Controls on Rb enrichment during granitic magma–hydrothermal evolution: Constraints from fluid–melt partitioning at 850 °C and 200 MPa

Rubidium (Rb) enrichment in granites and granitic pegmatites commonly reaches several thousand to tens of thousands of μg/g, far exceeding average crustal abundances, yet the relative roles of magmatic differentiation and fluid exsolution in generating such enrichment remain poorly constrained. Here, we experimentally determined Rb and Cl partitioning between hydrous granitic melts and supercritical Cl-bearing aqueous fluids over a range of melt and fluid compositions at 850 °C and 200 MPa, and integrated the partitioning results with Rayleigh-type modelling to track the coupled evolution of fluid chlorinity and Rb extraction efficiency during progressive magma crystallization. Rb fluid–melt partitioning is controlled primarily by fluid chlorinity rather than by melt composition, with fluid–melt partition coefficients for Rb increasing systematically from 0.3 to 1.2 as fluid chlorinity rises from ∼1 to 4 mol/kg H₂O, consistent with increasingly important Rb Cl complexing in saline fluids. However, comparison with previous in situ measurements indicates that the absolute magnitude of D Rb f / m at the highest chlorinity varies among experimental systems and should be interpreted cautiously. Combining Cl partitioning data, the modelling results indicate that the chlorinity of exsolved fluids rapidly decreases and stabilizes near ∼1 mol/kg H₂O (∼ 5 wt% NaCleq.) during crystallization, largely independent of initial melt H₂O content, melt composition, or initial fluid chlorinity. Under these conditions, fluid–melt partition coefficients for Rb converge rapidly to ∼0.3, causing magmatic fluids to extract only a minor fraction (<10% at 99% crystallization) of the total magmatic Rb budget even in initially saline systems. In contrast, residual melts continuously concentrate Rb to ore-forming levels during advanced crystallization. These results demonstrate that extreme Rb enrichment in granite systems is controlled primarily by prolonged magmatic differentiation, whereas magmatic fluid exsolution is generally inefficient for Rb concentration.