Lavas erupted at Indian Ocean spreading centers are isotopically distinct from those of the Pacific Ocean, reflecting a fundamental difference in the composition of the underlying upper mantle. Along the Southeast Indian Ridge (SEIR), the Indian Ocean and Pacific Ocean isotopic provinces are separated by a uniquely sharp boundary. This boundary has been located to within 25 km along the spreading axis of the SEIR within the Australian-Antarctic Discordance (AAD; Klein et al., 1989; Pyle et al., 1992; Christie et al., 1998), and subsequent off-axis sampling has shown that the Pacific mantle has migrated rapidly westward during at least the last 4 m.y. Specifically, Leg 187 investigations will delineate this boundary farther off-axis, allowing us to infer its history over the last 30 m.y.

Because of its proximity to the AAD, this project exploits a unique opportunity to quantify the dynamic behavior and composition of the Earth's upper mantle. In terms of the Ocean Drilling Program (ODP) Long Range Plan, this proposal addresses a fundamental problem in mantle dynamics, including relationships among ocean crustal composition, mantle composition, spreading, and magma supply rates. It also has strong ties to the U.S. Ridge Interdisciplinary Global Experiments (RIDGE) program and the international InterRidge program.

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