Composition and alteration of volcanic rocks of ODP Hole 119-738C

During ODP Leg 119 one basement hole was drilled at Site 738, on the Southern Kerguelen Plateau. The 38.2 m of basement rocks drilled comprises three basaltic aa-lava flows with basal and top breccias, overlain by Turanian marine carbonates. Site 738 basalts probably erupted near a fracture zone, and were emplaced during the plateau-forming stage of Kerguelen Plateau evolution under quiet, subaerial to shallow water conditions. The basalts are T-MORB, chemically resembling Mesozoic continental flood basalts of the southern hemisphere. Two slightly different magma batches are distinguished by Fe, Ti, Al, Zr, and REE concentrations. Prior to eruption, the magmas had undergone significant olivine and some clinopyroxene fractionation. Incompatible and immobile trace element concentrations and ratios point to a veined upper mantle source, where a refractory mineral assemblage retains Nb, Ta, and the HREE. The basaltic melts derived from this regionally veined, enriched upper mantle have high LREE, and especially Ba and Th concentrations and bear the DUPAL isotopic signature gained from deep- seated, recycled, old oceanic(?) crust. A saponite-celadonite secondary mineral assemblage confines the alteration temperature to <170°C. Alteration is accompanied by net gains of H2O, CO2, K2O, and Rb, higher oxidation, minor Na2O, SiO2 gains, and losses of V and CaO. Released Ca, together with Ca from seawater, precipitated as calcite in veins and vesicles, plumbed the circulation system and terminated the rock/open seawater interaction.

Distribution of calcareous nannofossils in upper Cretaceous and Cenozoic sediments of the Kerguelen Plateau

ODP Leg 119 drilled 11 sites on the Kerguelen Plateau (southern Indian Ocean) and Prydz Bay (East Antarctica). Upper Pliocene through Quaternary sediments were recovered at Site 736 on the northern Kerguelen Plateau; calcareous nannofossils occurred in only a few samples. Over 700 m of middle Eocene through Quaternary sediments was cored at Site 737 on the northern Kerguelen Plateau, and calcareous nannofossils are abundant in the middle Eocene through the middle Miocene sediments. Nearly 500 m of sediments ranging from the lower Turanian to the Quaternary was recovered at Site 738 on the southern Kerguelen Plateau; calcareous nannofossils are abundant from the Miocene downward. Calcareous nannofossils are also abundant in the upper Eocene through Miocene section from Site 744 on the southern Kerguelen Plateau. Except for Core 119-746A-13H, the Neogene sequences drilled at deep-water Sites 745 and 746 off the southern Kerguelen Plateau are devoid of calcareous nannofossils. Occurrences of calcareous nannofossils were generally rare and sporadic at Sites 739 and 742 in Prydz Bay and suggest that the diamictite sequences recovered is as old as middle Eocene-early Oligocene age. Other sites drilled in Prydz Bay (Sites 740, 741, and 743) did not yield calcareous nannofossils. Species diversity of calcareous nannofossils was low (about a dozen) in the southern Indian Ocean in the Late Cretaceous. High-latitude nanno floral characteristics are apparent after the Cretaceous/Tertiary boundary extinctions. Cold climatic conditions limited Oligocene calcareous nannofossil assemblages to fewer than a dozen species, and extinctions of species generally were not compensated by originations of new species. Only a few species of calcareous nannofossils were found in the Miocene sequences, in which Coccolithuspelagicus and one or two species of Reticulofenestra exhibit extreme (0%-100%) fluctuations in assemblage dominance, and these fluctuations may reflect rapid fluctuations in the surface-water temperatures. Further deterioration of climate in the late Neogene essentially excluded calcareous nannoplankton from the Southern Ocean. Significantly warmer water conditions during part of the early-middle Pleistocene were inferred by a few lower-middle Pleistocene calcareous nannofossil species found on the Kerguelen Plateau. The calcareous nannofossil zonation of Roth (1978 doi:10.2973/dsdp.proc.44.134.1978) can be applied to the Upper Cretaceous section recovered at Site 738, and the zonation of Okada and Bukry (1980 doi:10.1016/0377-8398(80)90016-X) can be applied without much difficulty to the Paleocene to middle Eocene sequences from the Kerguelen Plateau. However, some conventional upper Paleogene markers are not useful for southern high latitudes, whereas a few nonconventional species events are useful for subdividing the upper Paleogene sequences. The latter species events include the first occurrence (FO) of Reticulofenestra reticulata, the FO and last occurrence (LO) of Reticulofenestra oamaruensis, the LO of Isthmolithus recurvus, and the LO of Chiasmolithus altus. As the Neogene sequences from the southern Indian Ocean contain only a few long-ranging, cold-water species, or are devoid of coccoliths, calcareous nannofossil zonations remain virtually unworkable for the Neogene in the high-latitude southern Indian Ocean as in other sectors of the Southern Ocean.

Chemical and isotopic compositions of lavas from the Broken Ridge and Southern Kerguelen Plateau

Lavas from several major bathymetric highs in the eastern Indian Ocean that are likely to have formed as Early to Middle Cretaceous manifestations of the Kerguelen hotspot are predominantly tholeiitic; so too are glass shards from Eocene to Paleocene volcanic ash layers on Broken Ridge, which are believed to have come from eruptions on the Ninetyeast Ridge. The early dominance of tholeiitic compositions contrasts with the more recent intraplate, alkalic volcanism of the Kerguelen Archipelago. Isotopic and incompatible-element ratios of the plateau lavas are distinct from those of Indian mid-ocean ridge basalts; their Nd, Sr, 207Pb/204Pb and 2078b/204Pb isotopic ratios overlap with but cover a much wider range than measured for more recent oceanic products of the Kerguelen hotspot (including the Ninetyeast Ridge) or, indeed, oceanic lavas from any other hotspot in the world. Samples from the Naturaliste Plateau and ODP Site 738 on the southern tip of the Kerguelen Plateau are particularly noteworthy, with e-Nd(T) = -13 to -7, (87Sr/86Sr)T=0.7090 to 0.7130 and high 207Pb/204Pb relative to 206Pb/204Pb. In addition, the low-e-Nd(T) Naturaliste Plateau samples are elevated in SiO2 (>54 wt%). In contrast to "DUPAL" oceanic islands such as the Kerguelen Archipelago, Pitcairn and Tristan da Cunha, the plateau lavas with extreme isotopic characteristics also have relative depletions in Nb and Ta (e.g., Th/Ta, La Nb > primitive mantle values); the lowest e-Nd(T) and highest Th/Ta and La Nb values occur at sites located closest to rifted continental margins. Accepting a Kerguelen plume origin for the plateau lavas, these characteristics probably reflect the shallow-level incorporation of continental lithosphere in either the head of the early Kerguelen plume or in plume-derived magmas, and suggest that the influence of such material diminished after the period of plateau construction. Contamination of asthenosphere with the type of material affecting Naturaliste Plateau and Site 738 magmatism appears unlikely to be the cause of low-206Pb/204Pb Indian mid-ocean ridge basalts. Finally, because isotopic data for the plateaus do not cluster or form converging arrays in isotope-ratio plots, they provide no evidence for either a quickly evolving, positive ?Nd, relatively high-206Pb/204Pb plume composition, or a plume source dominated by mantle with e-Nd of -3 to ~0.