Oxygen and hydrogen isotope fractionation factors in the talc-water and serpentine-water systems have been determined by laboratory experiment from 250 to 450 ??C at 50 MPa using the partial exchange technique. Talc was synthesized from brucite + quartz, resulting in nearly 100% exchange during reaction at 350 and 450 ??C. For serpentine, D-H exchange was much more rapid than 18O-16O exchange when natural chrysotile fibers were employed in the initial charge. In experiments with lizardite as the starting charge, recrystallization to chrysotile enhanced the rate of 18O-16O exchange with the coexisting aqueous phase. Oxygen isotope fractionation factors in both the talc-water and serpentine-water systems decrease with increasing temperature and can be described from 250 to 450 ??C by the relationships: 1000 ln ??talc s(-) water18 O s(-) 16 O = 11.70 ?? 106/T2 - 25.49 ?? 103/T + 12.48 and 1000 ln ??serpentine s(-) water18 O s(-) 16 O = 3.49 ?? 106/T2 - 9.48 where T is temperature in Kelvin. Over the same temperature interval at 50 MPa, talc-water D-H fractionation is only weakly dependent on temperature, similar to brucite and chlorite, and can be described by the equation: 1000 ln ??talc s(-) waterD / H = 10.88 ?? 106/T2 - 41.52 ?? 103/T + 5.61 where T is temperature in Kelvin. Our D-H serpentine-water fractionation factors calibrated by experiment decrease with temperature and form a consistent trend with fractionation factors derived from lower temperature field calibrations. By regression of these data, we have refined and extended the D-H fractionation curve from 25 to 450 ??C, 50 MPa as follows: 1000 ln ??serpentine s(-) waterD s(-) H = 3.436 ?? 106/T2 - 34.736 ?? 103/T + 21.67 where T is temperature in Kelvin. These new data should improve the application of D-H and 18O-16O isotopes to constrain the temperature and origin of hydrothermal fluids responsible for serpentine formation in a variety of geologic settings. ?? 2009 Elsevier Ltd.
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Oxygen and hydrogen isotope fractionation in serpentine-water and talc-water systems from 250 to 450 ??C, 50 MPa