POST-ERUPTION OXIDATION OF HORNBLENDE PHENOCRYST FROM KAITAKU, SHODO-SHIMA, JAPAN

Abstract

The variation of hornblende phenocrysts in a single lava flow has been studied by the determination of optical properties and the partial chemical analyses. Average 100×Fe³⁺/total Fe value is estimated to be about 25 for the green common hornblende dominantly contained in the chilled base, whereas it is about 80 for the reddish oxyhornblende scattered in the interior of the lava. Oxidation, developed during the initial cooling, is at a maximum at points approximately 2m. above the base as a whole. The degree of the primary post-eruption oxidation seems in general to be a function of the rate of cooling and that of volatile loss, except for a case of reheating.