Untitled research work

Minerals occurring as groundmass, phenocrysts and constituents of cumulates are systematically different in chemical composition in an andesite flow at Katadomari, Kuro-shima Volcano, Kagoshima. The groundmass plagioclase contains more ∑ Fe than the phenocryst, which in turn, is richer in ∑ Fe than plagioclase of the cumulates. The groundmass pyroxenes (orthopyroxene and augite) have the highest Al and Ti contents, whereas pyroxenes of the three distinct generations are nearly identical in Na+K content. Oxidation accompanied by the lava eruption might have contributed a little to the increase in ∑ Fe content in plagioclase. However, most observations can be explained rationally in terms of different growth rate of the minerals (groundmass _??_ phenocrysts _??_ cumulates) combined with differences of cations in diffusion coefficient and crystal/liquid partition coefficient in the magma. A cation with a small diffusion coefficient tends to increase in concentration in portions of a liquid immediately adjacent to growing crystals if its partition coefficient is small; and it tends to decrease if the partition coefficient is large. This must result in that the more rapidly grown minerals contain the more amounts of cations with smaller partition coefficients and smaller diffusion coefficients among the exchangeable cations. For certain elements, the phenocryst/groundmass partition coefficient must be far from the equilibrium partition coefficient in magmatic processes.