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Melt-inclusion-hosted excess 40Ar in quartz crystals of the Bishop and Bandelier magma systems

Geology

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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 Tufts 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.

Additional Publication Details

Publication type:
Article
Publication Subtype:
Journal Article
Title:
Melt-inclusion-hosted excess 40Ar in quartz crystals of the Bishop and Bandelier magma systems
Series title:
Geology
Volume
29
Issue:
3
Year Published:
2001
Language:
English
Larger Work Type:
Article
Larger Work Subtype:
Journal Article
Larger Work Title:
Geology
First page:
275
Last page:
278
Number of Pages:
4