The Origin of Voluminous, Hydrous, High-SiO2 Rhyolites at Long Valley, CA: High-resolution Numerical Thermal Models and Dynamic, Hydrous Experiments

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High-silica rhyolite is the most differentiated silicate magma type on Earth and makes up some of the largest explosive eruptions (100's-1000's cubic km), including those at Yellowstone (WY) and Long Valley (CA) calderas in the last 1 Myr. Understanding the origin and transport of high-SiO2 rhyolites and what triggers their eruption in large volumes, often explosively, is of considerable interest because their formation must fundamentally re-constitute and differentiate continental crust Additionally, the western U.S. may be the site of future "supervolcano" eruptions, and so understanding the process of the eruptive process can yield additional insight into the hazard associated with these eruptions. Although the chances of a major catastrophic eruption in the western U.S. in the immediate future are low, one of the primary goals of this proposal is to test whether the timescales between rhyolite melt formation and rapid transport to the surface (via dikes) may be on the order of weeks and months (and thus human time scales) and not 10's to 100's of thousands of years.The overall purpose of this study is to combine dynamic, hydrous partial melting experiments with 1D and 2D high-resolution numerical models to examine how the influx of volatile-bearing basalt into continental crust drives the formation, segregation and eruption of high-SiO2 rhyolitic melts. The high-resolution, numerical models will focus on processes that occur over short spatial (meter) and temporal (days/years) scales, in addition to those that operate over crustal distances (kms) and longer times (Myrs). There are four important phenomena that are captured by high-resolution models, including: (1) The transfer of exsolved H2O-rich fluid from crystallizing basaltic sills (10-100 m) to adjacent wall rock, (2) The transient (100's of years) heating of the wall rock adjacent to newly emplaced basaltic sills, which leads to significant partial melting of surrounding crust (both granitoid and previously solidified mafic sills) under H2O-present conditions (where fluid-saturated conditions prevail at the solidus, but fluid-absent conditions develop at melt fractions over 20%). (3) The interaction of partial melts of granitoid and previously solidified mafic sills. (4) The role of pre-existing aplite dikes in granitoid wall rock in facilitating the rapid development of melt-filled dikes, and thus rapid transport out of the wall-rock source region. These short-term processes impact crustal-scale thermal profiles because of the rapid transfer of heat via advection (i.e., dikes). They also lead to significant and irreversible compositional re-working of the upper crust due to the transport of these partial melts from both granitoid and previously solidified mafic sills; thus the re-worked upper crust will carry the geochemical signature of this mixed heritage. Dynamic, hydrous experiments will be performed that simulate the release of H2O fluid from the basaltic sills into adjacent wall rock, and its role in driving partial melting. The new numerical models and software codes will be made widely available to facilitate scientific reproducibility and comprehension. Throughout the course of this work, several undergraduate and graduate students will be supported.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.