W. C. McIntosh N. W. Dunbar J. A. Winick 2001 <p>⁴⁰<span>Ar/</span>³⁹<span>Ar experiments on melt-inclusion–bearing quartz (MIBQ) from the Bishop and Bandelier Tuff Plinian deposits indicate high concentrations of excess&nbsp;</span>⁴⁰<span>Ar 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&nbsp;</span>⁴⁰<span>Ar 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&nbsp;</span>⁴⁰<span>Ar in MIBQ. Trapped melt inclusions in sanidine phenocrysts may contain excess&nbsp;</span>⁴⁰<span>Ar concentrations similar to those in MIBQ. This excess&nbsp;</span>⁴⁰<span>Ar has the potential to increase single-crystal laser-fusion ages of sanidine by tens of thousands of years, relative to the actual eruption age.</span></p> application/pdf 10.1130/0091-7613(2001)029<0275:MIHEAI>2.0.CO;2 en Geological Society of America Melt-inclusion-hosted excess 40Ar in quartz crystals of the Bishop and Bandelier magma systems article