Origin and Potential Geothermal Significance of China Hat and Other Late Pleistocene Topaz Rhyolite Lava Domes of the Blackfoot Volcanic Field, SE Idaho

摘要

The Snake River Plain and neighboring regions are well known for their high heat flow and robust Neogene-Quaternary tectonic and magmatic activity. Interestingly, however, there are comparatively few surficial manifestations of geothermal activity. This study is part of a renewed examination of this region as a possible hidden or blind geothermal resource. We present an integrated conceptual model for basalts and genetically derivative topaz rhyolite lava domes of the Blackfoot Volcanic Field (BVF), linking ultimate heat and mass sources rooted in the upper mantle to potential blind geothermal systems in the upper crust. In our model intracrustal magma transfer and storage respond opportunistically and interactively evolving tectonic processes. Production of rhyolite magma is dominated by mantle-derived mass and heat sources. Rhyolite formation processes via fractional crystallization, cyclic remelting, mixing and assimilation occur mainly in the mid- to upper-crust, contributing to incremental assembly of an evolving magma storage and pluton system. Pre-eruption storage and phenocryst formation for China Hat occurred at -14 km depth, and is inferred to be representative of the most evolved parts of the magmatic system. Approximately 1.2 km~3 of topaz rhyolite have been erupted since 1.6 Ma and up to another 350 km~3 of silicic rocks may have been intruded as hypabyssal dikes and sills at depths of 0.5 to 6 km. Average silicic magma production rate could therefore be on the order of ~1 to 200 km~3/m.y. Given simple assumptions of magma genesis, eruption rates, and initial and final volatile concentrations, we infer average H_2O and CO_2 volatile fluxes from the rhyolite source region of ～0.1 to 10 MT/year and 10 to 1000 T/ day, respectively. Lithium flux may be comparable to CO_2, and it may be both an effective tracer and resource in fluids derived from the inferred consolidating China Hat magma body.