9403583 Sempere An improved understanding of the systematics of mantle circulation is a high scientific priority in solid earth geophysics. By mapping the areal distribution of crustal compositions over key areas of ocean floor, quantitative information about the size, composition, and distribution of mantle reservoirs, variations in mantle temperatures, the efficiency of convective stirring in the mantle, and the longevity of individual mantle reservoirs can be obtained. One very important problem of this type, which can be addressed by surficial sampling and crustal drilling, concerns the origin and evolution of the distinctive isotopic and geochemical signature of the Indian Ocean upper mantle. Lavas erupted at Indian Ocean spreading centers are isotopically distinct from those of the Pacific. A unique and sharp boundary between Pacific and isotopic provinces has been identified within the Australian-Antarctic Discordance along the spreading ridge. The objective of the present project is to map this boundary away from the ridge crest, since limited available data tends to indicate a westward migration of the boundary over time. Seismic reflection, gravity and magnetic data and dredge samples will be collected in the off-axis area during a 40 day research cruise in early 1996. These data will also provide site-survey information in support of a proposal to the Ocean Drilling Program to sample old crust in an attempt to map the history of the boundary. The project involves scientists from the University of Washington, Oregon State University, and Woods Hole Oceanographic Institution. ***
