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

Abstract

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 α talc – water 18 O – 16 O = 11.70 × 10 6/T 2 − 25.49 × 10 3/T + 12.48 and 1000 ln α serpentine – water 18 O – 16 O = 3.49 × 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 ln α talc – water D / H = 10.88 × 10 6/T 2 − 41.52 × 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 °C, 50 MPa as follows: 1000 ln α serpentine – water D – H = 3.436 × 10 6/T 2 − 34.736 × 10 3/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.