Cesium partitioning between granitic melts and aqueous fluids: Is Cs in hydrothermal fluids an accurate proxy of the degree of fractionation of parental magmas?

The concentration of Cesium in the aqueous magmatic fluid is widely used as an indicator for the degree of differentiation of the parental magma, which is fundamental for estimating the ore-forming potential of the fluid and characterizing the overall magmatic system. However, the fluid-melt partitioning behavior of Cs has not yet been well-characterized experimentally, posing some challenges in the quantitative use of measured Cs concentrations. In this study, we determined the fluid-melt partition coefficient of Cs (D_Cs^f/m) at 800 °C, 150 MPa and 250 MPa and the oxygen fugacity of the Ni-NiO buffer as a function of total Cl concentration (m_Cl^total) and HCl/total Cl (molar) ratios in the fluid. The results show that D_Cs ^f/m increases linearly from 0.21 ± 0.06 (1σ) to 6.31 ± 0.18 (1σ) as m_Cl^total increases from 1 to 16 m (5.5–48 wt% NaCl equivalent). At a constant m_Cl^total = 2 m, D_Cs^f/m broadly remains constant at 0.75 when HCl/total Cl ratios increase from 0.09 to 0.33. At a constant m_Cl^total = 1 m, D_Cs^f/m is identical within error (0.21 ± 0.01) at 150 and 250 MPa confining pressure. The observed partitioning data suggest that Cs was dominantly present as CsCl in the Cl-rich aqueous fluids at the experimental conditions. The fluid-melt partition coefficients of alkali elements (M) increase in the order of Li < K < Rb < Cs corresponding to decreasing ionic potential (except Na), which is controlled by the strength of M–O bonds and the structure of the silicate melts. Numerical modeling indicates that the Cs concentration in the evolving fluid decreases initially and then increases during fractional crystallization (a ‘U-type’ curve) when the initial melt has a high Cl/H₂O ratio or differentiates under higher pressure. In contrast, the Cs/Cl and Cs/Na ratios of the fluid exhibit a monotonous growth trend during crystallization at all modeled conditions. Therefore, we suggest that rather than the Cs concentration in fluid, the Cs/Cl and Cs/Na ratios of the fluid are more appropriate to be used as indicators for the degree of fractionation of the parental melt. In addition, the new partitioning data will facilitate direct estimation of the Cs concentration in the parental melt of observed ore-forming fluids, and thus the estimation of the residual melt fraction in the parental magma body at the time of fluid extraction, which can facilitate better understanding of the physical mechanism of fluid extraction from crustal magma reservoirs.