Collaborative Research: A Uranium-Series and Hafnium Isotope Investigation of the Link Between Partial Melting and Mantle Heterogeneity Beneath the Southeast Indian Ridge

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Under this award, the PIs will measure 176Hf/177Hf and 238U-230Th-226Ra-232Th activities in basalt glasses from the Southeast Indian Ridge (SEIR)to test how the depth and rate of melting control the sampling of regional heterogeneity in the underlying mantle during ocean ridge magmatism. With these new data, it should be possible to sort out differences in the mineralogy of the mantle source, the depth and extent of melting, and the source compositions. The SEIR was chosen for the study, because along it a number of systematic variations occur at constant full spreading rate (70-75 mm/y) and in the absence of large transform offsets and nearby ocean island hotspots:(a)there is a large eastward increase in axial depth (2300 to 5000 m) and a morphological transition from axial high to axial valley; (b) there is evidence that melt production rate and crustal thickness decrease eastward along axis; and (c) lavas from this setting record systematic variations in temperature and mantle composition. It has also been the subject of previous petrologic and isotopic analyses of numerous basalt glasses. The PIs will search for and use correlations of 230Th/232Th, 230Th/238U, Th/U, 226Ra/230Th, Ba/Th and 176Hf/177Hf with these physical and geochemical variations to test two fundamental hypotheses:1) A regional increase in ridge depth at constant spreading rate is primarily controlled by the depth and rate of melting. 2) The distinctive Pb-Sr-Nd isotope signature of Indian Ocean MORBs is due to the presence of garnet pyroxenite in the shallow asthenosphere.