A Hornblende Basalt from Western Mexico: Water-saturated Phase Relations Constrain a Pressure-Temperature Window of Eruptibility |
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Authors: | BARCLAY J; CARMICHAEL I S E |
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Institution: | DEPARTMENT OF EARTH AND PLANETARY SCIENCE, UNIVERSITY OF CALIFORNIA, BERKELEY, CA 94720-4767, USA |
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Abstract: | Trachybasalt scoria from a cinder cone near the Mexican volcanicfront contain phenocrysts of olivine with chromite inclusions,apatite, augite and hornblende, with microphenocrysts of plagioclase.The water-saturated phase relations reproduce the phenocrystassemblage between 1040°C and 970°C with water contentsof between 2·5 and 4·5% (50150 MPa). Theabsence of biotite phenocrysts in the scoria places a tightconstraint on the pressuretemperature conditions of phenocrystequilibration, as there is only a small zone where biotite doesnot accompany hornblende in the experiments. Diluting the fluidphase with CO2 changes the composition of the olivine, indicatingthat 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 nickelnickel oxide buffer), butat lower oxygen fugacities, the upper limit is 990°C. Thereis a progressive increase in crystallinity in experimental runsas both pressure and temperature decrease. Isobaric plots ofcrystallinity show that the onset of hornblende crystallizationinvolves a reaction relation, and also results in a marked |
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Keywords: | : crystallization eruptibility limit experiments hornblende trachybasalt Mexico |
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