Tracht change of groundmass pyroxene crystals in decompression experiments

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