Experimental partitioning of F and Cl between olivine, orthopyroxene and silicate melt at Earth's mantle conditions

Abstract

Halogens have, because of their volatile behavior and incompatibility, the potential to act as key tracers of volatile transport processes within the Earth's mantle. A better understanding of halogen behavior during partial melting processes will improve our understanding of volatile input mechanisms into the Earth's mantle and give insight into its evolution over the Earth's history. This study introduces time of flight secondary ion mass spectrometry (TOF-SIMS) as an analytical method for the determination of halogen partition coefficients and significantly extends the available dataset for fluorine and chlorine partitioning between mantle minerals and silicate melts to conditions of partial melting processes in Ocean Island Basalt (OIB) source regions.Halogen partitioning between olivine, orthopyroxene and silicate melt has been determined in experiments at 1.0–2.3GPa and 1350–1600°C. Combining our data with results of recent studies (O'Leary et al., 2010; Beyer et al., 2012; Dalou et al., 2012, 2014; Rosenthal et al., 2015) shows that fluorine and chlorine partitioning between olivine and melt increases by about 1.5–2 orders of magnitude between 1350°C and 1600°C (fluorine: 0.005(3)–0.31(16); chlorine: 0.005(45)–0.17(9)) and does not show any pressure dependence between 1.0 and 2.3GPa. Chlorine partitioning between orthopyroxene and melt increases by about 1 order of magnitude between 1450°C and 1600°C (0.015(8)–0.16(9)) at a constant pressure of 2.3GPa. Fluorine partitioning between orthopyroxene and melt increases by 1.5 orders of magnitude between 1250°C and 1600°C (0.029(6)–0.20(14)) and does not show any pressure dependence.Transmission electron microscopy measurements show that halogens are not incorporated in the form of humite-type defects in olivine. The most reasonable incorporation mechanism for halogens is via point defects in the olivine and orthopyroxene lattice, where they are inferred to be charge-balanced via oxygen defects.By combining our partition coefficients with natural halogen concentrations in oceanic basalts, we are able to give estimates for fluorine and chlorine abundances in Mid Ocean Ridge Basalt (MORB) (F=3–14; Cl=0.6–14ppm) and OIB (F=34–76; Cl=21–71ppm) mantle source regions. Comparing these with estimates of bulk silicate Earth (BSE) concentrations (F=18±8ppm, Lyubetskaya and Korenaga, 2007; F=25±10ppm, Palme and O'Neill, 2003; Cl=30±12ppm Palme and O'Neill, 2003) indicates that the upper mantle is degassed by 22–88% in fluorine and 22–99% in chlorine relative to the primitive mantle. The OIB source mantle region has a chlorine concentration that is similar to primitive mantle estimates, but is enriched in fluorine by a factor of 1.4–4.2 relative to the primitive mantle. An explanation for the relative fluorine enrichment in the OIB source region is that compared to chlorine, fluorine may be incorporated to a greater extent into the crystal structure of minerals that are stable at high P–T conditions and may thus be recycled more efficiently into the deeper mantle through subduction of oceanic crust.