Geochemistry and geochronology of ancient southeast Indian and southwest Pacific seafloor

Abstract

Within the Australian‐Antarctic Discordance (AAD), a boundary exists between isotopically defined “Pacific‐type” and “Indian‐type” mid‐ocean ridge basalt (MORB) erupted along the Southeast Indian Ridge (SEIR). This boundary has migrated westward beneath the easternmost AAD spreading segment at a minimum rate of 25 mm/yr since 4 Ma; however, its long‐term history remains a matter of speculation. To determine if Pacific‐type upper mantle has migrated westward beneath the eastern Indian Ocean basin as Australia and Antarctica drifted apart during the last 70 m.y., we present new Sr‐Nd‐Pb isotope data, combined with trace element and 40 Ar‐ 39 Ar radiometric age determinations, for samples from Legs 28 and 29 of the Deep Sea Drilling Project (DSDP). Basaltic basement at these DSDP sites provides a record of their upper mantle source composition and shows regional variations consistent with upper mantle flow in this region. East of the South Tasman Rise, all DSDP basalts have 87 Sr/ 86 Sr (0.7025–0.7029) and 206 Pb/ 204 Pb (18.80–19.48) ratios typical of Pacific‐type MORB indicating that Pacific‐type upper mantle existed east of the Australian‐Antarctic continental margin and beneath the Tasman Sea during the early stages of seafloor spreading in this region. Basalts from DSDP sites west of the AAD have high 87 Sr/ 86 Sr (0.7030–0.7035), low 206 Pb/ 204 Pb (17.99–18.10) and trace element characteristics typical of present day Indian‐type SEIR MORB. Between these two regions, DSDP basalts recovered along the western margin of the South Tasman Rise have isotopic characteristics that are, in one case consistent with an Indian‐type MORB source (Site 280A) and, in the second case, transitional between Pacific‐type and Indian‐type mantle sources. The occurrence of seafloor basalts with transitional or Indian‐type isotopic characteristics well to the east of the present Indian‐Pacific MORB isotopic boundary within the AAD strongly implies that Pacific‐type upper mantle has migrated westward into the region since the South Tasman Rise separated from Antarctica circa 40 Ma.