(U-Th)/He dating of apatite as a thermal history tool

1.          Overview

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.

2.           Age determination

The basic equation governing the production of Helium in apatite is as follows:

⁴He = 8 [²³⁸U] (e^{l238 t} – 1)  + 7 [²³⁵U] (e^{l235 t} – 1) + 6 [²³²Th] (e^{l232 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 age. 

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.

3.           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 F_T) for a particular grain size.

4.          Thermal sensitivity

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).

5.          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 E_a and log (D_o), using best estimates of E_a = 33 ± 0.5 kcal/mol and log (D_o) = 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.  

6.          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.

7.          Compositional effects

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.

References

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, 4223-4229.

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), 112, 179-191.

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, 105-114.

Wolf, R.A., Farley, K.A. and Silver, L.T.  1996.  Helium diffusion and low-temperature thermochronometry of apatite.  Geochimica et Cosmochimica Acta, 60, 4231-4240.

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.