Abstract

The Miocene rhyolites of the Spor Mountain Formation hosts the world's largest beryllium deposit which produced 85% of the world's beryllium in 2010. The fresh lava is extremely enriched in Be (up to 75 ppm in matrix glass). We have examined the rhyolite to understand the Be enrichment. 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 rhyolite erupted within the Spor Mountain Formation, a less evolved magma (1150 ppm Rb, 42 ppm Be, 0.68 wt% F in matrix glass) and an evolved magma (1710 ppm Rb, 75 ppm Be, 1.56 wt% F in matrix glass). 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. Using the Ti-in-Qz equation of Huang and Audetat (2012), the pressure of the Spor Mountain rhyolite system is estimated to be around 2 kbar 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). Viscosity of the rhyolite was about 6.2 log Pa·s for the less evolved rhyolite and 5.8 log Pa·s for the evolved rhyolite. Magma viscosities calculated using the Einstein-Roscoe question suggest the evolved magma has a slightly higher viscosity than the less evolved magma (7.0 log Pa·s in the evolved magma vs 6.7 log Pa·s in the less evolved magma) because of higher phenocryst content. Fluorine lowered the melt viscosity, though not by a significant amount (less than 0.5 log units at 1.7 wt% F). Partition coefficients for 32 elements have been calculated for biotite, for 21 elements for sanidine and plagioclase, and for 6 elements for quartz, using data acquired by laser ablation inductively coupled plasma mass spectrometry. Partition coefficients for feldspars in the Spor Mountain rhyolite are generally higher than other silicic magmas, and lower for biotite. Beryllium is one of the most incompatible trace elements in the Spor Mountain rhyolite, with a bulk partition coefficient <0.1. Volatile content of the melt (specifically F), melt composition, and the low temperature of crystallization act as the major controls of trace element partitioning. Trace element models using these partition coefficients suggests that crystal fractionation is the dominant magmatic enrichment process within the rhyolite, requiring ~45% crystallization (f = 55%) of the observed phenocrysts to get compositions from the less evolved to evolved rhyolite. Accumulation of batches of melt formed by different degrees of partial melting cannot explain the great depletion of compatible elements.

Degree

College and Department

Physical and Mathematical Sciences; Geological Sciences

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