Evolution of the Horoman Peridotite (Hokkaido, Japan): Implications from pyroxene compositions

Processes occurring in the Earth's upper mantle are important in controlling evolution of the crust-mantle system. The effects of multiple igneous and metamorphic processes are recorded in upper-mantle peridotites, such as the Horoman Peridotite in Hokkaido, Japan. Geochemical studies of these peridotites and their minerals, combined with determination of the spatial geochemical variations, can be used to understand the sequence of processes that affected the peridotite. In this study we show that compositional zoning patterns of major and trace elements in clinopyroxene porphyroclasts reflect a sub-solidus, closed-system transition from garnet peridotite, equilibrated at 20-24 kbar and 1040-1160 degrees C, to plagioclase peridotite equilibrated at similar to 7 kbar and 850-950 degrees C. The preservation of compositionally zoned pyroxenes indicates that this transition was a relatively recent process that probably occurred as the Horoman Peridotite was emplaced into the Hidaka metamorphic belt, The clinopyroxene compositions also show that: (I) harzburgites and lherzolites reacted with a fluid/melt that resulted in relative enrichment of highly incompatible elements, such as the light rare-earth elements; and (2) this enrichment process preceded the sub-solidus breakdown of garnet.