Abstract
Hydrogen-isotope exchange between Mg-Al chlorite of sheridanite composition and water has been studied experimentally over the temperature range 300-700°C. Problems were encountered with changes in the bulk D/H ratios of capsule contents during high P-T experiments. Below 500°C, exchange was so slow that equilibrium fractionation factors could not be determined. Over the temperature range 500-700°C, no temperature dependence is observed, with 1000 In around -28. Chlorite-water fractionations are not simply related to octahedral cation composition; preferential fractionation of the light isotope into chlorite reflects the presence of hydrogen bonding in the chlorite structure.
A compilation of experimentally and empirically determined chlorite-water hydrogenisotope fractionation factors over a wide range of temperatures indicates that these are not measurably dependent on Fe/Mg in chlorite.
Hydrogen-isotope exchange reactions proceeded by volume diffusion of hydrogen in chlorite, for which activation energies in the range 166-172 kj/mol H are much higher than in any other hydrous mineral yet studied. Hydrogen-isotope exchange between chlorite and water in slowly cooling metamorphic rocks may nonetheless proceed to subgreenschist facies temperatures.
Hydrogen-isotope exchange during hydrous metamorphism of the oceanic crust in the temperature range 200-500°C may be dominated by fractionation factors in the range 1000 In = -30 to -40.
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