Dacite lava including fresh and oxidized biotite phenocryst

Oxygen fugacity in Terayama dacite magma, Osaka, Japan

Abstract

The Terayama dacite, which is middle Miocene in age, has a lava dome/flow about 3 kilometers in diameter in the Setouchi volcanic zone, Japan. The dacite contains quartz, plagio-clase, magnesium rich orthopyroxene, hornblende and biotite phenocrysts. The dacite is divided into three rock types by degrees of oxidation of biotite phenocryst. The first type, named F-type, includes fresh biotite. The second, named M-type, includes fine grain Fe-Ti oxide minerals
concentrated along its cleavage and a margin of biotite. The third, named O-type includes dark red biotite which has been completely replaced by Fe-Ti oxide minerals and secondary silicate minerals. These Fe-Ti oxide minerals are composed of ilmenite, magnetite, hematite and rutile. In the primary stages, though they were composed of magnetite+ilmenite, sometimes secondary hematite has replaced the margin of magnetite. Temperature and oxygen fugacity of both M and 0-types of magma are about 700°C and f_o2=10^-14 on the basis of chemical data of magnetite+ilmenite. This temperature and ƒo₂ fall between the FMQ and MH buffer curve, which is a stable location for biotite and/or amphibole phenocryst. Therefore Fe-Ti oxide minerals have been
formed from an alteration of biotite by dehydration with degassed dacite magma, not by oxida-tion. F-type lava erupted at the beginning and was followed by the extension of M and O-type lava. Dehydration in vent and/or magma chamber was started by an eruption of F-type magma.