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

Ayla S Pamukcu 1, Sarah M Hickernell1, Michael P Eddy2, Blair Schoene3, Travis Steiner-Leach4

  • Affiliations: 1Earth and Planetary Sciences, Stanford University, Stanford, USA; 2Department of Earth, Atmospheric, and Planetary Sciences, Purdue University, West Lafayette, USA; 3Department of Geosciences, Princeton University, Princeton, USA; 4Plutonium Science and Technology Group, Materials Science Division, Lawrence Livermore National Laboratory, Livermore, USA

  • Presentation type: Poster

  • Presentation time: Tuesday 16:30 - 18:30, Room Poster Hall

  • Poster Board Number: 273

  • Programme No: 1.3.18

  • Theme 1 > Session 3

Abstract

High-silica magmas (≥68 wt. % SiO2) source some of the most impactful volcanic eruptions on Earth. Crystallization timescales estimated for these magmas vary widely (101-106 a), with a particularly large gap between results from zircon geochronology and quartz geospeedometry. However, recent work re-examining the diffusivity of Ti in quartz has introduced new uncertainty into our understanding of this parameter and results from Ti-in-quartz geospeedometry. Here, we utilize constraints from field relations, geochronology, geobarometry, geochemistry, heat loss models, and crystal growth rates to establish limits on the crystallization timescales of high-silica magmas in a natural system. We use these limits to critically assess the Ti-in-quartz diffusivity in this system. The Searchlight Magmatic System (NV, USA) includes the tectonically tilted Searchlight pluton and coeval Highland Range volcanics. Prior work suggests high-silica magmas (rhyolite, leucogranite) in this system represent melts extracted from the pluton. Results from Ti-in-quartz diffusion chronometry of a rhyolite unit range from 102-108 a, depending on the diffusion law applied. Results using the two slowest diffusivities exceed timescales suggested by zircon geochronology of the plutonic source (100-200 ka), plagioclase sizes (100-<103 a), and heat loss models (101-103 a). Quartz growth rates estimated using the Ti-in-quartz timescales are 10-12-10-21 m/s; the slowest diffusivities produce rates that are slower than the slowest published rates for quartz and feldspars (10-15 m/s).  We conclude that crystallization of the high-silica rhyolite was substantially shorter (≤103 a) than its source mush (104-105 a), and only the fastest Ti-in-quartz diffusion laws produce timescales consistent with geological constraints.