Collaborative Research: Combined Cathodoluminescence Microscopy and Silicate Microsample Sr Isotopic Studies of Paleogroundwater Hydrology in the Western United States

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Intellectual merit: This project aims to evaluate the utility of Sr isotopic data from low-T silicate alteration phases in continental igneous and sedimentary rock as tracers for paleogroundwater hydrology, specifically tracking the distribution and timing of paleogroundwater flow. Cathodoluminescence petrography, physical microsampling, TIMS Sr isotopic, and oxygen isotope analyses will be combined to characterize secondary adularia formed during Tertiary low-T (200 C) fluid flow events in the western U.S. Two Tertiary volcanic suites were selected for study: the K-metasomatized Lemitar Tuff in southern New Mexico and K-metasomatized Tertiary volcanic rocks in the Harcuvar Mountains in western Arizona. The data for adularia precipitated from migrating fluids will be used to identify fluid events, and to understand the origin the fluids and the geometry of their pathways (e.g., whether they were density driven and related to the formation of closed basin brines, or meteoric fluids driven and related to the formation of surface topography during early Basin and Range extension). Preliminary studies of the Upper Lemitar tuff reveal that the metasomatizing fluids were isotopically distinct both from the phenocryst feldspars replaced by secondary adularia and from the tuff matrix, and so must have been derived from sources external to the tuff. Sr isotopic data are consistent with local Precambrian granitic source rocks and support models in which K-metasomatizing fluids were topographically driven meteoric waters. Further studies will test this hypothesis through Sr and O isotopic analyses of local Precambrian basement rocks, evaporitic minerals, and microsample analyses of adularia, matrix and other primary mineral phases within the Upper Lemitar Tuff. Lateral variations in fluid Sr isotopic compositions due to rock/water interactions along fluid pathways will be evaluated by sampling transects. Similar studies are planned for the Harcuvar Mountains. A third project involves microsampling and analysis of secondary cements and diagenetic phases within surface and well samples of arkoses from the Penn/Permian Fountain Formation (central Colorado); the goal is to investigate fluid interactions with these sediments. Broader impacts: This project involves a novel use of Sr isotopic data for paleohydrologic investigations, that if successful, could have wide applicability in many regions. This project will also provide an introduction to research methodology to undergraduates at Beloit College, who will conduct research activities both at their home institution and at the University of Colorado, Boulder. The project is planned as a joint research activity between the two institutions, with undergraduates being introduced to various laboratory techniques and their applications.