Behavior of halogens in the NHFMs of the upper-mantle

Halogens are abundant in components in volcanic gases and may be a key for understanding and predicting volcanic eruptions. The knowledge of the role of halogens in the system Earth can help to improve our understanding of the evolution of igneous systems and the migration of halogens in subduction zones, at which the most hazardous volcanism on Earth occurs. The distribution of halogens between aqueous fluids (brine), silicate melts, and mantle minerals are not well known and there is a lack on experimental data about it. In this study it is proposed to investigate experimentally the incorporation of halogens in Nominally Halogen-Free Minerals, NHFMs, e.g., olivine and pyroxenes. Experiments will be performed in the system MgO-SiO2-H2O + Halogens under upper-mantle conditions. In addition, the effect of the bulk composition will be investigated adding small amounts of Al2O3, CaO and FeO at the starting material. Runs will be carried out in high pressure solid media apparatuses between 2 GPa and 4 GPa and temperatures ranging from 1000 to 1300 C. Major-element and halogen concentrations will be determined by electron microprobe and LA-GD-TOFMS (Laser Ablation Glow Discharge Time-Of-Flight Mass Spectrometry). Runs are expected to produce an assemblage of crystals melt aqueous fluids (brine) allowing (1) to determine the solubility of halogens in olivine, garnet and pyroxene, (2) the partitioning of halogens between aqueous fluids (brine) and/or silicate melts and NHFMs, and (3) to estimate the halogens abundance and distribution in the upper-mantle. With the new experimental data the maximum amount of halogens that can be dissolved in the NHFMs will be defined, and the results will be compared to compositions of natural material to fill the gap of our knowledge about the halogens concentration and distribution in magma source regions. Results will be particularly interesting to organize future thermodynamic models for halogen- bearing melts and fluids (vapor, hydrosaline liquid, vapor + hydrosaline liquid) as a function of P and T, to clarify the role of silicate melts and aqueous fluids in sequestering and migrating halogens towards the crust, to identify the role of NHFMs acting as halogen reservoir in the upper mantle. Furthermore, results will improve our knowledge concerning volcanism and may help understanding and predicting volcanic eruptions.