This project will develop and apply an internally consistent thermodynamic dataset for fluid-rock equilibria at elevated temperatures and pressures. Geochemical modeling of fluid-rock reactions is an essential component of reactive transport simulations of shallow to deep crustal processes such as geothermal systems, formation of hydrothermal ore deposits, and fluid generation and migration during metamorphism, orogenesis and emplacement of hydrous magmas. The quality and robustness of geochemical phase equilibria modeling depends critically on the quality and consistency of the input thermodynamic datasets. Internally consistent thermodynamic data for fluid-mineral reactions derived from global fitting of experimental data on mineral solubility and aqueous speciation are required to advance the predictive capabilities of geochemical and reactive transport models. The proposed project will generate such a dataset for the core system Na-K-Al-Si-O-H-Cl, utilizing the large body of experimental mineral solubility and aqueous species stability data that was accumulated in recent years. The dataset will be developed using a strategy that relies on pre-correlated constraints for aqueous speciation equilibria and activity model parameters, and simultaneous global fitting of the standard state properties of aqueous species using all available data from mineral solubility experiments. This project will utilize and extend the existing code architecture of the GEM-Selektor equilibrium solver and the GEMSFIT generic framework for optimization of thermodynamic data. The dataset will be tested and verified by comparing modeled fluid compositions with those obtained from fluid inclusion microanalysis in two natural field laboratories, which are quartz fissure veins in very low-grade metamorphic rocks of the Swiss Alps, and fractured granite reservoirs of the Soultz-sous-Forets enhanced geothermal system. The experience gained through this project will guide future strategies for building large comprehensive internally consistent datasets that include minerals, fluids and aqueous solutions. The dataset will be widely used in future geochemical modeling and reactive transport applications that have relevance to fields as diverse as geothermal energy utilization, carbon dioxide sequestration, and fluid migration related to metamorphic and igneous processes in the Earth’s crust.