Increase of chromium with fractionation in early formed pyroxene: examples from boninite and Kasayama andesite

Abstract

Chromium in pyroxene generally decreases as magmatic fractionation proceeds because of its large octahedral site preference energy. Nevertheless, an increase in Cr with progressive fractionation was found in the early formed proto- and orthopyroxenes from the Bonin Island boninite and augites from the Kasayama olivine andesite. This unusual increase in Cr can be interpreted by the early removal of Cr from magma with Cr-spinel crystallization and the later reaction relations of the crystallized spinels with magma to form pyroxenes.