A hornblende-bearing basalt from Western Mexico: Water-saturated phase relations constrain a pressure-temperature window of eruptability

Barclay, J. ORCID: https://orcid.org/0000-0002-6122-197X and Carmichael, I. S. E. (2004) A hornblende-bearing basalt from Western Mexico: Water-saturated phase relations constrain a pressure-temperature window of eruptability. Journal of Petrology, 45 (3). pp. 485-506. ISSN 1460-2415

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Abstract

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.

Item Type:	Article
Faculty \ School:	Faculty of Science > School of Environmental Sciences
UEA Research Groups:	Faculty of Science > Research Groups > Geosciences Faculty of Science > Research Groups > Volcanoes@UEA (former - to 2018) Faculty of Science > Research Groups > Geosciences and Natural Hazards (former - to 2017)
Depositing User:	Rachel Snow
Date Deposited:	28 Mar 2011 10:10
Last Modified:	23 Jul 2024 12:30
URI:	https://ueaeprints.uea.ac.uk/id/eprint/27285
DOI:	10.1093/petrology/egg091

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