Melt-inclusion–hosted excess 40Ar in quartz crystals of the Bishop and Bandelier magma systems
- 1Department of Earth and Environmental Science, New Mexico Institute of Mining and Technology, Socorro, New Mexico 87801, USA
- 2New Mexico Bureau of Mines and Mineral Resources, Socorro, New Mexico 87801, USA
Abstract
40Ar/39Ar experiments on melt-inclusion–bearing quartz (MIBQ) from the Bishop and Bandelier Tuff Plinian deposits indicate high concentrations of excess 40Ar in melt inclusions. Two rhyolite glass melt inclusion populations are present in quartz; exposed melt inclusions and trapped melt inclusions. Air-abrasion mill grinding and hydrofluoric acid treatments progressively remove exposed melt inclusions while leaving trapped melt inclusions unaffected. Laser step-heating of MIBQ yields increasing apparent ages as a function of exposed melt inclusion removal, reflecting the higher nonatmospheric 40Ar concentrations hosted in trapped melt inclusions. Exposed melt inclusion–free MIBQ from the Bishop, Upper Bandelier, and Lower Bandelier Tuffs yield total-gas ages of 3.70 ± 1.00 Ma, 11.54 ± 0.87 Ma, and 14.60 ± 1.50 Ma, respectively. We interpret these old apparent ages as compelling evidence for the presence of excess 40Ar in MIBQ. Trapped melt inclusions in sanidine phenocrysts may contain excess 40Ar concentrations similar to those in MIBQ. This excess 40Ar has the potential to increase single-crystal laser-fusion ages of sanidine by tens of thousands of years, relative to the actual eruption age.
Footnotes
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↵*Present address: U.S. Geological Survey, P.O. Box 25046, Denver, Colorado 80225, USA.
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↵GSA Data Repository item 200126, Table 2, Results of 40Ar/39Ar analyses, is available on request from Documents Secretary, GSA, P.O. Box 9140, Boulder, CO 80301-9140, editing@geosociety.org, or at www.geosociety.org/pubs/ft2001.htm.
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- Accepted November 29, 2000.
- Received June 14, 2000.
- Revision received November 9, 2000.
- Geological Society of America
