Constraints on the upper pressure stability of blueschist facies metamorphism along the reaction: Glaucophane = talc + 2 jadeite in the Na2O-MgO-Al2O3-SiO2-H2O system

Corona, Juan Carlos and Jenkins, David M. and Holland, Timothy J. B. (2013) Constraints on the upper pressure stability of blueschist facies metamorphism along the reaction: Glaucophane = talc + 2 jadeite in the Na2O-MgO-Al2O3-SiO2-H2O system. American Journal of Science, 313 (10). pp. 967-995. ISSN 0002-9599 EISSN: 1945-452X DOI 10.2475/10.2013.01

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Abstract

Blueschist-facies metamorphism, which is associated with high pressure and low temperature subduction zone metamorphism, is usually recognized by the presence of the sodic-amphibole glaucophane. Determining the upper- and lower-pressure stability of end-member glaucophane places important constraints on the conditions of blueschist metamorphism. An experimental investigation into the upper-pressure stability of glaucophane has been done in the system Na2O-MgO-Al2O3-SiO2-H2O over the range of 600 to 750 °C and 2.5 to 4.5 GPa. Mixtures of synthetic glaucophane, jadeite, and talc were used to determine the location of the reaction boundary glaucophane = 2 jadeite + talc by reversing the sense of reaction direction. The upper-pressure stability of glaucophane is located over a pressure interval (0.2-0.3 GPa in width) whose midpoints lie at 2.6 GPa at 600 °C and at 3.1 GPa at 700 °C, across which the proportion of glaucophane decreases to zero as talc and jadeite increase. This band has a positive dP/dT slope of around 0.005 GPa/°C. Glaucophane showed the largest change in composition, consistent with the incorporation of significant amounts of the nyböite (17-34 mol%) and cummingtonite (10-18 mol%) components. Talc showed minor, but definite incorporation of Na (0.08 atoms per formula unit, apfu) and Al (0.15 apfu) consistent with incorporation of the components aspidolite and tschermak-talc. Jadeite displayed no clear change from its ideal composition. The results from this study were combined with the lower-pressure stability of glaucophane + quartz reported earlier by Corona and Jenkins (2007) to refine the thermodynamic values for glaucophane (ΔfH° = −11,956.36 kJ/mol), report an initial set of values for the amphibole nyböite (ΔfH° = −12,165.85 kJ/mol) and phyllosilicate aspidolite (ΔfH° = −6163.86 kJ/mol), and to develop activity-composition relationships for both the amphibole and talc in this chemical system. The results of this study place an upper-pressure limit to a key index mineral of the blueschist-facies at about 3.2 GPa at 700 °C, which is just above the quartz-coesite transition, allowing nearly end-member glaucophane to remain stable up to the conditions of ultra-high-pressure metamorphism.

Item Type:	Article
Uncontrolled Keywords:	2013AREP; IA67;
Subjects:	05 - Petrology - Igneous, Metamorphic and Volcanic Studies
Divisions:	05 - Petrology - Igneous, Metamorphic and Volcanic Studies
Journal or Publication Title:	American Journal of Science
Volume:	313
Page Range:	pp. 967-995
Identification Number:	10.2475/10.2013.01
Depositing User:	Sarah Humbert
Date Deposited:	21 Mar 2014 18:49
Last Modified:	07 Apr 2014 14:29
URI:	http://eprints.esc.cam.ac.uk/id/eprint/2998

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