Clumped fluoride-hydroxyl defects in forsterite:Implications for the upper-mantle

Céline Crépisson (1) , Marc Blanchard (1) , Hélène Bureau (1) , Chrystèle Sanloup (2, 3) , Anthony C. Withers (4) , Hicham Khodja (5) , Suzy Surblé (5) , Caroline Raepsaet (5) , Keevin Béneut (1) , Clémence Leroy (1) , Paola Giura (1) , Etienne Balan (1)

1 IMPMC - Institut de minéralogie et de physique des milieux condensés
2 iSTeP - Institut des Sciences de la Terre de Paris
3 SUPA, Centre for Science at Extreme Conditions and School of Physics and Astronomy
4 Department of Earth Sciences [London, ON]
5 LEEL - UMR 3685 - Laboratoire d'Etudes des Eléments Légers

The mechanism and magnitude of fluorine incorporation in H-bearing forsterite were investigated through a combined experimental and theoretical approach. Forsterite samples were synthesized in a piston cylinder press at 2 and 4 GPa, in hydrous conditions, with or without fluorine. High fluorine solubilities of 1715 and 1308 ppm F were measured by particle induced gamma-ray emission (PIGE) in forsterites synthesized at 2 and 4 GPa, respectively. In addition, first-principles calculations based on density functional theory were performed in order to investigate the coupled incorporation mechanisms of fluorine and water in forsterite. Our results demonstrate the close association of fluoride, hydroxyl groups and Si vacancies. Comparison of experimental and theoretical infrared absorption spectra enables assignment of the nine OH stretching bands (3500-3700 cm-1) observed in F-rich synthetic forsterite to clumped fluoride-hydroxyl defects in the forsterite crystal structure. Noteworthily, similar bands were previously recorded on some natural olivine with Mg/(Mg+Fe) molar ratio down to 0.86. Fluorine and water cycles are therefore strongly coupled through the nominally anhydrous minerals and the mantle fluorine budget can be entirely accommodated by these mineral phases.

Domaines: Sciences de la Terre

Céline Crépisson, Marc Blanchard, Hélène Bureau, Chrystèle Sanloup, Anthony C. Withers, et al.. Clumped fluoride-hydroxyl defects in forsterite:Implications for the upper-mantle. Earth and Planetary Science Letters, 2014, 390, pp.287-295. ⟨10.1016/j.epsl.2014.01.020⟩. ⟨hal-00959948⟩