Lateral variation of K/Hf ratios in Quaternary volcanic rocks of Northeastern Japan.

Abstract

K/Hf ratios of rocks from 39 Quaternary volcanoes in NE Japan have been examined. In most volcanoes, the ratio remains constant throughout the process of magmatic differentiation, suggesting that the ratios in volcanic rocks represent those in primary magmas produced in the upper mantle. The observed lateral variation of the ratios, increasing toward the back-arc side could be influenced by two factors. The first possibility could be the result of systematic differences in the degree of partial melting, decreasing toward the back-arc side, during which process minor minerals such as zircon, sphene, perovskite, and rutile buffer the concentration of Hf in the liquid produced; the second possibility is the result of differences in K/Hf ratios of fluid phases added to the mantle wedge from the downgoing lithosphere in which serpentine and phlogopite decompose beneath fore- and back-arc regions, respectively.