GEOTHERMOBAROMETRY OF THE FLUORINE- AND BERYLLIUM-RICH SPOR MOUNTAIN RHYOLITE, WESTERN UTAH

The Miocene rhyolites of the Spor Mountain Formation host the world’s largest beryllium deposit. We have examined the rhyolite to understand the magmatic Be enrichment (up to 75 ppm in matrix glass). The Spor Mountain rhyolite contains ~40% quartz, ~40% sanidine, ~10% biotite, and ~10% plagioclase, along with accessory fluorite, columbite, euxenite, fergusonite, monazite, thorite, and zircon. Two types of lava erupted, a less evolved magma (1150 ppm Rb, 42 ppm Be, 0.68 wt% F in matrix glass) and evolved magma (1710 ppm Rb, 75 ppm Be, 1.56 wt% F).

Eruption temperatures estimated using zircon saturation, feldspar-liquid, two feldspar, and Ti-in-quartz geothermometers converge on 718 °C for the less evolved magma and 682 °C for the evolved magma. Zircon saturation temperatures using whole-rock compositions were as much as 70°C higher than those based on glass compositions. Using the Ti-in-Qz equation of Thomas et al. (2010) and a temperature of 700°C gives unreasonably high P of 8-12 kb. Using the Huang and Audetat (2012) calibration, the pressure of the Spor Mountain rhyolite system is estimated to be ~ 2 kb at 700°C. Water content of the rhyolite melt was less than <5 wt%, based on the presence of all four major mineral phases at 700°C and the magma was water undersaturated (Webster et al., 1987) before eruption. The calculated viscosity of the melt is about 6.2 log Pa·s for the less evolved rhyolite and 5.8 log Pa·s for the evolved rhyolite. Fluorine lowered the melt viscosity, though not by a large amount (less than 0.5 log units at 1.7 wt% F compared to an F-free melt). Thus, it seems that the crystal fractionation may not have been greatly accentuated by a low viscosity melt. Instead, the principal role of F may have been to allow the melt to remain liquid to low temperature.

Trace element models using measured partition coefficients suggests that separate batches of melt formed by different degrees of partial melting cannot explain the great depletion of compatible elements in the evolved melts. Instead, crystal fractionation was the dominant magmatic enrichment process for Be, requiring ~45% crystallization of the observed phenocrysts to get compositions from the less evolved to evolved rhyolite and an estimated 95% crystallization of a melt with the composition of average continental crust.