THE HYDROUS PHASE-EQUILIBRIA (TO 3 KBAR) OF AN ANDESITE AND BASALTIC ANDESITE FROM WESTERN MEXICO - CONSTRAINTS ON WATER-CONTENT AND CONDITIONS OF PHENOCRYST GROWTH

We have conducted high pressure (to 3 kbar), water saturated melting e xperiments on an andesite (62 wt% SiO2) and a basaltic andesite (55 wt % SiO2) from western Mexico. A close comparison between the experiment al phase assemblages and their compositions, and the phenocryst assemb lages of the lavas, is found in water saturated liquids, suggesting th at the CO2 content was minimal in the fluid phase. Thus the historic l avas from Volcan Colima (with phenocrysts of orthopyroxene, augite, pl agioclase, and hornblende) were stored at a temperature between 950-97 5 degrees C, at a pressure between 700-1500 bars, and with a water con tent of 3.0-5.0 wt%. A hornblende andesite (spessartite) from Mascota, of nearly identical composition but with only amphibole phenocrysts, had a similar temperature but equilibrated at a minimum of 2000 bars p ressure with a dissolved water content of at least 5.5 wt% in the liqu id. Experiments on the basaltic andesite show that the most common nat ural phenocryst assemblages (olivine, +/-augite, +/-plagioclase) could have precipitated at temperatures from 1000-1150 degrees C, in liquid s with a wide range of dissolved water content (similar to 2.0-6.0 wt% ) and a corresponding pressure range. A lava of the same bulk composit ion with phenocrysts of hornblende, olivine, plagioclase, and augite i s restricted to temperatures below 1000 degrees C and pressures below 2500 bars, corresponding to <5.5 wt% water in the residual liquid. the re is some evidence for mixing in the (sporadic olivine phenocrysts), the broad theme of the history of both lava types is that the phenocry st assemblages for both the andesitic magmas and basaltic andesitic ma gmas are generated from degassing and reequilibration on ascent of ini tially hydrous parents containing greater than 6 wt% water. Indeed and esitic magmas could be related to a basaltic andesite parent by hornbl ende-plagioclase fractionation under the same hydrous conditions.