyses of the Rennes Nd standard yielded 143Nd/144Nd = 0.51196 ± 1 (2sm on 39 samples). Pb isotopic compositions were measured on single Re filaments using the H3PO4-silica gel technique. All Pb isotopic ratios were corrected for mass fractionation (0.12 ± 0.04% per amu) on the basis of 72 analyses of the NBS 981 Pb standard for a Geochemistry Geophysics Geosystems G3 weis et al.: trace of the kerguelen mantle plume 10.1029/2001GC000251 6 of 27 temperature range between 1090C and 1150C. Between-run precisions were better than 0.1% for 206Pb/204Pb and 207Pb/204Pb and better than 0.15% for 208Pb/204Pb. The quality of a complete replicate analysis can be evaluated by sample D4- 33, which was analyzed twice because of its unusual composition. 2.5. Hf Isotopic Compositions [15] Extraction of Hf was performed at the Uni- versité Libre de Bruxelles. Hf isotope analyses were carried out on the multicollector plasma- source mass spectrometer (P54) at the Ecole Natio- nale Supérieure de Lyon, following the analytical procedure of Blichert-Toft et al. [1997]. All meas- ured Hf isotopic ratios were corrected for W and Ta, Lu and Yb interferences at masses 180 and 176, respectively, by monitoring the isotopes 182W, 181Ta, 175Lu, 173Yb and normalized for mass frac- tionation to 179Hf/177Hf of 0.7325 using an expo- nential law. During the period of data collection, eighteen analyses of the JMC175 Hf standard gave an unweighted mean for 176Hf/177Hf of 0.28216 ± 1 (standard deviation). 2.6. He Isotopic Compositions [16] Sample MD109-07 D6 88 was crushed to less than 2 mm diameter using a disk mill and sieved to obtain a size fraction between 0.8 and 2 mm. A Frantz magnetic separator was used to concentrate the olivine before handpicking of the most pristine phenocrysts using a binocular microscope. The olivine separate was abraded for several hours in an air-abrader to remove adhering groundmass and alteration minerals. After a final microscopic inspec- tion, the abraded olivines were weighed and rinsed with 4N HNO3 and water in an ultrasonic bath. The sample was dried with a final rinse with acetone and loaded into a clean crusher for in vacuo analysis by the 90 sector mass spectrometer at the Woods Hole Oceanographic Institution [e.g., Kurz et al., 1996]. 3. Results 3.1. Age of the Seamounts [17] K-Ar and 40Ar-39Ar measurements were made on whole rock samples (Tables 1 and 2). The combined use of the two methods was essential to evaluate the loss of neutron-induced 39ArK owing to recoil from fine-grained phases or glass [Seidemann, 1977], which results in overestimates of the ages. Evaluating the geological meaning of K-Ar and 40Ar-39Ar dates for submarine basalts can be difficult. We use two criteria: first the compar- ison between the K-Ar conventional age and the 40Ar-39Ar integrated age, and second the shape of the 40Ar-39Ar release spectrum. [18] The 40Ar-39Ar release spectra are displayed in Figure 3. The salient features of the results are as follows: 1. Olivine basalts from D5, which contain a significant amount of glass in the matrix, give K-Ar ages from 16.5 to 17.7 Ma, whereas they yield Ar- Ar integrated ages of around 19.5 Ma, all within error of each other. Two of the samples (D5-44a and D5-44b) display inverse-shaped spectra (i.e., the high temperature steps correspond to the lowest ages, while the third (D5-42) reveals a hump-shaped one). Both types of release spectrum are commonly attributed to irradiation in the nuclear reactor, which causes 39ArK redistribution owing to recoil in glassy or cryptocrystalline matrix [Turner et al., 1997]. 2. Picritic basalts (D6) with a glassy matrix yield fairly concordant K-Ar ages from 13.8 to 14.8 Ma, except one sample that indicates a somewhat older date of 19.0 Ma. They yield fairly concordant Ar-Ar integrated ages, between 18.7 and 21.4 Ma. The shapes of the release patterns are variable: flat for the most part of D6-88 and D6-89, inverse for D6- 87, and hump-shaped for D6-85. 3. Basalt D4-37, which has olivine phenocrysts in a well-crystallized matrix, yields a K-Ar date of 21.0 ± 0.6 Ma. Its degassing pattern is characterized by an anomalously old age for the initial step, 220 ± 20 Ma, and a well-defined plateau with an age of 21.9 ± 0.2Ma. It indicates an integrated age of 23.9 ± 0.4 Ma, which if calculated without the initial step, becomes 21.3 ± 0.6 Ma. 4. Basalt D4-33, which has pyroxene and olivine phenocrysts in a fresh crystalline matrix, yields a K- Ar age of 0.72 ± 0.25 Ma and an Ar-Ar integrated age of 1.6 ± 0.7 Ma. The degassing pattern is staircase-type. [19] All 40Ar-39Ar ages are slightly older than the K-Ar ages for the same sample. Inspection of the Geochemistry Geophysics Geosystems G3 weis et al.: trace of the kerguelen mantle plume 10.1029/2001GC000251 7 of 27 inverse isochron diagrams reveals no significant occurrence of excess argon. The Ca/K release is normal for all types of basalt. The ratio is low in the initial step, rises more or less gradually with temperature, and rises dramatically in the final step, displaying high values. [20] Olivine basalts (D5) display 40Ar-39Ar inte- grated ages, which are 15% higher than the con- ventional K-Ar ages. This difference cannot be explained either by a systematic bias in conven- tional K-Ar measurement of standard rocks nor by heterogeneity of the standard LP6 in 40Ar-39Ar dating as the samples were measured against three different aliquots of that standard. Accordingly, we consider a 15% 39ArK loss during irradiation to be likely. The concordant K-Ar dates suggest that the samples are contemporaneous. Given the signifi- cant amount of glass, we view the highest value, 17.7 ± 0.5 Ma, as the minimum age for the time of emplacement of these basalts. The freshness of the samples and the moderate loss of 39Ar during irradiation, however, suggests that the 40Ar* loss by the matrix should not have been severe. There- fore we propose an age of emplacement bracketed between 17.7 and 20–21 Ma, plateau ages yielded by D5-44a and D5-42, respectively. [21] The difference between the conventional K-Ar ages and 40Ar-39Ar ages for three of the picritic basalts (D6) suggests that they underwent 30–40% 39Ar loss during irradiation, while sample D6-88 did not suffer appreciably. Therefore we focus on D6-88 to appraise the geological age of the picritic basalts. The last steps of the degassing pattern with low ages are indicative of 39ArK redistribution during irradiation. Nevertheless, this redistribution occurred without significant argon loss as exem- plified by the concordance of the K-Ar conven- tional age with the 40Ar-39Ar age. Consequently, we propose a minimum age of 19.1 ± 1.0 Ma for the emplacement. The freshness of sample D6-88 and Table 1. K-Ar Ages on the Kerimis Dredged Basaltsa Sample Rock Type K2O, weight % 40Ar*100 total 40Ar rad. 40Ar, 1011mole/g Age, Ma ±2s, Ma D4-33 basalt 0.823 4.7 0.0853 0.72 0.25 D4-37 basalt 2.091 40.2 6.360 21.0 0.6 D5-42 olivine basalt 2.331 48.5 5.558 16.5 0.5 D5-44a olivine basalt 2.035 48.0 5.199 17.7 0.5 D5-44b olivine basalt 2.141 57.1 5.232 16.9 0.5 D6-85 picritic basalt 1.077 12.2 2.173 14.0 0.8 D6-87 picritic basalt 1.363 25.0 2.925 14.8 0.6 D6-88 picritic basalt 0.904 12.7 2.492 19.0 1.0 D6-89 picritic basalt 1.282 11.1 2.557 13.8 0.9 a The 40K/K total = 1.167  104 mole/mole; lb = 4.962  1010 a1; le = 0.581  1010 a1. Table 2. Summary of 40Ar-39Ar Results on the Kerimis Dredged Basalts Sample Integrated Date in Ma ± 2s Plateau Date in Ma ± 2s 36Ar/40Ar versus 39Ar/40Ar date in Ma ± 2s Initial 40Ar/36Ar D4-33 1.60 ± 0.7 1.63 ± 0.42a 1.56 ± 0.74b 297 ± 3 D4-37 23.9 ± 0.4 21.9 ± 0.40 21.9 ± 0.70 296 ± 12 D5-42 19.1 ± 0.4 20.4 ± 0.30 20.2 ± 0.50 301 ± 8.8 D5-44a 20.0 ± 0.5 21.2 ± 0.50 21.0 ± 0.60 295 ± 4 D5-44b 19.3 ± 0.5 D6-85 18.7 ± 1.0 D6-87 21.4 ± 2.0 D6-88 19.2 ± 1.3 20.7 ± 1.30 20.3 ± 2.60 295 ± 3 D6-89 19.3 ± 1.6 a ‘‘Plateau age’’ determined with two steps representing 90% of the released 39ArK. b Isochron determined with five steps. Geochemistry Geophysics Geosystems G3 weis et al.: trace of the kerguelen mantle plume 10.1029/2001GC000251 8 of 27 the probable absence of 39ArK loss during irradiation are reasons to assume that the glass matrix of the basalt retained 40Ar* completely. Hence the age of emplacement should not exceed 20 Ma. [22] The anomalously high age of the first step of basalt sample D4-37 (220 ± 20 Ma), which amounts to 1.3% of the released 39ArK, is probably due to significant 39ArK loss by the surface of the grains. If we discard this step the integrated Ar-Ar age becomes 21.3 ± 0.6 Ma, in good agreement with the K-Ar conventional age of 21.0 ± 0.6 Ma, which is proposed as the time of emplacement, given the freshness and the minor amount of glass of the basalt. [23] Basalt D4-33 yields a conventional K-Ar age, 0.72 ± 0.25 Ma, somewhat different from the 40Ar-39Ar age, 1.6 ± 0.7 Ma. The well-crystallized nature of the matrix precludes any significant loss    20 10 21.2+ 0.5 Ma integrated age =  20.0 + -  0.5 Maap pa ren t a ge  (M a) ap pa ren t a ge  (M a) 0 20 40 60 80 100 cumulative % 39Ar released integrated age = 19.3 + -  0.5 Maap pa ren t a ge  (M a) ap pa ren t a ge  (M a) cumulative % 39Ar released 0 20 40 60 80 100 20 10 10 20 30 3737 ArAr Ca  Ca  / / 3 939 ArAr K 0 20 40 60 80 100 cumulative % 39Ar released 3737 ArAr Ca  Ca  / / 3 939 ArAr K ap pa ren t a ge  (M a) ap pa ren t a ge  (M a) 2 6 10 10 20 integrated age = 19.1 + -  0.4 Ma 20.3 + -  0.4 Ma 15 10 5 3737 ArAr Ca  Ca  / / 3 939 ArAr K - 5 10 integrated age = 1.6 + -  0.7 Ma ap pa ren t a ge  (M a) ap pa ren t a ge  (M a) 0 20 40 60 80 100 cumulative % 39Ar released 21.9 + -  0.4 Ma integrated age = 23.9 + -  0.4 Ma ap pa ren t a ge  (M a) ap pa ren t a ge  (M a) cumulative % 39Ar released 0 20 40 60 80 100 80 60 40 20 3 5 1 3737 ArAr Ca  Ca  / / 3 939 ArAr K 0 20 40 60 80 100 cumulative % 39Ar released 3737 ArAr Ca  Ca  / / 3 939 ArAr K ap pa ren t a ge  (M a) ap pa ren t a ge  (M a) 10 20 integrated age = 18.7 + -  1.0 Ma 20 10 50 10 30 3737 ArAr Ca  Ca  / / 3 939 ArAr K 30 20 10 integrated age = 21.4 + -  2.0 Maap pa ren t a ge  (M a) ap pa ren t a ge  (M a) 0 20 40 60 80 100 cumulative % 39Ar released 20.7 + -  1.3 Ma integrated age = 19.2 + -  1.3 Ma ap pa ren t a ge  (M a) ap pa ren t a ge  (M a) cumulative % 39Ar released 0 20 40 60 80 100 10 30 5 15 3737 ArAr Ca  Ca  / / 3 939 ArAr K 0 20 40 60 80 100 cumulative % 39Ar released 3737 ArAr Ca  Ca  / / 3 939 ArAr K ap pa ren t a ge  (M a) ap pa ren t a ge  (M a) 20 40 integrated age = 19.3 + -  1.6 Ma 20 10 15 10 5 3737 ArAr Ca  Ca  / / 3 939 ArAr K D4-37 D5-44a D4-33 D6-87 D5-44b D6-88 D5-42 D6-85 D6-89   +1.63 - 0.42 Ma Figure 3. 40Ar-39Ar age spectrum plots for Kerimis dredged basalts. The uncertainties are quoted at two standard deviation. Geochemistry Geophysics Geosystems G3 weis et al.: trace of the kerguelen mantle plume 10.1029/2001GC000251 9 of 27 of 39ArK during irradiation. The isochron diagram does not reveal an initial 40Ar/36Ar ratio different from the air ratio and precludes the existence of extraneous argon. Therefore we choose as a prob- able age of emplacement the ‘‘plateau age’’ given by the two consecutive steps contributing to 90% of the released 39ArK and showing the lowest errors, 1.63 ± 0.42 Ma. [24] In summary, the seamounts dredged between the Kerguelen Archipelago and Heard Island sampled three types of basalts of comparable age, 18–21 Ma, slightly younger than the last phases of alkalic flood basalt volcanism on the archipelago [Nicolaysen et al., 2000]. The abundance of glass in the matrix of the olivine basalts (D5) and picritic basalts (D6) does not allow for the establishment of a more detailed chronology. The 1.6 Ma basalt (D4-33) demonstrates the existence of recent activ- ity previously undocumented in this area. 3.2. Petrology and Mineral Chemistry of the Picritic Basalts and Other Dredged Basalts [25] All of the dredged basalts contain abundant phenocrysts of olivine ± clinopyroxene and those fromD4 contain plagioclase phenocrysts. Represen- tative olivine compositions are reported in Table A1 and representative clinopyroxene compositions are reported in Table A2. The general petrographic characteristics (rock type, vesicles, phenocryst mor- phology, and grain sizes) of the samples from the Kerimis MD109 dredges are summarized in Table A3. Modal abundances of phenocrysts and vesicles were determined by counting 1500 points per thin section. [26] The picritic basalts (D6) contain 28–35 vol % olivine phenocrysts, including xenocrysts, with minor augite phenocrysts (0–1.3 vol %) (Table A3). The olivine phenocrysts (Fo83–84) are large (up to 5mm), euhedral and unzoned (Figures 4a and 4b). In contrast, the olivine xenocrysts are rounded or have irregular shapes indicative of resorption (Figures 4c and 4d) and have slightly higher Fo contents (Fo85.5) than the equant olivine phenocrysts. Many of the xenocrysts display undulatory or patchy extinction (Figures 4c and 4d) suggesting that they have undergone solid-state deformation prior to being incorporated into the picritic lavas. None of the xenocrysts contain major dislocations or kink bands as have been documented for Hawai- ian picritic basalts [Garcia, 1996] and for the Réunion picritic basalts [Albarède et al., 1997]. Many of the olivine phenocrysts in the picritic basalts contain chromite inclusions (Figures 4a and 4e), and some phenocrysts contain melt inclu- sions that are typically altered, although local pristine patches of glass remain (Figure 4f ). The chemistry of these chromite and glass inclu- sions is discussed in detail by Borisova et al. [2002]. The rare, euhedral clinopyroxene phenoc- rysts in the picritic basalts from D6 are Mg-rich (En51Fs07Wo42) and Al-poor (1–3 wt % Al2O3), consistent with a low-pressure origin, and show only minor compositional zoning. [27] The olivine basalts (D5) and basalts (D4) are much less porphyritic than the D6 picritic basalts (Table A3). The olivine basalts (D5) contain 2–4 vol % of skeletal and euhedral olivine phenocrysts (Fo78–83) in a fine-grained groundmass of clinopyr- oxene and plagioclase microlites. The basalt (D4- 33) is highly vesicular and contains 10 vol % of zoned plagioclase (An44–64), 3 vol % of subhedral olivine (Fo59–70), and 2 vol % of subhedral clino- pyroxene (En48Fs16Wo36) phenocrysts. Finally, the alkali basalt (D4-37) is different from all other dredged lavas having clinopyroxene more abundant than olivine (Table A3). This sample contains 3.5 vol % of zoned, fractured clinopyrox- ene (En42–54Fs17–07Wo41–39), which appears to be xenocrystic in origin, 2 vol% of olivine phenocrysts, and 3 vol % of zoned plagioclase phenocrysts (An60–78). 3.3. Major and Trace Element Compositions [28] Major and trace element abundances were determined for nine whole rock samples (Table 3). On the basis of their typical OIB-like Ba/Rb abun- dance ratios (11.4–14.7) compared to the average oceanic basalt ratio of 11.6 [Hofmann and White, 1982], these samples have not experienced signifi- cant postmagmatic mobility of alkali metals. The D4 basalt and D6 picritic basalts plot near or on the Geochemistry Geophysics Geosystems G3 weis et al.: trace of the kerguelen mantle plume 10.1029/2001GC000251 10 of 27 0.5 mm A  B C  D E  F 0.5 mm 0.5 mm 0.5 mm 0.5 mm 0.25 mm Figure 4. Photomicrographs illustrating the different types of olivine phenocrysts and xenocrysts present in the D6 picritic basalts and their inclusions. The scale is noted on each photomicrograph. Photomicrographs A–E are in crossed-polarized light and F is in plain-polarized light. A. Euhedral olivine phenocryst from sample MD109-71 showing near-perfect double terminations indicative of equilibrium crystallization. The phenocryst contains two patches of altered brown glass and a small chromite inclusion (lower right). B. Euhedral olivine phenocryst from sample MD109-89 showing excellent development of external crystal faces. C. Irregular, resorbed olivine xenocryst from the same sample (MD109-89) as the previous euhedral phenocrysts. D. Very irregular, resorbed olivine xenocryst from MD109-71. This xenocryst is also slightly deformed with the development of small subgrains along the upper margin of the main grain. E. Two rounded chromite inclusions in a single euhedral olivine phenocryst from MD109-89. F. Numerous brown glass inclusions within a single olivine phenocryst from MD109-72. Geochemistry Geophysics Geosystems G3 weis et al.: trace of the kerguelen mantle plume 10.1029/2001GC000251 11 of 27 alkalic-tholeiitic dividing line [MacDonald and Katsura, 1964] in a total alkalis (Na2O + K2O) versus SiO2 plot (Figure 5), whereas olivine basalts (D5) are clearly alkalic and plot in the field of the Lower Miocene basalts of the Kerguelen Archipe- lago [Weis et al., 1993;Frey et al., 2000b]. However, the whole rock compositions of the picritic basalts do not represent melt compositions as they have accumulated substantial amounts of olivine. Figure 6 shows the whole rock Mg # of Kerimis samples plotted against the forsterite (Fo) content of olivine phenocrysts and the Mg # of clinopyroxene pheno- crysts. The whole rockMg # were calculated assum- ing 10% Fe3+, which corresponds to low-pressure crystallization conditions for oxygen fugacity of FMQ-1. The whole rock Mg # for the picritic basalts (D6) are too high for the measured olivine phenocryst compositions, based on an Fe/Mg Table 3. Major and Trace Element for Kerimis Dredged Basaltsa Dredges 4 4 5 5 5 6 6 6 6 Samples 33 37 42 44a 44b 85 88 87 89 BHVO-1 SiO2 49.44 50.97 47.41 46.43 45.92 46.09 46.07 46.52 46.19 49.58 TiO2 3.20 3.34 4.18 3.90 4.39 2.03 2.01 2.12 2.00 2.74 Al2O3 15.10 14.57 12.61 12.43 12.22 8.22 8.08 8.56 7.98 13.61 Fe2O3 12.69 10.86 13.04 12.94 13.92 13.16 13.35 13.20 13.30 12.17 MnO 0.15 0.13 0.15 0.22 0.14 0.16 0.17 0.17 0.16 0.17 MgO 5.78 5.40 7.48 7.53 7.14 19.45 20.04 18.50 20.16 7.09 CaO 8.23 8.79 9.01 10.24 9.82 7.18 6.99 7.39 6.99 11.35 Na2O 3.71 2.74 2.68 2.80 2.84 1.45 1.50 1.49 1.43 2.40 K2O 1.11 2.40 2.62 2.22 2.19 1.36 1.36 1.45 1.36 0.55 P2O5 0.62 0.57 .97 1.46 1.62 0.35 0.34 0.36 0.34 0.27 XRF Rb 17.8 45.7 46.2 33.8 34.6 25.3 26.7 25.4 25.6 9.2 Sr 577 660 882 842 907 422 415 409 406 388 Ba 210 468 529 498 478 321 311 310 307 139 V 183 198 209 221 213 145 152 148 144 285 Cr 108 119 281 307 321 1095 1118 1157 1155 284 Ni 87 105 177 187 163 799 754 837 847 126 Zn 140 126 167 172 229 132 137 135 132 113 Ga 27 24 23 22 22 14 14 13 13 21 Y 29.1 26.3 33.0 34.9 53.1 14.8 15.4 14.6 14.5 24.6 Zr 325 346 447 409 451 200 207 195 194 191 Nb 38.9 38.0 71.2 65.0 74.2 24.0 24.8 23.8 23.6 19.4 La 35 40 64 61 76 30 31 30 28 16 Ce 61 89 112 102 113 45 51 45 46 36 INAA Sc 17.5 20.5 22.8 20.7 19.2 19.3 19.1 19.0 37.2 Cr 119 272 299 285 1132 1143 1180 1130 290 Co 42.3 49.3 51.3 43.7 89.7 93.1 92.4 91.6 43.9 Hf 7.16 9.87 9.2 9.65 4.32 4.44 4.47 4.39 4.4 Ta 2.35 4.19 3.82 4.28 1.32 1.27 1.36 1.16 1.16 Th 3.17 6.76 6.0 6.7 2.40 2.28 2.41 2.36 1.07 La 29.5 59.7 57.7 71.7 25.0 24.6 24.6 23.8 15.1 Ce 69.7 129 121 131 54.7 52.2 53.4 52.0 39.7 Nd 38.3 62.0 59.6 69.4 27.0 26.2 24.9 24.8 24.4 Sm 8.88 12.4 11.6 13.4 5.25 5.20 5.18 5.13 6.1 Eu 2.88 3.84 3.55 4.08 1.70 1.69 1.67 1.64 2.05 Tb 1.06 1.47 1.33 1.71 0.75 0.70 0.66 0.61 0.90 Yb 1.97 1.93 2.18 2.94 0.95 0.95 1.02 1.02 1.98 Lu 0.27 0.255 0.32 0.40 0.136 0.139 0.146 0.132 0.282 Cs – 1.9 0.3 0.37 0.24 0.28 0.21 0.27 a BHVO-1 data for ME and TE determined by XRF are from Rhodes [1996] and for TE determined by INAA are from Ila and Frey [2000]; in each case the same analytical methods were used for analysis of KERIMIS samples. Geochemistry Geophysics Geosystems G3 weis et al.: trace of the kerguelen mantle plume 10.1029/2001GC000251 12 of 27 exchange partition coefficient between olivine and basaltic liquid of 0.30 ± 0.03 [Roeder and Emslie, 1970], which indicates accumulation of Fo84 oli- vine in a basaltic melt with an Mg # of 62. The equilibrium magma compositions for the picritic basalts were calculated by removing wt % incre- ments of average olivine core compositions from the whole rock analysis for each sample until Fe/Mg equilibrium was attained. The results are coherent for each of the four samples and indicate an average equilibrium magma composition that contains 6 vol % olivine phenocrysts, with a whole rock Mg # of 62 and MgO content of 8.5 wt %. The corrected average picritic basalt composition is shown in the total alkalis versus SiO2 plot (Figure 5) and plots along the alkalic-tholeiitic dividing line (as pre- dicted by simply removing olivine), but at signifi- cantly higher SiO2 contents, 50 wt %. Systematic removal of olivine phenocrysts also results in a more meaningful relationship between the whole rockMg # and the Mg # of the 1 vol % clinopyroxene phenocrysts in the picritic basalts; after correction, all of the clinopyroxene phenocrysts plot within the Fe/Mg equilibrium field (Figure 6b). [29] The alkalic affinity of the D5 olivine basalts is also reflected in their abundances of incompatible elements, such as TiO2, P2O5, and Nb, at a given MgO content (Figure 7). The olivine basalts (D5) are more enriched in incompatible elements than the lower Miocene alkalic basalts and the transi- tional Oligocene flood basalts of the Kerguelen Archipelago [Yang et al., 1998; Frey, unpublished manuscript, 1996]. Their TiO2 and P2O5 contents are most similar to the incompatible element-rich Upper Miocene basanites erupted in the southeast Province of the Kerguelen Archipelago [Weis et al., 1993]. Surprisingly, the two D4 basalts of very different age (Table 2) are similar in composition (Table 3). Their compositions overlap with the Na2O+K2O SiO2 wt.% Tholeiitic Alkalic 15 10 0 5 40 45 50 55 60 65 Kerguelen Oligocene flood basalts Kerguelen Lower Miocene Series Kerguelen Upper Miocene Series Kerguelen Plateau basalts Big Ben basanites D5 D4 D6 Kerimis D4 D5 D6 Figure 5. Total alkalis (Na2O + K2O) versus SiO2 (all in wt %) classification plot. The star corresponds to D6 lava after correction for olivine addition. Shown for comparison are fields for different ages of lavas in the Kerguelen Archipelago, data for Cretaceous lavas from the Kerguelen Plateau and data (+) for basanites from Big Ben Volcano on Heard Island. Data Sources: Kerguelen Archipelago [Weis et al., 1993; Yang et al., 1998; Frey et al., 2000b; unpublished manuscript, 1996]; Heard Island: [Barling et al., 1994]; Kerguelen Plateau: [Davies et al., 1989; Mahoney et al., 1995]. Geochemistry Geophysics Geosystems G3 weis et al.: trace of the kerguelen mantle plume 10.1029/2001GC000251 13 of 27 fields of the flood basalts erupted in the Kerguelen Archipelago (Figures 5 and 7). [30] Lavas associated with the Kerguelen mantle plume reveal a long-term systematic variation in Zr/Nb. Older lavas, 115 to 30 Ma, have Zr/Nb > 10, whereas <30 Ma lavas typically have Zr/Nb < 10 (Figure 8). The dredged lavas are consistent with this temporal trend; basalts (D4) and picritic basalts (D6) have higher Zr/Nb than olivine basalts (D5), but all of the dredged lavas have Zr/Nb < 10 (Figure 8). Lavas on the Kerguelen Archipelago show a long-term trend of increasing La/Yb with decreasing eruption age [Frey et al., 2000b]. Con- sistent with this trend, the olivine and picritic basalts have high La/Yb, 25–20, within the range D5: olivine basalt D6: picritic basalt D4: trachybasalt D4: basalt 70 75 80 85 90 95 45 50 55 60 65 70 75 80 xenocrysts KD(cpx-liq) = 0.25  0.05 MD109-37 MD109-33 55 60 65 70 75 80 85 90 95 45 50 55 60 65 70 75 80 KD(ol-liq) = 0.30  0.03 MD109-42 MD109-33 MD109-85 MD109-87 MD109-88 MD109-89 xenocrysts Fo o liv in e M g# cp x Mg#wholerock MD109-44 MD109-85 MD109-87 MD109-88 MD109-89 A. B. accumulation xe no cr ys ts equilibrium melt average cores 55 60 65 70 75 80 85 90 95 70 75 80 85 90 95 28-35 vol.% olivine ~1 vol.% cpx Figure 6. Plot of wholerock Mg #, where Mg # = (Mg/(Mg+Fe2+))*100, versus the forsterite content of olivine (A) and the Mg # of clinopyroxene (B) for basaltic lavas from the Kerimis dredges. A. The gray field indicates the equilibrium field calculated using an Fe/Mg exchange partition coefficient between olivine and basaltic liquid of 0.30 ± 0.03 [Roeder and Emslie, 1970]. Note that the olivine xenocrysts from the D6 picritic basalts have slightly higher Fo contents than the phenocrysts. Olivine phenocrysts from the D5 olivine basalts are in equilibrium (MD109- 44) or nearly in equilibrium (MD109-42) with their wholerock Mg #, whereas olivine microphenocrysts from the DR4 trachybasalt (MD109-33) fall well below the equilibrium field. B. The gray field indicates the equilibrium field calculated using an Fe/Mg exchange partition coefficient between clinopyroxene and basaltic liquid of 0.25 ± 0.05 [Grove et al., 1982]. Recalculating the wholerock Mg # after olivine removal shows that all of the clinopyroxene phenocrysts from the picritic basalts plot in the equilibrium field. Geochemistry Geophysics Geosystems G3 weis et al.: trace of the kerguelen mantle plume 10.1029/2001GC000251 14 of 27 of lower Miocene basalts; a D4 basalt, however, has much lower La/Yb of 15 (Figure 8). The two main lava series on Heard Island (Laurens Penin- sula and Big Ben) have different Ba/Nb ratios. These differences correlate with differences in Sr 0 1 2 3 4 5 6 7 0.0 0.5 1.0 1.5 0 20 40 60 80 100 0 5 10 15 20MgO Big Ben basanites Big Ben basalts Kerguelen Oligocene basalts Kerguelen Miocene basalts Kerguelen basanites Big Ben basanites Big Ben basaltsKerguelen Oligocene basalts Kerguelen Miocene basalts Kerguelen basanites D6 D4 D5 Nb P2O5 TiO2 Kerimis D4 D5 D6 Figure 7. TiO2, P2O5, and Nb versus MgO (oxides in wt % and Nb in ppm) in lavas forming the Kerguelen Archipelago and Heard Island. The incompatible element-rich D5 olivine basalts are most similar, but not identical, to basanites erupted during the upper Miocene in the Kerguelen Archipelago. The D6 picritic basalts are most similar to the high-MgO end of the recent Big Ben basanite sub-group erupted on Heard Island. Corrected for olivine addition (star), they overlap the Big-Ben basalt field and plot at the low P2O5 and TiO2 end of the D5 olivine basalt trend. Data sources given in caption are the same as for Figure 5. basalts 0 5 10 15 20 115/85  82/38   30/26  25/23   22/18     <10   0.2/0     Ma LPS Big Ben Big Ben basalts Kerguelen Oligocene basalts Kerguelen Miocene basalts Kerguelenbasanites basanites NER D6 D4 D4 D5 748 Kerguelen Plateau 0 4 8 12 16 0 5 10 15 20 25 30 35 0 5 10 15 20MgO Ba/Nb Zr/Nb La/Yb Big Ben basanites Kerguelen Oligocene basaltsKerguelen Miocene basalts Kerguelen basanites Laurens Peninsula basalts D6 D4 Dr 5 D6 D4 D5 Kerguelen basanites Big Ben basanites Big Ben basalts Kerguelen Oligocene basalts Kerguelen Miocene basalts D5 A B C Big Ben Figure 8. Ba/Nb (A) and La/Yb (B) versusMgO (wt%) and Zr/Nb versus eruption age for lavas associated with the Kerguelen plume (C). In panel A, which distin- guishes the two main lava series on Heard Island (Laurens Peninsula, LPS and Big Ben, BBS), the picrites (D6) overlap with the BBS and the D5 lavas overlap with the LPS. Data sources as for Figure 5. In panel B, the La/Yb of the basalt (D4) and of olivine lavas (D5) are in the range of Miocene alkalic flood basalts of the Kerguelen Archipelago. The picrites have similar La/Yb, and plot near the MgO-rich end of the Big Ben basanite field. In the bottom panel C, the time axis is not linear. The Cretaceous lavas drilled from the Kerguelen Plateau and Ninetyeast Ridge are tholeiitic basalts with relatively high Zr/Nb, with the exception of ODP Site 748 where they are alkalic [Storey et al., 1992; Frey et al., 1991]. The Oligocene flood basalts from the Kerguelen Archipelago have transitional major element compositions with intermediate Zr/Nb ratios [Yang et al., 1998] whereas the alkalic Miocene flood basalts from the Kerguelen Archipelago have lower Zr/Nb [Frey et al., 2000b]. The Kerimis dredged lavas have Zr/Nb similar to the latter as well as to the recent alkalic lavas erupted in the Kerguelen Archipe- lago and Heard Island. Geochemistry Geophysics Geosystems G3 weis et al.: trace of the kerguelen mantle plume 10.1029/2001GC000251 15 of 27 and Nd isotopic ratios, and the high Ba/Nb ratios are inferred to be derived from a continental crust component [Barling et al., 1994]. Olivine basalts (D5) have relatively low Ba/Nb within the range of the Laurens Peninsula series, but basalts (D4) and picritic basalts (D6) overlap with the Big Ben series and the Oligocene to Miocene flood basalts form- ing the Kerguelen Archipelago (Figure 8). [31] In summary, the major and trace element abun- dances of lavas from each dredge form distinct compositional groups (Figures 5, 7, and 8). The basalts (D4) have the lowest abundances of incom- patible elements; they are similar in composition to flood basalts erupted in the Kerguelen Archipelago. In contrast, D5 olivine basalts have significantly higher abundances of incompatible elements, exceeding those of the Kerguelen Archipelago flood basalts and approaching those of basanites erupted in the Kerguelen Archipelago and at Heard Island. The D6 picritic basalts are not typical of lavas associated with the Kerguelen plume; these picrites are most similar to the MgO-rich lavas of the Big Ben basanitic series erupted on Heard Island. 3.4. Isotope Ratios [32] All isotope ratios are reported in Table 4. 3.4.1. Sr-Nd-Hf-Pb isotopic variations [33] In a Sr-Nd diagram (Figure 9), all the 18–21Ma basalts (i.e., the olivine basalts (D5), the picritic pillow basalts (D6) and the alkali basalt (D4)) plot within the field defined by the UpperMiocene Series of the Kerguelen Archipelago [Weis et al., 1993]. The effect of age-correction for in situ 87Rb and 147Sm decay is negligible (between 3 and 5  105 and 1.1 and 1.5  105, respectively). D6 picritic basalts have slightly lower 143Nd/144Nd than D5 alkali basalts and plot within the field of the archi- pelago basanites. More than 90% of the Kerguelen Archipelago Miocene flood basalts have 87Sr/86Sr between 0.7050 and 0.7056 and 143Nd/144Nd between 0.5127 and 0.5125, while the Oligocene flood basalts present a larger range, extending toward more depleted isotopic compositions [Weis et al., 1998a; Yang et al., 1998]. Surprisingly, the slightly alkalic, younger D4 basalt which was ana- lyzed in duplicate (sample D4-33) plots within the field defined by the depleted basalts of MORB from the southeast Indian Ridge. This sample is similar in composition to D4-37 (Table 3). [34] In contrast to the relatively homogeneous Sr- Nd isotopic compositions, alkali (D4) and olivine (D5) basalts and picritic basalts (D6) have distinct Pb isotopic ratios (Figure 10). The D5 olivine basalts plot within the less radiogenic part of the field defined by the Kerguelen Archipelago Oligo- cene flood basalts [Yang et al., 1998]. They have slightly lower 207Pb/204Pb than lavas from Heard Island and the youngest lavas from the Kerguelen Archipelago, the Upper Miocene Series and basan- Table 4. Isotopic Ratios for the Kerimis Dredged Basalts Dredges Samples 87Sr/86Sr 2sm 143Nd/144Nd 2sm 206Pb/204Pba 207Pb/204Pba 208Pb/204Pba 176Hf/177Hf 2sm 4 33 0.703126 6 0.512946 14 19.669 15.678 39.315 0.282937 8 33b 0.703100 6 0.512942 9 19.677 15.686 39.347 4 37 0.705942 6 0.512525 9 18.451 15.551 39.009 5 42 0.705654 6 0.512582 10 18.179 15.511 38.576 5 44a 0.705625 7 0.512606 8 18.214 15.517 38.634 0.282752 8 5 44b 0.705658 8 0.512576 10 18.208 15.527 38.658 0.705676 11 18.197 15.524 38.662 6 85 0.705694 8 0.512543 8 17.875 15.536 38.266 6 87 0.705683 10 0.512533 6 17.874 15.540 38.274 6 88 0.705681 8 0.512547 7 17.872 15.538 38.265 0.282728 7 0.705703 7 17.869 15.531 38.266 6 89 0.705699 6 0.512526 9 17.876 15.541 38.278 0.705683 12 a Absolute 2sm errors for the Pb isotopic analysis are between 0.015 and 0.017 for 206Pb/204Pb and 207Pb/204Pb and between 0.045 and 0.050 for 208Pb/204Pb. b Full-procedural duplicate analysis, including leaching. Geochemistry Geophysics Geosystems G3 weis et al.: trace of the kerguelen mantle plume 10.1029/2001GC000251 16 of 27 ites of the southeast Province [Weis et al., 1993] and lavas from Mount Ross [Weis et al., 1998b]. Relative to the picritic basalts (D6), the olivine basalts (D5) have lower 207Pb/204Pb and higher 143Nd/144Nd. The D6 picritic basalts have very homogeneous Pb isotopic ratios, distinctly less radiogenic than those of the Kerguelen Archipelago basalts, but they overlap with the Pb isotopic ratios in dredged and drilled basalts from the Kerguelen Plateau [Weis et al., 1989a; Mahoney et al., 1995], as well as the least radiogenic end of the Heard Island trend [Barling et al., 1994]. [35] The two alkali basalts fromD4 have very differ- ent Pb isotopic compositions (Figure 10). Sample D4-33 has very radiogenic Pb isotopic compositions falling in the field defined by the Comores Archipe- lago [Class and Goldstein, 1997], whereas sample D4-37 plots within the field of Kerguelen Archipe- lago Miocene flood basalts (Figure 10). [36] The Hf isotopic composition of three basalts was determined, one from each dredge, in order to compare with basalts from the Kerguelen Archipe- lago and the Kerguelen Plateau [Mattielli et al., 2000]. The olivine (D5) and the picritic (D6) basalts have very comparable Hf isotopic ratios, while sample D4-33, because of its higher 206Pb/204Pb, again plots within the range of Comores basalts [Salters and White, 1998]. 3.4.2. He isotopes [37] Helium isotopic ratios have been used as a geochemical tracer to identify magmas derived  Figure 9. 87Sr/86Sr143Nd/144Nd. Isotope plots for Kerimis dredged basalts compared with fields for Kerguelen Archipelago and Kerguelen Plateau basalt fields as well as other volcanic features of the Indian Ocean. Data sources as in Figures 5 and 8, plus: Southeast Indian Ridge (SEIR): [Hamelin et al., 1985/1986; Michard et al., 1986; Price et al., 1986; Dosso et al., 1988; Mahoney et al., 1998]. Olivine and picritic basalts (D5, D6) have comparable 87Sr/86Sr but distinct 143Nd/144Nd with the olivine basalts being slightly less radiogenic (see detailed inset). All Kerguelen Plateau basalts as well as Kerguelen Archipelago flood basalts have been corrected for in situ 87Rb and 147Sm decay since their emplacement age. Geochemistry Geophysics Geosystems G3 weis et al.: trace of the kerguelen mantle plume 10.1029/2001GC000251 17 of 27 from relatively undegassed mantle [e.g., Kurz et al., 1983; Condomines et al., 1983; Graham et al., 1999]. Because of the relatively rare occurrence of olivine phenocrysts, very few samples from the Kerguelen mantle plume environment have been analyzed for 3He/4He; the exception is Heard Island [Hilton et al., 1995] where basanites and other MgO-rich lavas are much more common. A dredged picritic basalt (D6-88) was analyzed for its He isotopic compositions and yields a 3He/4He of 8.8 ± 0.5 R/RA (1s), where R/RA is the 3He/4He ratio of the sample relative to an atmospheric 3He/4He of 1.39  106. This value is more typical of southeast Indian Ridge basalts (7.73–9.70 R/RA [Graham et al., 1999]) than of higher 3He/4He ocean island basalts interpreted to have an unde- gassed mantle component [e.g., Kurz and Jenkins, 1981; Kurz et al., 1982; Poreda et al., 1986]. However, previous and ongoing studies show that the Kerguelen Archipelago lavas generally have 17.5 18.0 18.5 19.0 19.5 37 38 39 Kerguelen Archipelago Ascension HIMU Islands Comores Hawaii Walvis Ridge Amsterdam & St Paul  MORBs 17 18 19 20 21 Crozet 0.2835 0.2827 0.2825 0.2829 0.2831 0.2833 Dr 6 Dr 5 Dr 4 206Pb/204Pb 176Hf/177Hf Kerguelen Oligocene flood basalts SEIR Kerguelen Plateau 738 Big Ben basanite Kerguelen Lower Miocene basalts Kerguelen basanites Kerguelen Upper Miocene series Mt. Ross Heard Island Dr 6 Dr 5 Dr 4 Dr 4 208Pb/204Pb 15.4 15.5 15.6 15.7 17.5 18.0 18.5 19.0 19.5 207Pb/204Pb 206Pb/204Pb Heard Island Dr 6 Dr 5 Dr 4 Dr 4 Kerguelen Oligocene flood basalts SEIR Kerguelen Plateau 738 Big Ben basanite Kerguelen Lower Miocene basalts Kerguelen basanites Kerguelen Upper Miocene series 1137 Mt. Ross Figure 10. 207Pb/204Pb206Pb/204Pb and 208Pb/204Pb206Pb/204Pb. Data sources as in Figure 9, plus Site 1137 [Weis et al., 2001]. Pb isotope compositions discriminate between the various Kerimis dredge sites. The inset reports 176Hf/177Hf206Pb/204Pb and comparison with literature data [Salters and White, 1998]. Geochemistry Geophysics Geosystems G3 weis et al.: trace of the kerguelen mantle plume 10.1029/2001GC000251 18 of 27 low 3He/4He ratios (4.9– 10.2 R/RA; [Vance et al., 1989; Nicolaysen et al., 1998]) as do lavas of the Big Ben Series on Heard (3.9–8.4 R/RA [Hilton et al., 1995]). In contrast, olivines from Laurens Peninsula lavas on Heard Island give R/RA at 18.1, which Hilton et al. [1995] considered as represen- tative of the Kerguelen plume helium isotopic signature. 4. Discussion 4.1. Source Characteristics of Dredged Lavas on the Seamounts [38] Recent stratigraphic studies of volcanic sec- tions on the Kerguelen Archipelago indicate that the isotopic composition of the Kerguelen mantle plume is best represented by the 24 Ma mildly alkalic basalts from the Mt. Crozier section that have the most radiogenic Pb isotopic compositions [e.g., Weis et al., 1998a]. Younger lavas from the Upper Miocene series or the Ross volcano [Weis et al., 1993, 1998b] have distinctly lower 206Pb/204Pb ratios pointing towards the Kerguelen Plateau, while the Oligocene flood basalts, 29–25 Ma, have isotopic compositions indicative of interaction with the depleted mantle [Gautier et al., 1990; Yang et al., 1998; Weis et al., 1998a]. Except for sample D4-33, all Sr, Nd, and Hf isotopic ratios for the seamount basalts overlap the field of Kerguelen Archipelago lavas (Figure 9) and indi- cate an origin by partial melting of the Kerguelen mantle plume. The Pb ratios in the picritic basalts (D6), however, extend toward lower 206Pb/204Pb or higher 207Pb/204Pb and 208Pb/204Pb for a given 206Pb/204Pb, i.e., toward the field for Kerguelen Plateau lavas. In the extreme case of ODP Site 738 on the southern Kerguelen Plateau and of Elan Bank at ODP Site 1137 (Figure 10), the Pb isotope characteristics are indicative of incorporation of continental lithosphere by the ascending magma derived from the Kerguelen plume [Mahoney et al., 1995; Frey et al., 2000a; Weis et al., 2001]. None of the dredged samples from between the Kergue- len Archipelago and Heard Island have the extreme isotopic characteristics of Site 738 basalts, which include very high 87Sr/86Sr, low 143Nd/144Nd and very high 207Pb/204Pb and 208Pb/204Pb for a given 206Pb/204Pb (Figures 9 and 10), or those of Site 1137 basalts from Elan Bank that reflect increasing upper crust contamination with age [Ingle et al., 2000; Weis et al., 2001]. The lavas erupted on Heard Island show a distinctly larger range of isotopic compositions [Barling et al., 1994] than those erupted on the Kerguelen Archipelago, ranging to higher 87Sr/86Sr and lower 143Nd/144Nd and from high 206Pb/204Pb to the relatively low 206Pb/204Pb, typical of the Kerguelen Plateau basalts (Figures 9 and 10). [39] If the low 143Nd/144Nd and 206Pb/204Pb of some Kerguelen Plateau lavas can be attributed to a continental component, then the D6 picrites and some Heard Island lavas may contain some of this component, which is not present in the D5 olivine basalts nor in most of Kerguelen Archipelago basalts. The abundance ratio La/Nb has also been used to identify a continental component in lavas associated with the Kerguelen plume [Mahoney et al., 1995; Coffin et al., 2000]. None of these dredged samples are depleted in Nb. For example La/Nb varies from 0.84 to 0.97 and 0.99 to 1.03 in D5 and D6 lavas, respectively, compared to the primitive mantle estimate of 0.96 [Sun and McDonough, 1989]. The low 143Nd/144Nd and 206Pb/204Pb in D6 picrites may reflect interaction of the plume-derived magmas with the Creta- ceous Kerguelen Plateau, but there is no compel- ling compositional evidence for a component derived from continental crust. The modest helium isotopic ratio (3He/4He = 8.8 R/RA) of D6-88 may indicate that the picrites do not represent pristine plume melts. In the case of Heard Island where a wide range in 3He/4He (5 to 18 RA) occurs in lavas from a single island, Hilton et al. [1995] interpreted the low He R/RA values as reflecting shallow-level contamination by radiogenic He. [40] One, apparently <1 Ma, alkali basalt sample, D4-33, has a chemical composition similar to those of the Lower Miocene flood basalts of the Kergue- len Archipelago, but its isotopic compositions are MORB-like for Sr-Nd, while its Pb composition plots within the field defined by lavas from the Comores, another Indian Ocean Island, situated more than 4600 km to the west. Currently, we have Geochemistry Geophysics Geosystems G3 weis et al.: trace of the kerguelen mantle plume 10.1029/2001GC000251 19 of 27 no explanation for the puzzling isotopic character- istics or for the age of this sample. 4.2. Kerguelen Hot Spot Track [41] The exact location of the Kerguelen hot spot has long been a matter of debate. In tectonic reconstructions of the Indian Ocean basin, Müller et al. [1993], amongst others, required that the present location of the Kerguelen hot spot be beneath Skiff Bank, under the northwestern North- ern Kerguelen Plateau, in order to acceptably model the location fit of Ninetyeast Ridge using a fixed hot spot reference frame. Steinberger [2000] assumed a present location under the Kerguelen Archipelago, which improves the fit of Ninetyeast Ridge and the Rajmahal traps when compared to results from fixed hot spot models. According to Steinberger [2000], none of the mantle models gives an accept- able fit of Ninetyeast Ridge and the Rajmahal traps if the present location of the Kerguelen hot spot is assumed to be under Heard Island. All models nevertheless agree in predicting a southward surface motion for the Kerguelen hot spot. [42] Recent argon dating from ODP Site 1139 (Leg 183) clearly rules out Skiff Bank (68 Ma [Duncan and Pringle, 2000]) as the current location of the Kerguelen hot spot. More recent volcanic activity on the Kerguelen Archipelago and Heard Island leave them more viable can- didates for the location of the hot spot. [43] The age range of 21–18 Ma for the sea- mounts between the Kerguelen Archipelago and Heard Island is slightly younger than the main peak of volcanism on the Kerguelen Archipelago and intermediate between the peak of the volcan- ism on these two main islands. Although they are more enriched in incompatible elements, the D5 alkali basalts are similar in major element compo- sition to the 24–25 Ma Miocene alkalic flood basalts in the eastern part of the Kerguelen Archi- pelago and the D6 picrites are most similar to the high-MgO Big Ben basanites erupted on Heard Island. The geochemical and isotopic similarities of D5 and D6 lavas with Miocene to recent alkaline lavas erupted in the Kerguelen Archipe- lago and Heard Island indicate comparable sour- ces for these lavas. Given the evidence for a common source and the spatial variations in age and volume of magmatism (Figure 11), it is possible that the trend from ODP Site 1140 through the Kerguelen Archipelago to the dredged submarine volcanoes, and even possibly to Heard and McDonald Islands, corresponds to the hot spot track of the Kerguelen plume. [44] Another possibility is that the Miocene to Recent volcanism associated with the Kerguelen plume has become more diffuse because the plume is beneath the thick Kerguelen Plateau. In this case, definitive identification of the cur- rent location of the plume may be beyond our grasp. The thick Cretaceous lithosphere may have limited decompressional melting and forced melts to exploit lithosphere weaknesses for ascent and eruption [Barling et al., 1994]. Three observa- tions consistent with this interpretation are as follows: (1) the numerous mantle xenoliths in the alkalic lavas of the Kerguelen Archipelago which require rapid transport and exploitation of lithospheric fractures [Grégoire et al., 1998]. (2) Despite this evidence for rapid transport of mafic lavas, the trachytes, phonolites, and other evolved lavas erupted in the Kerguelen Archipelago and at Heard and McDonald Islands demonstrate that some magmas stagnated and fractionated within the lithosphere. (3) With decreasing eruption age in the Kerguelen Archipelago, lavas with similar Sr and Nd isotopic ratios increase in La/Yb (Figure 8). This trend is consistent with a tem- poral decrease in extent of melting and increase in depth of melt segregation. 5. Conclusions [45] The seamounts dredged between the Kergue- len Archipelago and Heard Island during the recent Kerimis survey cruise, in the Northern (previously unsampled) and Central Kerguelen Plateau, are formed by Miocene mildly alkalic basalts and picrites. The latter are the first dis- covered picritic basalts related to the Kerguelen plume activity. [46] The alkalic compositional characteristics of the dredged basalts are consistent with the temporal Geochemistry Geophysics Geosystems G3 weis et al.: trace of the kerguelen mantle plume 10.1029/2001GC000251 20 of 27 trends of Kerguelen plume magmatism, from >25 Ma tholeiitic/transitional basalts to <25 Ma alkalic basalts, a trend that probably reflects a decrease in degree of mantle melting [Weis et al., 1998b; Frey et al., 2000b]. Olivine and picritic basalts from D5 and D6 have Sr-Nd isotopic com- positions within the range defined by >90% of the Kerguelen Archipelago Miocene flood basalts and the Big Ben basaltic series on Heard Island suggest- ing that these lavas have been produced by partial melting of the Kerguelen plume. D5 and D6 basalts have distinct Pb isotopic compositions for compa- rable Sr isotopic ratios. D5 olivine basalts have Pb ratios comparable to those of the Kerguelen Archi- pelago Oligocene lavas, whereas D6 picritic basalts have Pb ratios plotting within the field of Kerguelen Plateau basalts, which are characterized by lower 206Pb/204Pb values. One D4 alkali basalt, sample D4-33, is unique in its isotopic ratios in the context of the Kerguelen plume magmatism; it has MORB- 1140 NORTH ARCHIPELAGO FOCH 0 -1000 -2000 -3000 -4000 1000 2000 ROSS D6 D4/D5 HEARD 0 10 20 30 40 AGE IN MA NORTHERN KERGUELEN PLATEAU CENTRAL KERGUELEN PLATEAUSSW NKP KA FLOOD BASALTS GEOG MTS BALLONS RDB UMS ROSS SEAMOUNTS HEARD NNE KA 0 100 200 300 400 500 600 700 800 900 1000 1100 DEPTH IN M KM VG Figure 11. Plot showing the relative position of magmatic products related to the Kerguelen mantle plume versus their age. Age data are from Duncan and Pringle [2000] for ODP Site 140 in the NKP, from Nicolaysen et al. [2000] for Kerguelen Archipelago flood basalts, from Weis and Giret [1994] for the Kerguelen Archipelago plutonic complexes (in italics) and from Clarke et al. [1983] and Quilty et al. [1983] for Heard Island. The relative distance is calculated along two straight lines (see Figure 2) along which the bathymetric profiles (bottom part) have also been drawn, one from ODP Site 1140 to the Mt. Ross volcano on the Kerguelen Archipelago and the other one from the Mt. Ross volcano to Heard Island, passing through the dredged seamount. This plot shows clearly the southwards migration with time of the location of the most voluminous emissions of basaltic magma related to the Kerguelen plume, as the volume of magma associated with the seamounts and Heard Island greatly exceeds that of the relatively small volcano-plutonic complexes on the Kerguelen Archipelago (Val Gabbro (VG), Mts Ballons, Rallier du Baty (RdB), Géographie) and of the basanite to phonolite plugs and domes of the Upper Miocene Series (UMS). Geochemistry Geophysics Geosystems G3 weis et al.: trace of the kerguelen mantle plume 10.1029/2001GC000251 21 of 27 T a b le A 1 . R ep re se n ta ti v e E le ct ro n p ro b e A n al y se s o f O li v in e F ro m K er im is M D 1 0 9 B as al ts [T h e fu ll A p p en d ix A 1 is av ai la b le in th e H T M L v er si o n o f th e ar ti cl e at h tt p :/ /w w w .g -c u b ed .o rg .] D re d g e D 4 D 5 S am p le M D 1 0 9 -3 3 M D 1 0 9 -4 2 M D 1 0 9 -4 4 a M D 1 0 9 -4 4 b R o ck ty p ea T B B B B T y p eb P h en . M ic . P h en . P h en . P h en . P h en . P h en . P h en . P h en . P h en . P h en . P h en . P h en . P h en . K in d c E S S K S S K E S S K E S K S K S K E E O x id es (w t % ) S iO 2 3 7 .7 5 3 6 .3 4 3 8 .8 8 3 9 .3 2 3 9 .4 2 3 9 .4 1 3 9 .1 1 3 8 .7 0 3 8 .6 9 3 9 .0 6 3 8 .3 5 3 8 .7 1 3 8 .6 4 3 8 .5 5 C r 2 O 3 0 .1 2 0 .0 0 0 .0 1 0 .0 4 0 .0 5 0 .0 3 0 .0 1 0 .0 2 0 .0 4 0 .0 0 0 .0 5 0 .0 0 0 .0 1 0 .0 3 F eO 2 6 .9 8 3 5 .2 9 1 8 .5 3 1 6 .6 8 1 7 .8 7 1 8 .1 6 1 8 .1 1 2 0 .1 4 1 8 .6 4 1 6 .2 2 1 7 .8 5 1 7 .1 0 1 7 .3 5 1 6 .3 3 M n O 0 .3 0 0 .4 8 0 .1 8 0 .2 5 0 .2 5 0 .1 9 0 .1 9 0 .2 3 0 .2 5 0 .1 9 0 .2 0 0 .1 8 0 .1 7 0 .2 4 M g O 3 5 .5 1 2 8 .6 0 4 2 .6 3 4 4 .0 5 4 2 .9 2 4 2 .9 2 4 2 .7 4 4 1 .4 2 4 2 .1 7 4 3 .9 8 4 2 .3 5 4 3 .7 1 4 3 .1 5 4 4 .6 2 C aO 0 .1 7 0 .1 7 0 .2 6 0 .2 5 0 .2 4 0 .1 6 0 .2 3 0 .3 1 0 .2 2 0 .1 6 0 .2 1 0 .2 3 0 .2 5 0 .1 5 N iO 0 .1 0 0 .0 8 0 .2 7 0 .2 8 0 .1 8 0 .1 6 0 .1 6 0 .1 7 0 .1 6 0 .3 0 0 .1 5 0 .2 6 0 .2 4 0 .2 9 T o ta l 1 0 0 .9 8 1 0 1 .0 3 1 0 0 .9 1 1 0 0 .8 7 1 0 0 .9 5 1 0 1 .0 3 1 0 0 .5 9 1 0 1 .1 0 1 0 0 .2 8 9 9 .9 6 9 9 .2 0 1 0 0 .2 2 9 9 .9 0 1 0 0 .2 2 C at io n s (p fu .) S i 0 .9 9 5 0 .9 9 9 0 .9 8 6 0 .9 8 9 0 .9 9 5 0 .9 9 4 0 .9 9 2 0 .9 8 7 0 .9 8 8 0 .9 9 0 0 .9 8 7 0 .9 8 3 0 .9 8 5 0 .9 7 6 C r 0 .0 0 2 0 .0 0 0 0 .0 0 0 0 .0 0 1 0 .0 0 1 0 .0 0 1 0 .0 0 0 0 .0 0 0 0 .0 0 1 0 .0 0 0 0 .0 0 1 0 .0 0 0 0 .0 0 0 0 .0 0 1 F e2 + 0 .5 9 5 0 .8 1 1 0 .3 9 3 0 .3 5 1 0 .3 7 7 0 .3 8 3 0 .3 8 4 0 .4 3 0 0 .3 9 8 0 .3 4 4 0 .3 8 4 0 .3 6 3 0 .3 7 0 0 .3 4 6 M n 0 .0 0 7 0 .0 1 1 0 .0 0 4 0 .0 0 5 0 .0 0 5 0 .0 0 4 0 .0 0 4 0 .0 0 5 0 .0 0 5 0 .0 0 4 0 .0 0 4 0 .0 0 4 0 .0 0 4 0 .0 0 5 M g 1 .3 9 6 1 .1 7 2 1 .6 1 2 1 .6 5 2 1 .6 1 5 1 .6 1 5 1 .6 1 6 1 .5 7 5 1 .6 0 6 1 .6 6 1 1 .6 2 5 1 .6 5 5 1 .6 4 0 1 .6 8 5 C a 0 .0 0 5 0 .0 0 5 0 .0 0 7 0 .0 0 7 0 .0 0 7 0 .0 0 4 0 .0 0 6 0 .0 0 8 0 .0 0 6 0 .0 0 4 0 .0 0 6 0 .0 0 6 0 .0 0 7 0 .0 0 4 N i 0 .0 0 2 0 .0 0 2 0 .0 0 6 0 .0 0 6 0 .0 0 4 0 .0 0 3 0 .0 0 3 0 .0 0 3 0 .0 0 3 0 .0 0 6 0 .0 0 3 0 .0 0 5 0 .0 0 5 0 .0 0 6 S u m 3 .0 0 3 3 .0 0 0 3 .0 1 1 3 .0 1 0 3 .0 0 4 3 .0 0 5 3 .0 0 7 3 .0 1 1 3 .0 1 0 3 .0 0 9 3 .0 1 1 3 .0 1 7 3 .0 1 3 3 .0 2 3 E n d m em b er s (% ) F o 7 0 .0 5 8 .9 8 0 .1 8 2 .2 8 0 .8 8 0 .6 8 0 .5 7 8 .2 7 9 .9 8 2 .7 8 0 .7 8 1 .8 8 1 .3 8 2 .8 F a 3 0 .0 4 1 .1 1 9 .9 1 7 .8 1 9 .2 1 9 .4 1 9 .5 2 1 .8 2 0 .1 1 7 .3 1 9 .3 1 8 .2 1 8 .7 1 7 .2 a R o ck ty p e: P B , p ic ri ti c b as al t; B , b as al t; T B , tr ac h y b as al t. b G ra in si ze : P h en . an d X en ., p h en o /x en o cr y st s > 0 .7 m m ; M ic ., m ic ro p h en o cr y st s > 0 .3 m m ; G lo m ., g lo m er o cr y st > 0 .7 m m . c P h en o cr y st m o rp h o lo g y : E , eu h ed ra l; S , su b h ed ra l; A , an h ed ra l; S K , sk el et al ; R , ro u n d ed . Geochemistry Geophysics Geosystems G3 weis et al.: trace of the kerguelen mantle plume 10.1029/2001GC000251 22 of 27 T a b le A 2 . R ep re se n ta ti v e E le ct ro n p ro b e A n al y se s o f C li n o p y ro x en e F ro m K er im is M D 1 0 9 B as al ts U n it D 4 D 6 D 6 D 6 D 6 S am p le M D 1 0 9 -3 7 M D 1 0 9 -8 5 M D 1 0 9 -8 7 M D 1 0 9 -8 8 M D 1 0 9 -8 9 R o ck ty p ea B P B P B P B P B T y p eb P h en . X en . P h en . P h en . P h en . P h en . P h en . P h en . K in d c E S E E E R E E Z o n e co re R im co re ri m co re ri m co re ri m co re ri m co re ri m co re m id co re ri m O x id es (w t. % ) S iO 2 5 1 .2 4 4 6 .2 3 5 1 .3 3 5 2 .1 9 4 9 .5 5 5 0 .9 9 5 1 .0 3 5 1 .2 2 5 2 .1 3 5 0 .6 3 5 1 .5 3 5 2 .1 9 5 2 .2 0 5 1 .6 8 5 2 .4 0 5 2 .3 1 T iO 2 1 .2 9 2 .9 7 0 .8 2 0 .9 0 1 .5 8 1 .0 9 0 .9 3 1 .1 4 0 .8 1 1 .2 6 0 .9 6 1 .0 1 0 .9 0 1 .2 4 0 .8 0 0 .8 9 A l 2 O 3 2 .6 0 6 .2 6 2 .4 0 2 .6 0 3 .4 7 1 .7 1 1 .7 2 2 .3 8 2 .0 7 2 .7 3 2 .4 4 2 .4 9 1 .9 1 2 .4 4 1 .8 2 1 .8 8 C r 2 O 3 0 .0 4 0 .1 5 0 .7 5 0 .8 1 0 .2 3 0 .4 4 0 .1 7 0 .4 3 0 .7 2 0 .5 1 0 .9 2 0 .8 4 0 .6 7 0 .2 5 0 .9 3 0 .7 9 F eO to t 7 .2 1 8 .7 5 5 .9 6 5 .6 7 6 .2 3 5 .9 2 5 .8 4 5 .5 7 4 .9 7 5 .8 2 5 .1 1 5 .4 4 4 .5 0 6 .2 8 4 .4 3 4 .6 6 M n O 0 .1 4 0 .1 9 0 .1 4 0 .0 1 0 .1 1 0 .0 5 0 .1 4 0 .0 7 0 .1 3 0 .0 8 0 .0 9 0 .1 3 0 .0 6 0 .1 4 0 .0 3 0 .0 7 M g O 1 6 .0 6 1 3 .6 8 1 7 .6 6 1 7 .3 6 1 5 .5 6 1 7 .2 0 1 6 .2 1 1 6 .5 0 1 6 .8 2 1 6 .4 2 1 6 .3 7 1 6 .4 9 1 7 .1 4 1 6 .4 7 1 6 .8 2 1 6 .8 4 C aO 2 1 .5 5 2 1 .0 9 2 0 .5 5 2 0 .4 7 2 1 .5 1 2 0 .9 6 2 2 .3 6 2 1 .4 7 2 1 .6 5 2 1 .5 7 2 2 .1 1 2 1 .9 8 2 2 .4 5 2 1 .4 4 2 2 .0 2 2 1 .6 7 N a 2 O 0 .3 1 0 .3 6 0 .3 5 0 .3 4 0 .3 1 0 .1 7 0 .2 5 0 .2 4 0 .2 6 0 .2 0 0 .2 6 0 .3 0 0 .2 7 0 .3 5 0 .2 0 0 .2 6 T o ta l 1 0 0 .4 4 9 9 .6 8 9 9 .9 6 1 0 0 .3 7 9 8 .5 5 9 8 .5 3 9 8 .6 5 9 9 .0 2 9 9 .5 4 9 9 .2 4 9 9 .7 8 1 0 0 .8 8 1 0 0 .0 9 1 0 0 .3 0 9 9 .4 4 9 9 .3 5 C at io n s (p fu .) S i 1 .8 8 2 1 .7 2 6 1 .8 8 2 1 .9 0 1 1 .8 5 0 1 .9 0 5 1 .9 0 7 1 .8 9 7 1 .9 1 7 1 .8 7 6 1 .8 9 5 1 .8 9 9 1 .9 0 9 1 .8 9 3 1 .9 2 9 1 .9 2 7 A lI V 0 .1 1 3 0 .2 7 4 0 .1 0 4 0 .0 9 9 0 .1 5 0 0 .0 7 5 0 .0 7 6 0 .1 0 3 0 .0 8 3 0 .1 1 9 0 .1 0 5 0 .1 0 1 0 .0 8 2 0 .1 0 5 0 .0 7 1 0 .0 7 3 A lV I 0 .0 0 0 0 .0 0 2 0 .0 0 0 0 .0 1 3 0 .0 0 2 0 .0 0 0 0 .0 0 0 0 .0 0 1 0 .0 0 7 0 .0 0 0 0 .0 0 0 0 .0 0 6 0 .0 0 0 0 .0 0 0 0 .0 0 8 0 .0 0 9 C r 0 .0 0 1 0 .0 0 4 0 .0 2 2 0 .0 2 3 0 .0 0 7 0 .0 1 3 0 .0 0 5 0 .0 1 3 0 .0 2 1 0 .0 1 5 0 .0 2 7 0 .0 2 4 0 .0 1 9 0 .0 0 7 0 .0 2 7 0 .0 2 3 F e3 + 0 .0 6 2 0 .1 2 7 0 .0 6 2 0 .0 3 7 0 .0 7 5 0 .0 1 3 0 .0 3 7 0 .0 4 4 0 .0 2 9 0 .0 4 8 0 .0 4 3 0 .0 3 7 0 .0 3 2 0 .0 5 5 0 .0 0 7 0 .0 1 1 T i 0 .0 3 6 0 .0 8 3 0 .0 2 3 0 .0 2 5 0 .0 4 4 0 .0 3 1 0 .0 2 6 0 .0 3 2 0 .0 2 2 0 .0 3 5 0 .0 2 7 0 .0 2 8 0 .0 2 5 0 .0 3 4 0 .0 2 2 0 .0 2 5 M g 0 .8 7 9 0 .7 6 2 0 .9 6 5 0 .9 4 3 0 .8 6 6 0 .9 5 8 0 .9 0 3 0 .9 1 1 0 .9 2 2 0 .9 0 7 0 .8 9 7 0 .8 9 4 0 .9 3 4 0 .8 9 9 0 .9 2 3 0 .9 2 4 F e2 + 0 .1 5 9 0 .1 4 6 0 .1 2 1 0 .1 3 5 0 .1 2 0 0 .1 7 2 0 .1 4 6 0 .1 2 9 0 .1 2 4 0 .1 3 2 0 .1 1 4 0 .1 2 8 0 .1 0 5 0 .1 3 8 0 .1 2 9 0 .1 3 3 M n 0 .0 0 5 0 .0 0 6 0 .0 0 4 0 .0 0 0 0 .0 0 4 0 .0 0 2 0 .0 0 5 0 .0 0 2 0 .0 0 4 0 .0 0 3 0 .0 0 3 0 .0 0 4 0 .0 0 2 0 .0 0 4 0 .0 0 1 0 .0 0 2 C a 0 .8 4 8 0 .8 4 4 0 .8 0 7 0 .7 9 9 0 .8 6 0 0 .8 3 9 0 .8 9 5 0 .8 5 2 0 .8 5 3 0 .8 5 6 0 .8 7 1 0 .8 5 7 0 .8 8 0 0 .8 4 1 0 .8 6 8 0 .8 5 5 N a 0 .0 2 2 0 .0 2 6 0 .0 2 5 0 .0 2 4 0 .0 2 2 0 .0 1 2 0 .0 1 8 0 .0 1 7 0 .0 1 8 0 .0 1 4 0 .0 1 8 0 .0 2 1 0 .0 1 9 0 .0 2 5 0 .0 1 5 0 .0 1 9 S u m 4 .0 0 6 4 .0 0 0 4 .0 1 4 4 .0 0 0 4 .0 0 0 4 .0 2 0 4 .0 1 7 4 .0 0 0 4 .0 0 0 4 .0 0 5 4 .0 0 0 4 .0 0 0 4 .0 0 8 4 .0 0 2 4 .0 0 0 4 .0 0 0 E n d m em b er s (% ) E n 4 9 .6 5 1 .9 5 4 .5 5 3 .6 5 1 .8 4 9 .3 4 7 .8 5 1 .3 5 1 .4 5 1 .4 5 1 .6 5 1 .1 5 1 .0 5 0 .8 5 0 .5 5 0 .6 F s 9 .0 1 0 .0 6 .8 7 .7 7 .2 8 .9 7 .7 7 .2 6 .9 7 .5 6 .5 7 .3 5 .7 7 .8 7 .1 7 .3 W o 4 1 .4 3 8 .1 3 8 .7 3 8 .7 4 1 .1 4 1 .9 4 4 .5 4 1 .5 4 1 .7 4 1 .1 4 1 .9 4 1 .6 4 3 .3 4 1 .5 4 2 .4 4 2 .1 M g # 8 4 .7 8 3 .9 8 8 .9 8 7 .5 8 7 .8 8 4 .8 8 6 .1 8 7 .6 8 8 .1 8 7 .3 8 8 .7 2 4 8 7 .4 7 6 8 9 .9 8 6 .7 8 7 .7 8 7 .4 a R o ck ty p e: P B , p ic ri ti c b as al t; B , b as al t; T B , tr ac h y b as al t. b G ra in si ze : P h en ., p h en o cr y st s > 0 .7 m m ; M ic ., m ic ro p h en o cr y st s > 0 .3 m m ; G ra in , g ro u n d m as s < 0 .3 m m . c P h en o cr y st m o rp h o lo g y : E , eu h ed ra l; S , su b h ed ra l; A , an h ed ra l. Geochemistry Geophysics Geosystems G3 weis et al.: trace of the kerguelen mantle plume 10.1029/2001GC000251 23 of 27 like Sr and Nd ratios and high 206Pb/204Pb, similar to lavas from the Comores Islands. [47] The last 34 myr of mildly alkalic volcanism in the north central region of the Kerguelen Plateau has been spatially diverse with coeval volcanism on the Kerguelen Archipelago, at Heard and McDo- nald Islands, and at submarine structures between these islands. Although there has been coeval vol- canism, there appears to be a general trend for recent volcanism to migrate in a southeasterly direction from the Northern Kerguelen Plateau to the Ker- guelen Archipelago and finally to Heard andMcDo- nald Islands. This trend may well represent the Tertiary hot spot track of the Kerguelen plume. Acknowledgments [48] The Kerimis program was supported by the Institut National des Sciences de l’Univers (INSU-CNRS) and by the Université Louis Pasteur (Ecole et Observatoire des Sci- ences de la Terre). The senior author of this paper thanks the FNRS for financing this research program (FRFC Grant 2.4579.99). D. Damasceno thanks the FRIA for financing his Ph.D. scholarship. Both Weis and Damasceno thank the FNRS for financing their participation in the Kerimis survey cruise. Part of this research program was also funded by the ARC convention 98/03-233. C. Maerschalk is thanked for carefully preparing the samples for isotope analyses and M. Lo Cascio for diligently crushing them. Research done at MIT was supported by NSF Grant EAR 98141313. We thank P. Ila (MIT) and M. Vollinger (University of Massachusetts) for obtaining INAA and XRF data, respectively, M. Kurz and J. Curtice for assistance in He isotope analysis, and F. Albarède and J. Blichert-Toft for introducing one of us (NM) to Hf isotopic analyses. J. Barling, C. Chauvel, A.D. Saunders, R. Rudnick, and W. M. White are thanked for their reviews that significantly improved the manuscript. Logistical support for the Kerimis cruise was provided by the Institut Français pour la Recherche et la Technologie Polaires (IFRTP) and the Institut Français de Recherche pour l’Exploitation de la Mer (IFREMER). We thank Commandant Gauthier and the crew of the M/V Marion Dufresne 2 and the GENAVIR and IFRTP teams for their outstanding contributions on board. EOST contribution 2002.11-UMR7516. References Albarède, F., Géochronologie comparée par la méthode 39Ar-40Ar de deux régions d’histoire post-hercynienne différ- ente; La Montagne Noire et les Pyrénées orientales. Thesis, Univ. Paris 7, 1976. Albarède, F., B. Luais, G. Fitton, M. Semet, E. Kaminski, B. G. J. Upton, P. Bachèlery, and J.-L. 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