Partitioning and equilibration of Rb and Sr between silicate melts and aqueous fluids

Trace element concentrations in aqueous fluids in equilibrium with haplogranitic melt were determined in situ at elevated P–T conditions using hydrothermal diamond-anvil cells and synchrotron-radiation XRF microanalyses. Time-resolved analyses showed that the Rb and Sr concentrations in the fluids became constant in less than 2000 s at all temperatures (500 to 780 °C). Although fluid–melt equilibration was very rapid, the change in the concentration of both elements in the fluid with temperature was fairly small (a slight increase for Rb and a slight decrease for Sr). This permitted partitioning data for Rb and Sr between haplogranitic melt and H2O or NaCl + KCl + HCl aqueous solutions at 750 °C and 200 to 700 MPa to be obtained from EMP analyses of the quenched melt and the in situ SR-XRF analyses of the equilibrated fluid. The resulting DRbf/m and DSrf/m were 0.01 ± 0.002 and 0.006 ± 0.001 for water as starting fluid, and increased to 0.47 ± 0.08 and 0.23 ± 0.03 for 3.56 m (NaCl + KCl) + 0.04 m HCl at pressures of 224 to 360 MPa. In the experiments with H2O as starting fluid, the partition coefficients increased with pressure, i.e. DRbf/m from 0.01 ± 0.002 to 0.22 ± 0.02 and DSrf/m from 0.006 ± 0.001 to 0.02 ± 0.005 with a change in pressure from 360 to 700 MPa. At pressures to 360 MPa, the Rb/Sr ratio in the fluid was found to be independent of the initial salt concentration (Rb/Sr = 1.45 ± 0.6). This ratio increased to 7.89 ± 1.95 at 700 MPa in experiments with chloride free fluids, which indicates different changes in the Rb and Sr speciation with pressure.