U.S.-Japan Cooperative Research: Study of Melt-Solid Segregation in the Upper Mantle as Represented by the Horoman Peridotite, Hokkaido, Japan

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This award will facilitate cooperation in earth science between Professors Frederick A. Frey and Nobumichi Shimizu, Department of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, and Professor Masaaki Obata, Department of Geology, Kumamoto University, Japan, and several Japanese colleagues. The scientists will collaborate in a study of the Horoman peridotite, a large, exposed igneous rock formation on the Japanese island of Hokkaido. It is an unusual formation, derived from the earth's upper mantle, and characterized by a well developed layer structure, with systematic changes in mineral composition and well-exposed rocks which are relatively free from crustal alteration. These properties make it ideal for studying certain geochemical processes that have occurred in the upper mantle, particularly the formation, segregation and migration of partial melts. The observed layering has been interpreted as reflecting "magma waves" in a partially molten, ascending mantle. The Horoman may thus actually be a piece of upper mantle where the processes of melt formation and segregation have been frozen in situ. The cooperating scientists will combine a quantitative field and petrologic study of the peridotite formation with geochemical analyses for major and trace element abundances and radiogenic isotope ratios in the rocks. A study of layered upper mantle rocks is important because it provides information on the partial melting and melt segregation occurring in the earth's upper mantle. Though poorly understood, these processes appear to be fundamental to the past, current, and future evolution of the earth. This investigation of the Horoman peridotite is aimed at contributing to a better understanding of these processes.