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; Jawinick@nmt.edu
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.