Aqueous Aluminosilicate Complexing in Deep-Crustal and Upper-Mantle Fluids

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The geochemical evolution of high-pressure, high-temperature igneous and metamorphic environments is strongly influenced by the effects of fluids on solid minerals in the Earth. Evidence for these effects can be found in fluid inclusions in minerals, elemental and isotopic shifts, and veins and metasomatic features in samples from deep settings, including mantle xenoliths, diamonds, arc magmas, and rocks associated with subduction that are exhumed, as well as a wide range of regional metamorphic terranes. Unfortunately, modeling of high-pressure fluid-rock interaction is difficult because the physical chemistry of deep, dense fluids remains highly uncertain. This project will support a set of experimental studies that target a complexing hypothesis for dense fluids in the interior of the Earth. It is expected that the results from this study will provide new insights into the novel chemistry of dense aqueous fluids, while helping support the mentoring of undergraduate and graduate students, continued international collaboration and incorporation of the results into the UCLA's Advanced Petrology class. A key problem is that in H2O-rich high-pressure fluids there is growing evidence for formation of polymerized oxygen coordinated polynuclear complexes. Examples include H6Si2O7 dimers, H5AlSiO6, and albite-like molecules. Such species are poorly understood, chiefly because their concentrations are negligible in most upper- to mid-crustal hydrothermal settings; however, at high P and T, polymerized species may be much more abundant. Polynuclear complexes are potentially important because they elevate solubility of all elements. In particular, they provide coordination environments that permit dissolution of elements that are otherwise relatively insoluble in H2O (e.g., Al, Ti, Zr). This project will support an experimental investigation into key aspects of the hypothesized complexing. Hydrothermal piston-cylinder experimental studies will target interactions between Si, Al, and alkalis. These components are present at the highest concentrations in dense fluids. The high-pressure (5-20 kbar) and high-temperature (600-900°C) experimental work will include: (1) calibration of Si polymerization at very low Si concentration using the zircon-baddeleyite silica buffer; (2) determination of Al hydrate speciation in H2O based on corundum solubility as a function of pH; (3) investigation of Al-Si complexing through measurement of corundum solubility changes with aqueous Si concentration; (4) characterization of fluid compositions in the haplogranite-H2O system; and (5) determination of rutile solubility in Na-Al-Si-H2O solutions.