| |
The
Nevada Isotope Geochronology Laboratory
houses two rare gas mass spectrometers, a
MAP215-50 and Nuclide 4.5-60-RSS. The MAP
215-50, its extraction line, and
auxiliary equipment are computer
automated using LABVIEW software,
initially developed by Bruce Idleman at
Lehigh University. The MAP is a modern
high sensitivity, low background machine
featuring a triple collector assembly
with Faraday cup, standard electron
multiplier, and quiet electron multiplier
with pulse counting electronics. This
mass spectrometer is connected to a high
vacuum extraction line with automated
pneumatic valves, a 4K cryogenic pump for
condensing water vapor or separating
noble gases, and a quadrapole mass
spectrometer for monitoring gas species
prior to admission to the rare gas mass
spectrometer. Sample manipulation is
accomplished by an automated x-y stage
with up to 221 individual samples for
laser analysis, or a motorized furnace
sample dropper capable of running up to
16 sequential samples. Argon extraction
can be accomplished by a double vacuum
resistance furnace capable of heating
samples to 1600 °C ± 1-2 °C, or a
Merchantek laser system consisting of a
20W CO2 laser, or an
ultraviolet laser capable of 5-10 µm
resolution. The old Nuclide will be
operated manually, at least initially,
and will accomplish sample heating using
a custom built furnace similar to that
installed on the MAP's line. The Nuclide
will be utilized for running older (e.g.,
Mesozoic-Paleozoic or older) samples
which do not require the performance of
the more sensitive MAP.
We
will eventually offer three isotopic
dating techniques; the 40Ar/39Ar
method (a variant of the conventional
K/Ar method) and the 21Ne and 3He
surface exposure dating method. The 40Ar/39Ar
dating method has a wide range of uses in
geochronology (defining eruption or
emplacement ages of igneous rocks with
applications in, e.g, volcanic hazards
assessment, ore genesis, and ages of
hominid fossils) and thermochronology
(constraining time-temperature histories
of crustal rocks, e.g., timing and rates
of crustal-scale tectonic events such as
uplift of the Himalayas, and subsidence
of sedimentary basins with implications
for petroleum genesis). The generation of
cosmic ray induced isotopes such as 21Ne
and 3He in rocks exposed at
the Earth's surface has recently opened
new areas of geologic research, most
notably in dating very young or recently
exposed rocks not amenable to other
dating methods. Surface exposure dating
has been applied to research aimed at
understanding climate change, development
of landforms, constraining erosion rates,
and nuclear waste site assessment, and is
a rapidly growing and exciting new field
in geochronology. The combination of
these isotopic dating techniques in a
single laboratory will permit visitors,
faculty, and students to conduct research
on a large variety of problems in
geoscience involving materials spanning
the entire range of geologic time. 40Ar/39Ar
analyses are are currently being
performed with the cosmogenic exposure
dating techniques expected in during
Summer of 2004. |
