New Experimental Insights Into the Geochemical Cycle of Halogens: Halogen Partitioning Between Amphibole, Phlogopite and Silicate Melts

Halogens are important volatile elements in the Earth's mantle [1], but little is known about their behavior in the different mantle reservoirs and little is known about how they are fractionated during mantle melting processes. Several studies recently published halogen partitioning data between major mantle minerals (olivine, pyroxene, garnet) and melts [2] but there is scant data on halogen partitioning between less abundant but nevertheless important mantle phases such as amphibole and mica [3].

We conducted partitioning experiments in the systems Na₂O-K₂O-CaO-MgO-Al₂O₃-SiO₂and Na₂O-FeO-CaO-K₂O-MgO-Al₂O₃-SiO₂-TiO₂, adding all four halogens (F, Cl, Br, I) in each. Experiments were run at 1.4 GPa and temperatures between 1015°C and 1300°C. The experimental products were analyzed with EMPA, with a custom made analytical protocol following [4, 5]. Mineral/melt partition coefficients (D^min/melt) for halogens were calculated for amphibole, phlogopite, clinopyroxene. These D values were then used in a melting model for phlogopite and amphibole-bearing mantle sources in order to determine the behavior and abundance of the halogens in mantle-derived melts. We modelled F/Cl of the melts and compared them to those of pristine melts from different settings. We found that they are concordant with those of a metasomatized mantle source, which therefore could be the potential source of certain ultrapotassic magmas.

[1] Koga and Hemley, The Role of Halogens in Terrestrial and Extraterrestrial Geochemical Processes, 2018. Spinger Geochemistry, [2] Klemme and Stalder, The Role of Halogens in Terrestrial and Extraterrestrial Geochemical Processes, 2018. Springer Geochemistry, [3] Sato et al., 1997, Earth Planet. Sci. Lett., 146, 511-526, [4] Zhang et al., 2015, Geostandards and Geoanalytical Research, 40, 351-363, [5] Zhang et al., 2017, Geostandards and Geoanalytical Research, 41, 449-457.