伊豆南崎火山ベイサナイト質溶岩のクロム・スピネル

Abstract

Spinels (sensu lato) in the Nanzaki basanitoid show a continuous compositional variation from chromian spinel (sense stricto) enclosed in the magnesian olivine and augite phenocrysts to titaniferous magnetite in the groundmass. The chromian spinel is one of the most aluminous found in microphenocrysts of volcanic rocks so far reported, suggesting that it may have crystallized at relatively high pressures. Judged by the present authors' results and data from the literature, the Cr/Al ratio in chrome-spinels from volcanic rocks tends to increase in the order of alkalic basalt, abyssal tholefite, island-arc tholefite, calcalkalic basalt, and magnesian andesite, which is also an increasing order of silica-saturation of the rocks. Chromium-rich chromite would crystallize from the silica-rich magma that lost a large amount of water at lower pressures, because the solubility of Cr3+ in magma decreases with increasing silica and decreasing water contents.