Extended Deep Earth Water Model for predicting major element mantle metasomatism

Abstract

Abstract Fluids in the deep crust and upper mantle appear to have played roles in the long-term evolution of the subcratonic lithospheric mantle and the stabilities of the continents, in the geochemical cycles of the elements from subduction zones to Earth’s surface environment, and in the formation of diamonds. Much evidence of the chemistry of deep fluids has accumulated from studies of fluid inclusions in diamonds and xenoliths. But the origins of the fluids and their behavior are still unclear. In part, this is due to the lack of a comprehensive theoretical model of aqueous, high-pressure fluids. Traditional models have used a C-O-H-type of model, which contains no major rock-forming elements or aqueous ions or metal-complexes. In the present study, we use experimentally measured solubility data for multicomponent K-free eclogite, K-free peridotite and K-bearing peridotite rocks at upper mantle conditions from the literature to construct aqueous speciation solubility models that enabled calibration of the thermodynamic properties of ions and metal-complex species involving the elements Na, K, Mg, Ca, Fe, Al, Si, and C in an extended Deep Earth Water (DEW) model. New equilibrium constants were retrieved for the aqueous bisilicate anion, a silica trimer, silicate complexes of Ca, Fe, and Al, a silicate complex of Mg and bicarbonate, and formate complexes of Fe and Ca. The aqueous speciation and solubility model also took account of decreases in the activity of water and aqueous activity coefficients of neutral dissolved gases and included consideration of H 2 CO 3 0 . Based on the temperature and pressure dependences of the equilibrium constants, and supporting data covering a wide range of conditions, we then developed aqueous equation of state characterizations of the ions and metal-complex species. Overall, the results form a basis for modeling fluid-rock interactions under upper mantle conditions consistent with experimental solubility measurements.