Coupled fluorine and hydroxyl incorporation into forsterite

Abstract The incorporation mechanism of F and OH in forsterite from a metasomatized marble was investigated by electron probe microanalyzer (EPMA) and Fourier transform infrared (FTIR) spectroscopy. The high-precision EPMA analyses show that the forsterite typically contains high F contents (158 ± 30 to 439 ± 30 μg/g, 3s) and exhibits a core-rim zonation with elevated F in the rims (rim-to-core ratios of 1.29 ± 0.18). The Fourier transform infrared (FTIR) spectroscopy indicates the existence of two characteristic OH absorption bands of 3673 cm−1 and 3640 cm−1 in the F-rich forsterite, consistent with those of synthesized F- and OH-bearing olivine. The calculated H2O contents and H2O distribution maps reveal that there is an increase of H2O contents from the forsterite cores (96–130 μg/g, ±15%) to their rims (114–180 μg/g, ±15%) with rim-to-core ratios of 1.20 ± 0.01. A positive correlation between F and H2O contents is observed, indicating a coupled incorporation of OH and F in forsterite. The presence of F thus enhances the incorporation of H2O in olivine. By comparison with previously reported experimental and theoretical data, we propose the presence of clumped fluoride-hydroxyl defects in forsterite at O sites surrounding a Si vacancy. Our results suggest coupled cycling and storage of F and H2O in olivine with implications for volatile budgets and rheology of the mantle.