The responses of the four main substitution mechanisms of H in olivine to H2O activity at 1050\xa0\xb0C and 3\xa0GPa

Abstract

The water solubility in olivine left({C}_{{mathrm{H}}_2mathrm{O}}right) has been investigated at 1050 °C and 3 GPa as a function of water activity left({a}_{{mathrm{H}}_2mathrm{O}}right) at subsolidus conditions in the piston-cylinder apparatus, with {a}_{{mathrm{H}}_2mathrm{O}} varied using H2O–NaCl fluids. Four sets of experiments were conducted to constrain the effect of {a}_{{mathrm{H}}_2mathrm{O}} on the four main substitution mechanisms. The experiments were designed to grow olivine in situ and thus achieve global equilibrium (G-type), as opposed to hydroxylating olivine with a pre-existing point-defect structure and impurity content (M-type). Olivine grains from the experiments were analysed with polarised and unpolarised FTIR spectroscopy, and where necessary, the spectra have been deconvoluted to quantify the contribution of each substitution mechanism. Olivine buffered with magnesiowüstite produced absorbance bands at high wavenumbers ranging from 3566 to 3612 cm−1. About 50% of the total absorbance was found parallel to the a-axis, 30% parallel to the b-axis and 20% parallel to the c-axis. The total absorbance and hence water concentration in olivine follows the relationship of {C}_{{mathrm{H}}_2mathrm{O}}propto {a_{{mathrm{H}}_2mathrm{O}}}^2 , indicating that the investigated defect must involve four H atoms substituting for one Si atom (labelled as [Si]). Forsterite buffered with enstatite produced an absorbance band exclusively aligned parallel the c-axis at 3160 cm−1. The band position, polarisation and observed {C}_{{mathrm{H}}_2mathrm{O}}propto {a}_{{mathrm{H}}_2mathrm{O}} are consistent with two H substituting for one Mg (labelled as [Mg]). Ti-doped, enstatite-buffered olivine displays absorption bands, and polarisation typical of Ti-clinohumite point defects where two H on the Si-site are charge-balanced by one Ti on a Mg-site (labelled as [Ti]). This is further supported by {C}_{{mathrm{H}}_2mathrm{O}}propto {a}_{{mathrm{H}}_2mathrm{O}} and a 1:1 relationship of molar H2O and TiO2 in these experiments. Sc-doped, enstatite-buffered experiments display a main absorption band at 3355 cm−1 with {C}_{{mathrm{H}}_2mathrm{O}}propto {a_{{mathrm{H}}_2mathrm{O}}}^{0.5} and a positive correlation of Sc and H, indicating the coupled substitution of a trivalent cation plus a H for two Mg (labelled as [triv]). Our data demonstrate that extreme care has to be taken when inferences from experiments conducted at {a}_{{mathrm{H}}_2mathrm{O}}=1 are applied to the mantle, where in most cases, a low {a}_{{mathrm{H}}_2mathrm{O}} persists. In particular, the higher exponent of the [Si] substitution mechanism means that the contribution of this hydrous defect to total water content will decrease more rapidly with decreasing {a}_{{mathrm{H}}_2mathrm{O}} than the contributions of the other substitution mechanisms. The experiments confirm previous results that the [Mg] mechanism holds an almost negligible amount of water under nearly all T-P-fO2-fH2O conditions that may be anticipated in nature. However, the small amounts of H2O we find in substituting by this mechanism are similar in the experiments on forsterite doped with either Sc or Ti to those in the undoped forsterite at equivalent {a}_{{mathrm{H}}_2mathrm{O}} (all buffered by enstatite), confirming the assumption that, thermodynamically, {C}_{{mathrm{H}}_2mathrm{O}} substituting by each mechanism does not depend on the water concentration that substitutes by other mechanisms.