Fluorine Diffusivity in Lunar Melts and Fluorine Content in the Lunar Interior

Lunar volcanic glass beads, as the closest approximations for primary lunar magma, have experienced a significant degree of degassing during eruption. To retrieve their initial fluorine contents requires fluorine diffusivity data, which have yet to be experimentally determined for lunar melts. We have performed diffusion experiments at 0.5 GPa and 1,673–1,883 K in a piston cylinder apparatus for four synthesized lunar melts with compositions corresponding to Apollo green glass (GG), yellow glass (YG), orange glass (OG) and red glass (RG). The fluorine diffusion profiles yield fluorine diffusivities increasing mildly from GG to YG to OG to RG melt. The fluorine diffusivities of lunar melts are greater than F diffusivities in terrestrial melts by at least a factor of 2–10. We develop a general model for F diffusivity in terrestrial and lunar melts as a function of X Si+Al (mole fraction of Si and Al combined among all cations), which can reproduce experimental data within a factor of 3. Using the new fluorine diffusivity data, modeling of the fluorine profile in Apollo GG bead recovers a pre‐eruptive fluorine content of 11.4–15.4 μg/g. Assuming minimal volatile loss before magma fragmentation, the fluorine content is 0.6–1.8 μg/g in its mantle source. The fluorine content in the bulk silicate Moon is inferred to be 4.4–7.4 μg/g, which is consistent with previous estimates. This consistency confirms diffusion degassing as the major mechanism of fluorine depletion for glass beads. Our calculation indicates a depletion degree of 70%–82% for fluorine compared to bulk silicate Earth.