PRE-ERUPTIVE TEMPERATURES OF A RUTILE-BEARING RHYOLITE ERUPTED FROM LONG CANYON DOME, SOUTHERN SIERRA NEVADA

Quantifying pre-eruption pressures and temperatures of magma advances our understanding of magmatic differentiation processes and allows for the comparison of petrologic and geophysical datasets, which is important to our ability to monitor volcanoes. Attempts to quantify pre-eruption conditions of magma, however, often rely on a single geothermobarometric model even though magmas are multiply saturated, which limits our ability to evaluate the validity of results. This research combines three geothermometers to constrain pre-eruptive temperatures of a rutile-bearing rhyolite erupted from Long Canyon Dome (LCD), located on the Kern Plateau in the southern Sierra Nevada. Pre-eruption temperatures based on the Ti-in-quartz geothermometer¹ yield a bimodal temperature distribution with one range of 600-680°C and another range of 935-955°C, whereas pre-eruption temperatures based on magnetite-ilmenite geothermometry² range from 655 to 945°C. Previous work³ determined pre-eruption temperatures of 660-780°C based on the Ti-in-Zr geothermometer⁴. One hypothesis is that the LCD magma reservoir was thermally zoned with a ~350°C gradient, however, the existence of such a large thermal gradient seems improbable due to the small eruption volume (0.1 km³) and the absence of whole-rock compositional variation. Therefore, the preferred hypothesis is that a rhyolite crystal mush that was stored at near-solidus temperatures (~600°C) before being evacuated following a prolonged intrusion of a warmer (~950°C) felsic magma, resulting in the eruption of a relatively uniform magma composition but with a wide range of pre-eruption temperatures recorded by mineral phases.

¹Wark, D.A., Watson, E.B., 2006, TitaniQ: a titanium-in-quartz geothermometer: Con Min Pet, v. 152, p. 743-754; ²Ghiorso, M.S., Evans, B.W., 2008, Thermodynamics of Rhombohedral Oxide Solid Solutions and a Revision of the Fe-Ti Two-oxide Geothermometer and Oxygen-barometer: Am J Sc, v. 308, p. 957-1039; ³Lewis, B., 2010, Pre-Eruptive Magmatic Temperature of Long Canyon Dome Rhyolite Based on Ti-in-Zircon Thermometry, Sierra Nevada [B.S. Thesis]: Fullerton, California State University, 40 p; ⁴Watson, E.B., Wark, D.A., Thomas, J.B., 2006, Crystallization thermometers for zircon and rutile: Con Min Pet, v. 151, p. 413-433.