Journal of Petrology

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Journal of Petrology | Volume 43 | Number 7 | Pages 1287-1309 | 2002
© Oxford University Press 2002

Submarine Basalts of the Northern Kerguelen Plateau: Interaction Between the Kerguelen Plume and the Southeast Indian Ridge Revealed at ODP Site 1140

D. WEIS^1,* and F. A. FREY²

¹DÉPARTEMENT DES SCIENCES DE LA TERRE ET DE L’ENVIRONNEMENT, UNIVERSITÉ LIBRE DE BRUXELLES, CP 160/02, AVENUE F. D. ROOSEVELT, 50, B-1050 BRUSSELS, BELGIUM
²DEPARTMENT OF EARTH, ATMOSPHERIC AND PLANETARY SCIENCES, MASSACHUSETTS INSTITUTE OF TECHNOLOGY, CAMBRIDGE, MA 02139, USA

Received June 20, 2001; Revised typescript accepted February 11, 2002

	ABSTRACT

TOP ABSTRACT INTRODUCTION GEOLOGICAL SETTING, BASEMENT... ANALYTICAL TECHNIQUES RESULTS DISCUSSION CONCLUSIONS REFERENCES

 
During Ocean Drilling Program Leg 183, basaltic cores were retrieved from the Northern Kerguelen Plateau (NKP) at Site 1140 on the extreme north of the plateau, 270 km north of the Kerguelen Archipelago. Amongst the six basement units recovered, five were pillow basalts with fresh glassy rims together with flow lobes. This is the first evidence for submarine eruption anywhere on the Kerguelen Plateau. Each flow unit has distinct geochemical and isotopic characteristics that span the range from Southeast Indian Ridge (SEIR) mid-ocean ridge basalt to tholeiitic–transitional basalts derived from the Kerguelen mantle plume. Relationships between element abundance ratios involving incompatible elements and isotopic ratios reflect mixing of near-primary melts from an SEIR source and the Kerguelen plume with an increasing role for an incompatible element-rich end-member in the order: Unit 1 < 6 5 < 2 < 3; that is, there is no systematic downcore geochemical variation. Modeling results show that the depleted SEIR component is dominant, ranging from 99–90% in Unit 1 to 76–63% in Unit 3. Comparison with Kerguelen Archipelago flood basalts with similar geochemical characteristics indicates that in the northern part of the archipelago, where 29–28 Ma tholeiitic–transitional basalts are present, incorporation of asthenospheric mantle by the plume on its way to the surface may have been significant. At 34 Ma, when Site 1140 basalts were formed, the SEIR was only 50 km away. Apparently, the proximity of the spreading center and plume allowed mixing of geochemically diverse magmas. In contrast to older Cretaceous parts of the Kerguelen Plateau, the Cenozoic Northern Kerguelen Plateau appears oceanic in origin and there is no evidence for a component derived from continental crust in Site 1140 basalts.

KEY WORDS: isotopic geochemistry; Kerguelen plume; Northern Kerguelen Plateau; plume–ridge interaction; trace elements

	INTRODUCTION

TOP ABSTRACT INTRODUCTION GEOLOGICAL SETTING, BASEMENT... ANALYTICAL TECHNIQUES RESULTS DISCUSSION CONCLUSIONS REFERENCES

 
The Northern Kerguelen Plateau (NKP) was not sampled until 1998 when the Kerimis survey cruise dredged Skiff Bank, also known as Leclaire Rise (Fig. 1). These dredges recovered basalts and picritic basalts. In addition, the dredges included granites and syenites, very unusual rocks for an oceanic environment; these were interpreted as ice-rafted material (Weis et al., 2002). Ocean Drilling Program (ODP) Leg 183 recovered cores of igneous basement from two NKP sites: Site 1139 on Skiff Bank, a bathymetric and gravimetric high 350 km west of the Kerguelen Archipelago, and Site 1140 at the extreme north end of the NKP, 270 km north of the Kerguelen Archipelago (Fig. 1). The objectives of drilling at Sites 1139 and 1140 were to establish the age, geochemical characteristics and eruption environment of the submarine igneous basement forming the NKP and to compare these results with the igneous basement forming other parts of the Kerguelen Plateau.

View larger version (139K): [in this window] [in a new window]   Fig. 1. Satellite-derived gravity field for the Northern Kerguelen Plateau (scale in 1/10 mgals) showing the MD109-Kerimis track chart, dredge sites (gray circle), proposed drilling sites (small dot) and ODP Leg 183 site locations (gray circle with black rim); from Weis et al. (2002).

 

Because formation of the Kerguelen Plateau has been attributed to volcanism from the Kerguelen mantle plume (e.g. Frey et al., 2000a), an important goal was to constrain magma production rates associated with the Kerguelen plume. The ages and lithologies recovered from the sites in the NKP are very different: at Site 1139, the igneous basement is formed by an 68 Ma (Ar–Ar ages) alkaline lava series ranging from trachybasalt to trachyte and rhyolite (Kieffer et al., 2002), whereas at Site 1140 the 34 Ma igneous basement is made of pillow lava flow units of tholeiitic basalt (Shipboard Scientific Party, 2000; Duncan, 2002). Drilling at Site 1140 recovered the first basaltic glass associated with volcanic products of the Kerguelen plume. This is also the first example of submarine volcanism on the Kerguelen Plateau, as all previous ODP sites have indicated subaerial eruption environments. The apparent scarcity of submarine volcanism on the Kerguelen Plateau contrasts strongly with the Ontong–Java Plateau in the western Pacific Ocean, which was entirely submarine (Mahoney et al., 1993).

The Kerguelen Archipelago is interpreted as an 30 Ma to recent (Nicolaysen et al., 2000) subaerial expression of the Kerguelen plume on the NKP. Weis et al. (2002) showed an apparent age and magma production trend from Site 1140 across the Kerguelen Archipelago to Heard Island on the Central Kerguelen Plateau, including seamounts between the archipelago and Heard Island that may represent the hotspot track of the Kerguelen plume. The oldest flood basalts on the northwestern part of the Kerguelen Archipelago contain a component derived from depleted mantle (Yang et al., 1998; Doucet et al., 2002). Storey et al. (1988) and Gautier et al. (1990) proposed that interaction between the Kerguelen plume and a depleted mantle reservoir [i.e. the source of Southeast Indian Ridge (SEIR) mid-ocean ridge basalt (MORB)] decreased as the distance between the SEIR and the archipelago increased with time. Detailed investigation of several stratigraphically controlled flood basalt sections demonstrates that this model was oversimplified (e.g. Yang et al., 1998; Doucet et al., 2002). The compositions of these basalts show that there was a general decrease in the amount of partial melting with time, as indicated by increasing alkalinity of the lavas with decreasing eruption age, and the absence of a depleted component (relatively low ⁸⁷Sr/⁸⁶Sr and high ¹⁴³Nd/¹⁴⁴Nd), in <25 Ma flood basalts (e.g. Frey et al., 2000b, 2002a; Weis et al., 1998b, 2002; Mattielli et al., 2002). Thus, the 34 Ma Site 1140 samples from the extreme north of the Kerguelen Plateau enable us to constrain the relative importance of interaction between the Kerguelen plume and the depleted upper mantle.

Finally, an important aspect of Kerguelen Plateau volcanism is the role of continental lithosphere. A role for continental lithosphere is evident in some of the Cretaceous basalts forming the Kerguelen Plateau (Mahoney et al., 1995; Frey et al., 2000a; Weis et al., 2001a; Ingle et al., 2002), but not in the Cenozoic lavas forming the Kerguelen Archipelago (Weis et al., 1998b; Yang et al., 1998; Frey et al., 2000b; Mattielli et al., 2002). On the Kerguelen Archipelago, the only evidence for a component of continental lithosphere has been found in a few ultrabasic xenoliths (Hassler & Shimizu, 1998; Mattielli et al., 1999). Hence, an important goal is to assess the potential role of continental lithosphere in the petrogenesis of Site 1140 lavas.

This paper presents geochemical and isotopic data for the basalts that were recovered at Site 1140 on the NKP. Eruption ages for Site 1140 lavas (Duncan, 2002) and for the flood basalts of the Kerguelen Archipelago (Nicolaysen et al., 2000; Doucet et al., 2002) show that the NKP is distinctly younger (Cenozoic) than the Cretaceous Central and Southern Kerguelen Plateau and Elan Bank. Site 1140 lavas show that mixing between SEIR depleted MORB and plume-derived magmas was important at 34 Ma at a distance of 50 km from the spreading SEIR axis, but there is no evidence for a continental component.

	GEOLOGICAL SETTING, BASEMENT LITHOLOGY AND PETROLOGY

TOP ABSTRACT INTRODUCTION GEOLOGICAL SETTING, BASEMENT... ANALYTICAL TECHNIQUES RESULTS DISCUSSION CONCLUSIONS REFERENCES

 
Site 1140 is at a water depth of 2394 m on the northern flank of the NKP at a distance of 270 km from the Kerguelen Archipelago (Fig. 1). The boundary between the NKP and the Australia–Antarctic Basin lies 5 km north of Site 1140 and offsets basement by 400 m. The site is at a location with simple basement structure and a thin sedimentary section in a region of sufficient water depth for drilling. The top of the acoustic basement is horizontal and covered by a sediment sequence 350 m thick (Fig. 2; Shipboard Scientific Party, 2000).

View larger version (101K): [in this window] [in a new window]   Fig. 2. Marion Dufresne MD109-06 multichannel seismic profile across Site 1140 (Shipboard Scientific Party, 2000). Vertical exaggeration 16·1 at the sea floor; twt, two-way travel time.

 

Drilling at Site 1140 penetrated 87·4 m of basement with 50% recovery. The basement is divided into six units based on interbedded sedimentary layers and changes in phenocryst abundance and type; these are five pillow basaltic units (Units 1, 2, 3, 5 and 6) and an interbedded sedimentary unit (Unit 4) of dolomite (Fig. 3). In Units 2 and 3, the pillows are small (40–100 cm), but in Units 1 and 6, <1 m pillows are accompanied by large 5 m lobes (Figs 3 and 4). Unit 5 is more massive and is composed of three large lobes (>3 m thick). Unit 4 is a pale brown to orange dolomite and only 0·8 m were recovered, including upper and lower contacts. Because of the presence of thin sedimentary layers (chalk between Units 2 and 3 and dolomite between Units 5 and 6), these basaltic units are interpreted as representing separate eruptions (Shipboard Scientific Party, 2000). Although the contact between Units 1 and 2 was not recovered, a probable sedimentary interlayer was recorded in the logging data (Fig. 3) and the two units have different magnetic polarity. Biostratigraphic and paleomagnetic constraints were used to infer eruption ages between 33·1 and 35·4 Ma (Shipboard Scientific Party, 2000). This age has been confirmed by Ar–Ar dating at 34 Ma (Duncan, 2002).

View larger version (50K): [in this window] [in a new window]   Fig. 3. Downcore stratigraphy and interpretative summary of the lithology, morphology, and mineralogy of basement units sampled at Site 1140 (Shipboard Scientific Party, 2000). Boundaries for igneous Units 1–6 are indicated. It should be noted that the contact between Units 1 and 2 was not recovered and was located using downhole logging data. The occurrence of plagioclase, olivine, and clinopyroxene phenocrysts is indicated, as well as the presence of glassy pillow margins. A reversal of the magnetic field occurred between the time of eruption of Units 1 and 2. The density log and its interpretation are shown schematically to provide information about the lithology of unrecovered Cores 183-1140A-29R and -30R.

 

View larger version (128K): [in this window] [in a new window]   Fig. 4. Drillcore photographs showing pillow margins and glassy rinds (Shipboard Scientific Party, 2000). Upper photograph: drillcore photograph of glassy pillow rind (interval 183-1140A-28R-3, 86–101 cm) with calcite filling vesicles and veins. Open space-filled dolomite and baked white sediment is along the margin with the glass. Lower photograph: drillcore photograph showing the consistent glassy rinds of 1 cm thickness typical of this site. Inwards from the glassy rind, there is a zone of 1 cm thickness with a very fine-grained to glassy groundmass and sparse vesicles (interval 183-1140A-26R-1, 0–10 cm). (Note the calcite band that separates the similar thickness of the two chilled margins from two different pillows of basement Unit 1.)

 

Submarine eruption has produced distinctive textures. The outermost 1–2 cm of pillow rims are composed of dark brown glass, some of which is unaltered (Wallace, 2002). Pillow margins are sparsely porphyritic with a very fine-grained to glassy groundmass that contains extremely fine-grained plagioclase microlites or small laths with ragged or swallow-tailed terminations. Olivine forms sub-equant, euhedral phenocrysts and, in the glassy rinds, some of the olivine phenocrysts contain numerous inclusions of unaltered glass and rare inclusions of chromite (Fig. 5). The massive interiors of pillows and lava lobes have the same mineral phases, but are more crystalline and coarser grained than the pillow margins, reflecting slower cooling rates in the flow interiors (especially in Units 5 and 6, where the maximum clinopyroxene and plagioclase grain size reaches 0·7 mm). Clinopyroxene and plagioclase are major groundmass phases in crystalline areas; minor phases are titanomagnetite (5%) and intersertal glass. In some samples, glass has partially crystallized to cryptocrystalline clinopyroxene and opaque minerals. Sulfide of possible magmatic origin occurs as minute (<0·01 mm) inclusions in primary minerals and fresh glass. The Site 1140 basalts are unique amongst those recovered on Leg 183 in that they contain unaltered olivine and glass (Figs 4 and 5). Anorthite-rich plagioclase (An_60–70) is the dominant phenocryst type in all units and is accompanied by olivine (Figs 3 and 5), except in Unit 6, and by clinopyroxene in Units 2 and 6. Units 1 and 2 are petrographically distinct: Unit 1 contains <1% phenocrysts (except in pillow margins), whereas Unit 2 contains 20% phenocrysts. Many phenocrysts of plagioclase and clinopyroxene are strongly zoned.

View larger version (120K): [in this window] [in a new window]   Fig. 5. Photomicrographs (plane-polarized light) of pillow lavas from Site 1140, Unit 1 (Sample 183-1140A-26R-1, piece 1A, 5–18 cm) (Shipboard Scientific Party, 2000). The width of each photograph is 1·02 mm. (a) Glass from a chilled pillow margin containing fresh euhedral olivine phenocrysts with inclusions of chromite and glass (left part of crystal). The presence of chromite inclusions in the olivine phenocryst indicates early spinel saturation. In (b), the glass contains fresh euhedral olivine phenocrysts (upper center), some of which contain spherical melt inclusions. The hair-like texture shown on the right consists of narrow channels that are the result of microbial degradation of the glass (e.g. Fisk et al., 1998).

 

Petrographically, Site 1140 basalts appear relatively fresh, except in the areas adjacent to pillow margins. The interstices between pillows are filled by Foraminifera-bearing dolomitic chalk and the glassy margins contain thin carbonate veins. In the freshest parts of Units 1 and 2, alteration is limited to replacement of interstitial glass by clay and the filling of vesicles by clay and carbonate (Shipboard Scientific Party, 2000). Units 2, 3, 5 and 6 contain secondary pyrite in veins and as small disseminated grains that replace residual glass.

	ANALYTICAL TECHNIQUES

TOP ABSTRACT INTRODUCTION GEOLOGICAL SETTING, BASEMENT... ANALYTICAL TECHNIQUES RESULTS DISCUSSION CONCLUSIONS REFERENCES

 
Abundances of the major elements and the trace elements Sr, V, Ni, Zn, Ga, Y and Zr were determined by X-ray fluorescence (XRF) at the University of Massachusetts, Amherst (Rhodes, 1996). All reported XRF data for major elements (in wt % oxide, Table 1) are the mean values of duplicate analyses. Abundances of the trace elements Sc, Cr, Co and Hf, and several rare earth elements (REE) were determined by instrumental neutron activation (Ila & Frey, 1984, 2000), and abundances of Rb, Ba, REE, Y, Zr, Hf, Nb, Ta and Th were determined by inductively coupled plasma mass spectrometry (ICP-MS) at MIT (Table 1). Analyses of major, trace and volatile elements in pillow-rim glasses have been presented by Wallace (2002).

View this table: [in this window] [in a new window]   Table 1: Major (wt % oxides) and trace element abundances (ppm) in Site 1140 basalts

 

Whole-rock samples for Sr, Nd and Pb isotope analyses were selected to encompass the entire range of chemical composition using samples with relatively low loss on ignition (LOI) values (Tables 1 and 2). The chemical procedure used was similar to that described by Weis et al. (1987) with improvements as discussed by Weis & Frey (1991). All of the samples were acid-leached to remove secondary or alteration phases. Total blank values were <1 ng for all three isotopic systems considered. Such values are negligible in view of the elemental concentrations in the 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 x 10^-5 unless specified in Table 2. Sr isotopic ratios were normalized to ⁸⁶Sr/⁸⁸Sr = 0·1194 and Nd isotopic ratios, measured as metal, to ¹⁴⁶Nd/¹⁴⁴Nd = 0·7219. The average ⁸⁷Sr/⁸⁶Sr value of the NBS 987 Sr standard was 0·710272 ± 17 (2_m on 48 samples) and analyses of the Rennes Nd standard yielded ¹⁴³Nd/¹⁴⁴Nd = 0·511962 ± 9 (2_m on 56 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 a.m.u.) on the basis of 72 analyses of the NBS 981 Pb standard for a temperature range between 1090°C and 1150°C. Between-run precision was better than 0·1% for ²⁰⁶Pb/²⁰⁴Pb and ²⁰⁷Pb/²⁰⁴Pb, and better than 0·15% for ²⁰⁸Pb/²⁰⁴Pb. The reproducibility of our Pb analyses is reflected by the complete replicate analysis of Sample 1140A-32R4, 0–6 cm, which had surprisingly high ²⁰⁶Pb/²⁰⁴Pb for its position in Unit 5 (see discussion in Fig. 12, below).

View this table: [in this window] [in a new window]   Table 2: Sr, Nd and Pb isotopic compositions in Site 1140 basalts

 

View larger version (116K): [in this window] [in a new window]   Fig. 12. Isotopic diagrams comparing Site 1140 samples with literature data for Southeast Indian ridges (Michard et al., 1986; Dosso et al., 1988; Johnson et al., 2000; Mahoney et al., 2002) and SEIR ASP (Amsterdam–St. Paul Plateau, Johnson et al., 2000; K. Nicolaysen, unpublished data, 1999); the Kerguelen Plateau drill sites (Sites 747, 749 and 750: Frey et al., 2002b; Site 1137: Weis et al., 2001a; Ingle et al., 2002; Site 1139: Kieffer et al., 2002); the Kerguelen Archipelago (29–25 Ma flood basalts: Yang et al., 1998; Doucet et al., 2002; Frey et al., 2002a; 25–22 Ma alkaline basalts: Weis et al., 1993; Frey et al., 2000b; D. Weis, unpublished data, 1998; and <10 Ma lavas: Weis et al., 1993, 1998b), Heard Island (Barling et al., 1994), St. Paul and Amsterdam Islands (Johnson et al., 2000; Weis et al., 2001a) and the Ninetyeast Ridge (NER; Weis & Frey, 1991; Frey & Weis, 1995). The crosses represent the end-members of the mixing model (dark cross, extreme; grey cross, average of component) and the dashed lines correspond to the mixing model of Table 3. (a) 87Sr/86Sr vs 143Nd/144Nd isotope variations. Data are age-corrected for in situ 87Rb and 147Sm decay, which accounts for most of the older sites on the Kerguelen Plateau plotting below the mantle array (Site 750 is an exception). The basalts of Site 1139, also located on the NKP, plot at the enriched end of the field for the Kerguelen Archipelago lavas. Each flow unit of Site 1140 presents a unique isotopic signature intermediate between the two extremes of Unit 1 within the field of SEIR, and Unit 3, within the field of Kerguelen flood basalts that are interpreted as having interacted with a depleted material. The inset shows the limited effect of the 34 Myr age-correction on individual 1140 pillow basalts. (b) 208Pb/204Pb vs 206Pb/204Pb isotope diagram. Kerguelen Plateau data are age-corrected for in situ 232Th and 238U decay, except for Site 1137. This plot is especially useful to distinguish the Cretaceous Kerguelen Plateau (Sites 747, 749, 750 and 1137) forming the Central, Southern Kerguelen Plateau and Elan Bank from Site 1140 in the Northern Kerguelen Plateau. The older Kerguelen Plateau is clearly characterized by lower 206Pb/204Pb, <18·2. Site 1140 pillow lavas have distinctly more radiogenic 206Pb/204Pb, ranging from typical SEIR basalt composition to the composition of the Kerguelen plume as represented by the 25–22 Ma alkaline basalts. This field also overlaps the field of Ninetyeast Ridge basalts, except that for Site 1140 the linear trend defines binary mixing between a depleted SEIR-type source and the Kerguelen plume. The two measured 1140 samples that plot off the trend have clearly been enriched in U by alteration (see Fig. 8)—age correction for in situ U and Th decay for 34 Ma brings them back into the linear mixing trend. (c) 87Sr/86Sr vs 206Pb/204Pb isotope diagram. Same comment for age-correction as for (b). The inset shows a comparison between individual 1140 samples; it should be noted that sample 1140A-32R4, 0–6 cm from Unit 5, which has a ‘too high’ measured 206Pb/204Pb ratio (18·59) for its 87Sr/86Sr, plots off the 1140 mixing trend. When corrected for in situ decay, this sample comes back into the linear mixing array—its 238U/204Pb is >60 (Table 2). Another sample from Unit 6 (Sample 1140–37R-3, 104–108 cm Piece 2) is also slightly off the linear trend. No concentration data are available for this shipboard sample, but we assume that the discrepancy reflects the same alteration feature.

 

View this table: [in this window] [in a new window]   Table 3: Mixing model for individual 1140 flow units

 

View larger version (37K): [in this window] [in a new window]   Fig. 8. Incompatible element abundances in the five lava units of Site 1140 normalized to the primitive mantle estimates of Sun & McDonough (1989). In general, the relative enrichment of highly incompatible elements increases in the order: Unit 1 < Unit 5 Unit 6 < Unit 2 < Unit 3.

 

	RESULTS

TOP ABSTRACT INTRODUCTION GEOLOGICAL SETTING, BASEMENT... ANALYTICAL TECHNIQUES RESULTS DISCUSSION CONCLUSIONS REFERENCES

 
Chemical composition
The five basaltic units at Site 1140 are tholeiitic basalts (Fig. 6). Two samples from Unit 5 show alkali enrichment, as well as moderate enrichment in Rb and U, relative to other samples from Unit 5 (Table 1 and Fig. 6). The LOI values are the highest in Unit 5, all >3 wt % (up to 5·3 wt % in one sample), whereas in the rest of the core, they vary between 1 and 2 wt %. Hence, we interpret the higher alkali content of the two Unit 5 samples as reflecting the presence of clays.

View larger version (25K): [in this window] [in a new window]   Fig. 6. Total alkalis (Na2O + K2O) vs SiO2 classification diagram with the alkalic–tholeiitic dividing line from Macdonald & Katsura (1964). Site 1140 flows are clearly tholeiitic basalts with a limited range of composition compared with other drilled sites of the Kerguelen Plateau (Frey et al., 2002b; Ingle et al., 2002). In detail, Unit 1 lavas have the lowest SiO2 contents. Apart from two samples from Unit 5 that have higher alkali contents as a result of alteration, Units 2 and 3 are distinctly richer in alkalis than the other units.

 

The MgO contents vary from 8·33 to 5·48 wt % and are fairly homogeneous for each unit (Table 1); Units 2 and 3 are characterized by lower MgO contents (<6·1 wt %; Fig. 7). Each flow unit consists of tholeiitic basalts with distinctive major and trace element contents (Figs 7 and 8). Unit 1 is the least enriched in incompatible elements and has the highest Ni and Cr contents (K₂O 0·09–0·12 wt %, P₂O₅ 0·11 wt %, Zr/Nb = 29–27, La/Sm = 1·02–1·15, Cr 280 ppm and Ni 90 ppm), whereas Units 2 and 3 are the most enriched in incompatible elements and have the lowest Ni and Cr contents (K₂O 0·32–0·58 wt %, P₂O₅ 0·25–0·39 wt %, Zr/Nb = 12·8–10·6, La/Sm = 2·36–2·70, Cr <120 ppm and Ni <70 ppm). Units 5 and 6 have intermediate concentrations and ratios. Therefore, there are no systematic downcore variations in abundance ratios of incompatible elements (Fig. 9). Differences in the relative abundances of incompatible elements in the five basement units are apparent in a mantle-normalized plot (Fig. 8). The abundances of the highly incompatible elements, such as Ba, Th, U, Nb, Ta and light REE (LREE), increase in the order: Unit 1 < Unit 5 Unit 6 < Unit 2 < Unit 3 (Fig. 8). Units 2 and 3 are characterized by negative Sr anomalies, especially Unit 3. At a given MgO content, lavas from Units 2 and 3 have lower Sr/Nd and CaO/Al₂O₃; and lower abundances of Sc and Cr than lavas from Units 1, 5 and 6 (Fig. 10). Such results are consistent with an important role for plagioclase and clinopyroxene fractionation during the petrogenesis of the Unit 2 and 3 basalts (Fig. 3 and Shipboard Scientific Party, 2000).

View larger version (13K): [in this window] [in a new window]   Fig. 7. K2O and P2O5 vs MgO (wt %) diagrams for 1140 basalts. The relative abundances of the incompatible elements K and P clearly distinguish Units 2 and 3 from Units 1, 5 and 6. Two samples from Unit 5 and one from Unit 6 show higher K contents that reflect submarine alteration.

 

View larger version (17K): [in this window] [in a new window]   Fig. 9. Nb/Zr and (La/Sm)N vs depth (in meters below sea floor; mbsf) for the five basalt units of Site 1140, where (La/Sm)N is the primitive mantle-normalized ratio. Units 2 and 3 are the most enriched in highly incompatible elements. As a result, there is no systematic downcore variation.

 

View larger version (22K): [in this window] [in a new window]   Fig. 10. Sc and Cr abundances and Sr/Nd and CaO/Al2O3 vs MgO variation plots. The offset of Units 2 and 3 to lower values at a given MgO content is consistent with more extensive fractionation of plagioclase (low Sr/Nd) and clinopyroxene (low Sc and CaO/Al2O3).

 

Incompatible element abundance ratios in Site 1140 lavas, such as Ce/Y and Nb/Zr, range from values that overlap with the oldest 30 Ma transitional flood basalts of the Kerguelen Archipelago (i.e. Units 2 and 3) to values similar to SEIR MORB (Fig. 11). The younger, mildly alkalic basalts in the Kerguelen Archipelago have ratios even higher than those of Units 2 and 3. In binary trace element ratio diagrams, Site 1140 pillow basalts plot on a mixing trend, intermediate between depleted mantle compositions for Unit 1 basalts and tholeiitic–transitional basalts from the Kerguelen Archipelago for Units 2 and 3 (Yang et al., 1998; Doucet et al., 2002; Frey et al., 2002a).

View larger version (29K): [in this window] [in a new window]   Fig. 11. Ce/Y vs Nb/Zr of 1140 basalts compared with Kerguelen Archipelago flood basalts and SEIR N-MORB (K. Nicolaysen, unpublished data, 1999). Units 2 and 3 overlap with the 29–25 Ma archipelago flood basalts that have tholeiitic–transitional compositions (•; Yang et al., 1998; Doucet et al., 2002; Frey et al., 2002a). The 25–22 Ma archipelago flood basalts have even higher ratios (; Frey et al., 2000b; D. Damasceno, unpublished data, 1998). Units 1, 5 and 6 have distinctly lower ratios and do not overlap with any of the Kerguelen Archipelago flood basalts. The dashed curve corresponds to mixing between an average SEIR MORB and average alkali basalt derived from the Kerguelen plume (Table 3).

 

Isotope geochemistry
In agreement with the distinct compositional characteristics for each flow unit, Site 1140 units have distinct Sr, Nd and Pb isotopic ratios (Table 2 and Fig. 12a–c). Unit 1 samples have very homogeneous measured ⁸⁷Sr/⁸⁶Sr (0·70341) and ¹⁴³Nd/¹⁴⁴Nd (0·51301). Unit 2 has higher ⁸⁷Sr/⁸⁶Sr (0·70426) and lower ¹⁴³Nd/¹⁴⁴Nd (0·51282), and Unit 3 has even higher ⁸⁷Sr/⁸⁶Sr (0·70444) and lower ¹⁴³Nd/¹⁴⁴Nd (0·51278). Units 5 and 6 have very comparable intermediate ⁸⁷Sr/⁸⁶Sr and ¹⁴³Nd/¹⁴⁴Nd (0·70375 and 0·51294), although Unit 5 seems to have slightly higher ⁸⁷Sr/⁸⁶Sr (averages are 0·70378 vs 0·70372). The corrections for 34 Myr of in situ ⁸⁷Rb and ¹⁴⁷Sm decay are small [(2–6) x 10^-5 for ⁸⁷Sr/⁸⁶Sr and (3–4) x 10^-5 for ¹⁴³Nd/¹⁴⁴Nd] and the units show even more homogeneous ratios when corrected for decay. In a Sr–Nd isotope diagram (Fig. 12a), Site 1140 basalts define a linear trend between the field of SEIR basalts not influenced by hotspots and the field for Kerguelen Archipelago flood basalts. Unit 1 basalts overlap with the SEIR field, whereas Unit 2 and 3 basalts overlap with the depleted end of the field for 29–25 Ma Kerguelen Archipelago flood basalts. Units 5 and 6 have intermediate compositions and plot close to the field of St. Paul Island (Frey & Weis, 1995; Weis et al., 2001b), i.e. in the field of SEIR basalts from the Amsterdam–St. Paul Plateau (Johnson et al., 2000; K. Nicolaysen, unpublished data, 1999).

In Pb–Pb isotope diagrams (Fig. 12b), Site 1140 lavas overlap the fields for Kerguelen Archipelago flood basalts and basalts forming the Ninetyeast Ridge, a hotspot track related to the Kerguelen plume (e.g. Weis & Frey, 1991). The linear trends for Site 1140 lavas can be interpreted as reflecting mixing between an SEIR-related component and the Kerguelen plume as represented by the Mt. Crozier series, the most radiogenic Pb flood basalts of the Kerguelen Archipelago (Mattielli et al., 2002; Weis et al., 2002). The two analyzed 1140 samples that plot off this trend have clearly been enriched in U by alteration at or just before their emplacement at 34 Ma (see Fig. 8); after age-correction for in situ ²³⁸U decay, they plot along the linear trend. Similarly, in a ⁸⁷Sr/⁸⁶Sr vs ²⁰⁶Pb/²⁰⁴Pb diagram (Fig. 12c), sample 1140A-32R4 0–6 cm from Unit 5 has a very high measured ²⁰⁶Pb/²⁰⁴Pb relative to its ⁸⁷Sr/⁸⁶Sr and plots off the 1140 mixing trend. However, after correction for in situ decay, this sample falls within the linear mixing array (²³⁸U/²⁰⁴Pb > 60) (Table 2). Another sample from Unit 6 (Sample 1140–37R-3, 104–108 cm Piece 2) is also slightly off the trend, but this shipboard sample has no U–Pb concentration data. Each of these three samples has high K₂O concentrations for their MgO contents (Fig. 7) and it is likely that they have very high ²³⁸U/²⁰⁴Pb as a result of alteration.

	DISCUSSION

TOP ABSTRACT INTRODUCTION GEOLOGICAL SETTING, BASEMENT... ANALYTICAL TECHNIQUES RESULTS DISCUSSION CONCLUSIONS REFERENCES

 
Mixing of depleted and plume components
The linear correlation defined by Site 1140 lavas in isotopic ratio plots and the range of variation in <90 m of core is remarkable (Fig. 12). For example, correlation coefficients for the calculated initial ratios are 0·992 for ⁸⁷Sr/⁸⁶Sr–¹⁴³Nd/¹⁴⁴Nd, 0·92 for ²⁰⁸Pb/²⁰⁴Pb–²⁰⁶Pb/²⁰⁴Pb, 0·64 for ²⁰⁷Pb/²⁰⁴Pb–²⁰⁶Pb/²⁰⁴Pb and 0·96 for ⁸⁷Sr/⁸⁶Sr–²⁰⁶Pb/²⁰⁴Pb. If unit averages are used, the respective correlation coefficients are 0·99, 0·96, 0·86 and 0·98. The correlations defined by Site 1140 basalts in Pb–Pb isotopic plots (Fig. 12b) extend from the field for SEIR N-MORB (Unit 1) into the field defined by flood basalts of the Kerguelen Archipelago (Unit 3). Such trends are consistent with two-component mixing between a depleted MORB component and a plume-derived component and are also observed in Hf–Nd isotopic diagrams (Mattielli et al., 2000). In a Sr–Pb isotope diagram (Fig. 12c), the data fall along a mixing hyperbola for two end-members with different Sr/Pb abundance ratios. The linear trend for Site 1140 lavas in the ⁸⁷Sr/⁸⁶Sr–¹⁴³Nd/¹⁴⁴Nd plot requires that the Sr/Nd abundance ratios were similar in the two mixing components (Langmuir et al., 1978). Similar Sr/Nd abundance ratios are expected in primary MORB and plume-derived magmas, but this ratio varies by a factor of two in Site 1140 lavas (Units 2 and 3 in comparison with Units 1, 5 and 6), largely as a result of plagioclase fractionation (Fig. 10). Also, simple mixing of magmas is inconsistent with the absence of linear trends in x–y plots (Figs 7 and 10). Consequently, we infer that the isotopic trends defined by Site 1140 lavas reflect a mixing process that occurred before shallow-level fractionation of plagioclase. Moreover, there is no obvious petrographic evidence in the Site 1140 lavas for magma mixing.

The Nb/Y vs Zr/Y diagram developed by Fitton et al. (1997) to distinguish lavas derived from the Iceland mantle plume from North Atlantic MORB is also useful in evaluating mixing components in the Kerguelen plume system. In this plot (Fig. 13), the position of the different basalt units of Site 1140 is also consistent with mixing components represented by N-MORB from the SEIR and the Kerguelen mantle plume: Unit 1 plots within the field of SEIR basalts, Units 5 and 6 plot on the line separating the plume field from the MORB field, as defined for Iceland and also valid for Kerguelen, and Units 2 and 3 plot in the plume field and overlap with the tholeiitic–transitional basalts of the Kerguelen Archipelago. In a linear Nb/Y vs Zr/Y plot (inset of Fig. 13), 1140 units plot on a linear trend. Trace element and isotope systematics indicate that mixing of melts is the most likely explanation because mixing of solid mantle components is not likely to generate such strong correlations.

View larger version (35K): [in this window] [in a new window]   Fig. 13. Nb/Y vs Zr/Y diagram (Fitton et al., 1997) for Site 1140 pillow basalts and comparison with other basalts from the Indian Ocean. Literature data as in Fig. 12 except for Indian ridges (Mahoney et al., 2002; K. Nicolaysen, unpublished data, 1999). Site 1140 basalts plot distinctly within the SEIR-MORB field for Unit 1, whereas Units 5 and 6 plot at the limit of the SEIR field and the plume-related magmatism field. Unit 2 and 3 basalts plot within the Kerguelen 29–25 Ma tholeiitic–transitional field, which also corresponds to the position of Site 756, the southernmost site of the Ninetyeast Ridge (Frey & Weis, 1995).

 

A mixing model for the 1140 basaltic units involving depleted MORB-like and plume-derived components is presented in Table 3. Inputs to the model are isotopic ratios and abundance of Sr, Nd and Pb in the end-members (SEIR MORB and Kerguelen plume magmas); outputs are the mixing proportions of the end-members required to match the observed isotopic ratios in each lava unit. Each isotopic system was fitted independently. For end-member components, two sets of data were used. For SEIR MORB, the two sets of isotopic data [SEIR-A and SEIR-B, representing respectively an Indian SEIR MORB composition calculated from literature data (Michard et al., 1986; Dosso et al., 1988; Johnson et al., 2000; Mahoney et al., 2002) and a depleted composition plotting near the depleted end of the SEIR MORB field, in Table 3] are within the field of N-MORB found by Mahoney et al. (2002). For the Kerguelen plume end-member, we used the average value of the Crozier basalt section in the Kerguelen Archipelago and the Crozier sample with the most radiogenic Pb (D. Weis, unpublished data, 1998). The modeling results based on Sr–Nd isotopic ratios indicate that the relative proportion of SEIR MORB component varies from 99 to 69% in the order: Unit 1 > 6 > 5 > 2 > 3. Similar mixing calculations in the Hf–Nd isotopic system indicate 92% of SEIR for Unit 1 and 65% in Unit 3 (Mattielli et al., 2000). For Pb isotopic systematics, the results are more variable (from 90 to 63% for ²⁰⁶Pb/²⁰⁴Pb, from 95 to 67% for ²⁰⁷Pb/²⁰⁴Pb, and from 94 to 82% for ²⁰⁸Pb/²⁰⁴Pb), using the most radiogenic Pb end-member for the Kerguelen plume. In detail, the mixing model does not exactly fit all the data for each unit, and the relative proportion of the SEIR and Kerguelen plume components is not the same for each isotope ratio (Table 3). Nevertheless, in all cases, a large proportion (>60%) of the SEIR component is required.

Our mixing model results have implications for the genesis of volcanic sections in the northern part of the Kerguelen Archipelago, where some of the oldest basalts (29–28 Ma) of tholeiitic–transitional composition show considerable isotopic variation, extending from the enriched Kerguelen plume composition to isotopic compositions that are comparable with those of Units 2 and 3 of Site 1140 (Yang et al., 1998; Doucet et al., 2002). Mixing calculations, using the same end-members as for Site 1140 basalts (an average SEIR MORB and the average of the Crozier basalt section), indicate up to 60–70% of a depleted asthenospheric component for the most depleted signatures in Kerguelen Archipelago basalts, the group D lavas (⁸⁷Sr/⁸⁶Sr 0·7040 and ¹⁴³Nd/¹⁴⁴Nd 0·5129). For these archipelago basalts, this component could represent depleted material that was incorporated into the plume during its ascent to the surface with the additional complexity that the Group D lavas have distinctive Sr/Nd and Ba/Th ratios and Eu anomalies, indicative of the involvement of a plagioclase-rich component (Yang et al., 1998). For the Tourmente section, which is slightly younger (26 Ma), and presents the transition from transitional to alkalic volcanism on the archipelago, the calculated values drop to 55% for Sr and Nd and 70–80% for Pb. This section is nearly homogeneous in isotopic composition and if the basalts of this section were formed by mixing, the mixing process most probably involved solids (mantle components) rather than melts and was extremely efficient (Frey et al., 2002a). We emphasize that the results of the mixing calculations are dependent on the end-member compositions as well as on the assumption that the end-members have homogeneous compositions. Recent SEIR MORB are fairly heterogeneous and the plume composition has probably not been homogeneous over the 115 Myr history of the Kerguelen plume. In addition, these modeled mixing proportions are strongly dependent on the Sr, Nd, Hf and Pb concentrations of the end-members and, especially in the case of the SEIR magmas, one could expect a large range of variations depending on where the magmas formed, i.e. close to or far from a hotspot or the Amsterdam–St. Paul Platform.

Mechanism for mixing plume-derived magmas and MORB-like magmas at Site 1140
Mixing between plume-derived and MORB magmas is typically manifested by along-axis geochemical gradients that reflect flow of a plume component to the spreading center axis (e.g. Schilling et al., 1985). There are both geophysical (Small, 1995; Yale & Phipps-Morgan, 1998) and geochemical indications (Dosso et al., 1988; Storey et al., 1989; Johnson et al., 2000; Mahoney et al., 2002) that the Kerguelen plume has locally affected MORB erupted along the SEIR axis. At 34 Ma, Site 1140 lavas erupted 50 km away from the ridge axis (Fig. 14). The location of the Kerguelen plume at 34 Ma is uncertain, but it is inferred that the plume was coincident with the axis of the newly formed SEIR at 40 Ma (e.g. Frey et al., 2000a). If this assumption is correct, the plume was probably within 200 km of Site 1140 at 34 Ma. It is possible that plume-derived melts at Site 1140 mixed with melts formed at the periphery of the melting regime that created oceanic crust at the ridge axis (e.g. Spiegelman, 1996).

View larger version (30K): [in this window] [in a new window]   Fig. 14. Plate reconstruction at 34 Ma (courtesy of the PLATES Project, University of Texas Institute for Geophysics). The isochrons on the Antarctic and Australian plates are dark and light grey, respectively, and the ridge crest at 34 Ma lies midway between the two (thicker black line). Also plotted are locations of the Kerguelen hotspot (Müller et al., 1993) assuming that either Kerguelen (white star) or Heard (dark star) is the current location of the hotspot. The location of Site 1140 is indicated by the circled cross.

 

Recent geophysical studies and modeling indicate that there may be a significant Kerguelen hotspot-to-ridge flow followed by along-ridge flow (Yale & Phipps Morgan, 1998). The Kerguelen hotspot is at present located beneath the stationary Antarctic Plate, 800 km to the SW of the Amsterdam–St. Paul Platform, which jumped closer to the hotspot than adjacent sections of the SEIR, which are 1200 km away from the Kerguelen hotspot (Small, 1995). In the past, the Indian Plate was moving northwards at 70 mm/yr, but is now moving more slowly, at 35 mm/yr. Satellite gravity data reveal that there are transitions in the structure of the spreading center where it is influenced by the Kerguelen and the Amsterdam hotspots and the Australian–Antarctic Discordance (AAD) (Small, 1995; Small et al., 1999). Changes in axial morphology and segmentation may be controlled by an upper-mantle thermal gradient, possibly a flux of asthenosphere from the hotspots to the AAD (Small et al., 1999). The rapid northward movement of the Indian Plate may also have allowed the preservation of preferential flow paths.

A possible mechanism for explaining the SEIR–Kerguelen plume mixing process observed in Site 1140 basalts and in the oldest flood basalts on the Kerguelen Archipelago (Yang et al., 1998; Doucet et al., 2002 is that the proximity of a spreading center to a plume raises the temperature of the surrounding upper mantle and its viscosity, which could lead to more entrainment by the ascending plume (Hauri et al., 1994). Once established, this mechanism may have allowed for the existence of preferential pathways between the SEIR and the plume lasting a few million years. Significant hotspot–ridge interactions have been documented along the SEIR. He isotopic compositions of dredged basalts along the SEIR indicate that there is either widespread contamination of the asthenosphere by the Kerguelen hotspot or localized input of some Kerguelen plume material along ridge segments to the SE of the Amsterdam–St. Paul Platform (Graham et al., 1999). Clearly, the Kerguelen hotspot has even today a significant role on the SEIR, especially around the ASP segment.

No role for a continental component in creating the northernmost Northern Kerguelen Plateau
The older Cretaceous Kerguelen Plateau (Sites 738, 747, 749, 750 and 1137) is characterized by significantly lower ²⁰⁶Pb/²⁰⁴Pb (<18·2) than the NKP (Fig. 12b and c). For several of these sites, the low ²⁰⁶Pb/²⁰⁴Pb and higher ²⁰⁷Pb/²⁰⁴Pb have been interpreted as reflecting contamination by a continental crust component (Mahoney et al., 1995; Frey et al., 2000a, 2002b; Weis et al., 2001a; Ingle et al., 2002). In contrast to the Central and Southern Kerguelen Plateau and Elan Bank, where evidence for a continental crust component has been found, there is no geochemical evidence for a continental component in Site 1140 basalts nor in the flood basalts that form the majority of the Kerguelen Archipelago (Weis et al., 1998a, 2002; Yang et al., 1998; Frey et al., 2000b, 2002a; Mattielli et al., 2002).

The temporal evolution of ¹⁴³Nd/¹⁴⁴Nd in lavas associated with the Kerguelen plume is complex, but reflects a change of tectonic setting for volcanism related to the Kerguelen plume (i.e. relative plume–ridge–continent proximity) (Fig. 15). From 115 to 95 Ma, six drill sites range widely in ¹⁴³Nd/¹⁴⁴Nd. Basalts from the three sites with the lowest ratios clearly contain a continental component, i.e. Sites 738 (Mahoney et al., 1995), 747 (Frey et al., 2002b) and 1137 (Weis et al., 2001a; Ingle et al., 2002). At each of these sites, low ¹⁴³Nd/¹⁴⁴Nd is also accompanied by (Nb/La)_PM < 1. Basalts from these sites are all >95 Ma and were emplaced ‘soon’ after the break-up of Gondwana, when continents were still relatively close to each other. These isotopic and geochemical data indicate that during the early stages of Kerguelen Plateau formation, small amounts of partially melted continental fragments were incorporated in Kerguelen plume magmas. Such low ratios (¹⁴³Nd/¹⁴⁴Nd and (Nb/La)_PM < 1) are not found in the younger basalts from the Ninetyeast Ridge, Northern Kerguelen Plateau and Kerguelen Archipelago. Within the last 34 Myr, ¹⁴³Nd/¹⁴⁴Nd has decreased in the NKP, from Site 1140 to the 25–22 Ma archipelago flood basalts, indicating that the plume component has become dominant as the distance between the plume and the SEIR has increased (Fig. 14). Although our sampling of the submarine NKP is still extremely limited, data for Site 1140 add to a growing body of evidence from the archipelago lavas that there was no involvement of continental crust during the formation of the NKP.

View larger version (30K): [in this window] [in a new window]   Fig. 15. 143Nd/144Nd vs age plot for the Kerguelen plume system starting at 120 Ma. There is a clear evolution in the Nd isotopic compositions of the Kerguelen Plateau samples reflecting a decrease of continental crust contamination with time, i.e. increase of 143Nd/144Nd, from Site 738 in the southern end of the plateau to Site 747 in the CKP. The Ninetyeast Ridge basalts (Sites 756, 757 and 758) show a different range of Nd isotopic compositions that have been interpreted as reflecting interaction between the Kerguelen plume and a nearby spreading ridge. At 34 Ma, Site 1140 basalts overlap the range of Ninetyeast Ridge variations and extend it towards more depleted values. Subsequently, the SEIR moved away from the Northern Kerguelen Plateau and in the Kerguelen Archipelago basalts there is a clear decrease of Nd isotopic compositions with time from 30 to 22 Ma. References as in Fig. 12.

 

	CONCLUSIONS

TOP ABSTRACT INTRODUCTION GEOLOGICAL SETTING, BASEMENT... ANALYTICAL TECHNIQUES RESULTS DISCUSSION CONCLUSIONS REFERENCES

 
Site 1140 in the NKP is the first drilled site on the entire Kerguelen Plateau where there is evidence for submarine eruption. Five tholeiitic basalt units were recovered with pillow lavas. These basalts are dated at 34 Ma, which indicates that the northern NKP is much younger than the Cretaceous Central and Southern Kerguelen Plateau (>95 Ma). All geochemical and isotopic characteristics of Site 1140 tholeiitic basalts are intermediate between those of SEIR MORB and those of basalts derived from the Kerguelen plume. At 34 Ma, the SEIR was only 50 km away from the NKP and its proximity to the plume-derived material allowed extensive interaction between melts derived from the Kerguelen plume and from those forming the SEIR. The proportion of the depleted component varies between 99–90% in Unit 1 and 76–63% in Unit 3. As in Kerguelen Archipelago basalts 270 km to the south, there is no evidence for a component derived from continental crust in Site 1140 basalts; thus the NKP appears oceanic in origin.

	ACKNOWLEDGEMENTS

 
We thank M. Lo Cascio for sample preparation, S. Ingle and J. S. Scoates for help in editing the manuscript, P. Ila for supervision of the MIT Neutron Activation Analysis Facility, C. Maerschalk for chemical processing and for isotopic analyses of 20 basalts, and S. Miller and S. Huang for their help in obtaining ICP-MS analyses. This paper benefited greatly from group discussions at Thursday seminars at ULB as well as from the numerous interactions that led to this special issue of Journal of Petrology. We thank D. Pyle and K. Johnson for their reviews. This research was supported by Belgium ARC Grant 98/03-233 from the Communauté Française de Belgique and Fonds National de la Recherche Scientifique (FNRS) Grant 2.4579.99, USSP Grant 418927-BA184 and US NSF EAR Grant 9814313. The FNRS funds the Belgian membership in the ODP program and supported the first author’s participation in ODP Leg 183.

	FOOTNOTES

 
^*Corresponding author. Present address: Earth and Ocean Sciences, University of British Columbia, Vancouver, B.C. V6T 1Z4, Canada.Telephone: 1-604-822.1697. Fax: 1-604-822.6088. E-mail: dweis{at}eos.ubc.ca

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