A hornblende basalt from western Mexico: Water-saturated phase relations constrain a pressure-temperature window of eruptibility

Trachybasalt scoria from a cinder cone near the Mexican volcanic front contain phenocrysts of olivine with chromite inclusions, apatite, augite and hornblende, with microphenocrysts of plagioclase. The water-saturated phase relations reproduce the phenocryst assemblage between 1040degreesC and 970degreesC with water contents of between 2.5 and 4.5% (50-150 MPa). The absence of biotite phenocrysts in the scoria places a tight constraint on the pressure-temperature conditions of phenocryst equilibration, as there is only a small zone where biotite does not accompany hornblende in the experiments. Diluting the fluid phase with CO2 changes the composition of the olivine, indicating that CO2 was only a minor component of the fluid of the scoria. Hornblende is stable to 1040degreesC at oxygen fugacities of NNO + 2 (where NNO is the nickel-nickel oxide buffer), but at lower oxygen fugacities, the upper limit is 990degreesC. There is a progressive increase in crystallinity in experimental runs as both pressure and temperature decrease. Isobaric plots of crystallinity show that the onset of hornblende crystallization involves a reaction relation, and also results in a marked similar to15-40 vol. % increase in crystallinity. Ascending hydrous magmas intersecting the cooler crust could be trapped there by the large increase in crystallinity accompanying the isobaric crystallization of hornblende.