Jeffrey S. Seewald
Wayne C. Shanks III
Peter J. Saccocia
1998
<p><span>Hydrogen and oxygen isotope fractionation factors between brucite and aqueous NaCl solutions (1000lnα</span><sub>br-sw</sub><span>) have been calibrated by experiment from 250 to 450°C at 0.5 Kb. For D/H fractionation, 1000lnα </span><sub>br-sw</sub><span> 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 </span><sup>18</sup><span>O/</span><sup>16</sup><span>O fractionation, 1000lnα</span><sub>br-sw</sub><span> 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 </span><sup>18</sup><span>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 </span><sup>18</sup><span>O/</span><sup>16</sup><span>O brucite-water fractionation factors from available calibrations of the salt-effect on </span><sup>18</sup><span>O/</span><sup>16</sup><span>O fractionation between coexisting phases. The resulting values were fit to the following equation that is valid from 250 to 450°C 1000ln α</span><sub>br-w</sub><span> = 9.54 × 10</span><sup>6</sup><i>T</i><sup>−2</sup><span> − 3.53 × 10</span><sup>4</sup><span>T</span><sup>−1</sup><span> + 26.58 where </span><i>T</i><span> is temperature in Kelvins. These new data have been used to improve the prediction of </span><sup>18</sup><span>O/</span><sup>16</sup><span>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</span><sup>18</sup><span>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 </span><sup>18</sup><span>O enrichment: talc > serpentine > brucite.</span></p>
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10.1016/S0016-7037(97)00346-3
en
Elsevier
Hydrogen and oxygen isotope fractionation between brucite and aqueous NaCl solutions from 250 to 450°C
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