New Insights into Granite Petrogenesis From Experimental Studies of Hydrous Melting, Water Solubility, and Supercritical Fluids

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Granites are one of our planet's most important rocks. They are a central component of the continental crust, which carries nearly all information in the geologic record older than the most recent 200 million years of earth history. These rocks help identify past sites of plate convergence. And they may have been among the first rocks to form during earth?s early evolution, perhaps as early as within 300 million years of Earth's origin. Yet despite their importance, there remains surprising controversy about the origin of granitic rocks. We know that water is required to create granitic magmas. But petrologists do not agree on the source of this water ? the widely accepted model is that water is delivered by hydrous minerals that become unstable in the melting region; however, this fails to explain key aspects of granite genesis and is inconsistent with more general models of convergent-margin magmatism in the continental crust. An alternative is that granites are generated during open-system melting in the continental crust, and that migrating deep-crustal fluids facilitate granite creation. However, the basic chemical properties of such fluids are poorly understood, hindering proper testing of the alternative hypothesis. This project seeks to address this problem through new experimental constraints on the role of water-rich fluids in the genesis of granites.The work will exploit hydrothermal piston-cylinder methods developed at UCLA, in which large volumes of H2O can be sealed and retained in noble metal capsules. Preliminary investigations demonstrate that textures diagnostic of each phase region in simple mineral-H2O or rock-H2O binaries can reproducibly be obtained. Where ambiguous, the textural interpretations can be checked with simple trace-element monitors that independently identify liquid, vapor, or liquid+vapor fields. When deployed together, accurate and precise isobaric phase relations across full T-XH2O binaries can now be obtained. Three lines of investigation will be pursued. The first will determine phase relations in albite-H2O and related binaries. We will first complete preliminary studies of albite-H2O described herein. The work maps isobaric temperature-composition relations across the full binary, including solidus, liquids, crystal solubility in H2O, and the liquid+vapor miscibility gap and its closure. When binaries at different pressures are combined, the critical curve and second critical end point can be established to previously unobtainable precision and accuracy. The studies will be expanded to the jadeite-H2O and nepheline-H2O binaries, forming a foundation for modeling liquid-H2O interactions in key parts of the system NaAlSiO4-SiO2-H2O. In the second experimental program, we will build on the mineral-H2O binaries by investigating the haplogranite-H2O system. Finally, we will examine the change in phase relations when H2O activity is reduced by addition of CO2 or alkali halides. Data from each set of experiments will be used as the basis for thermodynamic models of H2O-silicate mixing to establish a theoretical foundation for understanding, for the first time, the topologies of liquid-vapor miscibility gaps, their links to hydrous melting, and the thermodynamics of supercritical silicate melt-H2O fluid in granitic systems. When complete, the work will lead to new understanding of the role of aqueous fluids in granite petrogenesis, which will inform models of convergent margin magmatism and evolution of the continental crust.