D/H isotopic fractionation between brucite Mg(OH)2 and water from first-principles vibrational modeling

摘要: Abstract Isotopic fractionation between two phases can be calculated if the vibrational properties of isotopic end-members are fully characterized. We assessed a theoretical approach based on first-principles density-functional density (DFT) prediction frequencies by comparing with spectroscopic data isotopically substituted brucite, model mineral for which detailed experimental effects and brucite–water D/H partitioning exist. The deviation from values averages to less than 1% lattice modes, 4% OH stretching modes. reduced partition function ratio (RPFR) substitution in brucite was combined RPFR water calculate factor brucite. Results within harmonic approximation systematically deviate + 20–25‰. is very sensitive frequency shift high anharmonic important usually not explicitly taken introduced calculations minerals. asymmetry O–H potential DFT accounts measurements shifts mode overtones ratio. Calculations introducing corrections fitted expression 1000lnαD/H(brucite/H2O) = − 23.3 103 / T + 2.55 106 / T2 − 1.51 109 / T3, that yields an improved agreement experiments at temperature, but 25 °C − 30‰. Uncertainties factors arise dispersion or errors. For instance, change 3% could account discrepancy factors. pressure dependence RPRF calculated, given 1000lnГPbrucite = P (− 1.005 103 / T + 2.18 106 / T2 − 0.213 109 / T3), P GPa. large temperature suggests single curve partial consistent set as 1000lnαD/H(brucite/H2O) = − 27.9(30) 103 / T + 8.8(29) 106 / T2 − 2.24(63) 109 / T3. These equations allow calculating 300–900 K 0.1–100 MPa range when correction [Polyakov, V.B., Horita, J. Cole, D.R., 2006. Pressure hydrogen isotopes water. Geochimica Cosmochimica Acta, 70: 1904–1913.] geochemical applications. used here estimating extended other hydrous minerals using spectroscopy test accuracy prediction, proposed reference determining mineral–water