New Views on the Chemistry of Aqueous Fluids in the Lower Crust and Upper Mantle

Fluids play a fundamental role in a wide range of processes in the crust and mantle. Experimental results and theoretical modeling allow quantitative study of fluid-rock reactions to depths corresponding to the middle continental crust. However, extending such studies to higher P has been limited by a lack of experimental data. Indeed, it has not been possible to answer the fundamental question: How do high-P fluids differ from their low-P counterparts? New experimental results on salt- and silicate-bearing solutions provide three key insights. MgSO4-H2O solutions represent an important model planetary fluid. Formation of MgSO4 contact ion pairs (CIP) was studied to 2 GPa at ~23°C in diamond anvil cells by Raman spectroscopy. With increasing P, the CIP fraction at all compositions (0.5-2.25 molal) decreases to a minimum at ~0.5 GPa, but then increases to higher P towards higher CIP concentrations. The reversals correlate with slope changes in the frequencies, FWHM, and integrated intensities of deconvoluted Raman bands in the OH stretching region of H2O. This suggests that there is a relationship between and the liquid-liquid transition of water. Studies of the role of NaCl-H2O brines at high P and T reveal that solubilities of major rock forming components and volatiles can increase dramatically in brines. Of particular note are enhancements of Al in NaCl-H2O brines that contain SiO2±CaO. For example, at XNaCl=0.3 and grossular+corundum saturation, coexisting brine contains 2600 ppm Al. Such fluids are clearly powerful solvents for nominally insoluble components. Even in the absence of halides, high-P H2O can dissolve substantial silicate components by formation of polymerized aqueous complexes. Evidence for the existence of these polymers includes corundum solubilities in SiO2- H2O and Na2O-SiO2-H2O fluids that are higher than predicted based on uncomplexed aluminate species, as well as Raman spectroscopy and supercritical behavior in H2O-silicate systems. These species are important because they can elevate the concentrations of nominally insoluble trace elements, such as Ti, as demonstrated by rutile solubility in alibite-H2O fluids at 900°C, 1.5 GPa, which increases from 38 to 739 ppm from Xab=0-0.05. Thus, new experimental studies reveal three novel aspects of the physical chemistry of dense aqueous fluids: a tendency to reverse the isothermal salt dissociation trend with increasing P, strong solubility of major rock forming elements in NaCl-H2O brines, and the formation of polymerized aluminosilicate species that may enhance concentrations of nominally insoluble components.