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 |
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| Authors: | Gordon Moore I S E Carmichael |
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| Institution: | (1) Department of Geology and Geophysics, University of California, Berkeley, CA 94720, USA, US |
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| Abstract: | We have conducted high pressure (to 3 kbar), water saturated melting experiments on an andesite (62 wt% SiO2) and a basaltic andesite (55 wt% SiO2) from western Mexico. A close comparison between the experimental phase assemblages and their compositions, and the phenocryst
assemblages of the lavas, is found in water saturated liquids, suggesting that the CO2 content was minimal in the fluid phase. Thus the historic lavas from Volcan Colima (with phenocrysts of orthopyroxene, augite,
plagioclase, and hornblende) were stored at a temperature between 950–975 °C, at a pressure between 700–1500 bars, and with
a water content 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 pressure with a dissolved
water content of at least 5.5 wt% in the liquid. Experiments on the basaltic andesite show that the most common natural phenocryst
assemblages (olivine, ±augite, ±plagioclase) could have precipitated at temperatures from 1000–1150 °C, in liquids with a
wide range of dissolved water content (∼2.0–6.0 wt%) and a corresponding pressure range. A lava of the same bulk composition
with phenocrysts of hornblende, olivine, plagioclase, and augite is restricted to temperatures below 1000 °C and pressures
below 2500 bars, corresponding to <5.5 wt% water in the residual liquid. Although there is some evidence for mixing in the
andesites (sporadic olivine phenocrysts), the broad theme of the history of both lava types is that the phenocryst assemblages
for both the andesitic magmas and basaltic andesitic magmas are generated from degassing and reequilibration on ascent of
initially hydrous parents containing greater than 6 wt% water. Indeed andesitic magmas could be related to a basaltic andesite
parent by hornblende-plagioclase fractionation under the same hydrous conditions.
Received: 10 December 1996 / Accepted: 21 August 1997 |
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