Ore metal transport in hydrous silicate melts and silicate-rich aqueous fluids

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Granites and other felsic intrusive rocks may host rare-element ore deposits that are critical to high technology industries. Hydrous silicate melts and silicate-saturated hydrothermal fluids play a key role in the formation of intrusion-related ore deposits and are important agents of mass transfer in the lithosphere. The thermodynamic and transport properties of these liquids reflect their atomic or molecular scale structure; hence, knowledge of liquid structure is essential to understand physical and chemical processes such as ore metal mobilization, transport and deposition. The proposed research will directly investigate the structure of hydrous silicate melts, supercritical silicate-rich aqueous liquids, and silicate-saturated aqueous solutions at high temperatures and pressures in order to determine how changes in these parameters may affect the solution of the rare-elements, tantalum (Ta), niobium (Nb), and rare earth elements (REE) in evolving magmatic systems. To achieve these objectives, the study will utilize recent advances in analytical methods that enable in situ interrogation of volatile-rich silicate liquids in a Bassett-type hydrothermal diamond anvil cell over a range of pressure and temperature conditions that exist in the crust and upper mantle. A series of experiments are planned that will use high energy (~100 keV) x-ray scattering, x-ray absorption spectroscopy, and Raman spectroscopy to examine the topology (i.e., interconnections between atoms that describe the network) and the local structural environment surrounding Ta, Nb and REEs in samples at high temperatures and pressures. My students and I will also investigate the origin and composition of melt and fluid inclusions (tiny samples of fluid trapped in minerals) from major rare-element deposits using high-resolution three-dimensional textural and chemical mapping, laser-ablation inductively coupled plasma mass spectrometry, and Raman spectroscopy. The inclusion data will be used to characterize the composition of fluids involved in ore-forming processes and to help constrain some analog compositions that will be investigated experimentally. The proposed experimental and inclusion studies will fill the large gap in our understanding of the molecular-scale properties of hydrous silicate melts and silicate-rich aqueous fluids and the capacity of these liquids to mobilize and transport rare-elements in the lithosphere. The results are needed for testing theoretical methods that simulate the structure and dynamics of silicate-water systems, and for the development of quantitative predictive models of trace element partitioning that can be applied over a wide range of conditions. An improved understanding of silicate liquid structure and its effect on rare-element distribution is essential for modelling fractionation trends in magmatic systems and for developing more accurate genetic models of rare-element ore-forming processes upon which new exploration strategies can be based. Knowledge of the behavior of Ta, Nb and REEs in magmatic systems and in deep crustal fluids is also important for interpreting trace element patterns that are widely used to decipher geochemical processes and tectonic settings.