Halogens as a proxy for magma degassing at crustal depths

Arc magmatic systems are thought to attain fluid saturation already at mid to lower crustal depths, and magmatic fluids may play an important role in transporting ore metals and sulfur within the magma reservoir system and from the magma to the site of ore deposition. Therefore, tracking the migration of magmatic fluids within such systems is important for understanding magmatic-hydrothermal ore genesis and volcanic degassing.

We developed new geochemical tools to track magma degassing at crustal depths, by experimentally determining the partition coefficients of chlorine, bromine and iodine between aqueous fluids and silicate melts. We investigated a large range of pressures (P=150–835 MPa), temperatures (T=800–1000 ^oC), fluid salinities (from ~3 to ~62 wt% NaCl equivalent) and silicate melt composition (basalt to rhyolite following the calc-alkaline and alkaline magmatic trends) using a vector approach (i.e. changing only one variable at the time). The experimental phase assemblages were contained in Au capsules and run in externally heated René 41 or Molybdenum-Hafnium Carbide pressure vessels and a piston cylinder apparatus depending on the experimental P and T. The composition of the run product glasses was determined by Electron Probe Microanalysis (major elements + Cl), and Laser Ablation Inductively Coupled Plasma Mass Spectrometry (major elements + Cl, Br, I). The equilibrium fluid compositions were derived by mass balance calculation.

The results highlight that the fluid/melt partition coefficients (D^f/m) of Cl, Br and I increase with increasing halide ion radius and increasing fluid salinity leading to a drop in the Br/Cl and I/Cl ratios in the silicate melt during progressive degassing. Therefore, halogen ratios such as Br/Cl and I/Cl are good tracers for magma degassing and fluid fluxing in relatively evolved magmas, where traditional proxies such as CO₂/H₂O ratios have limited usefulness. Moreover, these ratios can be used as a proxy to quantify the fraction of the initial Cl budget degassed from a magmatic system, which closely relates to the extraction of ore metals. Importantly, pressure has a strong effect on the D^f/m of the three studied halogens, the extent of which depends on the fluid salinity. At low fluid salinities, all halogens increasingly partition into the fluid with increasing P up to 400-500 MPa and decreasing T; however, above 500 MPa their D^f/m decreases over the entire fluid salinity range. This has important implications for ore metal transport in deep crustal magmatic fluids. Finally, the D^f/mof all halogensrapidly drops as the silicate melt becomes more mafic. Model equations capable to predict D^f/m_halogens in P-T-melt and fluid compositional space were constructed.