dating of apatite as a thermal history tool
Helium is produced within apatite grains as a result
of alpha decay from uranium and thorium isotopes, present as impurities
at ppm levels. As reviewed by Lippolt et al. (1994), this
process formed the basis of the first attempts at geochronology (Rutherford,
1907a). However, it soon became clear (e.g. Rutherford, 1907b)
that at least a fraction of radiogenic Helium was lost from the host
crystal lattice, and with the advent of apparently more reliable methods
of geochronology (e.g. K-Ar, Rb-Sr, U-Pb), interest in the Helium systematics
of minerals waned.
More recently, however, the realisation that the partial
loss of radiogenic products could provide quantitative information on
the thermal history of mineral grains led to a resurgence of interest
in this topic (e.g. Zeitler, 1987; Lippolt et al., 1994). In particular,
efforts at Caltech through the 1990s led to the development of (U-Th)/He
dating of apatite as a rigorous, quantitative technique (Wolf et al.,
1996). Studies of the diffusion systematics of Helium in apatite
(Wolf et al., 1998; Farley, 2000) also revealed the unique temperature
sensitivity of the technique, with all Helium being lost over geological
timescales at temperatures as low as 90°C or less, and a closure
temperature as low as 75°C. A number of subsequent applications
of the method (e.g. House et al., 1997; Warnock et al., 1997; Wolf et
al., 1997) have illustrated the potential of the technique to provide
useful thermochronometric information at temperatures less than 100°C.
In principle, therefore, this technique provides a useful supplement
to the information provided by AFTA.
The basic equation governing the production of Helium
in apatite is as follows:
4He = 8 [238U] (el238 t 1) + 7 [235U] (el235 t 1) + 6 [232Th] (el232 t 1)
where 4He, [238U], [235U]
and [232Th] are the measured concentration of the respective
isotopes, the numeral before each term refers to the number of alpha
particles produced in the appropriate decay chain, each l represents the alpha-decay constants for the
respective isotopes and t is the time over which He has accumulated.
The three isotopes represented in the equation represent the only significant
contributors of helium in natural samples. By measurement of the
amounts of each isotope, the time t can be evaluated by solving this
equation iteratively. The resulting number is known as a (U-Th)/He
As with the case of fission track ages, in the absence
of other factors, this would provide a measure of the time over which
helium has accumulated in the apatite lattice. However, due to
a number of factors, outlined in the following Sections, a (U-Th)/He
age must be interpreted carefully before the true meaning of the measured
age can be evaluated.
Grain size correction
The ranges of alpha particles produced by decay of uranium
and thorium isotopes are typically between 12 and 34 mm (Farley et al., 1996). Since these stopping distances
are a significant fraction of the radius of typical accessory or detrital
apatite grains (between 30 and 100 mm),
a significant proportion of alpha particles produced with an apatite
grain may be emitted from the grain, resulting in loss of radiogenic
helium. Farley et al. (1996) showed how this effect can be corrected
for, by calculation of a correction factor (known as FT)
for a particular grain size.
Calculations of Helium retention over geological timescales,
based on laboratory diffusion measurements, suggest that Helium is progressively
lost at temperatures between 40 and 90°C (for timescales of tens of
millions of years), with this temperature range constituting a Helium
Partial Retention Zone or He PRZ.
More recently, measurements of (U-Th)/He ages in samples
from hydrocarbon exploration boreholes in the Otway Basin of S.E. Australia
(House et al., 1999) have confirmed this general pattern of behaviour.
Their results also suggest that, in general, helium diffusion systematics
derived from laboratory measurements can be extrapolated to geological
conditions with confidence, although the exact details details remain
to be quantitatively assessed.
Again analogous to the case of fission track ages in
apatite, the progressive reduction of (U-Th)/He ages with increasing
temperature means that a measured (U-Th)/He age from a sample of detrital
apatite from a sediiment cannot be interpreted as representing the timing
of a specific cooling episode (with the exception of the situation where
a sample cools very rapidly from above 90°C to less than 40°C).
Instead, the measured age must be interpreted in terms of the interplay
between production of Helium by alpha decay and loss due to thermally
controlled diffusion (as described below).
Effect of grain size on sensitivity
Detailed experimental measurements at Caltech have led
to further refinements in understanding the diffusion systematics of
Helium in apatite (Farley, 2000). This work, focussed on the much-studied
Durango apatite, has suggested that diffusion systematics are
controlled by the physical grain size. This key observation implies
that for any specified thermal history, modelled (U-Th)/He ages can
be produced for a particular sample using the measured mean grain size
together with single values of the key diffusion parameters Ea
and log (Do), using best estimates of Ea = 33
± 0.5 kcal/mol and log (Do) = 1.5
± 0.6 cm2/s. These values have
been used in modelling (U-Th)/He ages for this report.
Because of the greater diffusive loss expected from
smaller grains compared to larger grains, the helium closure temperatures
in apatite will also vary with grain radius. The overall variation
in closure temperature for samples with grain radii of 50-150 microns
is predicted to be only 5°C (Farley, 2000). However, effects related
to grain size may be significant in the interpretation of apatites from
sediments which have been heated to paleotemperatures within the He
PRZ, as grains of different radii will give different ages for a particular
thermal history. While this has yet to be demonstrated in natural
samples, this holds considerable promise for obtaining more precise
thermal history control in sedimentary basins.
Extraction of thermal history solutions
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.
Several studies suggest that the composition of
the apatite does not appear to affect the sensitivity of the He closure
temperature (Wolf et al., 1996; House et al., 1999), in contrast to
the effect of Cl contents on AFTA annealing kinetics. Further
studies of possible variation in diffusion rates between different apatite
species are currently being acrried out at Caltech.
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, 60,
House, M.A., Wernicke, B.P., Farley, K.A.
and Dumitru, T.A. 1997. Cenozoic thermal evolution of the
central Sierra Nevada, California, from (U-Th)/He thermochronometry.
Earth and Planetary Science Letters, 151, 167-179.
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),
Warnock, A.C., Zeitler, P.K., Wolf, R.A. and
Bergman, S.C. 1997. An evaluation of low-temperature apatite
U-Th/He thermochronometry. Geochimica et Cosmochimica
Acta, 61, 5371-5377.
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, 148,
Wolf, R.A., Farley, K.A. and Silver, L.T.
1996. Helium diffusion and low-temperature thermochronometry
of apatite. Geochimica et Cosmochimica Acta, 60,
Wolf, R.A., Farley, K.A. and Silver, L.T.
1997. Assessment of (U-Th)/He thermochronometry: the low
temperature history of the San Jacinto mountains, California.
Geology, 25, 65-68.