Fluorine partitioning between eclogitic garnet, clinopyroxene, and melt at upper mantle conditions

Abstract In this experimental study we obtained new mineral/melt (D F  = c mineral /c melt ) partitioning data for fluorine in a bimineralic hydrous eclogite under Earth's upper mantle conditions (4–6 GPa, 1460–1550 °C). Omphacitic clinopyroxene displays mineral/melt partition coefficients between D F  = 0.056 ± 0.005 and D F  = 0.074 ± 0.001. Garnet partition coefficients are consistently lower with an average partition coefficient of D F  = 0.016 ± 0.003. We found that omphacitic clinopyroxene is the dominant nominally fluorine-free phase in subducted oceanic crust and hence omphacite is expected to be the major fluorine carrier during subduction of crust into the deeper mantle. Together with previously obtained partitioning data we propose that the oceanic crust can host more fluorine per mass unit than the underlying depleted oceanic mantle. If the majority of entrained fluorine is recycled into Earth's transition zone it is possible that the fluorine is either incorporated into high-pressure transition zone phases or released during high-pressure phase transformations and forming fluorine-rich small degree partial melts. Both scenarios are supported by elevated fluorine concentration in ocean island basalts, kimberlites, and lamproites. Combining the fluorine partitioning data with water partitioning data yields a plausible process to generate lamproitic magmas with a high F/H 2 O ratio. The enrichment of fluorine relative to H 2 O is triggered by multiple episodes of small degree melting that deplete the residual more in H 2 O than in fluorine, caused by the approximately three times smaller mineral-melt partition coefficients of H 2 O.