The fluorine dichotomy: Amphibole control on fluorine availability during magmatic differentiation

Experimental and observational evidence suggests that water and fluorine (F) are incompatible in silicate minerals and originate from similar sources in the mantle. This, in turn, suggests that F and water should coevolve in magmatic systems. However, hydrous subduction zone (“arc”) magmas are depleted in F relative to anhydrous magmas. We explore this apparent dichotomy in fluorine’s behavior by examining amphibole, which preferentially incorporates F into its crystal structure. By developing a parametrized fluorine amphibole/silicate melt partition coefficient for F, we show that F concentrations in calculated equilibrium melts in arc systems do not increase with differentiation. This indicates that F behaves compatibly in hydrous arc magmas due to sequestration in amphibole-bearing arc cumulates. In dry magmatic systems, where amphibole does not crystallize, we show that F behaves incompatibly during differentiation, resulting in F enrichment in evolved magmas. These results have implications for the formation of ore deposits associated with elements that complex with F, such as rare-earth elements.