The Dissolution and Exsolution of Cl-Bearing Fluids in Mafic to Felsic Magmas

Abstract:

Although Cl is one of the lesser magmatic volatiles, it influences magma evolution and fluid exsolution, the solubility of other volatiles, fluid phase equilibria, and the fluid transport of ore metals. These Cl-mediated processes depend on the concentrations and solubilities of Cl in melts, timing of fluid exsolution, salinity of fluid, and tectonic environment. We review the Cl contents of magmatic systems and show how the ratio of (measured wt% Cl in melt/modeled wt% Cl solubility in melt) serves as a proxy for Cl activity in melt and provides useful insights into the behavior of magmatic Cl. Melt ratios of (measured Cl/modeled Cl solubility) that equal unity indicate equilibrium between silicate melt and molten brine.

The Cl contents of most MORB glasses are < 0.02 wt%, but numerous analyses of basaltic seafloor glasses report thousands of ppm Cl and maximum values near 0.5 wt%. Published, high maximum Cl contents in glasses and melt inclusions in basaltic rocks of subduction zones are up to 0.42 wt%, back arc basins ≤ 0.83 wt%, and ocean islands ≤ 2.4 wt%. Contamination of basaltic magma with brines sourced from boiling seawater or assimilation of brine-bearing gabbros or altered ocean crust have been called upon to explain such Cl enrichments in mafic melts. This range of 0.02 to 2.4 wt% Cl corresponds to (measured Cl/modeled Cl solubility) melt ratios of 0.007 to 0.85. Thus, some basaltic melts are not only contaminated with seawater-derived brine but some rare fractions of ocean island melts approach Cl concentrations reflecting brine saturation.

The Cl contents of melt inclusions from most rhyolitic to rhyodacitic rocks of these tectonic environments range from 0.01 to 0.75 wt%, and the lower Cl maxima in such felsic melts are a consequence of the reduced solubility of Cl in felsic melts and of the sequestration of Cl from melt by magmatic fluids. The corresponding (measured Cl/modeled Cl solubility) melt ratios are 0.009 and 1, respectively.