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

Abstract

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 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.

Contrasting partition behavior of F and Cl during hydrous mantle melting: implications for Cl/F signature in arc magmas

“…The discrepancy between these data is perhaps due to the presence of micro-inclusions, clinohumite lamellae, or clumped OH/F defects (Crépisson et al 2014) in the high F olivine ).…”

Is the transition zone a deep reservoir for fluorine?

“…F was measured using Particle Induced Gamma Ray Emission (PIGE) following the procedure described in Crépisson et al (2014). Hydrogen was measured using Elastic Recoil Detection Analysis (ERDA) using the procedure described in Before any analysis, the largest crystals were selected with SEM cartographies.…”

“…An additional possibility might be a competition process between OH and F for incorporation in the mineral lattices of Wd and Rw. Indeed, a recent study combining infrared characterization of experimentally F-and OH-enriched olivines and first-principle calculations, Crépisson et al (2014) have demonstrated a close association of fluoride, hydroxyl groups and Si vacancies. They found evidence for the presence of clumped fluoride-hydroxyl defects in the forsterite structure.…”

“…Like water, that is stored in silicate minerals as hydroxyl species, it has been proposed that the mantle fluorine budget can be entirely accommodated by these mineral phases (Beyer et al, 2012;Crépisson et al, 2014). Based on the observation of clumped fluoride-hydroxyl defects in pure-Mg olivine, the major upper mantle mineral, it is likely that fluorine and water cycles may be strongly coupled through the nominally anhydrous minerals (Crépisson et al, 2014). Like for water, F may be transferred at depth during subduction processes.…”

“…By comparison, experimental studies performed to determine fluorine solubility in these major mantle mineral phases yielded maximum contents of fluorine of 4500 ppm to 1900 ppm in olivine (Bromiley and Kohn, 2007;Bernini et al, 2012), 626 ppm in pyroxenes (Dalou et al, 2012), and 1110 ppm in pyrope (Bernini et al, 2012). Like water, that is stored in silicate minerals as hydroxyl species, it has been proposed that the mantle fluorine budget can be entirely accommodated by these mineral phases (Beyer et al, 2012;Crépisson et al, 2014). Based on the observation of clumped fluoride-hydroxyl defects in pure-Mg olivine, the major upper mantle mineral, it is likely that fluorine and water cycles may be strongly coupled through the nominally anhydrous minerals (Crépisson et al, 2014).…”