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Halogen ratios as tracers of magmatic fluid release in the crust

Mara Miranda, Alexandra Tsay and Zoltan Zajacz

Abstract

Porphyry-type Cu-(Mo-Au) ore deposits gain their metal budget from magmatic fluids, and chlorine is a key ligand facilitating metal extraction from magmas. Contrary to the conventional view, increasing amount of evidence suggests that significant masses of magmatic fluids are exsolved at greater crustal depths due to highly elevated dissolved water concentrations in arc magmas. As a corollary, significant fluid exsolution may also happen from relatively mafic melts. We present new data on the partitioning of chlorine, bromine and iodine between aqueous fluids and silicate melts as a function of pressure (P=150--835 MPa), temperature (T=800-1000 oC), fluid salinity (from ~ 3 to ~ 62 wt% NaCl equivalent)  and silicate melt composition (basalt to rhyolite). Fluids and silicate melts were equilibrated in externally heated pressure vessel assemblies and piston cylinder apparatus. The composition of the quenched glasses were determined by Electron Probe Microanalysis (major elements + Cl) and Laser Ablation Inductively Coupled Plasma Mass Spectrometry (major elements + Cl, Br, I), and the fluid compositions were derived by mass balance calculation. Results of key importance are: 1) the fluid/melt partition coefficients (Df/m) systematically increase with increasing halide ion radius, and therefore Br/Cl and I/Cl ratios are tracers of fluid loss; 2) At low fluid salinities, all halogens increasingly partition into the fluid with decreasing T and with P increasing up to 400-500 MPa; however, their Df/m decrease above 500 MPa; 3) Df/mhalogens rapidly drop as the silicate melt becomes more mafic. Model equations capable to predict Df/mhalogens in P-T-compositional space were constructed.