Sr-Nd-Pb-Hf isotope results from ODP Leg 187: evidence for mantle dynamics of the Australian-Antarctic Discordance

New high precision PIMMS Hf and Pb isotope data for 14-28 Ma basalts recovered during ODP Leg 187 are compared with zero-age dredge samples from the Australian-Antarctic Discordance (AAD) in order to extend the characterization of the Indian/Pacific mantle domain boundary from on-axis to older (14-28 Ma) off-axis crust and to address questions relating to the origin and evolution of the AAD. Based on our new Nd-Hf isotope data we demonstrate that Pacific MORB-source mantle (PMM) was present near the eastern margin of the AAD from as early as 28 Ma―its boundary with Indian MORB-source mantle (IMM) coinciding with the eastern edge of a basin-wide arcuate depth anomaly that is centered on the AAD. This observation rules out models requiring rapid migration of Pacific MORB mantle into the Indian Ocean basin since separation of Australia from Antarctica. Although PMM does not occur west of the fracture zone at 127^oE, IMM is recovered along with PMM up to 100 km east of this transform; at two localities, basalts derived from both mantle domains occur within the same site. Even here, however, IMM and PMM retain their compositional distinctiveness, indicating that the boundary between the two mantle domains is remarkably sharp. Hf isotope data also place constraints on the origin of the mantle reservoirs underlying the AAD. Nd-Hf isotope systematics can be explained by a model in which IMM basalts from the AAD are derived from mantle previously processed above a subduction zone. Such mantle could have been generated within the convergent margin that existed off the east coast of Gondwana throughout most of the Paleozoic and Mesozoic Eras and subsequently recycled into the upper mantle. Upwelling of a stagnated, subducted slab beneath the SEIR (Gurnis et al., 1998) is responsible for the progressive displacement of low-eHf (lithosphere-contaminated) IMM by high-eHf (uncontaminated but subduction-processed) IMM. Gurnis, M., R. D. Muller, R.D., &L. Moresi, Science, 279, 1499-1504, 1998.