Point defect populations of forsterite revealed by two-stage metastable hydroxylation experiments

Abstract

Hydroxylation is a method that allows “decoration” of the pre-existing point defect structure of nominally anhydrous minerals, such as olivine. We tested this method on synthetic forsterite (Fo: Mg2SiO4 ) crystals. To control starting point defect structures, Fo crystals were pre-annealed at different temperatures ( 1100−1500∘C ), silica activity conditions (forsterite–enstatite Fo–En and forsterite–periclase Fo–Per) and oxygen fugacity (0.21 and 10−6 bars). Then low-temperature hydroxylation (900 °C, 1.5 GPa) of the crystals successfully allowed the decoration with protons of pre-existing point defect structures, as subsequently revealed by infrared spectroscopy. Protons are arranged in three different point defect stoichiometries in Fo, related to Mg and Si vacancies ([Mg] and [Si], respectively) as well as to a trivalent cation-associated substitution mechanism ([triv]). Over the timescale and equilibrium conditions studied, hydroxylation does not reset the point defect structure inherited from pre-anneal. The data further show that the concentrations of [Mg]-, [Si]- and [triv]-hydrated defects are function of pre-anneal silica activity and temperature. Laser-ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) analysis of the crystals revealed diffusion of Al and Fe into the crystals during the pre-annealing, a phenomenon clearly promoted at high aSiO2 . The data confirm a very fast mechanism of Al diffusion in Fo during pre-annealing, and suggest a strong coupling between H+ and Al3+ during hydroxylation. Overall, they show the strong importance of aSiO2 and temperature in the incorporation of trace cations in forsterite, and the subsequent effects of incorporation of trace cations on Mg- and Si-related point defects in Fo. The dry point defect population of Fo is determined by interactions between the trace trivalent cations and dry Si and Mg vacancies. Without trace elements, T only has a limited effect on Mg- and Si-related point defect populations. Finally, approaching or potentially slightly exceeding the Fo–En solidus leads to strong changes in the trace element concentration and point defect population in Fo, which may be related to either partial melting or pre-melting effects.