Oxygen and hydrogen isotope fractionation in serpentine-water and talc-water systems from 250 to 450 °C, 50 MPa

Oxygen and hydrogen isotope fractionation factors in the talc-water and serpentine-water systems have been determined by laboratory experiment from 250 to 450 degrees 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 degrees C. For serpentine, D-H exchange was much more rapid than O-18-O-16 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 O-18-O-16 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 degrees C by the relationships: 1000 In alpha(18O-16O)(talc-water) = 11.70 x 10(6)/T-2 - 25.49 x 10(3)/T + 12.48 and 1000 In alpha(18O-16O)(serpentine-water) = 3.49 x 10(6)/T-2 - 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 In alpha(D/H)(talc-water) = 10.88 x 10(6)/T-2 - 41.52 x 10(3)/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 degrees C, 50 MPa as follows: 1000 In alpha(D-H)(serpentine-water) = 3.436 x 10(6)/T-2 - 34.736 x 10(3)/T + 21.67 where T is temperature in Kelvin. These new data should improve the application of D-H and O-18-O-16 isotopes to constrain the temperature and origin of hydrothermal fluids responsible for serpentine formation in a variety of geologic settings. (C) 2009 Elsevier Ltd. All rights reserved.