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First measurements of OH-C exchange and temperature-dependent partitioning of OH and halogens in the system apatite - silicate melt.

Riker, J. and Humphreys, M.C.S. and Brooker, R.A. and de Hoog, J.C.M. and EIMF, (2018) 'First measurements of OH-C exchange and temperature-dependent partitioning of OH and halogens in the system apatite - silicate melt.', American mineralogist., 103 (2). pp. 260-270.

Abstract

We present the first integrated study of carbonate, hydroxyl, fluoride, and chloride ion partitioning in the apatite-melt system. We determined volatile partitioning behavior between apatite and silicate melt for both haplobasaltic andesite and trachyte bulk compositions at 0.5–1 GPa and 1250°C using the piston-cylinder apparatus. All volatile species were analyzed directly in both apatite and glass using secondary ion mass spectrometry (SIMS) and electron probe microanalysis. Distribution coefficients for OH-halogen exchange are similar to those from previous studies, and together with literature data, reveal a significant log-linear relationship with temperature, while the effects of pressure and melt composition are minimal. Meanwhile, halogen-free experiments generate very high C contents (up to 5000 ppm) in apatite. Stoichiometry calculations and infrared spectra indicate that this C is mainly incorporated onto the channel volatile site together with hydroxyl. In halogen-bearing experiments, apatite crystals contain significantly lower C (≤500 ppm), which may be partly incorporated onto the phosphate site while the channel volatile site is filled by OH+F+Cl+C. Our experiments give the first constraints on H2O-CO2 exchange between apatite and silicate melt, with a KD of 0.355 ± 0.05 for the trachyte and 0.629 ± 0.08 for the haplobasaltic andesite. The new constraints on the temperature-dependence of partitioning will enable quantitative modeling of apatite-volatile exchange in igneous systems, while this new partitioning data and method for direct, in situ analysis of C in apatite mark a significant advance that will permit future studies of magmatic C and other volatiles. This has a broad range of potential applications including magmatic differentiation, fractionation, and degassing; quantification of volatile budgets in extraterrestrial and deep earth environments; and mineralization processes.

Item Type:Article
Full text:(AM) Accepted Manuscript
Available under License - Creative Commons Attribution.
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Full text:(VoR) Version of Record
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Status:Peer-reviewed
Publisher Web site:https://doi.org/10.2138/am-2018-6187ccby
Publisher statement:This article is published under a Creative Commons CC-BY licence.
Date accepted:07 October 2017
Date deposited:11 October 2017
Date of first online publication:01 February 2018
Date first made open access:06 February 2018

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