We have experimentally determined the specific D/H partitioning between pure water and serpentine (lysardite) from
100-200°C and 1 bar - 2,5 kbar of pressure. The purpose of these experiments is to estimate the pressure effect on hydrogen fractionation in ~50% exchanged samples as a possible reason of well-known discrepancy between different D/H fractionation factors for serpentine (Wenner and Taylor, 1974; Sakai and Tsutsumi, 1978; Grinenko et al., 1987; Mineev et al., 1989).
The 1031na values are given below as calculated by Northrop & Clayton method modified by Suzuoki & Epstein.
1031na %ex Pressure, bar
Serpentine-water (100°C) -46 16,5 1
Serpentine-water (100°C) +11 to +35 50-60 2500
Serpentine-water (200°C) -23 51 1
Serpentine-water (200°C) +1 to +6 46-47 1000
The experimental data obtained at low pressure are in a good agreement with the previous empirical data (Wenner and Taylor, 1974) and these one's obtained at high pressure - with the previous experimental data (Sakai and Tsutsumi, 1978). We are not ready to declare that presented data are the real D/H fractionation equilibrium factors in the serpentine-water system because of low and moderate percentage of exchange however we note that in all experiments (more than 10) have obtained the same results: at elevated pressure experiments the calculated 1031na values are much more positive than at normal pressure experiments and the difference between 'pressured' and atmospheric experiments decreases with the temperature increasing.
We discuss three possible ways for the explanation of our experimental data: 1) We have observed the direct pressure effect on the serpentine b-factor value; 2) We have seen the pressure depended ratio of the surface D/H fractionation and the
fractionation between OH-ion-hydroxyl of inner structural sites in serpentine at low and moderate isotope exchange; 3) In our experiments, we have detected the kinetic isotope effect between the external water and the serpentine interlayer water, the latter, in turn, isotopicaly exchanges with inner hydroxyl hydrogen of serpentine. This effect occurs only at low and moderate isotope exchange.
This study was funded by a ISF (grant MGH000).
Grinenko, Mineev et al., Geokhimiya 4, 574-579 (1987).
Mineev, Grinenko et al., In 5th working meeting 'Isotopes in nature', abstracts, Leipzig, 66-67 (1989).
Sakai & Tsutsumi, Earth Planet. Sci. Lett. 40, 231-242 (1978).
Wenner & Taylor, Geochim. Cosmochim. Acta 38, 1255-1286 (1974).