Abstract
Halogens show a range from moderate (F) to highly (Cl, Br, I) volatile and incompatible behavior, which makes them excellent tracers for volatile transport processes in the Earth’s mantle. Experimentally determined fluorine and chlorine partitioning data between mantle minerals and silicate melt enable us to estimate Mid Ocean Ridge Basalt (MORB) and Ocean Island Basalt (OIB) source region concentrations for these elements. This study investigates the effect of varying small amounts of water on the fluorine and chlorine partitioning behavior at 1280 °C and 0.3 GPa between olivine and silicate melt in the Fe-free CMAS+F–Cl–Br–I–H2O model system. Results show that, within the uncertainty of the analyses, water has no effect on the chlorine partitioning behavior for bulk water contents ranging from 0.03 (2) wt% H2O (DClol/melt = 1.6 ± 0.9 × 10−4) to 0.33 (6) wt% H2O (DClol/melt = 2.2 ± 1.1 × 10−4). Consequently, with the effect of pressure being negligible in the uppermost mantle (Joachim et al. Chem Geol 416:65–78, 2015), temperature is the only parameter that needs to be considered for the determination of chlorine partition coefficients between olivine and melt at least in the simplified iron-free CMAS+F–Cl–Br–I–H2O system. In contrast, the fluorine partition coefficient increases linearly in this range and may be described at 1280 °C and 0.3 GPa with (R2 = 0.99): D_{F}^{text{ol/melt}} = 3.6pm 0.4 times {{10}^{-3}} times {{X}_{{{text{H}}_{text{2}}}text{O}}}left( text{wt }!!%!!text{ } right) + 6 pm 0.4times ,{{10}^{-4}}. The observed fluorine partitioning behavior supports the theory suggested by Crépisson et al. (Earth Planet Sci Lett 390:287–295, 2014) that fluorine and water are incorporated as clumped OH/F defects in the olivine structure. Results of this study further suggest that fluorine concentration estimates in 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. Estimates for MORB source fluorine concentrations as well as chlorine abundances in both mantle source regions are within uncertainty not affected by the presence of water.
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
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
Sign-in/Register to view highlights & summary Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
