HYDROGEN AND OXYGEN-ISOTOPE FRACTIONATION BETWEEN BRUCITE AND AQUEOUSNACL SOLUTIONS FROM 250-DEGREES-C TO 450-DEGREES-C

Hydrogen and oxygen isotope fractionation factors between brucite and aqueous NaCl solutions (10001n alpha(br-sw)) have been calibrated by e xperiment from 250 to 450 degrees C at 0.5 Kb. For D/H fractionation, 10001n alpha(br-sw) values are as follows: -32 +/- 6 parts per thousan d (250 degrees C, 3.2 wt% NaCl), -21 +/- 2 parts per thousand (350 deg rees C, 10.0 wt% NaCl), and -22 +/- 2 parts per thousand (450 degrees C, 3.2 wt% NaCl), indicating that brucite is depleted in D relative to coexisting aqueous NaCl solutions. These results are in good agreemen t with previous D/H fractionation factors determined in the brucite-wa ter system, indicating that any effects of dissolved salt on D/H fract ionation are relatively small, particularly in solutions with near sea water salinity. The maximum salt effect (+4 parts per thousand) was ob served in 10.0 wt% NaCl solutions at 350 degrees C, suggesting that th e addition of dissolved NaCl increases the amount of deuterium fractio nated into mineral structures. For O-18/O-16 fractionation, 1001n alph a(br-sw) values in 3.0 wt% NaCl solutions are -6.0 +/- 1.3 parts per t housand, -5.6 +/- 0.7 parts per thousand and -4.1 +/- 0.2 parts per th ousand, at 250, 350, and 450 degrees C, respectively, and -5.8 +/- 0.6 parts per thousand in 10.0 wt % NaCl at 350 degrees C. These data ind icate that brucite is depleted in O-18 relative to coexisting aqueous NaCl solutions and that the degree of depletion decreases slightly wit h increasing temperature and is not strongly dependent on salinity. We calculated O-18/O-16 brucite-water fractionation factors from availab le calibrations of the salt-effect on O-18/O-16 fractionation between coexisting phases. The resulting values were fit to the following equa tion that is valid from 250 to 450 degrees C1001n alpha(br-w) = 9.54x1 0(6)T(-2) - 3.53 x 10(4)T(-1) + 26.58 where T is temperature in Kelvin s. These new data have been used to improve the prediction of O-18/O-1 6 fractionation factors in the talc-water and serpentine-water systems by modifying existing empirical bond-water models. The results of thi s analysis indicate that the delta(18)O composition of talc-brucite an d serpentine-brucite pairs could be used as a geothermometer and that these coexisting phases should display the following order of O-18 enr ichment: talc > serpentine > brucite. Copyright O 1998 Elsevier Scienc e Ltd.