Geology constrains the diffusivity of Ti in quartz and crystallization timescales of high-silica magmas in the Searchlight Magmatic System (NV, USA)

Abstract

Many methods and mineral phases have been used to estimate crystallization timescales of high-silica magmas (≥68 wt. % SiO₂), which source some of the most impactful volcanic eruptions on Earth and are a significant component of Earth’s continental crust. Results from these efforts vary widely (10¹-10⁶ a), with a particularly large disparity between results from zircon geochronology and quartz geospeedometry. Quartz is widespread in high-silica magmas, and Ti-in-quartz diffusion chronometry is a commonly applied geospeedometer. However, recent work re-examining the diffusivity of Ti in quartz has introduced substantial uncertainty into our understanding of this parameter, and estimates of the diffusivity now vary by many orders of magnitude for the same temperature. This has all but eliminated the utility of this technique until this discrepancy can be resolved. In this work, we leverage field relations, geochronology, geobarometry, geochemistry, heat loss models, and crystal growth rates to establish limits on the crystallization timescales and conditions of high-silica extrusive (rhyolite) and intrusive (leucogranite) magmas in the Searchlight Magmatic System (NV, USA), which is comprised of the tectonically tilted Searchlight pluton and coeval Highland Range volcanics. We use these limits to critically assess Ti-in-quartz diffusion laws and constrain the diffusivity of Ti in quartz in this system. We conclude that only the fastest Ti-in-quartz diffusion laws produce timescales consistent with other geological constraints and that the crystallization times of the high-silica magmas were substantially shorter (rhyolite: <10 ka, leucogranite: ≤51 ka) than the mushes (150-200 ka) from which they were segregated.