ALBERT

Description: The principal objective of Leg 187 was to locate the Indian/Pacific mantle boundary by sampling and analyzing 8- to 28-Ma seafloor basalts to the north of the Australian Antarctic Discordance (AAD). In this paper we present Sr and Nd isotopic data from basaltic glasses recovered from the 13 sites drilled during Leg 187. Our data show that the boundary region is characterized by a gradual east-west increase in 87Sr/86Sr, with a corresponding decrease in 143Nd/144Nd across a 150-km-wide zone located east and west of the 127°E Fracture Zone. The Sr-Nd isotopic composition of glasses therefore confirms the general conclusions derived by the Leg 187 shipboard scientific party in that the mantle boundary follows a west-pointing, V-shaped depth anomaly that stretches across the ocean floor from the Australian to the Antarctic continental margins. We document that two systematic trends of covariation between 87Sr/86Sr and 143Nd/144Nd can be distinguished, suggesting that the basalts sampled during Leg 187 formed through the interaction of three contrasting source components: (1) a component that lies within the broad spectrum of Indian-type mantle compositions, (2) a boundary component, and (3) a Pacific-type mantle component. The variations in elemental and isotopic compositions indicate that the boundary component represents a distinct mantle region that is associated with the boundary between the Pacific and the Indian mid-ocean-ridge basalt (MORB) sources rather than a dispersed mantle heterogeneity that was preferentially extracted in the boundary region. However, the origin of the boundary component remains an open question. The three components are not randomly intermixed. The Indian and the Pacific mantle sources both interacted with the boundary component, but they seem not to have interacted directly with each other. Large local variability in isotopic compositions of lavas from the mantle boundary region demonstrates that magma extraction processes were unable to homogenize the isotopic contrasts present in the mantle source in this region. Systematic variations in rare earth element (REE) concentrations across the depth anomaly cannot be explained solely by variations in source composition. The observed variations may be explained by an eastward increase and westward decrease in the degree of melting toward the mantle boundary region, compatible with a cooling of the Pacific mantle and a heating of the Indian mantle toward the mantle boundary.

Description: 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.