Journal of Petrology Volume 41 Number 1 Pages 149-174 2000
© Oxford University Press 2000
Rhyolite Thermobarometry and the Shallowing of the Magma Reservoir, Coso Volcanic Field, California
1DEPARTMENT OF GEOLOGY, UNIVERSITY OF NORTH CAROLINA, CHAPEL HILL, NC 27599-3315, USA
2US GEOLOGICAL SURVEY, 345 MIDDLEFIELD ROAD MS-910, MENLO PARK, CA 94025-3591, USA
The compositionally bimodal Pleistocene Coso volcanic field is located at the western margin of the Basin and Range province 
60 km north of the Garlock fault. Thirty-nine nearly aphyric high-silica rhyolite domes were emplaced in the past million years: one at 1 Ma from a transient magma reservoir, one at 0·6 Ma, and the rest since 0·3 Ma. Over the past 0·6 My, the depth from which the rhyolites erupted has decreased and their temperatures have become slightly higher. Pre-eruptive conditions of the rhyolite magmas, calculated from phenocryst compositions using the two-oxide thermometer and the Al-in-hornblende barometer, ranged from 740°C and 270 MPa (2·7 kbar; 10 km depth) for the 0·6 Ma magma, to 770°C and 140 MPa (1·4 kbar; 5·5 km) for the youngest (0·04 Ma) magma. Results are consistent with either a single rhyolitic reservoir moving upward through the crust, or a series of successively shallower reservoirs. As the reservoir has become closer to the surface, eruptions have become both more frequent and more voluminous.
KEY WORDS: Al-in-hornblende; caldera; eruption; geothermal; rhyolite
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