The validity of plagioclase-melt geothermometry for degassing-driven magma crystallisation.

Humphreys, Madeleine C. S. and Edmonds, M and Klöcking, Marthe S. (2016) The validity of plagioclase-melt geothermometry for degassing-driven magma crystallisation. American Mineralogist, 101 (4). pp. 769-779. ISSN 0003-004X DOI 10.2128/am-2016-5314

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Abstract

Any quantitative interpretation of the formation conditions of igneous rocks requires methods for determining crystallisation temperature. Accurate application of such thermobarometers relies on the attainment of equilibrium in the system to be studied. This may be particularly difficult in silicic magmas, where diffusivities are low and crystallisation kinetics sluggish. Moreover, progressive degassing of volatileTrich magmas during ascent can result in continuous changes in effective undercooling, causing particular problems in achieving equilibrium between melt and crystals that grow in response to decompression. We consider these problems in the context of plagioclase-melt equilibria for magmas undergoing decompression and degassing-driven crystallisation, using two published thermometers. The two thermometers show similar trends with key parameters but absolute temperatures can vary by > 200 °C. Analysis of decompression experiments conducted at constant temperature shows systematic variations in calculated temperature and equilibrium constant with varying decompression rate and quench pressure. This indicates that an unrecognised lack of equilibration could result in significant temperature overestimates and potentially spurious results. This highlights the need to assess for equilibrium, and we discuss problems associated with some commonly used indicators of equilibration. Finally, retrospective analysis of published plagioclase-hosted melt inclusion suites from five subduction zone volcanoes shows systematic increases in calculated temperature and equilibrium constant with decreasing H₂O concentration. While this could represent the signature of latent heat of crystallisation, we suggest that such patterns should be treated with caution unless there is clear evidence of sustained equilibrium between plagioclase and melt during decompression.

Item Type:	Article
Uncontrolled Keywords:	2016AREP; IA70; weekly list
Subjects:	05 - Petrology - Igneous, Metamorphic and Volcanic Studies
Divisions:	05 - Petrology - Igneous, Metamorphic and Volcanic Studies 06 - Part-III Projects 08 - Green Open Access
Journal or Publication Title:	American Mineralogist
Volume:	101
Page Range:	pp. 769-779
Identification Number:	10.2128/am-2016-5314
Depositing User:	Sarah Humbert
Date Deposited:	21 Apr 2016 16:40
Last Modified:	20 May 2016 16:56
URI:	http://eprints.esc.cam.ac.uk/id/eprint/3608

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