Mid-ocean-ridge basalt (MORB) erupted along the Indian Ocean spreading center are isotopically distinct from those erupted along the Pacific and Atlantic Ocean spreading centers (Dupré and Allègre, 1983; Hart, 1984). Indian Ocean MORB is characterized by high 87Sr/86Sr, 207Pb/204Pb, and 208Pb/204Pb for a given value of 206Pb/204Pb (e.g., Klein et al., 1988). Within the Australian Antarctic Discordance (AAD), located along the Southeast Indian Ridge (SEIR) south of Australia (Fig. F1A), a uniquely sharp isotopic boundary between the Indian Ocean and Pacific Ocean mantle provinces has been recognized (Klein et al., 1988). Based on isotopic analyses of MORB samples dredged from the present spreading axis in the AAD, only basalts younger than 3-4 Ma exhibit Pacific-type isotope signatures (Pyle et al., 1992; Christie et al., 1998). These data form the basis for a hypothesis that proposes rapid westward migration of Pacific mantle during the last 4 m.y. (Christie et al., 1998).

During Ocean Drilling Program (ODP) Leg 187 (Shipboard Scientific Party, 2001), 23 holes were drilled at 13 sites across the hypothesized isotope boundary forming the eastern terminus of the AAD along a swath of 10- to ~30-Ma seafloor interpreted from magnetic anomalies (Fig. F1B). Isotope mantle types, Indian, Pacific, and transitional-Pacific, were assigned to basalts collected during Leg 187 based on shipboard geochemical analysis of Ba and Zr contents of basalt glasses of younger lavas from the AAD and east of the AAD (Shipboard Scientific Party, 2001). The following relationships are summarized from the shipboard identifications: (1) there is a plausible mantle province boundary east of the regional residual depth anomaly (difference between the depth of seafloor and that expected from depth-age relations found for other ocean basins: e.g., Marks et al., 1990), and (2) basalts derived from Indian and Pacific mantle types alternate on a timescale of a few million years between 28 and 14 Ma along a flowline in western Zone A (Shipboard Scientific Party, 2001).

In this study, I will report chemical compositions of minerals in basalts collected from the AAD during Leg 187. Complemented with whole-rock major element compositions, I will discuss petrogenetic conditions, particularly pressure (depth) for magma segregation and estimate primary magma compositions for Indian- and Pacific-type MORB.