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 1040 C and 970 C with water contents of between 2 5 and 4 5%(50---150MPa).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 1040 C at oxygen fugacities of NNO ‡ 2 (where NNO is the nickel---nickel oxide buffer), but at lower oxygen fugacities, the upper limit is 990 C. 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 15---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.