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 2.5 GPa 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 6–15 μg/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 12 μg/g. However, using the partition coefficients on primary melt inclusions from ocean island basalts (OIB), suggest slightly higher fluorine contents between 8 and 31 μg/g for OIB mantle sources.