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Effect of oxygen fugacity and water on phase equilibria of a hydrous tholeiitic basalt

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Feig, ST and Koepke, J and Snow, JE (2010) Effect of oxygen fugacity and water on phase equilibria of a hydrous tholeiitic basalt. Contributions to Mineralogy and Petrology, 160 (4). pp. 551-568. ISSN 1432-0967

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Abstract

The influence of oxygen fugacity and water on phase equilibria and the link between redox conditions and water activity were investigated experimentally using a primitive tholeiitic basalt composition relevant to the ocean crust. The crystallization experiments were performed in internally heated pressure vessels at 200 MPa in the temperature range 940–1,220C. The oxygen fugacity was measured using the H2-membrane technique. To study the effect of oxygen fugacity, three sets of experiments with different hydrogen fugacities were performed, showing systematic effects on the phase relations and compositions. In each experimental series, the water content of the system was varied from nominally dry to water-saturated conditions, causing a range of oxygen fugacities varying by *3 log units per series. The range in oxygen fugacity investigated spans *7 log units. Systematic effects of oxygen fugacity on the stability and composition of the mafic silicate phases, Cr–spinel and Fe–Ti oxides, under varying water contents were recorded. The Mg# of the melt, and therefore also the Mg# of olivine and clinopyroxene, changed systematically as a function of oxygen fugacity. An example of the link between oxygen fugacity and water activity under hydrogen-buffered conditions is the change in the crystallization sequence (olivine and Cr–spinel) due to a change in the oxygen fugacity caused by an increase in the water activity. The stability of magnetite is restricted to highly oxidizing conditions. The absence of magnetite in most of the experiments allows the determination of differentiation trends as a function of oxygen fugacity and water content, demonstrating that in an oxidefree crystallization sequence, water systematically affects the differentiation trend, while oxygen fugacity seems to have a negligible effect.

Item Type: Article
Keywords: Oxygen fugacity - Water - Basalt - Phase equilibria - Differentiation - Crystallization experiments
Journal or Publication Title: Contributions to Mineralogy and Petrology
Page Range: pp. 551-568
ISSN: 1432-0967
Identification Number - DOI: 10.1007/s00410-010-0493-3
Additional Information: The final publication is available at http://www.springerlink.com
Date Deposited: 23 Feb 2012 00:56
Last Modified: 18 Nov 2014 04:09
URI: http://eprints.utas.edu.au/id/eprint/9664
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