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Journal of Petrology | Volume 45 | Number 3 | Pages 485-506 | 2004
Journal of Petrology 45(3) © Oxford University Press 2004; all rights reserved.
A Hornblende Basalt from Western Mexico: Water-saturated Phase Relations Constrain a Pressure–Temperature Window of Eruptibility
DEPARTMENT OF EARTH AND PLANETARY SCIENCE, UNIVERSITY OF CALIFORNIA, BERKELEY, CA 94720-4767, USA
* Corresponding author. Present address: School of Environmental Sciences, University of East Anglia, Norwich NR4 7TJ, UK. Fax: +44 (0)1603 507719. E-mail: j.barclay{at}uea.ac.uk
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% (50–150 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 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.
KEY WORDS: crystallization; eruptibility limit; experiments; hornblende trachybasalt; Mexico
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