Experimental constraints on plagioclase liquidus relations and decompression induced crystallization in hydrous basalt.

Experiments are used to constrain the onset of plagioclase crystallization in basalt during decompression crystallization starting from depths equivalent to 200 MPa (2 kb, where P H2O = P total). TZM experiments were run using starting material based on the 1921 Kilauea basalt and range from dry to supersaturated (up to 6% H2O). Water solubility and plagioclase liquidus curves are each determined for the basalt with 2% and 6% water content. In all cases, the liquidus phase is aluminous spinel, followed by clinopyroxene, then plagioclase. Plagioclase liquidus temperatures determined experimentally are 59-81 C hotter than predicted by MELTS. With 2% H2O, plagioclase-in temperatures vary from nearly 1200 C at low pressures to 1090 C at 150 MPa. With 6% H2O, the plagioclase liquidus is approximately 50 C lower. Decompression experiments are also compared with cooling experiments conducted at and below 200 MPa. Plagioclase crystals in cooling experiments at 100 MPa are larger (up to 1mm length) and have higher aspect ratios than decompression experiments from the same starting conditions. Applications of this work include better understanding the groundmass crystallization of hydrous basalt as it traverses the conduit during eruption. Distinguishing crystal morphologies in decompression and cooling experiments is also important for characterizing groundmass crystallinities resulting from pre- and post- eruption conditions. These data will be useful in quantifying processes associated with a'a and pahoehoe flow morphologies. (1) Ghiorso & Sack, 1995.