Reports: UNI850152-UNI8: Sandstone Silicification in a Caldera Lake: Implications for Cementation at High Geothermal Gradients

Jade Star Lackey, PhD , Pomona College

This narrative describes progress in year two 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. Our findings have continued to yield insight into how the Long Valley hydrothermal systems was cemented, thus providing insight on how such systems may self “seal”. The findings are applicable to understanding how to develop and sustain geothermal reservoirs and lend to understanding of cementation in hydrocarbon reservoirs. Project activities and major findings, personnel who were supported by the grant, and the research plan for next year, a zero cost extension of the grant, are given below.

The primary objectives for year 2 included: (1) ion microprobe analysis of δ18O variations of the cements to increase the spatial resolution of our understanding of the fluid flow system; (2) Major and trace element analysis of cements, to evaluate diagenetic alteration; (3) ion microprobe U-Th dating of opaline cements to establish the timing and rate of cementation; (4) analysis of fluid inclusion in silica veins from sinter deposits to better constrain fluid composition and temperature.

The Lackey Lab group made key new findings in first two objective areas and progress in the latter two. First, objectives 1 and 2 were completed and highly informative (discussed below). Objective 3, U-Th dating of the cements continued to be problematic; cements in this system appear to have relatively low U concentration and potentially will have to be analyzed in bulk, thus losing spatial resolution, but potentially can be dated. Previously published U-Th dates on some of the deposits provide useful age context. Objective 4, fluid inclusion analysis, has begun but is complicated by lack of primary fluid inclusions. In contrast, bulk D/H isotope ratios of water in opaline and crystalline silica have yielded useful information about paleo water sources and are being integrated with other isotope data for the initial publication of the research findings.

SIMS analyses conducted this year allowed the thin (<50µm) pervasive cements from throughout the caldera to be analyzed to test if fluids were homogeneous or heterogeneous and relative temperature of precipitation. The new δ18O values from the sandstones greatly expand the known δ18O values from 19–35‰ (Fig. 1), doubling the known range of δ18O and showing cements formed lower temperatures than opaline cements (Beeler, 2010). The new δ18O values also support a model of caldera paleofluid flow in which sandstones were cemented over a relatively narrow range of temperatures (±10 ° C) caldera geothermal system mixed with cooler lake waters above thereby causing cement precipitation in sandstone pore spaces. Multiple generations of cements are rare, but have distinct δ18O values where recognized; therefore cements appear to have formed as single pass fluid events, but occasionally new pulses of fluids infiltrated the fluid areas.  Thus, the model of fluids being expunged into a pile of sediments appears mostly accurate, with distinct variation showing renewed precipitation events at various periods. Cement values as high as 35‰ require that some cements were formed almost exclusively from water that was ~0‰ and potentially signifying an evaporative environment in Long Valley Lake (Fig. 2).

Two undergraduates worked on the project in Year 2, including one underrepresented student. Gustavo Ruiz (Pomona B.A. 2012) conducted the major and trace element analyses for the project; James Muller (Pomona B.A. 2012) assisted with SIMS oxygen isotope analyses and traveled to the University of Wisconsin to conduct the analyses with the PI. Muller is co-author on a presentation at the Fall 2011 western regional meeting of ACS.

The research plan for year three, on zero cost extension of the grant, is to conduct additional analyses where needed and sample additionally where needed. Also, given the isotopic variability we have observed, we will re-visit outcrops to look for cryptic fluid flow features that may record late fluid infiltration. We also continue the feasibility of U-Th dating of cements. A student will be invited to join the research team to complete the fluid inclusion studies of the deposits. At the time of the submission of this report, the PI is synthesizing the isotopic and textural data into the first manuscript that will publish the major findings of this work.

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