Kerguelen Plume Research Articles

Geochemical features of the quaternary lamproitic lavas of Gaussberg Volcano, East Antarctica: Result of the impact of the Kerguelen plume

Petrological-geochemical data were obtained on the lamproites of the Gaussberg Volcano located at the eastern Antarctic coast and compared with data on the magmatic rocks developed in the East Antarctica and Indian Ocean in relation with the Kerguelen plume. It was shown that the lamproites were derived from the ancient Gondwana lithosphere repeatedly modified at the early stages of its evolution, including significant enrichment in volatiles, lithophile elements, and radiogenic Sr and Pb isotopes.

Lead isotope signatures of Kerguelen plume-derived olivine-hosted melt inclusions: Constraints on the ocean island basalt petrogenesis

The nature of magmatic sources reflected by isotopic composition of the ocean island basalt (OIB) remains an on-going question in igneous geochemistry. To constrain the magmatic sources for OIB related to the Kerguelen plume activity, we performed detailed microanalytical investigation of the 21.4 Ma picritic basalt (MD109-D6-87) dredged during the “Marion Dufresne” cruise on a seamount between Kerguelen Archipelago and Heard Island. Lead isotope compositions of olivine-hosted melt inclusions and matrix glasses were measured by Laser Ablation Multiple Collector Inductively Coupled Plasma Mass Spectrometry (LA-MC-ICP-MS) and Secondary Ion Mass Spectrometry (SIMS). We also performed major and trace element microanalyses and mapping of the inclusions and the host olivine phenocrysts by electron microprobe (wavelength-dispersive X-ray spectroscopy, WDS). The observed significant major element (K 2 O/P 2 O 5 , Al 2 O 3 /TiO 2 ) and Pb isotope ( 207 Pb/ 206 Pb and 208 Pb/ 206 Pb) heterogeneities of parental melts (MgO = 7–10 wt.%) during early high pressure crystallisation stage (200–300 MPa, Fo 82–86 mol%), and relative homogeneity at later lower-pressure crystallisation stage (< 100 MPa, Fo 75–80 mol%) are interpreted by mixing between “Plume” and “Assimilant” melts during magma residence and transport. Lead isotope composition of the parental basaltic melts was inherited from both heterogeneous mantle and the Kerguelen Plateau crust. High K 2 O/P 2 O 5 (> 4), Al 2 O 3 /TiO 2 (> 4) ratios are attributed to assimilation of the plateau basaltic crust (≥ 50 wt.%) by the melts in the magma chamber at palaeodepths from 6 to 9 km. The crustal assimilation may have happened through plagioclase dissolution. The large chemical and isotopic heterogeneity of the parental OIB melts found by in situ microanalyses in this study suggests that the bulk rock chemistry alone cannot provide enough information to constrain the nature of the magmatic sources. • The new data on the Kerguelen melts are important for understanding the OIB genesis. • The plume mantle composition cannot be known from the OIB bulk rock chemistry. • Heterogeneous sources affect composition of the OIB parental melts. • Crustal assimilation happens through plagioclase dissolution. • Magma mixing is a dominant and fast process in the evolution of the OIB.

Structure of the Earth’s crust and tectonic evolution history of the Southern Indian Ocean (Antarctica)

The Southern Indian Ocean comprises large sedimentary basins of the Riiser-Larsen Sea (western sector); the Cosmonauts, Cooperation (Commonwealth), Davis seas (central sector); and the Mawson-d’Urville seas (eastern sector). The main tectonic provinces of the Southern Indian Ocean (Antarctica) have been outlined as a result of comprehensive interpretation of the geophysical data. Special attention is paid to determining the boundary between the rifted continental and oceanic crust. The basin of the Riiser-Larsen Sea was formed in the Early Jurassic under the action of the Karoo mantle plume. The intrusive complex, as a remote manifestation of the mantle plume, occurs along the inner boundary of the marginal rift. Opening of the ocean in the basin of Riiser-Larsen Sea started about 160 Ma ago and was characterized by rearrangement of plate motion and intense volcanic activity at the early stage. In the basin of the Cosmonauts, Cooperation, and Davis seas, the final stage of rifting was accompanied by the rise of the lithospheric mantle and by intrusive magmatism. The opening of the ocean started here 134 Ma ago. Emplacement of the Kerguelen plume resulted in jumping of ridges and detachment of continental crustal blocks from the Indian margin with the formation of the Kerguelen Plateau (microcontinent). The basin of the Mawson-d’Urville seas has evolved under conditions of long-term rifting since the Late Jurassic and is characterized by an extended zone of mantle unroofing. Breakup of the lithosphere between Australia and Antarctica developed asynchronously over a time interval of 95–65 Ma ago with propagation of MOR from the west eastward. The research was carried out using a great body of geophysical information (∼140000 km of CDP seismic profiling, more than 250 stations of seismic refraction sounding, and more than 250000 km of magnetic and gravity profiles) obtained by expeditions from many countries over more than 30 years.

Petrology and geochemistry of high-titanium and low-titanium mafic dykes from the Damodar valley, Chhotanagpur Gneissic Terrain, eastern India and their relation to Cretaceous mantle plume(s)

The Damodar valley within the Chhotanagpur Gneissic terrain at the northern-most margin of the Singhbhum craton, eastern India, is perhaps the only geological domain in the entire Indian shield which hosts the early Cretaceous Rajmahal as well as the late Cretaceous Deccan igneous activities. A number of Cretaceous mafic dykes intrude the Gondwana sedimentary formations and are focus of the present study. One set of these dykes strike NNE to ENE, are very fresh and mainly exposed within the Jharia, Bokaro and Karanpura basins; whereas the other set of dykes (including the well-known Salma mega dyke) trend NW to NNW, intrude mainly the Raniganj basin and show meagre hydrothermal alteration. Majority of the samples from both these dyke groups display ophitic or sub-ophitic textures and are essentially composed of augite/titan augite and plagioclase. On the basis of petrographic and geochemical characteristics the NNE to ENE dykes are identified as high-Ti dolerite (HTD) dykes and the NW to NNW dykes are referred to as low-Ti dolerite (LTD) dykes. Apart from the first-order distinction on their titanium contents, both these groups also show conspicuous geochemical differences. The HTD dykes contain relatively high values of iron, and high-field strength elements than those from the LTD dykes with an overlapping MgO contents.Although available field, paleomagnetic and limited geochronological data for most of the studied dykes suggests their emplacement during early Cretaceous period (110–115Ma), the Salma dyke, dated to be of Deccan-age at ∼65Ma, is an exception. Geochemically all the studied samples show an undoubted plume-derived character but their unequivocal affinity to either the early Cretaceous Kerguelen (Rajmahal) or the late-Cretaceous Reunion (Deccan) plume is not straightforward since they share bulk-rock characteristics of rocks derived from both these plumes. Even though, the spatial and temporal association of the mafic dykes of present study with the Rajmahal Traps are suggestive of their linkage to the Kerguelen plume activity, robust geochronological and paleomagnetic constraints are clearly required to understand the relative contributions of the two Cretaceous mantle plumes in the genesis of the mafic igneous activity in this interesting domain.

Geochemical evolution of Indian Ocean basaltic magmatism

A comparison of new and published geochemical characteristics of magmatism in the western and eastern Indian Ocean at the initial and recent stages of its evolution revealed several important differences between the mantle sources of basaltic melts from this ocean. 1. The sources of basalts, from ancient rises and from flanks of the modern Central Indian Ridge within the western Indian Ocean contain an enriched component similar in composition to the source of the Reunion basalts (with radiogenic Pb and Sr and unradiogenic Nd), except for basalts from the Comores Islands, which exhibit a contribution from an enriched HIMUlike component. 2. The modern rift lavas of spreading ridges display generally similar geochemical compositions. Several local isotopic anomalies are characterized by the presence of an EM2�like component. However, two anomalous areas with distinctly different enriched mantle sources were recognized in the westernmost part of the South� western Indian Ridge (SWIR). The enriched mantle source of the western SWIR tholeiites in the vicinity of the Bouvet Triple Junction has the isotopic ratios indicating a mixture of HIMU + EM2 in the source. The rift anomaly distinguished at 40° E displays the EM1 signature in the mantle source, which is character� ized by relatively low 206 Pb/ 204 Pb (up to 17.0) and high 207 Pb/ 204 Pb, 208 Pb/ 204 Pb and 87 Sr/ 86 Sr. This source may be due to mixing with material from the continental lithosphere of the ancient continent Gondwana. The material from this source can be distinguished in magmas related to the Mesozoic plume activity in Ant� arctica, as well as in basalts from the eastern Indian Ocean rises, which were formed by the Kerguelen plume at 100-90 Ma. 3. The geochemical heterogeneities identified in the ancient and presentday magmatic products from the western and eastern Indian Ocean are thought to reflect the geodynamic evolution of the region. In the east� ern part of the ocean, the interaction of the evolving Kerguelen plume with the rift zones produced magmas with specific geochemical characteristics during the early opening of the ocean; such a dispersion of magma composition was not recognized in the western part of the ocean.

Revised tectonic evolution of the Eastern Indian Ocean

Published plate tectonic models for the Australian‐Antarctic plate pair imply geologically improbable scenarios at either or both ends of the Cretaceous rift and spreading system. Controversy also exists around the location of and motion at the plate boundary extending west of Australia‐Antarctica, through the Kerguelen Plateau region. We present a plate tectonic model of relative motions among India, Australia, and Antarctica from the onset of continental rifting to the establishment of rapid seafloor spreading, at ~43 Ma. The model conforms to a wide range of geological/geophysical evidence and reconstructs the formation of both the western Kerguelen region and the eastern Tasman region. The incorporation of spatiotemporally continuous plate boundaries reveals the presence of a plate boundary beneath the contiguous Central Kerguelen Plateau and Broken Ridge for ~65 Ma. To investigate the relationship between the plate boundary system and the Kerguelen plume, we test three alternative absolute reference frames. Using a fixed hot spot reference frame, the Indian Ocean mid‐ocean ridge system remains within 500 km of the Kerguelen plume, while the proximity of the plate boundaries and the plume is more variable with a moving hot spot reference frame. Proximity between the plume, plate boundaries, and the Central Kerguelen Plateau/Broken Ridge for ~65 Myr suggests that these specific features were not formed by a single, short‐lived (5–10 Myr) pulse of magmatic activity, but rather by a ~25 Myr period of relatively high magma flux followed by ~40 Myr period of lower volume magmatic activity, an interpretation not excluded by the relatively sparse dredge and drill ages.

Isotopic and trace element geochemistry of alkalic–mafic–ultramafic–carbonatitic complexes and flood basalts in NE India: Origin in a heterogeneous Kerguelen plume

The Archean East Indian cratonic margin was affected by the Kerguelen plume (KP) ∼117Ma, causing flood-basalt eruptions of the Rajmahal–Bengal–Sylhet Traps (RBST). The RBST cover ∼one million km2 in and around the Bengal Basin as alkalic–ultrabasic intrusives in the west and Sikkim in the north, and Sylhet basalts and alkalic–carbonatitic–ultramafic complexes in the Shillong plateau – Mikir hills farther east of the Rajmahal–Bengal Traps. We provide new Nd–Sr–Pb-isotopic and trace element data on 21 unreported discrete lava flows of the Rajmahal Traps, 56 alkalic–carbonatitic–mafic–ultramafic rocks from four alkalic complexes, and three dikes from the Gondwana Bokaro coalfields, all belonging to the RBST. The data allow geochemical correlation of the RBST with some contemporaneous Kerguelen Plateau basalts and KP-related volcanics in the southern Indian Ocean. Specifically, the new data show similarity with previous data of Rajmahal group I–II basalts, Sylhet Traps, Bunbury basalts, and lavas from the southern Kerguelen Plateau, indicating a relatively primitive KP source, estimated as: εNd(I)=+2, 87Sr/86Sr(I)=0.7046, with a nearly flat time-integrated rare earth element (REE) pattern. We model the origin of the uncontaminated RBST basalts by ∼18% batch melting with a 2× chondritic KP source in the spinel-peridotite stability depths of 60–70km in the mantle. The new geochemical data similar to the Rajmahal group II basalts indicate a light REE enriched average source at εNd(I)=−5, 87Sr/86Sr(I)=0.7069. Our geochemical modeling indicates these lavas assimilated granulites of the Eastern Ghats, reducing the thickness of the continental Indian lithosphere. Lack of an asthenospheric MORB component in the RBST province is indicated by various trace element ratios as well as the Nd-Sr isotopic ratios.Three alkalic complexes, Sung, Samchampi, and Barpung in NE India, and one in Sikkim to the north are of two groups: carbonatites, pyroxenites, lamproites, nephelinites, sovites, melteigite in the first group and syenites and ijolites in the second. The Nd–Sr–Pb-isotopic and trace element geochemistry of the first group of carbonatitic–ultrabasic rocks are consistent with similar data of the RBST lavas of the present and previous studies, and are modeled as derived from a relatively primitive carbonated garnet peridotite source in the KP. In contrast, the syenites and ijolites of the second group show a wide range of Nd–Sr–Pb isotopic compositions, modeled by low-degree melts of an ancient recycled carbonated eclogite also in the KP. The KP thus reflects heterogeneities in the lower mantle-derived plume with carbonated components yielding ultrabasic melts at greater depths with low-degree melting, followed by rise of the plume at shallower depths causing tholeiitic flood basalt volcanism. Collectively, these data imply a zone of influence of the plate-motion-reconstructed KP head for ∼1000km around the Bengal Basin, as represented by the widely scattered and diverse rock types of the RBST.

Vestiges of the Kerguelen plume in the Sylhet Traps, northeastern India

The 117 Ma Sylhet Traps, exposed on the southern edge of the Shillong Plateau in northeastern India, are separated from the Rajmahal Traps ~ 550 km to the west by the Gangetic-Brahmaputra alluvium of the Bengal basin. On the basis of their similar age, Sylhet and Rajmahal Traps are correlated. We report Nd–Sr–Pb-isotopic and multiple trace element data for 18 discrete and consecutive lava flows from two sections of the Sylhet Traps. Thirteen of the analyzed lavas are from the Cherrapunji-Shella (CH) Bazaar section and five from the Mawsynram-Balot (MB) section. In major, trace elements and Nd–Sr–Pb isotopes, most of these lavas show similarity with Rajmahal Traps, Bunbury basalts and lavas from Naturaliste and parts of the Kerguelen Plateaus, allowing reconstruction of a ~ 800 km Kerguelen plume-head in the Bengal basin aligned with the Ninetyeast Ridge. The combined geochemical data and their correlation with the Rajmahal Traps, Bunbury basalts, and some Kerguelen Plateau lavas, imply a relatively less depleted plume source for CH basalts. We assess the average composition of this source at 117 Ma to be: ε Nd(I) = 2, 87Sr/ 86Sr(I) = 0.7046, with relatively flat rare earth element patterns, similar to the basalts from the Ocean Drilling Sites 1138, 1141 and 1142 on the Kerguelen Plateau. The Nd–Sr-isotopic data for the Sylhet basalts are modeled with two end members, an 18% partial melt from a chondritic garnet peridotite source, and a granulitic contaminant of the Eastern Ghats Belt. Most of the Sylhet lavas are close to the proposed plume end-member. The contaminated Sylhet basalts reflect as much as 20% of the granulite component caused by the incorporation of lower-continental crust in the Kerguelen plume-derived melt. Combined Nd–Sr–Pb-isotopic evidence, and, in particular, Ce/Pb vs. ε Nd(I) mixing-models among different reservoirs indicate more primitive CH lavas to be mixture of bulk-chondritic Earth and E-MORB, without apparent mixing with N-MORB, continental crust, or non-chondritic bulk Earth. However, Sr–Pb-isotopic ratios of these lavas fall in the estimated ranges of non-chondritic bulk Earth. The least contaminated Kerguelen plume component may be common to other large igneous provinces.

The Kerguelen plateau: Records from a long-living/composite microcontinent

The Kerguelen Plateau lies in the Southern Indian Ocean and is classically considered as a Large Igneous Magmatic Province (LIP) emplaced above an oceanic crust. The interpretation of the gravimetric/bathymetric anomalies shows that the Plateau is structured by NNW- and NW-trending en échelon fractures along its north-eastern border and by WNW-trending faults along its western side which separate very differentiated zones (i.e. Elan Bank and Enderby Basin). A geochemical contamination by continental material has been evidenced for all the studied magmatic rocks of the southern and central part of the Plateau, only the basalts of the northern zone recording the signature of ridge emitted magmas. The facies evolution of the overlying sediments also shows that the deposition began in a subaerial environment for the southern part of the Plateau and that the subsidence notably increased after 60 Ma. All these evidences suggest that the Southern part of the Kerguelen Plateau is underlain by stretched continental crust as it is the case for Elan Bank, and that the Central Kerguelen Plateau is most probably also underlain by attenuated continental crust. In reconstructions, the South and Central Kerguelen Plateaus resulted from the interaction between these remnants of continental crust with the Kerguelen Plume while the latter interacted with the South East Indian Ridge to form the North Kerguelen Plateau. We thus propose a cinematic scenario characterised by the formation of a composite microcontinent composed of fragments of stretched continental crust, separated by ridge propagators which were emplaced successively or contemporaneously between India and Antarctica, during the rifting process, being triggered by the Kerguelen plume emplacement.

Cretaceous potassic intrusives with affinities to aillikites from Jharia area: Magmatic expression of metasomatically veined and thinned lithospheric mantle beneath Singhbhum Craton, Eastern India

Cretaceous potassic dykes and sills at the Jharia area intrude the Permo-carboniferous coal-bearing Gondwana sediments of the Eastern Damodar Valley, Singhbhum craton. These intrusives are widely regarded as a part of the Mesozoic alkaline and Rajmahal flood basalt magmatism in the Eastern Indian shield. Jharia intrusives display a wide petrographic diversity; olivine, phlogopite and carbonate are the predominant phases whereas apatite and rutile constitute important accessories. Impoverishment in sodium, silica and alumina and enrichment in potassium, titanium and phosphorous are the hallmark of these rocks and in this aspect they are strikingly similar to the rift-related aillikites (ultramafic lamprophyres) of Aillik Bay, Labrador. Crustal contamination of the Jharia magmas is minimal and the incompatible trace element ratios demonstrate (i) their generation by greater degrees of partial melting of a sub-continental lithospheric mantle (SCLM) source similar to that of the kimberlites of Dharwar craton, southern India, and (ii) retention of long-term memories of ancient (Archaean) subduction experienced by their source regions. We infer that a metasomatically veined and thinned lithosphere located at the margin of the Singhbhum craton and the inheritance of an ancient (Archaean) subducted component has played a significant role in deciding the diverging petrological and geochemical characters displayed by the Jharia potassic intrusives: those of kimberlites (orangeites) and lamproites (cratonic signature) and those of aillikites (rift-related signature). A substantial melt component of Jharia potassic intrusives was derived from the SCLM and the melt contribution of the Kerguelen plume is inferred to be minimal.

An integrated study of microstructural, geochemical, and seismic properties of the lithospheric mantle above the Kerguelen plume (Indian Ocean)

Peridotite xenoliths brought up to the surface by the volcanism of the Kerguelen Islands represent a mantle that has been affected by a high degree of partial melting followed by intense melt percolation above the Kerguelen plume. These xenoliths are therefore particularly suitable to investigate effects of melt‐rock interaction on crystallographic fabrics (lattice‐preferred orientation (LPO)) of peridotite minerals and on the LPO‐induced seismic properties of peridotites above a mantle plume. We have studied a suite of 16 ultramafic samples representative of different degrees of partial melting and magma‐rock interaction among which the protogranular harzburgites are the least metasomatised xenoliths and dunites are the ultimate stage of metasomatism. Olivine LPO is characterized by high concentration of [010] axes perpendicular to the foliation and [100] axes close to the lineation or distributed in the foliation plane in harzburgites, whereas the high concentration of [100] axes is parallel to the lineation and [010] axes is perpendicular to the assumed foliation in dunites. Olivine LPO in harzburgites is interpreted as being due to a deformation regime in axial compression or transpression. The fabric strength of olivine decreases progressively from protogranular to poikilitic harzburgites and finally to dunites, for which it remains nevertheless significant (J index 3.8). Seismic properties calculated from LPO of minerals indicate that metasomatism at higher melt/rock ratio lowers the P wave velocities. The most significant difference between harzburgites and dunites corresponds to the distribution of S wave anisotropy. Harzburgites display the maximum of anisotropy within the foliation plane and the minimum of anisotropy perpendicular to the foliation plane, whereas the lowest anisotropy is parallel to the lineation for dunites. These modifications of seismic properties as a result of metasomatic processes may induce seismic heterogeneities in the mantle above the Kerguelen plume. In addition, assuming a lithospheric mantle primarily harzburgitic and structured with a horizontal foliation, the seismic properties calculated for the Kerguelen xenoliths reconcile the rather high anisotropy evidenced by the horizontally propagating surface waves with the apparent isotropy revealed by the absence of splitting of vertically propagating teleseismic SKS waves recorded by the GEOSCOPE Kerguelen station.

The Val Gabbro Plutonic Suite: A Sub-volcanic Intrusion Emplaced at the End of Flood Basalt Volcanism on the Kerguelen Archipelago

The age, petrology, major and trace element geochemistry, and Sr^Nd^Hf^Pb isotopic geochemistry of basic and felsic rocks from theVal gabbro plutonic suite on the Kerguelen Archipelago, Southern Indian Ocean, are used to constrain the temporal and compositional relationships between sub-volcanic intrusions and flood basalt volcanism during the formation of a major oceanic island. The 4 km Val gabbro plutonic suite was emplaced at 24 25 0 15Ma (U^Pb zircon) into 25Ma volcanic rocks of the Southeast Province, locally producing a large zone of overlying basaltic breccia. Cumulate basic^ultrabasic rocks are the dominant lithology in the intrusion, with horizontally layered peridotites at the base of the exposed part of the intrusion, overlain by vertically layered, coarsegrained plagioclase-bearing peridotites, melagabbros and equigranular gabbros. The intrusion was formed by repeated injections of relatively crystal-rich and crystal-poor magmas into an open-system magma reservoir. Strong geochemical and isotopic similarities between the fine-grained marginal microgabbros and cross-cutting felsic rocks and the hosting mildly alkalic basalts and trachytes of the Southeast Province indicate that they were derived from similar alkalic basaltic parental magmas, which were dominated by the enriched component of the Kerguelen mantle plume source. At 25Ma, the change from tholeiitic^transitional to mildly alkalic basalts marks the terminal stage of flood basalt volcanism on the Kerguelen Archipelago. This compositional change was associated with deeper melting within the Kerguelen plume source, lower extents of melting, a decrease in magma supply, and the emplacement of high-level intrusions such as theVal gabbro plutonic suite.

Petrogenesis of the earliest Early Cretaceous mafic rocks from the Cona area of the eastern Tethyan Himalaya in south Tibet: Interaction between the incubating Kerguelen plume and the eastern Greater India lithosphere?

The relationship between the breakup of eastern Gondwanaland and the Kerguelen plume activity is a subject of debate. The Cona mafic rocks are widely exposed in the Cona area of the eastern Himalaya of south Tibet, and are studied in order to evaluate this relationship. Cona mafic rocks consist predominantly of massive basaltic flows and diabase sills or dikes, and are divided into three groups. Group 1 is composed of basaltic flows and diabase sills or dikes and is characterized by higher TiO 2 and P 2O 5 content and OIB-like trace element patterns with a relatively large range of ɛNd( T) values (+ 1.84 to + 4.67). A Group 1 diabase sill has been dated at 144.7 ± 2.4 Ma. Group 2 consists of gabbroic sills or crosscutting gabbroic intrusions characterized by lower TiO 2 and P 2O 5 content and “depleted” N-MORB-like trace element patterns with relatively higher, homogeneous ɛNd( T) values (+ 5.68 to + 6.37). A Group 2 gabbroic diabase dike has been dated at 131.1 ± 6.1 Ma. Group 3 basaltic lavas are interbedded with the Late Jurassic–Early Cretaceous pelitic sediments; they have compositions transitional between Groups 1 and 2 and flat to slightly enriched trace element patterns. Sr–Nd isotopic data and REE modeling indicate that variable degrees of partial melting of distinct mantle source compositions (enriched garnet–clinopyroxene peridotite for Group 1 and spinel-lherzolite for Group 2, respectively) could account for the chemical diversity of the Cona mafic rocks. Geochemical similarities between the Cona mafic rocks and the basalts probably created by the Kerguelen plume based on spatial–temporal constraints seem to indicate that an incubating Kerguelen plume model is more plausible than a model of normal rifting (nonplume) for the generation of the Cona mafic rocks. Group 1 is interpreted as being related to the incubating Kerguelen plume–lithosphere interaction; Group 2 is likely related to an interaction between anhydrous lithosphere and rising depleted asthenosphere enriched by a “droplet” originating from the Kerguelen plume, while Group 3 may be attributed to thermal erosion resulting in the partial melting of lithosphere during the long-term incubation of a magma chamber/pond at a shallow crustal level. The Cona mafic rocks are probably related to a progressively lithospheric thinning beneath eastern Gondwanaland from 150–145 Ma to 130 Ma. Our new observations seem to indicate that the Kerguelen plume may have started its incubation as early as the latest Jurassic or earliest Cretaceous period and that the incubating Kerguelen plume may play an active role in the breakup of Greater India, eastern India, and northwestern Australia.

Breakup and early seafloor spreading between India and Antarctica

SUMMARY We present a tectonic interpretation of the breakup and early seafloor spreading between India and Antarctica based on improved coverage of potential field and seismic data off the east Antarctic margin between the Gunnerus Ridge and the Bruce Rise. We have identified a series of ENE trending Mesozoic magnetic anomalies from chron M9o (∼130.2 Ma) to M2o (∼124.1 Ma) in the Enderby Basin, and M9o to M4o (∼126.7 Ma) in the Princess Elizabeth Trough and Davis Sea Basin, indicating that India–Antarctica and India–Australia breakups were roughly contemporaneous. We present evidence for an abandoned spreading centre south of the Elan Bank microcontinent; the estimated timing of its extinction corresponds to the early surface expression of the Kerguelen Plume at the Southern Kerguelen Plateau around 120 Ma. We observe an increase in spreading rate from west to east, between chron M9 and M4 (38–54 mm yr–1), along the Antarctic margin and suggest the tectono-magmatic segmentation of oceanic crust has been influenced by inherited crustal structure, the kinematics of Gondwanaland breakup and the proximity to the Kerguelen hotspot. A high-amplitude, E–W oriented magnetic lineation named the Mac Robertson Coast Anomaly (MCA), coinciding with a landwards step-down in basement observed in seismic reflection data, is tentatively interpreted as the boundary between continental/transitional zone and oceanic crust. The exposure of lower crustal rocks along the coast suggests that this margin formed in a metamorphic core complex extension mode with a high strength ratio between upper and lower crust, which typically occurs above anomalously hot mantle. Together with the existence of the MCA zone this observation suggests that a mantle temperature anomaly predated the early surface outpouring/steady state magmatic production of the Kerguelen LIP. An alternative model suggests that the northward ridge jump was limited to the Elan Bank region, whereas seafloor spreading continued in the West Enderby Basin and its Sri Lankan conjugate margin. In this case, the MCA magnetic anomaly could be interpreted as the southern arm of a ridge propagator that stopped around 120 Ma.

Extensive basaltic magmatism on the naturaliste plateau, offshore SW Australia

The large submarine Naturaliste Plateau (NP), lying off the southwestern tip of Australia, is a poorly known part of Australia’s offshore jurisdiction. Only two dredges, one from the northern margin and one from the southern margin, have recovered rocks from this feature, which is approximately the size of Tasmania. Based on the presence of distinctive 130 m.y. old basalts at Bunbury, on the adjacent coast of Western Australia, it has been claimed that the NP may form the easternmost part of the massive Kerguelen Plateau Large Igneous Province (LIP). In this case, the NP would be underlain by thickened oceanic-type crust, and dredged rocks will provide substantial spatial and age constraints on one of the largest LIPs on Earth. However, a recent Geoscience Australia assessment suggested that much of the NP may be underlain by continental rocks. A recently completed swath mapping and dredging cruise using R/V Southern Surveyor investigated the nature and origin of the NP. Pillowed, but often highly vesicular and autobrecciated basalts were sampled along the entire length of the S margin of the NP, and at several locations on its N and NW margins. Quartz + feldspar-phyric glassy rhyolitic lavas were present in several dredges with basalts, suggesting they are part of the same igneous event. Crustal rocks, including garnet-bearing gneisses and granitic rocks, were also dredged along the S margin, including at the most westerly extent of the NP. We will present new ageand geochemical data for the extensive volcanic rocks of the Naturaliste Plateau, and evaluate possible links with the Kerguelen plume.

Geochemical constraints on the ∼120 myr history of the Kerguelen plume; new insights from high-precision Pb isotopes

Geochemical constraints on the ∼120 myr history of the Kerguelen plume; new insights from high-precision Pb isotopes

Primitive neon and helium isotopic compositions of high-MgO basalts from the Kerguelen Archipelago, Indian Ocean

The geochemical characteristics of mildly alkalic basalts (24–25 Ma) erupted in the southeastern Kerguelen Archipelago are considered to represent the best estimate for the composition of the enriched Kerguelen plume end-member. A recent study of picrites and high-MgO basalts from this part of the archipelago highlighted the Pb and Hf isotopic variations and suggested the presence of mantle heterogeneities within the Kerguelen plume itself. We present new helium and neon isotopic compositions for olivines from these picrites and high-MgO basalts (6–17 wt.% MgO) both to constrain the enriched composition of the Kerguelen plume and to determine the origin of isotopic heterogeneities involved in the genesis of Kerguelen plume-related basalts. The olivine phenocrysts have extremely variable 4He / 3He compositions between MORB and primitive values observed in OIB (∼90,000 to 40,000; i.e., R / Ra ∼8 to 18) and they show primitive neon isotopic ratios (average 21Ne / 21Neext ∼0.044). The neon isotopic systematics and the 4He / 3He ratios that are lower than MORB values for the Kerguelen basalts clearly suggest that the Kerguelen hotspot belongs to the family of primitive hotspots, such as Iceland and Hawaii. The rare gas signature for the Kerguelen samples, intermediate between MORB and solar, is apparently inconsistent with mixing of a primitive component with a MORB-like source, but may result from sampling a heterogeneous part of the mantle with solar 3He / 22Ne and with a higher (U, Th) / 3He ratio compared to typically high R / Ra hotspot basalts such as those from Iceland and Hawaii.

Platinum-group element constraints on source composition and magma evolution of the Kerguelen Plateau using basalts from ODP Leg 183

Seventeen basalts from Ocean Drilling Program (ODP) Leg 183 to the Kerguelen Plateau (KP) were analyzed for the platinum-group elements (PGEs: Ir, Ru, Rh, Pt, and Pd), and 15 were analyzed for trace elements. Relative concentrations of the PGEs ranged from ∼0.1 (Ir, Ru) to ∼5 (Pt) times primitive mantle. These relatively high PGE abundances and fractionated patterns are not accounted for by the presence of sulfide minerals; there are only trace sulfides present in thin-section. Sulfur saturation models applied to the KP basalts suggest that the parental magmas may have never reached sulfide saturation, despite large degrees of partial melting (∼30%) and fractional crystallization (∼45%). First order approximations of the fractionation required to produce the KP basalts from an ∼30% partial melt of a spinel peridotite were determined using the PELE program. The model was adapted to better fit the physical and chemical observations from the KP basalts, and requires an initial crystal fractionation stage of at least 30% olivine plus Cr-spinel (49:1), followed by magma replenishment and fractional crystallization (RFC) that included clinopyroxene, plagioclase, and titanomagnetite (15:9:1). The low Pd values ([Pd/Pt] pm < 1.7) for these samples are not predicted by currently available K d values. These Pd values are lowest in samples with relatively higher degrees of alteration as indicated by petrographic observations. Positive anomalies are a function of the behavior of the PGEs; they can be reproduced by Cr-spinel, and titanomagnetite crystallization, followed by titanomagnetite resorption during the final stages of crystallization. Our modeling shows that it is difficult to reproduce the PGE abundances by either depleted upper or even primitive mantle sources. Crustal contamination, while indicated at certain sites by the isotopic compositions of the basalts, appears to have had a minimal affect on the PGEs. The PGE abundances measured in the Kerguelen Plateau basalts are best modeled by melting a primitive mantle source to which was added up to 1% of outer core material, followed by fractional crystallization of the melt produced. This reproduces both the abundances and patterns of the PGEs in the Kerguelen Plateau basalts. An alternative model for outer core PGE abundances requires only 0.3% of outer core material to be mixed into the primitive mantle source. While our results are clearly model dependent, they indicate that an outer core component may be present in the Kerguelen plume source.

Rapid emplacement of the Kerguelen plume–related Sylhet Traps, eastern India: Evidence from 40Ar‐39Ar geochronology

We report for the first time 40Ar‐39Ar plateau ages for the Sylhet Traps of eastern India. Our results provide concordant ages for two samples, vertically separated by ∼200 m, from a tholeiite lava flow sequence. The ages are indistinguishable at 2σ confidence level indicating a rapid emplacement of these lavas. The weighted mean of the plateau ages associated with least errors, 116.0 ± 3.5 Ma, most likely represents the age of eruption. Clearly, the Sylhet Traps are contemporaneous with the Kerguelen plume generated Rajmahal and Bengal Traps. Our results in conjunction with the existing age data in the Rajmahal‐Bengal‐Sylhet igneous province suggest that the latter experienced widespread, and rapid emplacement of flood basalts at ∼118 ± 2 Ma. Such a large‐scale volcanism would have required a direct involvement of the Kerguelen plume, suggesting that the Kerguelen hotspot was located close to the eastern Indian margin during its initiation.

Constraining the components of the Kerguelen mantle plume: A Hf‐Pb‐Sr‐Nd isotopic study of picrites and high‐MgO basalts from the Kerguelen Archipelago

We report geochemical and Hf‐Pb‐Sr‐Nd isotopic analyses of a suite of picrites and associated high‐MgO volcanic rocks (6–17 wt.% MgO) from the Kerguelen Archipelago, which are rare compared to other oceanic islands, to better constrain the nature and the origin of components present in the Kerguelen mantle plume source. The Sr and Nd isotopic compositions of the transitional to mildly alkalic picrites and high‐MgO basalts closely match those of the 24 Ma mildly alkalic basalts from the Courbet Peninsula, whose compositions are considered to reflect the present geochemical expression of the enriched component of the Kerguelen mantle plume. However, linear trends in Pb isotopic compositions in the studied samples reflect involvement of a component with lower 206Pb/204Pb and 208Pb/204Pb than that inferred for the enriched Kerguelen plume. Contamination of the MgO‐rich magmas by the Kerguelen Plateau cannot account for the observed Hf‐Pb‐Sr‐Nd isotopic variations. Isotopic systematics in the picrites and the high‐MgO basalts are inconsistent with simple binary mixing between two distinct end‐members and indicate the presence of small‐scale heterogeneities within the Kerguelen plume itself as has been observed in other hot spot environments such as Hawaii and Iceland. The 34 to 26 Ma Kerguelen plume‐related basalts that formed when the archipelago was close to the ridge axis (∼50 to 250 km) show geochemical evidence for significant involvement of a Southeast Indian Ridge (SEIR)‐like source. In contrast, the 24–25 Ma mildly alkalic basalts from the eastern and southeastern parts of the archipelago, which erupted about 400 km away from the SEIR, and the picrites and high‐MgO basalts from this study show little or no contribution from a SEIR‐like component. Thus chemical interaction between the SEIR axis and the Kerguelen plume effectively ceased prior to 25 Ma.