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

Ar-40/Ar-39 experiments on melt-inclusion-bearing quartz (MIBQ) from the Bi shop and Bandelier Tuff Plinian deposits indicate high concentrations of ex cess Ar-40 in melt inclusions. Two rhyolite glass melt inclusion population s are present in quartz; exposed melt inclusions and trapped melt inclusion s. Air-abrasion mill grinding and hydrofluoric acid treatments progressivel y remove exposed melt inclusions while leaving trapped melt inclusions unaf fected. Laser step-heating of MIBQ yields increasing apparent ages as a fun ction of exposed melt inclusion removal, reflecting the higher nonatmospher ic Ar-40 concentrations hosted in trapped melt inclusions. Exposed melt inc lusion-free MIBQ from the Bishop, Upper Bandelier, and Lower Bandelier Tuff s 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 Ar-40 in MIBQ, Trapped melt inclusions in sanidine phenocrysts may contain excess Ar-40 concentrations similar to those in MIBQ. This excess Ar-40 has the potential to increase single-crys tal laser-fusion ages of sanidine by tens of thousands of years, relative t o the actual eruption age.