Hydrogen isotope fractionation in the system brucite-water +/- NaCl to elevated temperatures and pressures: Implications for the isotopic property of NaCl fluids under geologic conditions

摘要

A series of hydrothermal experiments have been conducted to determine equilibrium D/H fractionations between brucite and water as a function of temperature (200-600 degrees C), pressure (2.1-800 MPa), and dissolved NaCl (0-5 molal or 0-22.6 wt %). Along with our previous study, a total of 39 data points show that pressure and dissolved NaCl both increase the D/H fractionation factor at a given temperature. Theoretical treatment allowed a first direct comparison of experimental and theoretical results of brucite-water D/H fractionation at a zero pressure limit. After correcting the pressure effects on brucite D/H partition function ratios (beta-factor), the isotope effects of pressure and dissolved NaCl on the D/H beta-factor of water were for the first time rigorously evaluated. There is a good linear relationship between the D/H beta-factor of the aqueous NaCl solutions and their densities of at a given temperature. A good, pseudo-linear relationship observed between the density of aqueous NaCl solutions and the maximum intensity of the asymmetric O-H vibrational frequency of water in turn suggests that this frequency is a fundamental property that determines the beta-factor of water at elevated temperatures and pressures. Our study suggests that the density of aqueous solutions (rho(aq-soln)) could be used as a primary parameter to predict the effects of pressure and NaCl compositions on the beta-factor of water under geologic conditions encountered in the crust and upper mantle: Delta 10(3) ln beta(wat)/Delta rho(aq-soln) = -0.0111 (+/- 0.0012), where Delta is a difference in the value of 10(3) ln beta(wat) or rho(aq-soln), and rho(aq-soln) in kg m(-3). (C) 2018 Elsevier Ltd. All rights reserved.