Other
Scientific paper
Feb 1980
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1980gecoa..44..353g&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 44, Issue 2, pp.353-364
Other
10
Scientific paper
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 ( e ) between epidote and water is independent of temperature and Fe content of the epidote, and is given by 1000 In epidote - H 2 O e = -35.9 ± 2.5, while below 300°C 1000 In 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 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.
Graham Colin M.
Heaton Timothy H. E.
Sheppard Simon M. F.
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