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Trace of the Kerguelen mantle plume: Evidence from seamounts between the Kerguelen Archipelago and Heard.. Weis, Dominique; Scoates, James S. 2002

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Geochemistry Geophysics Geosystems  3  G  AN ELECTRONIC JOURNAL OF THE EARTH SCIENCES Published by AGU and the Geochemical Society  Article Volume 3, Number 6 20 June 2002 10.1029/2001GC000251 ISSN: 1525-2027  Trace of the Kerguelen mantle plume: Evidence from seamounts between the Kerguelen Archipelago and Heard Island, Indian Ocean Dominique Weis De´partement des Sciences de la Terre et de l’Environnement, Universite´ 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 De´partement des Sciences de la Terre et de l’Environnement, Universite´ 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 De´partement des Sciences de la Terre et de l’Environnement, Universite´ Libre de Bruxelles CP160/02, Avenue F.D. Roosevelt, 50, B-1050 Brussels, Belgium (knicol@ksu.edu)  James S. Scoates De´partement des Sciences de la Terre et de l’Environnement, Universite´ 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 Copyright 2002 by the American Geophysical Union  1 of 27  Geochemistry Geophysics Geosystems  3  G  weis et al.: trace of the kerguelen mantle plume 10.1029/2001GC000251  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 Â 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 generating major tectonic features in the Indian Ocean [e.g., Weis et al., 1991; Frey et al., 2000a] and subsequently in the genesis of geochemical anomalies 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 province (the giant oceanic plateau Kerguelen PlateauBroken 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 northwards 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 abun2 of 27  Geochemistry Geophysics Geosystems  3  G  weis et al.: trace of the kerguelen mantle plume 10.1029/2001GC000251  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.  dant evolved lavas (trachytes and phonolites). To the south, Heard Island (Figure 2) also has a record of volcanism from middle Eocene to Recent; however, 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 summary illustrates why there is significant uncertainty in the location of the Kerguelen hot spot and therefore 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 Kerguelen hot spot is of fundamental importance, not only for understanding the history of the Kerguelen 3 of 27  Geochemistry Geophysics Geosystems  3  G  weis et al.: trace of the kerguelen mantle plume 10.1029/2001GC000251  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.  plume, but also for reconciling plate tectonic reconstructions 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 subsequent complex mid-ocean ridge reorganizations, which may have included one or more ridge jumps. Importantly, the Kerguelen 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 heterogeneities. This paper presents the first geochronologic and geochemical study of seamounts  between Kerguelen Archipelago and Heard Islands. Notably the first picritic basalts related to the Kerguelen 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 an Oligoceneto-Recent hot spot track of the Kerguelen plume.  1.1. Kerguelen Plume and Kerguelen Plateau [5] The 2 Â 106 km2 Kerguelen Plateau (Figure 1) has been divided into several distinct domains [Houtz et al., 1977; Schlich, 1982; Coffin et al., 4 of 27  Geochemistry Geophysics Geosystems  3  G  weis et al.: trace of the kerguelen mantle plume 10.1029/2001GC000251  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 (Raggatt 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 (>3500 m), 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 characteristics of these basalts are consistent with derivation 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 component [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 submarine basement of the northern Kerguelen Plateau had not been sampled prior to this study and the subsequent ODP drilling Leg 183 [Frey et al., 2000a].  guelen, Imagerie Multifaisceau et Imagerie Sismique) 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 southwest-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 submarine 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 surface, which may indicate that throughout the NKP subaerial erosion occurred before subsequent subsidence 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].  1.2. Kerimis Survey Cruise: Dredge Locations and Descriptions  [8] More than 1 ton of basalt was recovered from four dredge sites (Figures 1 and 2). Dredge 1 (D1: 50°210S-63°480E) on the Northern Kerguelen Plateau, 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, microgabbroic to alkali granitic intrusive rocks, and sedimentary 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].  [7] New multichannel seismic reflection data were collected in March 1998 during the Kerimis (Ker-  [9] Dredges 4 (D4: 51°310S-71°090E) and 5 (D5: 51°310S-71°080E) were on the northern part of the 5 of 27  Geochemistry Geophysics Geosystems  3  G  weis et al.: trace of the kerguelen mantle plume 10.1029/2001GC000251  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 features reflecting an in situ origin (fresh broken surfaces, columnar shapes, minimal Mn crusts). These basalts have abundant centimeter-size olivine/clinopyroxene phenocrysts that distinguish them from the basalts of the Kerguelen Archipelago where plagioclase phenocrysts are relatively common [Gautier et al., 1990]. [10] Dredge 6 (D6: 51°010S-071°040E) 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 compositions (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  40  Ar-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 constants recommended by Steiger and Ja¨ger [1977] was used for age calculation. The 120–150 mg aliquots of standard biotite LP6 (129 Ma) were utilized as flux monitors. Uncertainties representing 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 Albare`de [1976] for 40 Ar-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 fluorescence (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 acidleached 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 samples. Sr and Nd isotopic compositions were measured on single Ta filaments and triple Re-Ta filaments, respectively, in the dynamic mode on a VG Sector 54 multicollector mass spectrometer with an internal precision better than 1 Â 10À5 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 143 Nd/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 6 of 27  Geochemistry Geophysics Geosystems  3  G  weis et al.: trace of the kerguelen mantle plume 10.1029/2001GC000251  temperature range between 1090°C and 1150°C. Between-run precisions were better than $0.1% for 206 Pb/ 204 Pb and 207 Pb/ 204 Pb and better than $0.15% for 208Pb/204Pb. The quality of a complete replicate analysis can be evaluated by sample D433, which was analyzed twice because of its unusual composition.  2.5. Hf Isotopic Compositions [15] Extraction of Hf was performed at the Universite´ Libre de Bruxelles. Hf isotope analyses were carried out on the multicollector plasmasource mass spectrometer (P54) at the Ecole Nationale Supe´rieure de Lyon, following the analytical procedure of Blichert-Toft et al. [1997]. All measured Hf isotopic ratios were corrected for W and Ta, Lu and Yb interferences at masses 180 and 176, respectively, by monitoring the isotopes 182W, 181 Ta, 175Lu, 173Yb and normalized for mass fractionation to 179Hf/177Hf of 0.7325 using an exponential 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).  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 comparison between the K-Ar conventional age and the 40 Ar-39Ar integrated age, and second the shape of the 40Ar-39Ar release spectrum.  3.1. Age of the Seamounts  [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 ArAr 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 D687, 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 ± 0.2 Ma. 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 KAr 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.  [17] K-Ar and 40Ar-39Ar measurements were made on whole rock samples (Tables 1 and 2). The  [19] All 40Ar-39Ar ages are slightly older than the K-Ar ages for the same sample. Inspection of the  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 inspection, 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  7 of 27  3  Geochemistry Geophysics Geosystems  G  weis et al.: trace of the kerguelen mantle plume 10.1029/2001GC000251  Table 1. K-Ar Ages on the Kerimis Dredged Basaltsa Sample  Rock Type  K2O, weight %  D4-33 D4-37 D5-42 D5-44a D5-44b D6-85 D6-87 D6-88 D6-89  basalt basalt olivine basalt olivine basalt olivine basalt picritic basalt picritic basalt picritic basalt picritic basalt  0.823 2.091 2.331 2.035 2.141 1.077 1.363 0.904 1.282  a  The  40  Ar*Â100 total  40  Ar  rad. 40Ar, 10À11mole/g  Age, Ma  ±2s, Ma  0.0853 6.360 5.558 5.199 5.232 2.173 2.925 2.492 2.557  0.72 21.0 16.5 17.7 16.9 14.0 14.8 19.0 13.8  0.25 0.6 0.5 0.5 0.5 0.8 0.6 1.0 0.9  4.7 40.2 48.5 48.0 57.1 12.2 25.0 12.7 11.1  K/K total = 1.167 Â 10À4 mole/mole; lb = 4.962 Â 10À10 aÀ1; le = 0.581 Â 10À10 aÀ1.  40  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.  samples and the moderate loss of 39Ar during irradiation, however, suggests that the 40Ar* loss by the matrix should not have been severe. Therefore we propose an age of emplacement bracketed between 17.7 and 20–21 Ma, plateau ages yielded by D5-44a and D5-42, respectively.  [20] Olivine basalts (D5) display 40Ar-39Ar integrated ages, which are 15% higher than the conventional K-Ar ages. This difference cannot be explained either by a systematic bias in conventional 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 significant 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  [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% 39 Ar 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 exemplified by the concordance of the K-Ar conventional 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 2. Summary of Sample D4-33 D4-37 D5-42 D5-44a D5-44b D6-85 D6-87 D6-88 D6-89 a ‘‘Plateau b  40  Ar-39Ar Results on the Kerimis Dredged Basalts  Integrated Date in Ma ± 2s 1.60 23.9 19.1 20.0 19.3 18.7 21.4 19.2 19.3  ± ± ± ± ± ± ± ± ±  0.7 0.4 0.4 0.5 0.5 1.0 2.0 1.3 1.6  36  Plateau Date in Ma ± 2s 1.63 21.9 20.4 21.2  ± ± ± ±  39  0.42a 0.40 0.30 0.50  1.56 21.9 20.2 21.0  20.7 ± 1.30  age’’ determined with two steps representing 90% of the released Isochron determined with five steps.  Ar/40Ar versus Ar/ Ar date in Ma ± 2s ± ± ± ±  0.74b 0.70 0.50 0.60  20.3 ± 2.60 39  Initial  40  Ar/36Ar  40  297 296 301 295  ± ± ± ±  3 12 8.8 4  295 ± 3  ArK.  8 of 27  Geochemistry Geophysics Geosystems  3  G  15  weis et al.: trace of the kerguelen mantle plume 10.1029/2001GC000251  30  D5-44a  D5-44b  D5-42  10  5  37Ar / 39Ar Ca K  37Ar / 39Ar Ca K  37Ar / 39Ar Ca K  10 20  10  6  2  10 20.0 +- 0.5 Ma  integrated age = 0  20  40  60  80  20  10  0  cumulative % 39Ar released  30  60  20  40  60  80  100  cumulative % 39Ar released 20  3  D6-85  10  1.63 +- 0.42 Ma  20  40  60  80  23.9 +- 0.4 Ma  integrated age =  80  apparent age (Ma)  5  15  60 40  21.9 +- 0.4 Ma  20  100  0  20  40  60  80  D6-87  20 10 21.4 +- 2.0 Ma  integrated age = 40  60  80  cumulative % 39Ar released  100  apparent age (Ma)  30  20  40  60  80  100  cumulative % 39Ar released 20  37Ar / 39Ar Ca K  5  D6-89  10  19.2 +- 1.3 Ma 40  30  apparent age (Ma)  37Ar / 39Ar Ca K  5  18.7 +- 1.0 Ma  integrated age = 0  D6-88  10  20  10  100  15  0  20  cumulative % 39Ar released  integrated age =  apparent age (Ma)  0  D4-37  cumulative % 39Ar released  40  19.1 +- 0.4 Ma  integrated age =  100  apparent age (Ma)  1.6 +- 0.7 Ma  10  0  Figure 3. deviation.  80  10  1  integrated age =  apparent age (Ma)  40  37Ar / 39Ar Ca K  37Ar / 39Ar Ca K  37Ar / 39Ar Ca K  5  10  37Ar / 39Ar Ca K  20  20  cumulative % 39Ar released  D4-33  50  19.3 +- 0.5 Ma  integrated age =  100  apparent age (Ma)  20.3 +- 0.4 Ma  apparent age (Ma)  apparent age (Ma)  21.2 +- 0.5 Ma 20  20.7 +- 1.3 Ma  10  0  20  40  60  80  cumulative % 39Ar released  100  20 19.3 +- 1.6 Ma  integrated age = 0  20  40  60  80  100  cumulative % 39Ar released  Ar-39Ar age spectrum plots for Kerimis dredged basalts. The uncertainties are quoted at two standard  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 40 Ar-39Ar age, 1.6 ± 0.7 Ma. The well-crystallized nature of the matrix precludes any significant loss 9 of 27  Geochemistry Geophysics Geosystems  3  G  weis et al.: trace of the kerguelen mantle plume 10.1029/2001GC000251  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 probable 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 activity 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 from D4 contain plagioclase phenocrysts. Representative olivine compositions are reported in Table A1 and representative clinopyroxene compositions are reported in Table A2. The general petrographic characteristics (rock type, vesicles, phenocryst morphology, 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 5 mm), 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 Hawaiian picritic basalts [Garcia, 1996] and for the Re´union picritic basalts [Albare`de et al., 1997]. Many of the olivine phenocrysts in the picritic basalts contain chromite inclusions (Figures 4a and 4e), and some phenocrysts contain melt inclusions that are typically altered, although local pristine patches of glass remain (Figure 4f ). The chemistry of these chromite and glass inclusions is discussed in detail by Borisova et al. [2002]. The rare, euhedral clinopyroxene phenocrysts 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 clinopyroxene and plagioclase microlites. The basalt (D433) is highly vesicular and contains 10 vol % of zoned plagioclase (An44 – 64), 3 vol % of subhedral olivine (Fo59 – 70), and 2 vol % of subhedral clinopyroxene (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 clinopyroxene (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 abundance ratios (11.4–14.7) compared to the average oceanic basalt ratio of 11.6 [Hofmann and White, 1982], these samples have not experienced significant postmagmatic mobility of alkali metals. The D4 basalt and D6 picritic basalts plot near or on the 10 of 27  Geochemistry Geophysics Geosystems  3  G  weis et al.: trace of the kerguelen mantle plume 10.1029/2001GC000251  A  0.5 mm  B  0.5 mm  C  0.5 mm  D  0.5 mm  E  0.5 mm  F  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.  11 of 27  3  Geochemistry Geophysics Geosystems  G  weis et al.: trace of the kerguelen mantle plume 10.1029/2001GC000251  Table 3. Major and Trace Element for Kerimis Dredged Basaltsa Dredges  4  4  5  5  5  6  6  6  6  Samples SiO2 TiO2 Al2O3 Fe2O3 MnO MgO CaO Na2O K2O P2O5  33 49.44 3.20 15.10 12.69 0.15 5.78 8.23 3.71 1.11 0.62  37 50.97 3.34 14.57 10.86 0.13 5.40 8.79 2.74 2.40 0.57  42 47.41 4.18 12.61 13.04 0.15 7.48 9.01 2.68 2.62 .97  44a 46.43 3.90 12.43 12.94 0.22 7.53 10.24 2.80 2.22 1.46  44b 45.92 4.39 12.22 13.92 0.14 7.14 9.82 2.84 2.19 1.62  85 46.09 2.03 8.22 13.16 0.16 19.45 7.18 1.45 1.36 0.35  88 46.07 2.01 8.08 13.35 0.17 20.04 6.99 1.50 1.36 0.34  87 46.52 2.12 8.56 13.20 0.17 18.50 7.39 1.49 1.45 0.36  89 46.19 2.00 7.98 13.30 0.16 20.16 6.99 1.43 1.36 0.34  BHVO-1 49.58 2.74 13.61 12.17 0.17 7.09 11.35 2.40 0.55 0.27  XRF Rb Sr Ba V Cr Ni Zn Ga Y Zr Nb La Ce  17.8 577 210 183 108 87 140 27 29.1 325 38.9 35 61  45.7 660 468 198 119 105 126 24 26.3 346 38.0 40 89  46.2 882 529 209 281 177 167 23 33.0 447 71.2 64 112  33.8 842 498 221 307 187 172 22 34.9 409 65.0 61 102  34.6 907 478 213 321 163 229 22 53.1 451 74.2 76 113  25.3 422 321 145 1095 799 132 14 14.8 200 24.0 30 45  26.7 415 311 152 1118 754 137 14 15.4 207 24.8 31 51  25.4 409 310 148 1157 837 135 13 14.6 195 23.8 30 45  25.6 406 307 144 1155 847 132 13 14.5 194 23.6 28 46  9.2 388 139 285 284 126 113 21 24.6 191 19.4 16 36  INAA Sc Cr Co Hf Ta Th La Ce Nd Sm Eu Tb Yb Lu Cs  17.5 119 42.3 7.16 2.35 3.17 29.5 69.7 38.3 8.88 2.88 1.06 1.97 0.27 –  20.5 272 49.3 9.87 4.19 6.76 59.7 129 62.0 12.4 3.84 1.47 1.93 0.255 1.9  22.8 299 51.3 9.2 3.82 6.0 57.7 121 59.6 11.6 3.55 1.33 2.18 0.32 0.3  20.7 285 43.7 9.65 4.28 6.7 71.7 131 69.4 13.4 4.08 1.71 2.94 0.40 0.37  19.2 1132 89.7 4.32 1.32 2.40 25.0 54.7 27.0 5.25 1.70 0.75 0.95 0.136 0.24  19.3 1143 93.1 4.44 1.27 2.28 24.6 52.2 26.2 5.20 1.69 0.70 0.95 0.139 0.28  19.1 1180 92.4 4.47 1.36 2.41 24.6 53.4 24.9 5.18 1.67 0.66 1.02 0.146 0.21  19.0 1130 91.6 4.39 1.16 2.36 23.8 52.0 24.8 5.13 1.64 0.61 1.02 0.132 0.27  37.2 290 43.9 4.4 1.16 1.07 15.1 39.7 24.4 6.1 2.05 0.90 1.98 0.282  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.  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 Archipelago [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 phenocrysts. The whole rock Mg # were calculated assuming 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 12 of 27  Geochemistry Geophysics Geosystems  3  G  weis et al.: trace of the kerguelen mantle plume 10.1029/2001GC000251  Na2O+K2O 15  Kerimis D4  A l kalic  Kerguelen Upper Miocene Series  D5 D6 10  Kerguelen Lower Miocene Series  D5 5  Tholeiitic  D6  Big Ben basanites  0 40  D4  Kerguelen Plateau basalts  Kerguelen Oligocene flood basalts 45  50  SiO2 wt.% 55  60  65  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].  exchange partition coefficient between olivine and basaltic liquid of 0.30 ± 0.03 [Roeder and Emslie, 1970], which indicates accumulation of $Fo84 olivine in a basaltic melt with an Mg # of $62. The equilibrium magma compositions for the picritic basalts were calculated by removing wt % increments 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 predicted by simply removing olivine), but at significantly higher SiO2 contents, $50 wt %. Systematic removal of olivine phenocrysts also results in a more meaningful relationship between the whole rock Mg  # 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 transitional 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 13 of 27  Geochemistry Geophysics Geosystems  3  G  45  weis et al.: trace of the kerguelen mantle plume 10.1029/2001GC000251  50  55  60  65  70  75  80 95  95  xenocrysts  KD(ol-liq) = 0.30 0.03 equilibrium melt  90 MD109-42  85  85  average cores  80  80 28-35 vol.% olivine  75  MD109-44  70  MD109-85 MD109-87 MD109-88 MD109-89  xenocrysts  Foolivine  90  65 MD109-33  60  75 70 65  accumulation  A.  55 95  60 55 95  KD(cpx-liq) = 0.25 0.05 MD109-37  Mg#cpx  90  90  xenocrysts 85  85 ~1 vol.% cpx  80 D4: trachybasalt D4: basalt D5: olivine basalt  75 MD109-33  MD109-85 MD109-87 MD109-88 MD109-89  80  75  D6: picritic basalt  B.  70 45  50  55  60  65  70  75  70 80  Mg#wholerock 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 (MD10944) 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.  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]. Consistent with this trend, the olivine and picritic basalts have high La/Yb, 25–20, within the range 14 of 27  3  Geochemistry Geophysics Geosystems  G  weis et al.: trace of the kerguelen mantle plume 10.1029/2001GC000251  7  TiO2  Ba/Nb Kerguelen  Big Ben basanites  6 5  Kerguelen Oligocene basalts  16  Big Ben basalts  Miocene D4 basalts  Kerguelen Big Ben basanites basalts  12 D6  4  8  Big Ben basanites  Dr 5 D5  3  Kerguelen basanites  4 Laurens P e ninsula basalts  2 0  1 0  Kerguelen Oligocene basalts  Kerguelen Miocene basalts  Kerguelen basanites  La/Yb 35  Big Ben basanites  30  P2O5  Kerimis D5  D5 Kerguelen Miocene basalts  25  D4  A  D5  15  D6  Big Ben basalts  20  1.5  D4  D6 10  1.0  Kerguelen Oligocene basalts  5  B  0  0.5  D6 D4  0  5  20  C  15  0.0 100  15 MgO  NER  Zr/Nb  Nb  10  20  Big Ben basalts  Kerguelen Miocene basalts  10  Kerguelen basanites  80  Kerguelen Oligocene basalts  5  Big Ben basanites  60  Kerguelen Plateau  748  0  Kerguelen Miocene D4 basalts  D6  LPS D4  D5 Big Ben Kerguelenbasanites basanites  115/85 82/38 30/26 25/23 22/18  <10 0.2/0  Ma  40 20 Kerguelen Oligocene basalts 0 0 5  Big Ben basalts  10  15  MgO  20  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.  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 Peninsula and Big Ben) have different Ba/Nb ratios. These differences correlate with differences in Sr  Figure 8. Ba/Nb (A) and La/Yb (B) versus MgO (wt %) and Zr/Nb versus eruption age for lavas associated with the Kerguelen plume (C). In panel A, which distinguishes 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 Archipelago and Heard Island. 15 of 27  Geochemistry Geophysics Geosystems  3  G  weis et al.: trace of the kerguelen mantle plume 10.1029/2001GC000251  Table 4. Isotopic Ratios for the Kerimis Dredged Basalts Dredges  Samples  4 4 5 5 5  33 33b 37 42 44a 44b  6 6 6  85 87 88  6  89  87  Sr/86Sr  2sm  0.703126 0.703100 0.705942 0.705654 0.705625 0.705658 0.705676 0.705694 0.705683 0.705681 0.705703 0.705699 0.705683  6 6 6 6 7 8 11 8 10 8 7 6 12  143  Nd/144Nd  2sm  0.512946 0.512942 0.512525 0.512582 0.512606 0.512576  14 9 9 10 8 10  0.512543 0.512533 0.512547  8 6 7  0.512526  9  206  Pb/204Pba  19.669 19.677 18.451 18.179 18.214 18.208 18.197 17.875 17.874 17.872 17.869 17.876  207  Pb/204Pba  15.678 15.686 15.551 15.511 15.517 15.527 15.524 15.536 15.540 15.538 15.531 15.541  208  Pb/204Pba  39.315 39.347 39.009 38.576 38.634 38.658 38.662 38.266 38.274 38.265 38.266 38.278  176  Hf/177Hf  2sm  0.282937  8  0.282752  8  0.282728  7  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 Pb/204Pb. b Full-procedural duplicate analysis, including leaching.  208  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 forming the Kerguelen Archipelago (Figure 8). [31] In summary, the major and trace element abundances of lavas from each dredge form distinct compositional groups (Figures 5, 7, and 8). The basalts (D4) have the lowest abundances of incompatible 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 18–21 Ma basalts (i.e., the olivine basalts (D5), the picritic  pillow basalts (D6) and the alkali basalt (D4)) plot within the field defined by the Upper Miocene Series of the Kerguelen Archipelago [Weis et al., 1993]. The effect of age-correction for in situ 87Rb and 147 Sm decay is negligible (between 3 and 5 Â 10À5 and 1.1 and 1.5 Â 10À5, respectively). D6 picritic basalts have slightly lower 143Nd/144Nd than D5 alkali basalts and plot within the field of the archipelago basanites. More than 90% of the Kerguelen Archipelago Miocene flood basalts have 87Sr/86Sr between 0.7050 and 0.7056 and 143 Nd/ 144 Nd 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 analyzed 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 SrNd 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 Oligocene 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 basan16 of 27  Geochemistry Geophysics Geosystems  3  G  weis et al.: trace of the kerguelen mantle plume 10.1029/2001GC000251  Figure 9. 87Sr/86Sr À 143Nd/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 87 Sr/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 147 Sm decay since their emplacement age.  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 143 Nd/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 from D4 have very different 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 Archipelago 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 Archipelago 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 17 of 27  Geochemistry Geophysics Geosystems  3  G  weis et al.: trace of the kerguelen mantle plume 10.1029/2001GC000251  Kerguelen Upper Miocene series  Heard Island  208Pb/204Pb  Dr 4  Dr 4  738  39  Kerguelen Lower Miocene basalts Kerguelen Oligocene flood basalts  Kerguelen basanites Mt. Ross Kerguelen Dr 6 Plateau  Big Ben 0.2835 basanite Dr 5  38  MORBs  176Hf/177Hf  0.2833  Amsterdam & St Paul HIMU Crozet Islands  0.2831 Hawaii  Ascension Walvis Ridge  0.2829  SEIR  Dr 4 Comores  Dr 6  Dr 5  0.2827  0.2825 17  37  17.5  18.0  18.5  Kerguelen Archipelago  19  18  19.0  206Pb/204Pb  20  21  19.5  207Pb/204Pb 738 15.7 1137  Kerguelen Upper Miocene series Big Ben basanite Kerguelen Lower Miocene basalts  Kerguelen basanites  15.6  Dr 4  Kerguelen Plateau Dr 5  Dr 6 15.5  Dr 4 Kerguelen Oligocene flood basalts  Mt. Ros s  SEIR Heard Island 15.4 17.5  18.0  18.5  19.0  19.5  206Pb/204Pb  Figure 10. 207Pb/204Pb À 206Pb/204Pb and 208Pb/204PbÀ 206Pb/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/177Hf À 206Pb/204Pb and comparison with literature data [Salters and White, 1998].  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 3 He/4He of 1.39 Â 10À6. 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 undegassed 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 18 of 27  Geochemistry Geophysics Geosystems  3  G  weis et al.: trace of the kerguelen mantle plume 10.1029/2001GC000251  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 representative 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 sections 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 indicate 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 206 Pb/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 Kerguelen 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  206  Pb/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 206 Pb/204Pb in D6 picrites may reflect interaction of the plume-derived magmas with the Cretaceous Kerguelen Plateau, but there is no compelling 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 Kerguelen 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 19 of 27  Geochemistry Geophysics Geosystems  3  G  weis et al.: trace of the kerguelen mantle plume 10.1029/2001GC000251  no explanation for the puzzling isotopic characteristics 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, Mu¨ller et al. [1993], amongst others, required that the present location of the Kerguelen hot spot be beneath Skiff Bank, under the northwestern Northern 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 acceptable 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 candidates for the location of the hot spot. [43] The age range of 21–18 Ma for the seamounts 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 volcanism on these two main islands. Although they are more enriched in incompatible elements, the D5 alkali basalts are similar in major element composition to the 24–25 Ma Miocene alkalic flood basalts in the eastern part of the Kerguelen Archipelago 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 Archipelago 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 current 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 observations 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 [Gre´goire 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 temporal decrease in extent of melting and increase in depth of melt segregation.  5. Conclusions [45] The seamounts dredged between the Kerguelen 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 discovered picritic basalts related to the Kerguelen plume activity. [46] The alkalic compositional characteristics of the dredged basalts are consistent with the temporal 20 of 27  3  Geochemistry Geophysics Geosystems  G  NNE 40 AG E IN MA  weis et al.: trace of the kerguelen mantle plume 10.1029/2001GC000251  NORTHERN KERGUELEN PLATEAU  CENTRAL KERGUELEN PLATEAUSSW  NKP KA FLOOD BASALTS  30  VG 20 MTS BALLONS GEOG 10  R DB UMS  SEAMOUNTS HEARD  ROSS 0 1140  D6 D4/D5  FOCH NORTH ARCHIPELAGO  DEPTH IN M ROSS  2000  HEARD  KA  1000 0 -1000 -2000 -3000 -4000  KM 0  100  200  300  400  500  600  700  800  900 1000 1100  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), Ge´ographie) and of the basanite to phonolite plugs and domes of the Upper Miocene Series (UMS).  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 compositions within the range defined by >90% of the Kerguelen Archipelago Miocene flood basalts and the Big Ben basaltic series on Heard Island suggesting that these lavas have been produced by partial  melting of the Kerguelen plume. D5 and D6 basalts have distinct Pb isotopic compositions for comparable Sr isotopic ratios. D5 olivine basalts have Pb ratios comparable to those of the Kerguelen Archipelago Oligocene lavas, whereas D6 picritic basalts have Pb ratios plotting within the field of Kerguelen Plateau basalts, which are characterized by lower 206 Pb/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 MORB21 of 27  Phen. E  37.75 0.12 26.98 0.30 35.51 0.17 0.10 100.98  0.995 0.002 0.595 0.007 1.396 0.005 0.002 3.003  70.0 30.0  Typeb Kindc  Oxides (wt %) SiO2 Cr2O3 FeO MnO MgO CaO NiO Total  Cations ( pfu.) Si Cr Fe2+ Mn Mg Ca Ni Sum  End members (%) Fo Fa  58.9 41.1  0.999 0.000 0.811 0.011 1.172 0.005 0.002 3.000  36.34 0.00 35.29 0.48 28.60 0.17 0.08 101.03  Mic. S  80.1 19.9  0.986 0.000 0.393 0.004 1.612 0.007 0.006 3.011  38.88 0.01 18.53 0.18 42.63 0.26 0.27 100.91  Phen. SK  MD109-42 B  82.2 17.8  0.989 0.001 0.351 0.005 1.652 0.007 0.006 3.010  39.32 0.04 16.68 0.25 44.05 0.25 0.28 100.87  Phen. S  80.8 19.2  0.995 0.001 0.377 0.005 1.615 0.007 0.004 3.004  39.42 0.05 17.87 0.25 42.92 0.24 0.18 100.95  Phen. SK  80.6 19.4  0.994 0.001 0.383 0.004 1.615 0.004 0.003 3.005  39.41 0.03 18.16 0.19 42.92 0.16 0.16 101.03  Phen. E  MD109-44a B  80.5 19.5  0.992 0.000 0.384 0.004 1.616 0.006 0.003 3.007  39.11 0.01 18.11 0.19 42.74 0.23 0.16 100.59  Phen. S  78.2 21.8  0.987 0.000 0.430 0.005 1.575 0.008 0.003 3.011  38.70 0.02 20.14 0.23 41.42 0.31 0.17 101.10  Phen. SK  b Grain c  79.9 20.1  0.988 0.001 0.398 0.005 1.606 0.006 0.003 3.010  38.69 0.04 18.64 0.25 42.17 0.22 0.16 100.28  Phen. E  Rock type: PB, picritic basalt; B, basalt; TB, trachybasalt. size: Phen. and Xen., pheno/xenocrysts > 0.7 mm; Mic., microphenocrysts > 0.3 mm; Glom., glomerocryst > 0.7 mm. Phenocryst morphology: E, euhedral; S, subhedral; A, anhedral; SK, skeletal; R, rounded.  MD109-33 TB  Sample Rock typea  D5  82.7 17.3  0.990 0.000 0.344 0.004 1.661 0.004 0.006 3.009  39.06 0.00 16.22 0.19 43.98 0.16 0.30 99.96  Phen. SK  80.7 19.3  0.987 0.001 0.384 0.004 1.625 0.006 0.003 3.011  38.35 0.05 17.85 0.20 42.35 0.21 0.15 99.20  Phen. SK  MD109-44b B  81.8 18.2  0.983 0.000 0.363 0.004 1.655 0.006 0.005 3.017  38.71 0.00 17.10 0.18 43.71 0.23 0.26 100.22  Phen. SK  81.3 18.7  0.985 0.000 0.370 0.004 1.640 0.007 0.005 3.013  38.64 0.01 17.35 0.17 43.15 0.25 0.24 99.90  Phen. E  82.8 17.2  0.976 0.001 0.346 0.005 1.685 0.004 0.006 3.023  38.55 0.03 16.33 0.24 44.62 0.15 0.29 100.22  Phen. E  G  3  a  D4  Dredge  Table A1. Representative Electronprobe Analyses of Olivine From Kerimis MD109 Basalts [The full Appendix A1 is available in the HTML version of the article at http://www.g-cubed.org.]  Geochemistry Geophysics Geosystems  weis et al.: trace of the kerguelen mantle plume 10.1029/2001GC000251  22 of 27  Phen. E  core  51.24 1.29 2.60 0.04 7.21 0.14 16.06 21.55 0.31 100.44  1.882 0.113 0.000 0.001 0.062 0.036 0.879 0.159 0.005 0.848 0.022 4.006  49.6 9.0 41.4 84.7  Typeb Kindc  Zone  Oxides (wt. %) SiO2 TiO2 Al2O3 Cr2O3 FeOtot MnO MgO CaO Na2O Total  Cations ( pfu.) Si AlIV AlVI Cr Fe3+ Ti Mg Fe2+ Mn Ca Na Sum  End members (%) En Fs Wo Mg# 54.5 6.8 38.7 88.9  1.882 0.104 0.000 0.022 0.062 0.023 0.965 0.121 0.004 0.807 0.025 4.014  51.33 0.82 2.40 0.75 5.96 0.14 17.66 20.55 0.35 99.96  53.6 7.7 38.7 87.5  1.901 0.099 0.013 0.023 0.037 0.025 0.943 0.135 0.000 0.799 0.024 4.000  52.19 0.90 2.60 0.81 5.67 0.01 17.36 20.47 0.34 100.37  rim  51.8 7.2 41.1 87.8  1.850 0.150 0.002 0.007 0.075 0.044 0.866 0.120 0.004 0.860 0.022 4.000  49.55 1.58 3.47 0.23 6.23 0.11 15.56 21.51 0.31 98.55  core  Phen. E  49.3 8.9 41.9 84.8  1.905 0.075 0.000 0.013 0.013 0.031 0.958 0.172 0.002 0.839 0.012 4.020  50.99 1.09 1.71 0.44 5.92 0.05 17.20 20.96 0.17 98.53  rim  47.8 7.7 44.5 86.1  1.907 0.076 0.000 0.005 0.037 0.026 0.903 0.146 0.005 0.895 0.018 4.017  51.03 0.93 1.72 0.17 5.84 0.14 16.21 22.36 0.25 98.65  core  Phen. E  51.3 7.2 41.5 87.6  1.897 0.103 0.001 0.013 0.044 0.032 0.911 0.129 0.002 0.852 0.017 4.000  51.22 1.14 2.38 0.43 5.57 0.07 16.50 21.47 0.24 99.02  rim  MD109-87 PB  D6  PB, picritic basalt; B, basalt; TB, trachybasalt. Phen., phenocrysts > 0.7 mm; Mic., microphenocrysts > 0.3 mm; Grain, groundmass < 0.3 mm. Phenocryst morphology: E, euhedral; S, subhedral; A, anhedral.  51.9 10.0 38.1 83.9  1.726 0.274 0.002 0.004 0.127 0.083 0.762 0.146 0.006 0.844 0.026 4.000  46.23 2.97 6.26 0.15 8.75 0.19 13.68 21.09 0.36 99.68  core  Xen. S  MD109-85 PB  D6  51.4 6.9 41.7 88.1  1.917 0.083 0.007 0.021 0.029 0.022 0.922 0.124 0.004 0.853 0.018 4.000  52.13 0.81 2.07 0.72 4.97 0.13 16.82 21.65 0.26 99.54  core  Phen. E  51.4 7.5 41.1 87.3  1.876 0.119 0.000 0.015 0.048 0.035 0.907 0.132 0.003 0.856 0.014 4.005  50.63 1.26 2.73 0.51 5.82 0.08 16.42 21.57 0.20 99.24  rim  51.6 6.5 41.9 88.724  1.895 0.105 0.000 0.027 0.043 0.027 0.897 0.114 0.003 0.871 0.018 4.000  51.53 0.96 2.44 0.92 5.11 0.09 16.37 22.11 0.26 99.78  core  Phen. R  51.1 7.3 41.6 87.476  1.899 0.101 0.006 0.024 0.037 0.028 0.894 0.128 0.004 0.857 0.021 4.000  52.19 1.01 2.49 0.84 5.44 0.13 16.49 21.98 0.30 100.88  rim  MD109-88 PB  D6  51.0 5.7 43.3 89.9  1.909 0.082 0.000 0.019 0.032 0.025 0.934 0.105 0.002 0.880 0.019 4.008  52.20 0.90 1.91 0.67 4.50 0.06 17.14 22.45 0.27 100.09  core  Phen. E  50.8 7.8 41.5 86.7  1.893 0.105 0.000 0.007 0.055 0.034 0.899 0.138 0.004 0.841 0.025 4.002  51.68 1.24 2.44 0.25 6.28 0.14 16.47 21.44 0.35 100.30  mid  MD109-89 PB  D6  50.5 7.1 42.4 87.7  1.929 0.071 0.008 0.027 0.007 0.022 0.923 0.129 0.001 0.868 0.015 4.000  52.40 0.80 1.82 0.93 4.43 0.03 16.82 22.02 0.20 99.44  core  Phen. E  50.6 7.3 42.1 87.4  1.927 0.073 0.009 0.023 0.011 0.025 0.924 0.133 0.002 0.855 0.019 4.000  52.31 0.89 1.88 0.79 4.66 0.07 16.84 21.67 0.26 99.35  rim  G  3  a Rock type: b Grain size: c  MD109-37 B  Sample Rock typea  Rim  D4  Unit  Table A2. Representative Electronprobe Analyses of Clinopyroxene From Kerimis MD109 Basalts Geochemistry Geophysics Geosystems  weis et al.: trace of the kerguelen mantle plume 10.1029/2001GC000251  23 of 27  3  Geochemistry Geophysics Geosystems  G  weis et al.: trace of the kerguelen mantle plume 10.1029/2001GC000251  Table A3. Petrographic Characteristics of Samples From the Kerimis MD 109 Dredge Sites Rock Namea Sample MD109-33 MD109-37 MD109-42 MD109-44a MD109-44b MD109-85 MD109-87 MD109-88 MD109-89  Phenocrysts,  b  vol %  Phenocryst Sizes, mm  Dredge  PB  B  TB  Vesicles, vol %  Ol  Cpx  Plag  Ox  Total  Ol  Cpx  Plag  4 4 5 5 5 6 6 6 6  – – – – – X X X X  – X X X X – – – –  X – – – – – – – –  8.3 – – – – – – – –  3.8 2.3 6.9 7.8 7.3 34.2 28.0 30.4 35.3  0.9 3.7 – – – 1.0 0.4 1.3 0.8  8.7 2.7 – – – – – – –  – – – – – 0.6 0.1 0.1 –  13.3 8.7 6.9 7.8 7.3 35.7 28.4 31.9 36.1  1–2 0.7 – 3 0.5 – 2 0.5 – 2 0.5 – 2 2–5 2–5 2–5 2–5  1–2 1–4 – – – $2 $2 $2 $2  1–5 0.7 – 1.5 – – – – – – –  a Rock b  names: picritic basalt (PB), basalt (B), and trachybasalt (TB). Phenocryst abundances were determined by point counting with 1500 points/slide.  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 McDonald Islands, and at submarine structures between these islands. Although there has been coeval volcanism, there appears to be a general trend for recent volcanism to migrate in a southeasterly direction from the Northern Kerguelen Plateau to the Kerguelen Archipelago and finally to Heard and McDonald 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 Universite´ Louis Pasteur (Ecole et Observatoire des Sciences 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. Albare`de 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. 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