Geochemical characteristics of the uppermost mantle beneath the Japan island arcs: implications for upper mantle evolution

Geochemical characteristics of clinopyroxene in peridotite xenoliths from three volcanos, Megata, On-yama and Kurose, in the Japan arcs are important for understanding igneous and metasomatic processes within the mantle wedge. The clinopyroxenes in peridotite xenoliths from the Japan arcs are clearly different from those in abyssal peridotites and the peridotite xenoliths from other tectonic settings, such as continental rifts and oceanic hotspots. Geochemical characteristics of the sub-are clinopyroxenes are not apparently related either to degree of hydration or to degree of refractoriness, but are consistent from one sample to another. The REE patterns vary from LREE-depleted pattern to flat or slightly LREE-enriched patterns. Then their (Ce/Yb)(N) (subscript N = chondrite-normalized) vary widely from 0.04 to 4.0. On the other hand the Ti/Zr ratio is rather constant in each sample, around 100. Clinopyroxenes in the Japan arcs peridotite xenoliths are intermediate for Ce and Sr contents, and (Ce/Yb), and Ti/Zr ratios. Furthermore, the most fertile peridotites from Japan arcs are similar for the clinopyroxene chemistry to the most fertile abyssal peridotites. The peridotite xenoliths from Japan arcs had possibly evolved through different process from common source peridotite to abyssal peridotite. The Japan are mantle peridotites had been polluted by the metasomatic agent with consistent chemical characteristics due to regional mantle wedge metasomatism. (C) 1998 Elsevier Science B.V. All rights reserved.