Partitioning of rare earth elements between an aqueous chloride fluid phase and melt during the decompression-driven degassing of granite magmas

This paper reports the results of numerical simulation for the behavior of rare earth elements (REE) during decompression degassing of H₂O- and Cl-bearing granite melts at pressures decreasing from 3 to 0.5–0.3 kbar under near isothermal conditions (800 ± 25°C). Fluid phase in equilibrium with the melt contains mainly chloride REE complexes, and their behavior during magma degassing is, therefore, intimately related to the behavior of chlorine. It was shown that the contents and distribution patterns of REE in the aqueous chloride fluid phase formed during decompression vary considerably depending on (1) the contents of volatiles (Cl and H₂O) in the initial melt, (2) the redox state of the magma, and (3) the dynamics of fluid phase separation from magmas during their ascent toward the Earth’s surface. During decompressiondriven degassing, the contents of both Cl and REE in the fluid decrease, especially dramatically under opensystem conditions. The REE patterns of the fluid phase compared with those of the melt are characterized by a higher degree of light to heavy REE fractionation. A weak negative Eu anomaly may be present in the REE patterns of Cl-rich fluids formed during the early stages of degassing at relatively high pressures. At a further decrease in pressure and Cl content in the fluid, it is transformed into a positive Eu anomaly increasing during decompression degassing. Such an anomalous behavior of Eu during degassing is related to its occurrence in magmatic melts in two valence states, Eu³⁺ and Eu²⁺, whereas the other REE occur in melts mainly as (REE)³⁺. The Eu³⁺/Eu²⁺ ratio of melt is controlled by the redox state of the magmatic system. The higher the degree of melt reduction, the more pronounced the anomalous behavior of Eu during decompression degassing. The amount of REE extracted by fluid from melt during various stages of degassing does not significantly influence the content and distribution patterns of REE in the melt.