Tracht change of groundmass pyroxene crystals in decompression experiments

Groundmass pyroxene crystals in pumice from the 1914 eruption of Sakurajima Japan show varied combinations crystallographic faces (i.e., ‘tracht’). To investigate whether groundmass tracht depends on magma decompression conditions, we performed isothermal single–step experiments hydrous dacite magma. The was held under water–saturated conditions at 920 °C, 120 MPa, and oxygen fugacity no more oxidizing than one log unit above Ni–NiO equilibrium for 24 h. Then, a control experiment immediately quenched, whereas others were decompressed to final pressures 20, 10, or 5 MPa 3 h before quenching. pyroxenes that quenched 20 showed octagonal shapes, those lower characteristically had hexagonal shapes. Some near plagioclase because crystallization locally increased supersaturation melt. We conclude changes as degree effective undercooling increases thus reflects history during its ascent volcanic conduit.