Zircon-scale insights into the history of a Supervolcano,Bishop Tuff,Long Valley,California, with implications for the Ti-in-zircon geothermometer |
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Authors: | Mary R Reid Jorge A Vazquez Axel K Schmitt |
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Institution: | (1) School of Earth Sciences and Environmental Sustainability, Northern Arizona University, Flagstaff, AZ 86011-4099, USA;(2) Department of Geological Sciences, California State University, Northridge, Los Angeles, CA 91330-8266, USA;(3) Present address: United States Geological Survey, 345 Middlefield Road, Mail Stop 910, Menlo Park, CA 94025, USA;(4) Department of Earth and Space Sciences, University of California, Los Angeles, CA 90095-1567, USA |
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Abstract: | Zircon has the outstanding capacity to record chronological, thermal, and chemical information, including the storage history
of zoned silicic magma reservoirs like the one responsible for the Bishop Tuff of eastern California, USA. Our novel ion microprobe
approach reveals that Bishop zircon rims with diverse chemical characteristics surround intermediate domains with broadly
similar compositions. The highest Y, REE, U, and Th concentrations tend to accompany the largest excesses in Y + REE3+:P beyond what can be explained by xenotime substitution in zircon. Apparent Ti-in-zircon temperatures of <720°C for zircon
rims are distinctly lower than most of the range in eruption temperatures, as estimated from FeTi-oxide equilibria and zircon
solubility at quench. While permissive of crystallization of zircon at near-solidus conditions, the low Ti-in-zircon temperatures
are probably better explained by sources of inaccuracy in the temperature estimates. After apparently nucleating from different
melts, zircons from across the Bishop Tuff compositional spectrum may have evolved to broadly similar chemical and thermal
conditions and therefore it is possible that there was no significant thermal gradient in the magma reservoir at some stage
in its evolution. There is also no compelling evidence for punctuated heat ± chemical influxes during the intermediate stages
of zircon growth. Judging by the zircon record, the main volume of the erupted magma evolved normally by secular cooling but
the latest erupted portion is characterized by a reversal in chemistry that appears to indicate perfusion of the magma reservoir
by—or zircon entrainment in—a less evolved melt from the one in which the zircons had previously resided. |
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