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Hydrogen and oxygen isotope fractionation between brucite and aqueous NaCl solutions from 250 to 450°C

Peter J. Saccocia·Jeffrey S. Seewald·Wayne C. Shanks
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Abstract

Hydrogen and oxygen isotope fractionation factors between brucite and aqueous NaCl solutions (1000lnα br-sw ) have been calibrated by experiment from 250 to 450°C at 0.5 Kb. For D/H fractionation, 1000lnα br-sw values are as follows: −32 ± 6‰ (250°C, 3.2 wt% NaCl), −21 ± 2‰ (350°C, 10.0 wt% NaCl), and −22 ± 2‰ (450°C, 3.2 wt% NaCl), indicating that brucite is depleted in D relative to coexisting aqueous NaCl solutions. These results are in good agreement with previous D/H fractionation factors determined in the brucite-water system, indicating that any effects of dissolved salt on D/H fractionation are relatively small, particularly in solutions with near seawater salinity. The maximum salt effect (+4‰) was observed in 10.0 wt% NaCl solutions at 350°C, suggesting that the addition of dissolved NaCl increases the amount of deuterium fractionated into mineral structures. For 18 O/ 16 O fractionation, 1000lnα br-sw values in 3.0 wt% NaCl solutions are −6.0 ± 1.3‰, −5.6 ± 0.7‰ and −4.1 ± 0.2‰, at 250, 350, and 450°C, respectively, and −5.8 ± 0.6‰ in 10.0 wt % NaCl at 350°C. These data indicate that brucite is depleted in 18 O relative to coexisting aqueous NaCl solutions and that the degree of depletion decreases slightly with increasing temperature and is not strongly dependent on salinity. We calculated 18 O/ 16 O brucite-water fractionation factors from available calibrations of the salt-effect on 18 O/ 16 O fractionation between coexisting phases. The resulting values were fit to the following equation that is valid from 250 to 450°C 1000ln α br-w = 9.54 × 10 6 T −2 − 3.53 × 10 4 T −1 + 26.58 where T is temperature in Kelvins. These new data have been used to improve the prediction of 18 O/ 16 O fractionation factors in the talc-water and serpentine-water systems by modifying existing empirical bond-water models. The results of this analysis indicate that the δ 18 O composition of talc-brucite and serpentine-brucite pairs could be used as a geothermometer and that these coexisting phases should display the following order of 18 O enrichment: talc > serpentine > brucite.

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Peter J. Saccocia, Jeffrey S. Seewald, Wayne C. Shanks. 1998. Hydrogen and oxygen isotope fractionation between brucite and aqueous NaCl solutions from 250 to 450°C. https://doi.org/10.1016/s0016-7037(97)00346-3

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