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            This narrative describes initial progress by the PI and his research group to decipher the mode of silica cementation of volcaniclastic sandstones that were deposited in Pleistocene Long Valley Lake following climatic the eruption of the Bishop Tuff from the Long Valley Caldera 760,000 years ago. We anticipate that our findings will yield fresh insight into how hydrothermal systems self "seal", thereby aiding development of geothermal resources. Additionally, the findings may inform studies of hydrocarbon reservoirs. Project activities and major findings, personnel who were supported by the grant, and the research plan for next year are described below.

The primary objectives for year 1 included: (1) to sample and describe volcaniclastic sandstones in the field area; (2) to characterize cements; (3); analysis of oxygen isotope ratios in cements as a measure of homogeneity of heterogeneity of fluid composition and temperatures of cement precipitation. As a whole these objectives were designed to test the hypothesis that cementation of the sandstones was under the same fluid and temperature conditions for all rocks (i.e., a single rapid cementation event), or that it was punctuated.

The Lackey Lab group made key findings in the first study phase. First, field observations show that sandstones distal from their resurgent dome source increase in degree of sorting, but show greater variation differences in porosity (and degree of cementation) at cm scales (Fig. 1 A-C). Second, x-ray diffraction indicates most cement is quartz or chalcedony, indicating maturation and recrystallization (Beeler, 2010), but Opal-A cement persists. In addition, multiple generations of cement are common (Fig. 1D). Oxygen isotope ratios of cements reveal the highest d¹⁸O values (14-18ä) yet found in the Long Valley geothermal deposits (Fig. 2), this finding is with lower temperatures of precipitation (Beeler, 2010). Values of d¹⁸O among sinter deposits that cross cut sandstone deposits are lower d¹⁸O (2–12ä; Williams, 2010). Such values overlap with modern hot spring deposits and suggest slightly lower d¹⁸O values for ancient water assuming geothermal waters were not boiling. Together, these findings suggest that hot, siliceous fluids from the deep caldera geothermal system mixed with cooler lake waters above thereby causing cement precipitation in sandstone pore spaces. Given that opal d¹⁸O values are bimodal (Fig. 2), we infer that multiple generations of silica were precipitated at different temperatures, possibly as geothermal exhalation "events" that punctuated the cementation process. 

Six undergraduates worked on the project in Year 1, including two senior thesis students had research expenses (thin sections, oxygen isotope analyses, micro saws and cores) and travel related to their theses supported by the grant. Katherine Beeler (Pomona B.A. 2010) conducted a study of low temperature cements in the sandstones; Mark Williams (Pomona B.A. 2010) studied epithermal fluid flow networks in silica veins preserved in a erosionally exposed sinter deposit that cuts across the sandstones after deposition and cementation. Two other students (Adam Curry, Pomona B.A. 2010 and Giustina de Waal, Pitzer, anticipated 2012) received stipends to assist in the lab with analyses and another student (Stephani Shusta, Harvey Mudd College, B.S. 2010) conducted an independent study of diatoms from intracaldera marls deposited below the sandstones. Gabriel Romero (Pomona, 2012) assisted in generating a GIS model of the caldera in which possible lake stand levels are depicted. These lake stand models will help us evaluate possible lake stand deposits. Results of Beeler's thesis were presented at the Spring 2010 Meeting of the Cordilleran Section of the Geological Society of America.

The research plan for year two is to fully describe the stratigraphy of key sits in the field area, and to sample them in detail. We also plan to conduct ion microprobe U-Th dating of opaline cements to establish the timing and rate of cementation. The potential for dating opaline cements was explored this year in a feasibility study. In addition, we will conduct the phase-two ion microprobe study of d¹⁸O variations of the cements to increase the spatial resolution of our understanding of the fluid flow system. Other planned activities include XRF analysis, to evaluate diagenetic alteration, and a study of fluid inclusions in silica veins from sinter deposits to better constrain fluid composition and temperature.