Water in Magmas: Experimental Constraints on the Stability of Hornblende in Basalts and Basaltic Andesites

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9706129 Carmichael Previous work has determined that the phenocryst assemblages of andesitic lavas in western Mexico are strongly influenced by the concentrations of water that the magmas contain. Basalts and basaltic andesites from central Mexico may contain hornblende phenocrysts and it is proposed to quantify the P-T conditions, together with the water concentration in the liquids, in the regimes in which hornblende is stable. In addition, phenocrysts of pyroxene and olivine, often without plagioclase, will be used to constrain the water concentrations in basalts and basaltic andesites without hornblende. Ferric-ferrous ratios in the hornblendes will be calibrated with oxygen fugacity in an internally heated pressure vessel in which the rapid quench preserves the liquid as a glass. In addition, the geological age and environment of andesites-basalts in central Mexico are so poorly known that mapping and sampling will be completed in Zitacuaro region of Michoacan, Mexico.