GEOCHEMICAL CHARACTERISTICS OF THE UPPERMOST MANTLE BENEATH THE JAPANISLAND ARCS - IMPLICATIONS FOR UPPER-MANTLE EVOLUTION

Geochemical characteristics of clinopyroxene in peridotite xenoliths f rom three volcanos, Megata, On-yama and Kurose, in the Japan arcs are important for understanding igneous and metasomatic processes within t he mantle wedge. The clinopyroxenes in peridotite xenoliths from the J apan arcs are clearly different from those in abyssal peridotites and the peridotite xenoliths from other tectonic settings, such as contine ntal rifts and oceanic hotspots. Geochemical characteristics of the su b-are clinopyroxenes are not apparently related either to degree of hy dration or to degree of refractoriness, but are consistent from one sa mple to another. The REE patterns vary from LREE-depleted pattern to f lat or slightly LREE-enriched patterns. Then their (Ce/Yb)(N) (subscri pt N = chondrite-normalized) vary widely from 0.04 to 4.0. On the othe r 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, t he most fertile peridotites from Japan arcs are similar for the clinop yroxene chemistry to the most fertile abyssal peridotites. The peridot ite xenoliths from Japan arcs had possibly evolved through different p rocess 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 meta somatism. (C) 1998 Elsevier Science B.V. All rights reserved.