Petrogenesis of the Flood Basalts Forming the Northern Kerguelen Archipelago: Implications for the Kerguelen Plume

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1 JOURNAL OF PETROLOGY VOLUME 39 NUMBER 4 PAGES Petrogenesis of the Flood Basalts Forming the Northern Kerguelen Archipelago: Implications for the Kerguelen Plume HUAI-JEN YANG 1, FREDERICK A. FREY 1, DOMINIQUE WEIS 2, ANDRE GIRET 3, DOUG PYLE 4,5 AND GILBERT MICHON 3 1 DEPARTMENT OF EARTH, ATMOSPHERIC AND PLANETARY SCIENCES, MASSACHUSETTS INSTITUTE OF TECHNOLOGY, CAMBRIDGE, MA 02139, USA 2 DÉPARTEMENT DES SCIENCES DE LA TERRE ET DE L ENVIRONNEMENT, UNIVERSITÉ LIBRE DE BRUXELLES, BRUXELLES, B-1050, BELGIUM 3 LABORATOIRE DE GÉOLOGIE PÉTROLOGIE, UNIVERSITÉ JEAN MONNET, CNRS UMR 6524, SAINT-ÉTIENNE, FRANCE 4 MARINE GEOLOGY AND GEOPHYSICS, WOODS HOLE OCEANOGRAPHIC INSTITUTION, WOODS HOLE, MA 02543, USA 5 DEPARTMENT OF GEOLOGY, SAN DIEGO STATE UNIVERSITY, SAN DIEGO, CA 92182, USA RECEIVED APRIL 5, 1997; REVISED TYPESCRIPT ACCEPTED NOVEMBER 28, 1997 The thick, >20 km, crust of the Kerguelen Archipelago formed as Ba/Th and positive Eu anomalies. However, plagioclase phenocrysts the tectonic setting of the Kerguelen Plume changed from an oceanic are absent in most of these lavas; therefore a plagioclase-rich ridge-centered location at 43 Ma to its present location beneath the component is required in their source. A plausible interpretation is Antarctic plate. The uppermost crust is dominantly flood basalt that plagioclase-rich cumulates formed in the lower oceanic crust with a thickness of up to 10 km. Inverse isochron 40 Ar/ 39 Ar ages when the Southeast Indian Ridge was coincident with the plume for upper and lower lavas in a 630 m section of basalt flows from at ~43 Ma; subsequently these cumulates were melted by the plume Mont Bureau are 30 4 and 29 0 Ma; Re Os isotopic systematics and the melts contributed to a small proportion of the flood basalts. are consistent with this age. Most of the lavas in two stratigraphic Previously it was proposed that as the distance between the archipelago sections ( Mont Bureau and Mont Rabouillère) from the northern and Southeast Indian Ridge increased, there was a systematic part of the archipelago have Sr, Nd and Pb isotopic characteristics decrease in the proportion of mid-ocean ridge basalt ( MORB)- similar to the youngest (Upper Miocene to Pleistocene) lavas erupted related component in the source of archipelago lavas. The new data 87 in the southeast part of the archipelago, i.e. initial Sr/ 86 Sr show that: (1) there is no systematic temporal trend in the proportion >0 7050, 143 Nd/ 144 Nd < and 206 Pb/ 204 Pb <18 3. The of MORB to plume source components and (2) the MORB dominance of this isotopic signature in archipelago lavas for 30 my component was derived from cumulate rocks in the oceanic crust and its presence in ~40 Ma gabbros is consistent with the previous rather than as melts derived directly from the asthenosphere. Finally, interpretation that these are isotopic characteristics of the Kerguelen there is no evidence of a continental lithosphere component in the Plume. Although this component occurs in high (>10%) MgO source of Kerguelen Archipelago lavas. alkalic lavas in the Southeast Province of the archipelago, in these northern sections it is confined to transitional lavas with <6% MgO. A low plume flux and extensive crustal processing are inferred. In contrast to the plume-derived lavas, ~15% of the flood basalts in these sections have lower initial 87 Sr/ 86 Sr (to ), higher 143 Nd/ 144 Nd (to ), and they have some compositional KEY WORDS: Kerguelen Plume; basalt petrogenesis; ocean island basalt; characteristics of plagioclase-rich cumulates, i.e. high Sr/Nd and buffered differentiation; plume lithosphere interaction Corresponding author. Fax: fafrey@mit.edu Oxford University Press 1998

2 JOURNAL OF PETROLOGY VOLUME 39 NUMBER 4 APRIL 1998 INTRODUCTION Ma for these flood basalts. White & Hofmann (1982) and W. M. White (unpublished data, 1985) ob- The 8500 km 2 Kerguelen Archipelago is located on the tained Sr, Nd and Pb isotopic data for four of the Mont submarine Kerguelen Plateau within the Antarctic plate Bureau lavas studied by Watkins et al. (1974). These in the southern Indian Ocean (Fig. 1). The Kerguelen results show large isotopic variations within 100 m of the Plateau and its conjugate Broken Ridge are a very section, and three of the lavas have the lowest 87 Sr/ 86 Sr large igneous province that is interpreted to represent 143 and highest Nd/ 144 Nd found in archipelago lavas. voluminous Cretaceous volcanism associated with arrival Gautier et al. (1990) studied flood basalts and younger of the Kerguelen Plume below a newly formed Indian lavas from different localities in the Kerguelen Ar- Ocean (Duncan & Storey, 1992). As a result of the rapid chipelago (2 5 samples from each locality) and Weis et northward movement of the Indian plate over the plume, al. (1993) studied a group (six) of Lower Miocene flood a 5000 km long, ~82 38 Ma hotspot track formed basalts from the Southeast Province. These workers conthe Ninetyeast Ridge. Subsequently, the newly formed cluded that the rock types, age and Sr and Nd isotopic Southeast Indian Ridge (SEIR) intersected the plume ratios of Kerguelen Archipelago lavas are correlated, position, and as this spreading center moved northward with the lowest 87 Sr/ 86 Sr ratios in ~26 Ma lavas from the plume became detached from the hotspot track and Mont Bureau on Foch Island (White & Hofmann, 1982) was isolated beneath the Antarctic plate. From ~39 to 0 1 and the highest 87 Sr/ 86 Sr ratios in the relatively young, Ma, the plume constructed the Kerguelen Archipelago on highly alkaline lavas. The relatively high 87 Sr/ 86 Sr and the nearly stationary, relative to hotspots, Antarctic plate. low 143 Nd/ 144 Nd ratios in young alkalic lavas were con- Thus, there is a very long term, ~115 my, record of firmed by studies of lavas from the Southeast Province volcanism attributed to the Kerguelen Plume, and the and Mont Ross (Weis et al., 1993, 1998). Gautier et Kerguelen Archipelago reflects the last 39 my of this al. (1990) postulated an SEIR mid-ocean ridge basalt volcanic activity (Weis et al., 1992). (MORB) Kerguelen Plume interaction model to explain The occurrence of plutonic rocks, ranging from gabbro the correlation of isotopic ratios with age. Storey et al. to syenite and granite, in the archipelago is unusual for (1988) proposed a similar model, but they also suggested an oceanic island, and these plutonic rocks have been that lavas with low 87 Sr/ 86 Sr ratios may contain a comstudied in detail (e.g. Dosso et al., 1979; Giret & Lameyre, ponent derived from the Kerguelen Plateau lithosphere. 1983; Weis & Giret, 1994). They range widely in age Because some of the plutonic rocks in the archipelago and size, but the oldest known rocks in the archipelago are similar to those occurring on continents, initial disare gabbros, such as the Val Gabbro in the Southeast cussions on the origin of the Kerguelen Archipelago Province (Fig. 1). The oldest K Ar age, 39 ± 3Maof focused on a continental vs oceanic origin (e.g. Watkins the Val Gabbro (Giret & Lameyre, 1983), is slightly less et al., 1974; Lameyre et al., 1976). However, the Sr, Nd than the youngest magnetic anomaly in the surrounding and Pb isotopic ratios of these plutonic complexes are oceanic basement, that is, anomaly 18, ~43 Ma (Munschy similar to those of the lavas which were derived from et al., 1994). More than 85% of the archipelago is the Kerguelen Plume. None of the plutonic rocks have composed of nearly horizontal flood basalt lavas with the extreme isotopic ratios typical of old continental no obvious vents and few dikes (Fig. 1). Because of a crust; consequently, most recent studies have concluded regional dip of 2 5 to the southeast and extensive glacial that a continental origin is unlikely (e.g. Dosso et al., erosion, the surficial lavas decrease in age from northwest 1979; Weis & Giret, 1994). However, recent Os and Pb to southeast (Giret et al., 1992). Locally, the dips of the isotopic data for a few mantle xenoliths in alkalic lavas lavas are affected by nearby intrusions. Especially in the from the Southeast Province show continental affinities; Southeast Province, younger volcanic features have been that is, relatively low 187 Os/ 188 Os (Hassler & Shimizu, constructed on the flood basalts; for example, a ) and high 207 Pb/ 204 Pb (Mattielli et al., 1996). Also, Ma Upper Miocene Series ranging from basanite to lavas dredged from the east end of Broken Ridge (dredge phonolite and the very young Ross volcano, <1 Ma, 8) and recovered from drill Site 738 at the southern end which ranges from trachybasalt to trachyte. These rel- of the Kerguelen Plateau (Fig. 1) have Sr, Nd and Pb atively young lavas have been studied in considerable isotopic ratios and trace element ratios showing the detail (Weis et al., 1993, 1998). effects of a continental component (Mahoney et al., 1995). In contrast, there have been only survey studies of Therefore, a continental component, perhaps minor in the flood basalts which form the major portion of the volume and sporadic in location, occurs in this igneous archipelago (Fig. 1). Watkins et al. (1974) analyzed major province. and some trace elements for sequences of lava flows on In this study we focus on the geochemical charthe Courbet peninsula and lavas from the lower 150 m acteristics, primarily major and trace element comof Mont Bureau on Foch Island (Fig. 1). An important positions and isotopic ratios of Sr, Nd and Pb, of the result was their K/Ar dating, which indicated ages of flood basalts from stratigraphic sections in the northern 712

3 YANG et al. KERGUELEN ARCHIPELAGO FLOOD BASALTS Fig. 1. Map of the Kerguelen Archipelago showing the major geologic units and the location of Mont Bureau on Foch Island and Mont Rabouillère on the Joffre Peninsula. Inset is a map of the eastern Indian Ocean showing the volcanic structures related to the Kerguelen Plume; i.e. the large igneous province formed by the now separated Kerguelen Plateau and Broken Ridge; the hotspot track formed by the Ninetyeast Ridge, and the Ma Kerguelen Archipelago on the northern Kerguelen Plateau (Χ, basement sampling sites). these variations occur? Are variations in major and trace element abundances and isotopic ratios correlated? What is the relative importance of different source compositions such as asthenosphere, plateau lithosphere, continental lithosphere and plume, and how did the relative proportions of these components change with time? Answers to these questions will bear on first-order problems such as: What are the geochemical characteristics of the Ker- guelen Plume and how have they changed with time? How did the proportions of source components change in plume-related volcanism as the plume location changed part of the archipelago, specifically the 630 m section at Mont Bureau on Foch Island and the 400 m section at Mont Rabouillère on the Joffre Peninsula (Fig. 1). Based on inverse isochrons derived from 40 Ar/ 39 Ar data, the age range exposed in the Mont Bureau section ranges from 30 5 ± 0 4 to 29 0 ± 0 5 Ma and a lower flow in the Rabouillère section is 29 0 ± 0 2 Ma (Nicolaysen et al., 1996). Our goal is to understand the recent history of the Kerguelen Plume by addressing the following questions. How did the geochemical characteristics of the magmas vary with time? Over what time scale did 713

4 JOURNAL OF PETROLOGY VOLUME 39 NUMBER 4 APRIL 1998 Fig. 2. Locations of studied samples in the Mont Bureau and Mont Rabouillère stratigraphic sections. Arrows indicate locations of Mont Bureau samples studied by Watkins et al. (1974) and analyzed for isotopic ratios by White & Hofmann (1982). (Table 1). Abundances of major elements and V, Cr, Ni, Zn, Ga, Rb, Sr, Y, Zr, Nb, Ba, La and Ce were determined by X-ray fluorescence (XRF) at the Uni- versity of Massachusetts (Rhodes, 1983). Abundances of Sc, Co, Hf, Ta, Th and rare earth elements (REE) were determined by instrumental neutron activation analysis (INAA) at MIT (Ila & Frey, 1984). The accuracy and precision of these analytical techniques were discussed by Frey et al. (1990). Thirty samples, 20 from Mont Bureau and 10 from Mont Rabouillère, were acid leached from a small newly opened oceanic basin to a ridgecentered location to its present intraplate oceanic location? ANALYTICAL PROCEDURES Twenty-eight lava flows and an ankaramite dike from Mont Bureau and 27 lavas from Mont Rabouillère (Fig. 2) were analyzed for major and trace element abundances 714

5 YANG et al. KERGUELEN ARCHIPELAGO FLOOD BASALTS Table 1: Abundances of major (wt %) and trace elements (ppm) of Mont Bureau and Mont Rabouillère lavas Mt Bureau GM92-59 GM92-58 GM92-57 GM92-56 GM92-55 GM92-54 GM92-53 GM92-52 GM92-51 GM92-50 Height (m) SiO TiO Al 2 O Fe 2 O MnO MgO CaO Na 2 O K 2 O P 2 O Total Rb Sr Ba Sc 27 8± ± ± ± ± V Cr 155± ± ± ±1 655±11 92 Co 40 7± ± ± ± ± Ni Zn Ga Y Zr Nb Hf 3 62± ± ± ± ± Ta 0 92± ± ± ± ± Th 1 70± ± ± ± ± U Pb La 15 1± ± ± ± ± Ce 35 0± ± ± ± ± Nd 20 1± ± ± ± ± Sm 5 03± ± ± ± ± Eu 1 73± ± ± ± ± Tb 0 81± ± ± ± ± Yb 1 76± ± ± ± ± Lu 0 26± ± ± ± ±

6 JOURNAL OF PETROLOGY VOLUME 39 NUMBER 4 APRIL 1998 Table 1: continued Mt Bureau GM92-48 GM92-46 GM92-45 GM92-44 GM92-43 GM92-42 GM92-41 GM92-40 GM92-39 GM92-38 GM92-37 Height (m) SiO TiO Al 2 O Fe 2 O MnO MgO CaO Na 2 O K 2 O P 2 O Total Rb Sr Ba Sc 29 5± ± ± ±0 2 V Cr 271±1 290± ± ±1 Co 51 4± ± ± ±0 1 Ni Zn Ga Y Zr Nb Hf 2 52± ± ± ±0 11 Ta 0 57± ± ± ±0 08 Th 0 65± ± ± ±0 08 U Pb La 8 80± ± ± ±0 1 Ce 21 2± ± ± ±1 6 Nd 12 9± ± ± ±3 8 Sm 3 33± ± ± ±0 11 Eu 1 24± ± ± ±0 036 Tb 0 63± ± ± ±0 04 Yb 1 88± ± ± ±0 07 Lu 0 31± ± ± ±

7 YANG et al. KERGUELEN ARCHIPELAGO FLOOD BASALTS Mt Bureau Mt Rabouillère GM92-36 GM92-35 GM92-34 GM92-33 GM92-32 GM92-31 GM92-30 GM92-29 GM GM GM Height (m) SiO TiO Al 2 O Fe 2 O MnO MgO CaO Na 2 O K 2 O P 2 O Total Rb Sr Ba Sc ± V Cr ± Co ± Ni Zn Ga Y Zr Nb Hf ± Ta ± Th ± U Pb La ± Ce ± Nd ± Sm ± Eu ± Tb ± Yb ± Lu ±

8 JOURNAL OF PETROLOGY VOLUME 39 NUMBER 4 APRIL 1998 Table 1: continued Mt Rabouillère GM GM GM GM GM GM GM GM GM GM GM GM Height (m) SiO TiO Fe 2 O MnO MgO CaO Na 2 O K 2 O P 2 O Total Rb Sr Ba Sc V Cr Co Ni Zn Ga Y Zr Nb Hf Ta Th U Pb La Ce Nd Sm Eu Tb Yb Lu

9 YANG et al. KERGUELEN ARCHIPELAGO FLOOD BASALTS Mt Rabouillère GM GM GM GM GM GM GM GM GM GM GM GM Height (m) SiO TiO Al 2 O Fe 2 O MnO MgO CaO Na 2 O K 2 O P 2 O Total Rb Sr Ba Sc V Cr Co Ni Zn Ga Y Zr Nb Hf Ta Th U Pb La Ce Nd Sm Eu Tb Yb Lu Major oxides and the trace elements Rb, Sr, Ba, V, Ni, Zn, Ga, Y, Zr and Nb determined by duplicate analyses using X-ray fluorescence (XRF). Abundances of U and Pb determined by isotope dilution mass spectrometry. Abundances of Sc, Cr, Hf, Ta, Th and rare earth elements La to Lu determined by instrumental neutron activation analysis (INAA). The mean is indicated for 10 duplicate INAA analyses (see text for discussion of accuracy and precision). Some samples were not analyzed by INAA (those lacking Sc, Co and complete REE data); in these cases, Cr, La and Ce data are by XRF. 719

10 JOURNAL OF PETROLOGY VOLUME 39 NUMBER 4 APRIL 1998 following the method of Weis & Frey (1991) before being Five samples (GM92-29, -30, -41, -43 and -44) have analyzed for Sr, Nd and Pb isotopic compositions and ~10% phenocrysts and microphenocrysts in a ground- abundances of U and Pb at the Brussels Free University mass similar to that of the sparsely phyric samples; (ULB). The analytical procedures and precision were except for sample GM92-29, plagioclase is the dominant described by Weis et al. (1993). Four samples were ana- phenocryst and microphenocryst phase (Table 2) samples lyzed for Os isotopic ratios following standard NiS fire- (GM92-46, -48, -50, -51, -52, -55, -57 and -59) contain essay preconcentration techniques (Hoffman et al., 1978; >15% phenocrysts and microphenocrysts (Table 2). Hauri & Hart, 1993; Hauri et al., 1996) with isotopic Sample GM92-59 has ~10% plagioclase phenocrysts. characterization by negative thermal ionization mass Plagioclase (~6%) accompanied by olivine is also the spectrometry (N-TIMS) (Creaser et al., 1991; Volkening dominant phenocryst in three samples (GM92-48, -50 et al., 1991) at the Woods Hole Oceanographic Institution. and -52) whereas olivine accompanied by clinopyroxene is Approximately 20 g of each sample was combined 1:1 the main phenocryst in the other four samples; plagioclase with a flux mixture having a blank contribution of 4 8 phenocrysts are rare in these samples. An ankaramite ng Os and 187 Os/ 186 Os of Rhenium concentrations dike (GM92-53) contains ~16% clinopyroxene and ~5% were determined on separate sample powder splits by olivine phenocrysts. isotope dilution inductively coupled plasma mass spectrometry (ICP-MS) at Oregon State University. Rhenium Mont Rabouillère was purified from ~1 g acid-dissolved samples aliquots Five lavas (GM92-153, -147, -139, -138 and -135) are by standard ion-exchange techniques which yield a total aphyric, and nine (GM92-133, -134, -136, -140, -142, - procedural blank contribution of ~2 pg Re (Hauri & 143, -144, -148, and -152), are sparsely plagioclase phyric Hart, 1993). (Ζ2%). Three other sparsely phyric samples (GM92-151, -137 and -128) have olivine and plagioclase phenocrysts. Similar to five of the Mont Bureau lavas, four of these Mont Rabouillère lavas (GM92-135, -137, -151 and -152) have large amounts, ~20%, of opaque minerals. Ten samples contain >2% phenocrysts (Table 2). All RESULTS of these samples contain olivine and plagioclase phenocrysts or microphenocrysts, and four also have small Petrography amounts of clinopyroxene phenocrysts. Plagioclase is the The textures of Mont Bureau and Mont Rabouillère main phenocryst or microphenocryst phase in seven samples are similar, ranging from aphyric to moderately samples (GM92-127, -130, -131, -132, -145, -146 and - phyric typically with groundmasses (<0 1 mm) of fine- 150) with the largest amount (17 2%) in GM grained plagioclase, clinopyroxene and opaques plus oli- Olivine is the dominant phenocryst in three samples vine in some samples. Six samples (GM92-46, -51, -55, - (GM92-129, -141 and -149). 129, -145 and -146) have coarser groundmasses (<0 3 mm). Most plagioclase phenocrysts (>0 7 mm) are slightly resorbed and some are zoned. In general, olivine and clinopyroxene phenocrysts or microphenocrysts ( Major element abundances mm) are subhedral and many olivine phenocrysts or Most of the samples have basaltic compositions; the two microphenocrysts are altered. All Mont Bureau and most evolved lavas, GM and GM with Mont Rabouillere samples contain secondary minerals, ~59% SiO 2 and 2 5% MgO, are in the Mont Rabouillère dominantly zeolites, formed after eruption (Giret et al., section (Table 1). If FeO is assumed to be 85% of 1992). total iron, all of the lavas are hypersthene normative. Previously, lavas from Mont Bureau and the Loranchet Mont Bureau Peninsula in the northwest of the archipelago (Fig. 1) were classed as tholeiitic lavas (Watkins et al., 1974; Storey Fifteen of the 29 Mont Bureau samples are sparsely et al., 1988). However, based on a silica total alkalis phyric with <2% plagioclase phenocrysts in a groundmass diagram, all of these lavas and our new analyses of Mont of subequal amounts of plagioclase laths and fine-grained Bureau and Mont Rabouillère lavas lie close to the clinopyroxene, opaques and olivine (<5%). Some of dividing line between alkalic and tholeiitic basalts (Fig. these samples also contain sparse and altered olivine 3). Therefore, we classify these flood basalts as transmicrophenocrysts, and five samples (GM92-34, -35, - itional in composition. 36, -40 and -42) contain abundant, ~20%, opaque min- Abundance of MgO does not vary systematically with erals. Two samples (GM92-40 and -42) also contain fine- stratigraphic height, but in both sections most of the grained apatite. lavas have 4 5% MgO (Fig. 4a and b). At Mont Bureau 720

11 YANG et al. KERGUELEN ARCHIPELAGO FLOOD BASALTS Table 2: Modes for Mont Bureau and Mont Rabouillère samples Sample Group olivine plagioclase clinopyroxene opaque AP GM Ph Mph Ph Mph Ph Mph Mph Mont Bureau GM92-29 P GM92-30 P 2 4 (1 1) 0 9 (0 3) GM92-41 P 1 5 (1 5) 0 2 (0 1) GM92-43 P 0 2 (0 2) GM92-44 P 0 3 (0 2) GM92-46 D 14 3 (3 3) 7 9 (0 8) GM92-48 D (1 6) GM92-50 D 0 9 (0 2) GM92-51 D 9 8 (1 1) 7 4 (1 2) GM92-52 D 2 6 (1 0) GM92-53 dike 4 9 (3 5) 3 1 (1 9) GM92-55 D 7 5 (2 6) 14 4 (2 0) GM92-57 D GM92-59 P 1 1 (1 1) Mont Rabouillère GM P GM D 5 9 (0 1) GM P 1 9 (1 1) 0 7 (0 4) GM P GM P (0 1) GM P 4 3 (4 3) 0 3 (0 3) GM D 0 3 (0 3) 1 7 (0 5) GM D 0 2 (0 2) GM P 5 5 (5 0) 0 7 (0 3) GN P 0 6 (0 3) 0 6 (0 2) Modes are based on counts per sample. See text for grouping samples. AP indicates alteration products (>0 1 mm) whose original minerals cannot be identified. GM indicates groundmass. Except for GM92-46, GM92-48, GM92-51, GM92-55, GM92-129, GM92-145, and GM whose groundmasses are coarser, <0 3 mm, other samples have groundmasses <0 1 mm. Ph and Mph indicate phenocryst (>0 7 mm) and microphenocryst ( mm for seven samples indicated in previous footnote, and mm for other samples), respectively. Number in parentheses is the proportion of altered olivine. lavas from the upper 330 m are surprisingly uniform in composition with MgO ranging only from 5 12 to 3 37%, whereas those from the lower 300 m range in MgO content from to 4 47% (Fig. 4a). Lavas from both sections overlap in MgO variation plots, and the evolved lavas with <6% MgO form well-defined trends; for example, as MgO content decreases the abundance of CaO decreases whereas Al 2 O 3 /CaO and abundances of SiO 2,Na 2 O, K 2 O and P 2 O 5 increase. The abundance of TiO 2 peaks at 4 4% MgO (Fig. 5). Trace element abundances First series transition metals All samples define positive correlations of Ni and Cr abundances with MgO content (Fig. 6), but like MgO content, Ni abundances were apparently buffered; e.g. most lavas have ppm Ni (Fig. 4b). As in MgO major oxide plots, lavas with MgO < 6% define the most coherent MgO Sc variation, with Sc being depleted in the most evolved lavas (Fig. 6). Lavas in this group also 721

12 JOURNAL OF PETROLOGY VOLUME 39 NUMBER 4 APRIL 1998 Fig. 3. SiO 2 vs Na 2 O + K 2 O (all in wt %). Ferrous iron calculated as 85% of total iron. The tholeiitic alkalic dividing line is from Macdonald & Katsura (1964). In (a) data points are for Mont Bureau and Mont Rabouillère samples; shown for comparison are data fields for an Upper Miocene basanite to phonolite suite (Weis et al., 1993), the Pleistocene alkalic basalt to trachyte suite forming Mont Ross (Weis et al., 1998), a Lower Miocene alkalic basalt to trachyte suite from the Southeast flood basalt province (Weis et al., 1993), and samples from the Loranchet Peninsula in the northwest (Storey et al., 1988). In (b) the data points for Mont Bureau samples are divided into the groups defined in Fig. 14 (also see text). The continuous lines connect measured data for altered samples and adjusted compositions whose K 2 O contents were increased so that K/Nb ratio was 375. define an inflected V MgO trend peaking at ~4% MgO (Fig. 6); i.e. similar to the TiO 2 MgO trend (Fig. 5). Incompatible elements Abundances of immobile incompatible elements such as Th, high field strength elements and REE are highly correlated in these flood basalts; even Ba, which is often mobile during late-stage alteration, defines a good correlation with Nb (Fig. 7). In contrast, Sr contents are nearly constant in lavas with % MgO, i.e. Sr ranges only from 321 to 382 ppm over a nearly two-fold increase in Nb with the lowest Sr contents in the two highly evolved (~2 5% MgO) lavas from Mont Rabouillère (Fig. 7). Sample GM92-57 from Mont Bureau has an anomalously high Sr content (Fig. 7) and Al 2 O 3 / CaO ratio (Fig. 5) possibly indicating plagioclase accumulation, but this sample does not contain plagioclase phenocrysts (Table 2). When normalized to primitive mantle the most con- spicuous feature of the low-mgo (<5 8%) lavas is relative depletion in Sr (Fig. 8a). Despite this relative depletion in Sr, only the two most evolved lavas (GM and 722

13 YANG et al. KERGUELEN ARCHIPELAGO FLOOD BASALTS Fig. 4. (a) MgO (wt %) and initial 87 Sr/ 86 Sr variations with stratigraphic height in meters. Symbol size is larger than the 2σ uncertainty. In the Mont Bureau section 40 Ar/ 39 Ar ages range from 30 4 Ma (lower flow) to 29 0 Ma (upper flow) (Nicolaysen et al., 1996). (b) MgO and Ni histograms for Mont Bureau and Mont Rabouillère sections from Mont Rabouillère) are relatively depleted The effects of post-magmatic alteration in Eu (Fig. 8b). In contrast, the lavas with relatively high (zeolitization) on chemical compositions MgO contents (>6 1%) have (Sr/Nd) N = (N indicates normalized to primitive mantle) and most Many of the flood basalts contain zeolites. Giret et al. (eight of 10) have small but obvious positive Eu anomalies (1992) defined a vertical zoneography for flood basalts (Fig. 8b). These high-mgo lavas have relatively low in the archipelago by identifying five zeolite zones contents of incompatible elements, and they have conuppermost representing different temperature ranges. The spicuous relative deficiencies in Th (Fig. 8a). An obvious zone contains phillipsite and chabazite, which complexity is the crossing REE pattern of GM92-46 (Fig. are stable at C, and the lowest zone contains 8b). laumontite and chlorite, which formed at C. 723

14 JOURNAL OF PETROLOGY VOLUME 39 NUMBER 4 APRIL 1998 Fig. 5. Abundances of major oxides and Al 2 O 3 /CaO vs MgO content (all in wt %). This zoneography indicates a geothermal gradient of ( C) and 4 ( C). Giret et al. (1992) inferred C/km, comparable with that, C/km, that zeolitization was associated with Ma inferred from zeolite zones in the Tertiary basalts of plutonism. Iceland (Walker, 1960). The lava sections at Mont Bureau What compositional changes were caused by formation and Mont Rabouillère contain zeolite zones 3 of zeolites and other alteration phases? Abundances of 724

15 YANG et al. KERGUELEN ARCHIPELAGO FLOOD BASALTS Fig. 6. Abundance of MgO (%) vs Sc, V, Ni and Cr contents (ppm). Fig. 7. Abundances of Ba, Th, La, and Sr vs Nb abundance (all in ppm). 725

16 JOURNAL OF PETROLOGY VOLUME 39 NUMBER 4 APRIL 1998 Fig. 8. four normally incompatible elements, K, Rb, U and Pb, are poorly correlated with relatively immobile incompatible elements, such as Nb (Fig. 9). Because it is well established that these four elements are mobile during post-magmatic alteration of ocean island basalts (OIB; e.g. Wood et al., 1976; Kennedy et al., 1991), deviations from the general trends in Fig. 9 are interpreted in terms of loss or gain of these elements. Based on deviations from the K 2 O and Rb vs Nb trends, 20 of the 56 flood basalt samples have lost K 2 O and Rb (Fig. 9). The most significant effect was Rb loss. Like young ocean island basalt, most of the archipelago lavas have Ba/ Rb < 20 (Hofmann & White, 1983), but in the 13 most altered samples Ba/Rb and K/Rb increase from 20 to 43 and from 550 to 1200, respectively. Obviously, mobility of K 2 O is important in classifying lavas as tholeiitic, alkalic or transitional. If K 2 O contents are adjusted to a K/Nb ratio of 375 which is typical of the unaltered lavas, the normative quartz decreases from 2 24 to 0 05% for the Mont Bureau lava with the lowest K/Nb ratio, GM92-43, and from 5 13 to 2 65% for the Mont Rabouillère lava with the lowest K/Nb, GM In the silica alkalis diagram, loss of K 2 O moves a sample toward the tholeiitic field. For example, with the adjusted K 2 O contents, Mont 726

17 YANG et al. KERGUELEN ARCHIPELAGO FLOOD BASALTS Fig. 8. (a) Incompatible element abundances in representative samples from both sections normalized to primary mantle estimates (Sun & McDonough, 1989). For each section the upper legend is for group P samples ( % MgO) and the lower legend is for group D ( % MgO) samples [groups are defined in text and Fig. 14 (below)]. (b) Chondrite-normalized REE patterns of representative lavas. Most of the group D samples have small positive Eu anomalies. Chondritic values are from Sun & McDonough (1989). Bureau samples GM92-41, GM92-43 and GM92-44 move to near the alkalic tholeiitic boundary (Fig. 3b), and nine of the 12 Mont Rabouillère samples with anomalously low K 2 O contents move from the tholeiitic to the alkalic field. Despite these shifts the lavas are best described as transitional basalts; i.e. they are not tholeiitic, and they are not as alkaline as Lower Miocene and recent archipelago lavas (Fig. 3a). 727

18 JOURNAL OF PETROLOGY VOLUME 39 NUMBER 4 APRIL 1998 Fig. 9. Abundance of Nb vs abundances of the mobile elements, K, Rb, U and Pb (all in ppm). Some altered samples are labeled and groups are as defined in Fig. 14 (below). In the Nb K panel, the continuous line indicates K/Nb = 375, which is typical for the relatively unaltered lavas. The post-magmatic mobility of Pb and U is more all 10 samples from the Mont Rabouillère section (Table difficult to evaluate because the samples were acid- 3). We find that the age-corrected 206 Pb/ 204 Pb and 207 Pb/ 204 leached before analyses for Pb and U. In general, Pb ratios for the six Mont Bureau samples with 238 U/ 204 abundances of Pb and U are positively correlated with Pb outside this range form a tighter cluster, if we use Nb contents, but the offset of Mont Bureau samples the average 238 U/ 204 Pb of 15 1 in the other 13 Mont GM92-55 and GM92-46 to higher U and Pb probably Bureau samples, rather than the extreme measured ratios reflects addition of these elements during late-stage alin these six samples (Table 3). Age corrections for 208 Pb/ 204 teration (Fig. 9). Pb ratios are more complex because Th data are for unleached whole rocks. Not surprisingly, in the 30 samples 232 Th/ 204 Pb ratios range widely from 4 8 to 278, but in 26 samples the range is from 67 7 to 163. Again we find Sr, Nd, Pb and Os isotopes a tighter cluster of age-corrected data, if the average The measured isotopic ratios of Sr, Nd and Pb were 232 Th/ 204 Pb ratio of 89 7 is used for the samples with corrected for in situ Rb, Sm, U and Th decay since 28 extreme 232 Th/ 204 Pb. The greater coherency obtained by Ma (Table 3 and Figs 10, 11 and 12), which is an estimate using the average parent/daughter abundance ratios for of the mean age for lavas in these sections (Nicolaysen samples with outlier 238 U/ 204 Pb and 232 Th/ 204 Pb is shown et al., 1996). For the Sr and Nd isotopic systems the in Fig. 11a. With this approach the maximum age age corrections use parent/daughter abundance ratios corrections of 0 45% for 206 Pb/ 204 Pb and 0 57% for 208 Pb/ measured by XRF or INAA techniques on unleached 204 Pb are less than the measured variations in 206 Pb/ 204 Pb 206 powders. However, Pb/ 204 Pb and 207 Pb/ 204 Pb ratios (1 9%) and 208 Pb/ 204 Pb (1 5%). The age correction for were corrected using Pb and U concentrations measured 207 Pb/ 204 Pb is insignificant because of the low abundance by isotope dilution on acid-leached samples. Twentyratios of 235 U. In all subsequent discussion we use these initial nine samples were analyzed for U and Pb; the 238 U/ 204 Pb for Sr, Nd and Pb isotopic ratios. ratio ranges from 10 to 27 in 28 of these samples and The flood basalts from Mont Bureau and Mont Ra- from only 10 9 to 19 3 in 23 of the 29 samples, including bouillère define an inverse correlation between initial 728

19 YANG et al. KERGUELEN ARCHIPELAGO FLOOD BASALTS Table 3: Sr, Nd and Pb isotopic ratios and parent/daughter abundance ratios of Mont Bureau and Mont Rabouillère Mt Bureau 87 Rb/ 86 Sr ( 87 Sr/ 86 Sr) m 2σ ( 87 Sr/ 86 Sr) i 147 Sm/ 143 Nd ( 143 Nd/ 144 Nd) m 2σ ( 143 Nd/ 144 Nd) i GM GM GM GM GM GM GM GM GM GM GM GM GM GM GM GM GM GM GM GM Mt Bureau 238 U/ 204 Pb 235 U/ 204 Pb 232 Th/ 204 Pb ( 206 Pb/ 204 Pb) m ( 207 Pb/ 204 Pb) m ( 208 Pb/ 204 Pb) m ( 206 Pb/ 204 Pb) i ( 207 Pb/ 204 Pb) i ( 208 Pb/ 204 Pb) i GM GM GM GM GM GM GM GM GM GM GM GM GM GM GM GM GM GM GM GM

20 JOURNAL OF PETROLOGY VOLUME 39 NUMBER 4 APRIL 1998 Table 3: continued Mt Rabouillère 87 Rb/ 86 Sr ( 87 Sr/ 86 Sr) m 2σ ( 87 Sr/ 86 Sr) i 147 Sm/ 143 Nd ( 143 Nd/ 144 Nd) m 2σ ( 143 Nd/ 144 Nd) i GM GM GM GM GM GM GM GM GM GM Mt Rabouillère 238 U/ 204 Pb 235 U/ 204 Pb 232 Th/ 204 Pb ( 206 Pb/ 204 Pb) m ( 207 Pb/ 204 Pb) m ( 208 Pb/ 204 Pb) m ( 206 Pb/ 204 Pb) i ( 207 Pb/ 204 Pb) i ( 208 Pb/ 204 Pb) i GM GM GM GM GM GM GM GM GM GM The 2σ uncertainties for ( 87 Sr/ 86 Sr) m and ( 143 Nd/ 144 Nd) m (m indicates measured) indicate variation in last significant digits. Initial ratios (subscript i) are calculated for 28 Ma. Parent/daughter abundance ratios are calculated from data in Table 1. These ratios were used to calculate the initial isotopic ratios except that a 238 U/ 204 Pb of 15 1 was used for the seven samples with measured 238 U/ 204 Pb <10 92 or >19 30 and a 232 Th/ 204 Pb of 89 7 was used for the six samples with measured 232 Th/ 204 Pb <67 69 or >162 7 (see discussion in text). 87 Sr/ 86 Sr and 143 Nd/ 144 Nd, which range from ratios typical these ratios overlap with those of the Upper Miocene of enriched Indian Ocean MORB to 87 Sr/ 86 Sr greater samples from the Southeast Province (Fig. 10), which than the bulk Earth estimate (Fig. 10). Although there were suggested to be the representative of the Kerguelen are no systematic variations in isotopic ratios with stratigraphic Plume (Weis et al., 1993). Four other samples from low height (Fig. 4a), the flood basalts form distinct in the Mont Bureau section have much lower initial 87 Sr/ isotopic groups. The eight uppermost lavas in the Mont 86 Sr ( , Fig. 10). A group of six lavas, Bureau section, from 300 to 620 m, and seven of the 10 three from each section, has intermediate initial 87 Sr/ analyzed Mont Rabouillère lavas span a narrow range of 86 Sr ( ), but the three lavas from Mont initial 87 Sr/ 86 Sr ( and , Rabouillère have lower initial 143 Nd/ 144 Nd (Fig. 10). As respectively, Fig. 4a). This range overlaps with that of a group, the Sr and Nd isotopic ratios in these ~28 30 lavas from the Pleistocene Mont Ross volcano and the Ma flood basalts encompass nearly the entire range Lower Miocene flood basalts from the Southeast Province previously found in archipelago lavas of widely varying of the Kerguelen Archipelago (Fig. 10). Among these age (Fig. 10). This is an important result because previous flood basalts, three samples from the bottom of the Mont isotopic data for lavas of varying age from different parts Bureau section, GM92-59, GM92-58 and GM92-56, of the archipelago led to the conclusion that the isotopic have the highest initial 87 Sr/ 86 Sr ratios ( ); ratios are correlated with eruption age and degree of 730

21 YANG et al. KERGUELEN ARCHIPELAGO FLOOD BASALTS Fig. 10. Initial 87 Sr/ 86 Sr vs initial 143 Nd/ 144 Nd for flood basalt lavas from Mont Bureau and Mont Rabouillère divided into the groups shown in Fig. 14 (below). Φ, ankaramite dike in the Bureau section;, four Mont Bureau samples analyzed by White & Hofmann (1982). SE LMS field is for Lower Miocene flood basalts from the Southeast Province (Weis et al., 1993). Shown for comparison are fields for the youngest archipelago lavas; i.e. for Mont Ross (Pleistocene) and the Southeast Upper Miocene Series (SE UMS) (Weis et al., 1993, 1998). Field for SEIR MORB from Hamelin et al. (1986), Michard et al. (1986), Dosso et al. (1988) and J. J. Mahoney (unpublished data, 1996). The stippled sub-field for SEIR MORB is for 29 samples distant from the St Paul Amsterdam Island platform, whereas the larger field extending to higher Sr and lower Nd isotopic ratios includes 14 samples dredged from the SEIR near these islands [see fig. 1 of Dosso et al. (1988)]. Inset shows fields for mafic and ultramafic xenoliths found in recent archipelago lavas (Mattielli, 1996; Mattielli et al., 1996) compared with field for all archipelago lavas. alkalinity (Storey et al., 1988; Gautier et al., 1990; Weis 87 Sr/ 86 Sr, > , and low 206 Pb/ 204 Pb, <18 3, and et al., 1993). These flood basalt data show that large some of the oldest flood basalts have isotopic ratios isotopic variations occurred on a short time scale and are similar to those of the much younger, Upper Miocene not correlated with alkalinity as measured in a silica total to Pleistocene, lavas erupted in the southeast part of the alkalis plot (Fig. 3). archipelago; and (2) there are no systematic correlations In Pb Pb plots, the flood basalts are offset from the between isotopic ratios and eruption age or the alkalinity Indian MORB field to high 207 Pb/ 204 Pb and 208 Pb/ 204 Pb of the lavas. (Fig. 11b and c). Most of the samples, 21 of 30, have Three lavas and an ankaramite dike with relatively 206 Pb/ 204 Pb ratios within a narrow range, , high MgO ( %) and Ni ( ppm) contents similar to those of the Ma Upper Miocene lavas were selected for Os isotopic analyses (Table 4). Three from the Southeast Province; thus compared with the samples are from the Mont Bureau section and one is a ~22 Ma Lower Miocene flood basalts from the Southeast flood basalt from the southeast part of the archipelago. Province, most of the ~28 30 Ma flood basalt lavas These samples cover a substantial range in Os and Re have lower 206 Pb/ 204 Pb ratios (Figs 11b and c and 12). concentration ( ppt and ppt, re- Therefore, as with 87 Sr/ 86 Sr and 143 Nd/ 144 Nd, archipelago spectively, Table 4). If these new data are considered lavas do not show a systematic temporal variation in with previous Os isotopic data for archipelago lavas 206 Pb/ 204 Pb ratio. There is no correlation between 206 Pb/ (Reisberg et al., 1993), the data show a trend to more 204 Pb and 87 Sr/ 86 Sr ratios (Fig. 12). In detail, lavas from radiogenic and more variable 187 Os/ 186 Os with decreasing the Mont Bureau and Mont Rabouillère sections overlap Os content as observed in other oceanic island suites in 206 Pb/ 204 Pb vs 208 Pb/ 204 Pb (Fig. 11c), but lavas from (Reisberg et al., 1993; Marcantonio et al., 1995; Roy- the Mont Bureau section have lower 207 Pb/ 204 Pb at a Barman & Allègre, 1995; Widom & Shirey, 1996). Present-day given 206 Pb/ 204 Pb ratio (Fig. 11b). 187 Os/ 186 Os values for these samples vary from In summary, the Sr, Nd and Pb isotopic data show to ( 187 Os/ 188 Os from to 0 239), and that: (1) most of the flood basalts have relatively high 187 age corrections (Table 4) indicate initial Os/ 186 Os 731