Rb and Sr partitioning between haplogranitic melts and aqueous solutions

摘要

Rubidium and strontium partitioning experiments between haplogranitic melts and aqueous fluids (water or 1.16–3.56 m (NaCl + KCl) ± HCl) were conducted at 750–950 C and 0.2–1.4 GPa to investigate the effects of melt and fluid composition, pressure, and temperature. In addition, we studied if the applied technique (rapid and slow quench, and in-situ determination of trace element concentration in the fluid) has a bearing on the obtained data. There is good agreement of the data from different techniques for chloridic solutions, whereas back reactions between fluid and melt upon cooling have a significant effect on results from the experiments with water. The Rb fluid–melt partition coefficient shows no recognizable dependence on melt composition and temperature. For chloridic solutions, it is 0.4, independent of pressure. In experiments with water, it is one to two orders of magnitude lower and increases with pressure. The strontium fluid–melt partition coefficient does not depend on temperature. It increases slightly with pressure in Cl free experiments. In chloridic fluids, there is a sharp increase in the Sr partition coefficient with the alumina saturation index (ASI) from 0.003 at an ASI of 0.8 to a maximum of 0.3 at an ASI of 1.05. At higher ASI, it decreases slightly to 0.2 at an ASI of 1.6. It is one to two orders of magnitude higher in chloridic fluids compared to those found in H2O experiments. The Rb/Sr ratio in non-chloridic solutions in equilibrium with metaluminous melts increases with pressure, whereas the Rb/Sr ratio in chloridic fluids is independent of pressure and decreases with fluid salinity. The obtained fluid–melt partition coefficients are in good agreement with data from natural cogenetic fluid and melt inclusions. Numerical modeling shows that although the Rb/Sr ratio in the residual melt is particularly sensitive to the degree of fractional crystallization, exsolution of a fluid phase, and associated fluid–melt partitioning is not a significant factor controlling Rb and Sr concentrations in the residual melt during crystallization of most granitoids.