The fluid/melt partitioning of chlorine, bromine and iodine in felsic magmas and the utility of halogen ratios to track the devolatilization and fluid fluxing of magma reservoirs

Abstract

The release of fluids from magmas at crustal depths is an essential process for the formation of magmatic-hydrothermal ore deposits. However, assessing the extent of volatile loss by magma degassing or volatile gain by fluid fluxing from deeper magmas remains challenging. To develop a new tool to quantitatively track these processes, we experimentally determined the partition coefficients of Cl, Br, and I between aqueous fluids and haplogranitic melts ($D_{\text{i}}^{f/m}$) as a function of fluid salinity and the aluminum saturation index [ASI = Al₂O₃/(Na₂O + K₂O)] of the silicate melt. The experiments were conducted in externally heated rapid-quench René 41 cold-seal pressure vessel apparatus at 200 MPa and 790 ± 10 °C. The Br and I concentrations in the run product glasses were determined by laser ablation inductively coupled plasma mass spectrometry and secondary ion mass spectrometry, whereas the concentration of Cl was determined by electron probe microanalysis. The results show that the partition coefficients of the three halogens in a system with chloride-dominated fluids increase with fluid salinity increasing from 1.49 to 60.6 wt% total NaCl equivalent. Specifically, $D_{\text{Cl}}^{f/m}$ increases from 23 ± 5 to 168 ± 7 (1σ), $D_{\text{Br}}^{f/m}$ increases from 57 ± 13 to 271 ± 14, and $D_{\text{I}}^{f/m}$ increases from 198 ± 61 to 736 ± 159. As for the influence of melt composition, $D_{\text{Cl}}^{f/m}$ and $D_{\text{Br}}^{f/m}$ attain maximum values at ASI = 1, whereas $D_{\text{I}}^{f/m}$ appears to be independent of ASI within error. Bromine and iodine partition into the fluid more strongly in the presence of significant Cl, indicating that these halogens compete for the same structural sites in the silicate melt. Empirical equations were derived to predict the $D^{f/m}$ of Cl, Br, and I in felsic magmatic systems as a function of fluid salinity and silicate melt ASI. These equations were in turn implemented in numerical models simulating the degassing of granitic magma reservoirs emplaced in the Earth’s upper crust. The results of these calculations show that the Br/Cl and I/Cl ratios in the silicate melt and the released fluid rapidly decrease during progressive magma degassing at depth due to the significantly increasing fluid/melt partition coefficients with increasing halide ion radius. On the other hand, a sudden increase of Br/Cl and I/Cl in the silicate melt indicates fluid fluxing of the magma. Therefore, halogen ratios may become particularly useful tools to track crystallization-driven degassing if the record of the variation of the silicate melt composition can be recovered from silicate melt inclusions in minerals. In addition, the evolving Br/Cl and I/Cl ratios of magmatic fluids feeding magmatic-hydrothermal ore-forming systems, as recorded by fluid inclusions in minerals, may inform about the evolution of the underlying magma reservoir.