9406105 Frey The 8 x 10 Km Horoman Peridotite (Hokkaido, Japan) is a layered upper mantle peridotite that contains a record of melt segregation, migration and reaction processes that have created considerable geochemical heterogeneity. In a collaborative effort with Japanese colleagues, F. Frey (MIT), graduate student E. Takazawa (MIT) and N. Shimizu (WHOI) are determining the geochemical characteristics of Horoman rocks and their constituent clinopyroxene. These data have been used to establish that: (a) the main rock type, plagioclase lherzolite, is a suitable source rock for mid-ocean ridge basalts but that (b) the plagioclase-free lherzolites and harzburgites which are relatively depleted in CaO and Al2O3 have reacted with an incompatible element-rich melt. This enrichment of incompatible elements, expressed by high light REE/heavy REE abundance ratios, is commonly known as "mantle metasomatism". By studying the spatial extent of the melt- peridotite reaction in three dimensions (an objective that cannot be attained by studying mantle xenoliths), the mechanism and scale of this process is being determined. The results of this study will identify the sequences of events and processes that create heterogeneous upper mantle in an arc and oceanic tectonic settings.
