Experimental crystallization of a High-K arc basalt: the golden pumice, Stromboli volcano

The near-liquidus crystallization of a high-K basalt (PST-9 golden pumice, 494 wt % SiO2, 185 wt % K2O, 796 wt % MgO) from the present-day activity of Stromboli (Aeolian Islands, Italy) has been experimentally investigated between 1050 and 1175C, at pressures from 50 to 400MPa, for melt H2O concentrations between 12 and 55 wt % and DNNO ranging from 007 to þ232. A drop-quench device was systematically used. AuPd alloys were used as containers in most cases, resulting in an average Fe loss of 13 % for the 34 charges studied. Major crystallizing phases include clinopyroxene, olivine and plagioclase. Fe–Ti oxide was encountered in a few charges. Clinopyroxene is the liquidus phase at 400MPa down to at least 200MPa, followed by olivine and plagioclase. The compositions of all major phases and glass vary systematically with the proportion of crystals. Ca in clinopyroxene sensitively depends on the H2O concentration of the coexisting melt