Effect of water on the fluorine and chlorine partitioning behavior between olivine and silicate melt

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 (DCl ol/melt = 1.6 ± 0.9 × 10−4) to 0.33 (6) wt% H2O (DCl ol/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 (R 2 = 0.99): . 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.

First-principles modeling of chlorine isotope fractionation between chloride-bearing molecules and minerals

“…Several studies have pointed out the key role of hydroxylated defects in the incorporation of fluorine in forsterite, the Mg olivine end-member (Crépisson et al, 2014;Joachim et al, 2015). Compared with fluorine, partition coefficients of chlorine between olivine and melt are smaller, with a lower dependence on the olivine water content (Joachim et al, 2017). Nonetheless, in absence of definite conclusions concerning the incorporation mechanism of chlorine in olivine, we considered that the Cl for OH substitution in hydroxylated defects of olivine still is a viable incorporation mechanism.…”

Halogen Heterogeneity in the Lithosphere and Evolution of Mantle Halogen Abundances Inferred From Intraplate Mantle Xenoliths

“…As stated earlier, although the elemental ratios are uniform (Kendrick, Jackson, et al, , ; Kendrick et al, ), the halogen concentrations in the mantle are heterogeneous (Joachim et al, , ). The OIB source mantle is expected to be relatively halogen rich compared to the depleted MORB source mantle (Joachim et al, , ). The depleted domain in the mantle can be assumed to be about 70% of the whole mantle on the basis of the rarity of OIB with high (primitive) 3 He/ 4 He ratio (e.g., Graham, ).…”

“…While halogen elemental ratios are uniform in the mantle (Kendrick, Jackson, et al, , ; Kendrick et al, ), their abundances may be heterogeneous. High‐pressure and high‐temperature experiments have suggested that Cl abundance in the OIB source mantle may be several times higher than that in the MORB source mantle (Joachim et al, , ). However, the uniform halogen elemental ratios in the mantle make it possible to evaluate the evolution rate of I/Cl ratio in the convecting mantle using a ratio of average concentrations of Cl and I in the whole mantle.…”

The effect of fluorine on reaction-rim growth dynamics in the ternary CaO-MgO-SiO2 system

“…A major constraint is the use of a matrix in powder form, which introduces, even if it is predried, a significant amount of adsorbed water to the sample (Joachim et al, 2012). These findings are supported by Joachim et al (2017), who demonstrate that the amount of adsorbed water introduced to a dry setup varies between 0.03 and 0.18 wt% H 2 O, depending on the hygroscopicity of the used starting materials. The water adsorbed to the powder matrix alone is likely sufficient to reach the water saturated grain boundary regime (Milke et al, 2013).…”

“…American Mineralogist, in press. DOI: https://doi.org/10.2138/am-2022-8123CCBY http://www.minsocam.org/ surface water into the capsule (Joachim et al 2017). Standard deviations in terms of 1 are denoted in the figure if they exceed the size of the symbol.…”