11:15 AM - 11:30 AM
[SIT20-09] Fluorine sink in Earth’s lower mantle: solubility and substitution mechanism in bridgmanite
Keywords: fluorine, bridgmanite, substitution mechanism
In the present study, high-pressure experiments were performed in the system MgO-SiO2 + MgF2 (± Al2O3 ± H2O) at 2073 K and 25 GPa to determine the F solubility in bridgmanite. Electron microprobe measurements show that F concentrations in bridgmanite occasionally exceed 1 wt%, although the F content in the Al- and H2O-free system is below detection limit (50 ppm). F solubility in bridgmanite is higher than that in wadsleyite. The solubility of F in bridgmanite shows a strong compositional dependence with a positive correlation between Al3+ and F-, suggesting a coupled substitution of Al3+ and F- with Si4+ and O2- to incorporate F in bridgmanite. When the system contains H2O, bridgmanite can incorporate larger amounts of F even for the Al-free system. In this case, F substitution mechanism would be similar to the water incorporation mechanism. Since the concentration of hydrated Si vacancies is extremely low at all water concentrations (Muir & Brodholt, 2018), F may incorporate into Mg site. Our experiments show that nominally fluorine-free bridgmanite has ability as the entire fluorine budget of the lower mantle.