Experimental hydrogen isotope studies—I. Systematics of hydrogen isotope fractionation in the systems epidote-H2O,zoisite-H2O and AlO(OH)-H2O期刊界 All Journals 搜尽天下杂志传播学术成果专业期刊搜索期刊信息化学术搜索

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Experimental hydrogen isotope studies—I. Systematics of hydrogen isotope fractionation in the systems epidote-H₂O,zoisite-H₂O and AlO(OH)-H₂O

Authors:	Colin M Graham Simon MF Sheppard Timothy HE Heaton

Institution:	1. Department of Geology, Edinburgh University, West Mains Road, Edinburgh EH9 3JW, U.K.;1. Centre de Recherches Petrographiques et Geochemiques, Case Officielle No. 1, 54500 Vandoeuvre-les-Nancy, France

Abstract:	$H D$ Fractionation factors between epidote minerals and water, and between the AlO(OH) dimorphs boehmite and diaspore and water, have been determined between 150 and 650°C. Small water mineral ratios were used to minimise the effect of incongruent dissolution of epidote minerals. Waters were extracted and analysed directly by puncturing capsules under vacuum. Hydrogen diffusion effects were eliminated by using thick-walled capsules. $H D$ Exchange rates are very fast between epidote and water (and between boehmite and water), complete exchange taking only minutes above 450°C but several months at 250°C. Exchange between zoisite and water (and between diaspore and water) is very much slower, and an interpolation method was necessary to determine fractionation factors at 450 and below.For the temperature range 300–650°C, the $H D$ equilibrium fractionation factor (α^e) between epidote and water is independent of temperature and Fe content of the epidote, and is given by 1000 In α_{epidote-H₂O}^e = ?35.9 ± 2.5, while below 300°C 1000 In $α_{epidote-H_{2}O}^{e} = 29.2(10^{6} T^{2}) ? 138.8$ , with a ‘cross-over’ estimated to occur at around 185°C. By contrast, zoisite-water fractionations fit the relationship 1000 In $α_{zoisite-H_{2}O}^{e} = ? 15.07 (10^{6} T^{2}) ? 27.73$ .All studied minerals have hydrogen bonding. Fractionations are consistent with the general relationship: the shorter the O-H -- O bridge, the more depleted is the mineral in D.On account of rapid exchange rates, natural epidotes probably acquired their H-isotope compositions at or below 200°C, where fractionations are near or above 0%.; this is in accord with the observation that natural epidotes tend to concentrate D relative to other coexisting hydrous minerals.

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