The CSIRO He extraction and analysis facility
comprises an all-metal He extraction and gas-handling line
connected to a dedicated on-line Balzers Prisma 200 quadrupole
mass spectrometer. Gas extraction is performed by using
either of the 2 identical single vacuum resistance furnaces,
where samples are heated to ~900°C for ~15 minutes. The line and furnace
are evacuated to ~10-8 mbar via ion, turbo and
backing pumps. Active gases, particularly hydrogen,
are removed using SAES getters. The analysis procedure
is operated by LabVIEW automation software supplied by Prof.
Ken Farley, Caltech.
4He abundances are determined
by isotope dilution using a pure 3He spike, which
is calibrated on a regular basis against an independent 4He
standard tank. 4He hot blanks (or re-extracts)
are performed routinely before and after each sample.
If the 4He standard and blank levels are acceptable
(<0.05ncc 4He), a sample capsule is dropped
into a ceramic crucible within the furnace. After the
heating and purification procedures, the extracted gas is
handled and measured via the fully automated computer controlled
Uranium and thorium Concentration
The U and Th content of degassed apatite
samples are determined on a Perkin Elmer Sciex 5000a ICP-MS
using the Isotope Ratio application. 100µl of each 235U
and 230Th spike solution (about 5ng and 6ng, U
and Th respectively) and 200µl of concentrated nitric acid
are added to a vial containing the capsule and degassed apatite.
100µl of 0.25ppm U and Th standard solutions (Johnson Matthey)
are similarly spiked and acidified. We have determined
the 235U/238U ratio of the Johnson Matthey
U-standard solution to be 135, close to the natural value
Blanks are prepared by adding an equivalent
amount of nitric acid to washed, empty capsules. The
blanks, standards and samples are all diluted to 5% nitric
solution with Alpha
Q water prior to analysis. Based on replicate analysis
of spiked standard solutions, precision for 235U/238U
and 230Th/232Th determination is 0.77%
and 0.41%, respectively.
Sample selection and grain size measurement
Apatite grains are carefully handpicked
in order to avoid U- and Th-rich mineral inclusions that may
produce excess He (eg. zircon). Images of selected grains
are captured by a CCD video camera mounted on the microscope
and measured using image analysis techniques for the purposes
of alpha ejection correction calculation. This correction
is mathematically calculated using the estimated dimensions
of each grain and is applied directly to the final age (discussed
in more detail below). Aliquots of ~5-30 grains are
sealed into stainless steel capsules and then up to 6 capsules
are loaded into the furnace sample holders.
Software provided by Prof. Ken Farley
of Caltech, based on the systematics presented in Farley (2000)
and references therein, allows modelling of the (U-Th)/He
age expected from any inputted thermal history, in grains
of any specified radius. By modelling ages through a
variety of different thermal history scenarios, it is possible
to define the range of histories giving predictions which
are consistent with measured ages. The thermal history
framework provided by AFTA forms a solid basis for this procedure.
By incorporating both AFTA and (U-Th)/He ages into the modelling,
a more restricted range of thermal history solutions can be
extracted. For an example of this procedure, check our
website at www.geotrack.com.au and follow the links to the
(U-Th)/He dating page.
Farley, K.A. 2000. Helium diffusion from
apatite: general behaviour as illustrated by Durango
fluorapatite. Journal of Geophysical Research,
105 (B2), 2903-2914.
Farley, K.A., Wolf, R.A. and Silver, L.T.
1996. The effects of long alpha-stopping distances on
(U-Th)/He ages. Geochimica et Cosmochimica Acta,
House, M.A., Farley, K.A. and Kohn, B.P.
1999. An empirical test of helium diffusion in apatite:
borehole data from the Otway Basin, Australia. Earth
and Planetary Science Letters, 170, 463-474.
Lippolt, H.J., Leitz, M., Wernicke, R.S.
and Hagedorn, B. 1994. (Uranium + thorium)/helium dating
of apatite: experience with samples from different geochemical
environments. Chemical Geology (Isotope Geoscience
Section), 112, 179-191.
Wolf, R.A., Farley, K.A. and Kass, D.M.
1998. Modeling of the temperature sensitivity of the
apatite (U-Th)/He thermochronometer. Chemical Geology,
Wolf, R.A., Farley, K.A. and Silver, L.T.
1996. Helium diffusion and low-temperature thermochronometry
of apatite. Geochimica et Cosmochimica Acta,