OH in mantle olivine: experiment vs. nature

Olivine is the principal rock-forming mineral in the mantle lithologies. Its defect structure depends on conditions of crystallization or re-equilibration and thus can be used as an indicator of magmatic and metasomatic processes. Protonated defect structure of olivine, i.e. defect structure in which cation vacancies are charge-balanced by H⁺ bonded to oxygen, is conventionally studied using FTIR (Fourier Transform Infra Red) spectroscopy. It has been shown experimentally that the frequency of OH^- IR absorption bands depends on a number of parameters, the most important of which have been proposed to be: aSiO₂, aTiO₂, and fO₂. Variation of these parameters may have similar effects on OH- IR absorption, hampering interpretation of the spectroscopic results.

In order to isolate the major parameters controlling OH^- IR absorption at the conditions of the lithospheric mantle we studied olivines from mantle peridotitic xenoliths and kimberlitic phenocrysts using FTIR and EPMA. We did not observe correlation between OH^- IR absorption and TiO₂ content in olivine. Atomic proportion of H typically exceeds atomic proportion of Ti. We assume that Ti does not control the position and concentrations of H in olivine from the lithospheric mantle.

At increasing fO₂, concentration of ferric iron increases and one anticipates stabilization of hydrous defects associated with Fe³⁺ (lower frequency OH^- IR absorption). However, spectra indicative of OH- associated with Me³⁺ were measured in olivines from contrasting fO₂ environments: spinel peridotites, relatively reduced diamondiferous garnet peridotites, as well as kimberlitic pheno- and xenocrysts. Thus at relatively oxidized conditions of the lithospheric mantle fO₂ does not significantly affect OH^- occurrence in mantle olivine. At the same time reduced conditions of the adiabatic upper mantle may cause preferential stabilization of the hydrogarnet type defects (higher frequency OH^- IR absorption) rather than defects associated with Me³⁺. Hydrogarnet-type defects are significantly more capable of storing OH^-. In that case oxidation of mantle material at the asthenosphere can cause partial dehydrogenation of olivine and thus assist melting.

We propose that the primary factor controlling OH solubility in olivine at conditions of the lithospheric mantle is aSiO₂. Peridotitic olivine in equilibrium with orthopyroxene (high aSiO₂) should show lower frequency OH^- IR absorption bands. However many olivine samples from mantle xenoliths show higher frequency OH^- IR absorption bands suggesting that these samples are not in equilibrium with orthopyroxene. This implies that a significant proportion of available mantle material may have been modified by transporting melts and/or metasomatic fluids, which were characterized by lower silica activities.