Fluorine in nominally fluorine-free mantle minerals: Experimental partitioning of F between olivine, orthopyroxene and silicate melts with implications for magmatic processes

Abstract

We present new experimentally determined partition coefficients for fluorine (F) between nominally fluorine-free minerals and corresponding basaltic melts in the systems CaO–MgO–Al2O3–SiO2+F, Na2O–CaO–MgO–Al2O3–SiO2+F, and in natural compositions at pressures up to 2.5GPa and temperatures between 1285°C and 1445°C. Fluorine is incompatible in olivine with F partition coefficients DFol/melt varying between 0.0007 and 0.0033, whereas the F in orthopyroxene is more compatible with DFopx/melt between 0.031 and 0.037. These findings agree well with new analyses of F in natural olivines and orthopyroxene from spinel peridotites and oceanic basalts, and indicate that F is less incompatible in olivine and orthopyroxene than hydrogen (DF/DH∼3.3 for olivine, DF/DH∼2.4 for orthopyroxene). Based on high-resolution TEM images of one of the samples, we argue that the F incorporation into the olivine structure is mass balanced via oxygen defects. Furthermore, we present some analyses of Cl in olivine and orthopyroxene from spinel peridotites and basalts.Hence, fluorine, and to a lesser extend chlorine, may be effectively stored in nominally halogen-free mantle minerals such as olivine and orthopyroxene. Considering their high modal proportion in the upper mantle, both phases are the major hosts for F in the Earth's mantle, and must be taken into account when calculating the Earth's budget of halogens or global cycles of halogens in the deeper Earth. Other F-bearing minerals such as amphibole or phosphates are not needed to balance the F concentrations between crust and mantle.Applying our new partition coefficients to primitive olivine hosted melt inclusion data from mid-ocean ridge basalt (MORB), we calculate a concentration of 6–15μg/g F for the depleted MORB mantle source. Our results also show that typical spinel lherzolites from the subcontinental lithosphere show virtually identical concentrations of about 12μg/g. However, using the partition coefficients on primary melt inclusions from ocean island basalts (OIB), suggest slightly higher fluorine contents between 8 and 31μg/g for OIB mantle sources.