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

C Beyer,S Klemme,T Grützner,T.R Ireland,C.W Magee,D.J Frost

doi:10.1016/j.chemgeo.2016.05.032

Abstract

In this experimental study we obtained new mineral/melt (DF=cmineral/cmelt) partitioning data for fluorine in a bimineralic hydrous eclogite under Earth's upper mantle conditions (4–6GPa, 1460–1550°C). Omphacitic clinopyroxene displays mineral/melt partition coefficients between DF=0.056±0.005 and DF=0.074±0.001. Garnet partition coefficients are consistently lower with an average partition coefficient of DF=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/H2O ratio. The enrichment of fluorine relative to H2O is triggered by multiple episodes of small degree melting that deplete the residual more in H2O than in fluorine, caused by the approximately three times smaller mineral-melt partition coefficients of H2O.

Highlights

Owing to their incompatibility and volatility, the distribution of H2O and halogens in the Earth’s mantle is influenced by processes such as fluid mobility, oxygen fugacity, fractionation, degassing, and partial melting
Results of this study further suggest that fluorine concentration estimates in Ocean Island Basalt (OIB) source regions are at least 10% lower than previously expected (Joachim et al Chem Geol 416:65–78, 2015), implying that consideration of the effect of water on the fluorine partitioning behavior between Earth’s mantle minerals and silicate melt is vital for a correct estimation of fluorine abundances in OIB source regions
The only available approach to estimate bulk halogen concentrations, such as fluorine and chlorine, in Mid Ocean Ridge Basalt (MORB) and OIB mantle source region was based on the analysis of element ratios, such as F/P, F/Sr and F/ Nd, or Cl/K and Cl/Nb obtained from natural samples that were used as a proxy (Schilling et al 1980; Ito et al 1983; Michael and Schilling 1989; Déruelle 1992; Jambon et al 1995; McDonough and Sun 1995; Newsom 1995; Wedepohl 1995; Saal et al 2002; Salters and Stracke 2004; Le Roux et al 2006; Workman et al 2006; Shaw et al 2008; Pyle and Mather 2009; Palme and O’Neill 2014)

Summary

Introduction

Owing to their incompatibility and volatility, the distribution of H2O and halogens in the Earth’s mantle is influenced by processes such as fluid mobility, oxygen fugacity, fractionation, degassing, and partial melting. With quantification of their distribution between different mantle phases, this makes halogens excellent tracers of volatile transport. Comparing OIB source halogen concentrations with primitive mantle estimates enables us to better understand and quantify any volatile transport processes during recycling of oceanic crust. Fluorine OIB source region concentration estimates range from 8 ppm (Beyer et al 2012) to 55 ppm (Kovalenko et al 2006), which covers a range from depleted to enriched abundances relative to primitive mantle estimates

Methods

Results

Conclusion