The New Mexico Geochronology Research Laboratory
(NMGRL) in Socorro is a state-of-the-art 40Ar/39Ar
dating facility administered by the New Mexico Bureau of Mines
at the New Mexico Institute of Mining and Technology. 40Ar/39Ar
research at NMGRL is directed by Bill McIntosh and Matt Heizler
who have over 25 years combined experience in the field of argon
geochronology and thermochronology. The lab also employs two full-time
technicians, Lisa Peters and Richard Esser, who supervise mineral
separation, sample irradiation and analyses and data reduction.
Since its christening in the winter of 1992, the NMGRL laboratory
has performed over 60,000 single sample analyses. Necessary in
this regard is the software developed by Al Deino at the Institute
of Human Origins in Berkeley, California which allows for full
automation of extraction line and mass spectrometer functions
. The software controls all crucial valves which are operated
via pneumatic air drivers. Additional automation comes from laser
and furnace sample "packages" which can contain up to 221 and
17 individual samples per load, respectively. Once under vacuum,
sample analyses do not require the physical presence of laboratory
personnel for extended periods of time.
Argon gas extraction at NMGRL is accomplished either by resistance
furnace or CO2 laser heating. The first method consists of a double
vacuum resistance furnace with molybdenum crucible and crucible
liner. Temperatures accuracy is ±10°C; precision is ±1°C; maximum
temperature is 1750°C. Argon blanks for the entire furnace extraction
line (dual stage) are on the order of 5E-16 moles of 40Ar. The
addition of a gate valve between the crucible and sample holder
in the near future will likely decrease system blank as well as
reduce turn around time for sample changes.
The second argon extraction method employs a 10 watt CO2 laser.
The CO2 laser provides many of the advantages of other lasers
used in argon extraction (e.g. Ar-ion and Nd-Yag) but requires
only a fraction of the cost to set up. The CO2 laser at NMGRL
is used mainly for the total-fusion of sanidine crystals which
include J-value determinations from Fish Canyon Tuff (FC-1) sanidine
monitors. System blanks for the laser (single stage) are on the
order of 2xE-16 moles of 40Ar.
Future innovations at NMGRL include the implementation of a cold-finger
to trap H2O from hydrous mineral phases. A new laser system with
smaller spot-size capabilities will be able to examine within-grain
variations in argon distribution. An ion-counter on the mass spectrometer
will allow smaller sample sizes to be used without decreasing
accuracy or precision. Also, future integration of a Vacscan Quadrupole
mass spectrometer will allow pre-screening of potentially harmful
gas before it is equilibrated with the mass spectrometer.
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