Investigating the mechanics of fluorine speciation in low-pressure silicate melts using ¹⁹F magic angle spinning nuclear magnetic resonance spectroscopy

Halogens are highly reactive non-metallic elements of the periodic table. Fluorine is the lightest of the halogens, the most enriched in the bulk silicate Earth, and above the predicted trend for all moderately volatile lithophile elements relative to condensation temperature, including the other halogens. To address fluorine’s distinction among the halogens we focus on the incorporation mechanism for fluorine in silicates. We investigate the speciation of F in low-pressure (1 atm) F-enriched CMAS silicate melts (CaO + MgO + Al₂O₃ + SiO₂). Speciation was analysed using solid-state magic angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy.

One possible mechanism is the substitution fluorine and hydroxyl anions in mafic minerals frameworks. However, this process alone fails to explain the differences in compatibility between fluoride and hydroxyl anions in mafic phases. Our data show that fluorine bonds in silicate melts in a model CMAS system (Fig. 1). These data suggest a potential preference for F incorporation in Mg-rich phases during melt crystallisation and for F sequestration in Mg-rich silicate melts, which speaks to fluorine’s relative enrichment in enstatite-type chondrite relative to CI-type chondrite.