Melt inclusion hosted excess argon (40ArE) in the quartz crystals of the Bishop and Bandelier magma systems

WINICK, Jeffrey A., Dept. of Earth and Environmental Science, New Mexico Institute of Mining and Technology, Socorro, NM 87801; DUNBAR, Nelia W.; and MCINTOSH, William C., New Mexico Bureau of Geology and Mineral Resources, Socorro, NM 87801;

Residence timescales of silicic magma chambers in the earth's crust are poorly known. Some recent determinations of these timescales, based on 40Ar/39Ar analysis of quartz hosted melt inclusions, may be flawed due to the problem of excess argon (40ArE) in trapped melt. In order to quantitatively assess the influence of 40ArE on estimated magma chamber ages, we sampled melt inclusion bearing quartz (MIBQ) from plinian pumice deposits of the Bishop Tuff (0.76 ± 0.01 Ma) in Long Valley, California, and the Lower (1.61 ± 0.01 Ma) and Upper Bandelier Tuffs (1.22 ± 0.02 Ma) from the Jemez Mountains west of Los Alamos, New Mexico. Three types of glass are observed in the MIBQ. First, non-vesicular glass adhered to the outside of the crystal; second, hourglass inclusions (inclusions that are not fully enclosed within the crystal and may have been in free exchange with atmosphere at the time of eruption); and third, fully trapped melt inclusions. Each sample was prepared for analysis in three ways: one aliquot of sample was left untreated, a second aliquot was run in a mill grinder for 1 hour, and a third aliquot was treated with hydrofluoric acid (HF) for 1 hour. Backscattered electron imaging with the electron microprobe shows that in the first case, all three glass populations are present, that mill grinding removes adhered glass, and that the HF treatment leaves only fully trapped inclusions.
Aliquots of MIBQ were step heated with a 40 W CO2 laser and measured for argon isotopes at the New Mexico Geochronology Research Laboratory. 40Ar/39Ar ages consistantly increase with decreasing K/Qtz ratios (attributed to the removal of rhyolite glass as observed on the electron microprobe). MIBQ from the Bishop Tuff yields a plateau age as old as 3.70 ± 0.16 Ma, with the oldest ages calculated from HF treated samples which again contain only trapped melt inclusions. Plateau ages for MIBQ from the Lower and Upper Bandelier Tuffs are as old as 15.92 ± 0.87 Ma and 11.41 ± 0.16 Ma respectively, again with the oldest ages representing the HF treated samples. This data suggests that significant quantities of 40ArE are present in trapped melt inclusions, with lesser amounts hosted by the hourglass inclusions and non-vesicular glass coatings.
Previous studies have cited 40Ar/39Ar single crystal laser fusion isochron ages of 1.93 ± 0.12 Ma on Bishop Tuff MIBQ as independent evidence for a ~2 Ma Rb-Sr magma differentiation event (Christensen and DePaolo, 1993; Christensen and Halliday, 1996; Van den Bogaard and Schirnick, 1995). Such an event necessitates a long-lived (>1 Ma) thermally equilibrated magma chamber; a difficult scenario to physically reconcile. Clearly the 3.70 ± 0.16 Ma age of the Bishop Tuff in addition to the respective 15.92 ± 0.87 Ma and 11.41 ± 0.16 Ma ages for the Lower and Upper Bandelier Tuffs are erroneous within the context of all known physical and geological constraints on these magma systems. We interpret this data as direct evidence for the presence of 40ArE and suggest that the results of Van den Bogaard and Schirnick (1995) are not geologically meaningful. As a result of this work, existing models of upper crustal magma residence times may need to be revised.