Winick, J A, McIntosh, W.C., and Dunbar, N.W. :
New Mexico Institute of Mining and Technology, 801 Leroy Place, Socorro, NM 87801 United States
40Ar/39Ar experiments on melt inclusion bearing quartz (MIBQ) from the Bishop and Bandelier plinian deposits indicate high concentrations of excess argon (40ArE) in trapped melt inclusions. Two rhyolite glass melt inclusion populations are present in quartz; exposed melt inclusions (EMI) and trapped melt inclusions (TMI). Air-abrasion mill grinding and hydrofluoric acid treatments progressively remove EMI while leaving TMI unaffected. Laser step-heating of MIBQ yields increasing apparent ages as a function of EMI removal, providing evidence of high 40ArE concentrations hosted in TMI. TMI-only MIBQ from the Bishop Tuff yield a total gas age of 3.70 +/- 1.00 Ma. Total gas ages for similar TMI-only MIBQ from the Upper and Lower Bandelier Tuffs are 11.54 +/- 0.87 Ma and 14.60 +/- 1.50 Ma respectively. Single-crystal laser-fusion analyses of MIBQ represent mixtures of EMI and TMI argon reservoirs yielding spuriously old ages that are significantly older than any crystallization or eruption event in the Bishop and Bandelier magma systems determined from Rb/Sr and Nd isotopic data, but that are younger than apparent ages of TMI. Single-crystal laser-fusion weighted mean apparent ages of Bishop Tuff and Lower Bandelier Tuff sanidines are consistent with previously published data from these deposits. However, our analyses of sanidine from the Upper Bandelier Tuff display a weighted mean age that is both imprecise and $\sim$40 ka older than its previously determined 40Ar/39Ar age. Trapped melt inclusions in these sanidine phenocrysts may contain 40ArE concentrations similar to those in MIBQ. Potential effects on single-crystal laser-fusion ages, caused by 40ArE in melt inclusions, are modeled from TMI 40ArE concentrations and measurements of melt inclusion abundances in sanidine. The modeling results are consistent with the 40Ar/39Ar single-crystal laser-fusion sanidine apparent age data and indicate that 40ArE has the potential to offset single crystal laser-fusion sanidine ages by tens of thousands of years.