Brief description to the (U-Th)/He thermochronological method

The He-thermochronology is an isotope geochronological method that is based on the alpha-decay of natural radioactive isotopes of 238U, 235U, 232Th and 147Sm. The method is applied usually on apatite and in this mineral phase the closure temperature is around 60 °C which is practically the lowest known mineral closure temperature for geological purposes. The (U-Th)/He method is rather young and a limited number of applications has been published on dating the thermal overprint of sedimentary basins.

History of the method

The history of He-chronology can be subdivided into three periods. This method was used as the first attempt to date rocks and minerals by the decay of radioactive isotopes. Ernest Rutherford's (1905) pioneering experiments indicated for the first time by a radioactive method that the dimension of Earth's history is millions of years. However, his results were not widely accepted because the rapid diffusion of helium in the minerals made the He-ages much younger than the other radiometric ages determined several years later (e.g. by U/Pb method). Except some trials (e.g. Fanale and Kulp, 1963) the He-method was not used, and it was put down as a meaningless method among the geoscientists for decades (see e.g. Zeuner, 1951).

The second period took place after ca. eighty years long break and actually can be linked to two persons and their laboratories. Hans Lippolt and his co-workers have started a new series of experiments with different minerals. They recognised also the rapid diffusion of helium and they called the result of the age determination process not as an age, just carefully as a 'He-number' (Lippolt et al., 1994). They made successful experiments with apatite and also with the dating of different iron oxides (Lippolt and Weigel, 1987; Bähr et al., 1994; Lippolt et al., 1995, 1997; Wernicke and Lippolt, 1995; Mankopf and Lippolt, 1997; Hautmann et al., 1999). Independently Peter Zeitler has performed experiments with apatites in the eighties. Their work (Zeitler et al., 1987) established the currently widespread thermochronological use of He-dating of apatites.

The third period of He-chronology has started in the nineties and is based on the precise determination of alpha-ejection and He diffusion in apatite both by laboratory experiments and in natural sites (Farley et al., 1996; Farley, 2000; Wolf et al., 1996, 1997, 1998). The lion's share of this work was done in Ken Farley's laboratory. The current trend is using mainly apatite to date low temperature events in the very shallow part of the crust. The rapid expansion of the application of (U-Th)/He method in the last decade can be related to its low closure temperature in apatite (e.g. Warnock et al., 1997; House et al., 1997a, 1997b, 1998, 1999; Spotila et al., 1998; McInnes et al., 1999).

Methodology

Physical background, decay

The major sources of the alpha particles (He gas) are the decay of U-238, U-235, Th-232 and Sm-147 isotopes. The decay chains of U and Th are composed of 6 to 8 stages, thus the complete decay of these isotopes until the stable lead isotope results in 6, 7 or 8 helium atoms. Thus, the accumulation rate in the U and Th-bearing minerals is relatively fast. This allows dating very young materials with acceptable precision (even the ca. 2000 years old Plinian explosion of the Vesuvius was dated correctly by the He method: Aciego et al., 2003). At the evaluation of the method the second important factor is the low He concentration in the air (only 5 ppm). Thus, the air contamination, further the excess or inherited helium in the samples usually cause negligible bias in age determination.

Ejection correction

The accumulation of He in actinide-bearing crystals is not quantitative. During the decay the alpha particles get significant kinetic energy and a part of them is ejected from the parent crystals instantaneously. This ejection is dominant along the external boundaries and plays no role in the interior of the crystals. Thus, the size of the grains (or by other words the surface-area-to-volume ratio) determines the ejection loss. The fraction of the total (abbreviated as Ft - unfortunately this expression resembles to fission track dating) expresses the proportion of the radiogenic helium that remains in the crystal after the radioactive decay. This factor is controlled by the size and shape of the crystal, the internal structure, the density and the U/Th ratio. Fractures and open cleavages are acting as diffusion channels and can contribute to the increase of the surface-area-to-volume ratio. That is why each dated crystal needs to be selected carefully and documented precisely, because both the Ft factor and the closure temperature are dependent on the geometry of the crystal.

Major biasing factors

Apatite is the dominantly dated mineral, and the crystallography, mineral chemistry and diffusion parameters of this mineral are the most studied. However, among others zircon, sphene, garnet, hematite, fluorite and calcite have been dated already. The selection criterion is very strict; no crystals with any visible inclusions can be dated precisely. Actinide-bearing inclusions (like zircon or monazite) can significantly contribute to the helium production, but they are usually not dissolved at the standard chemical procedure (HNO3) and the radioactive element content of these inclusions will not be considered at the age calculation. Fluid inclusions may contain 'motherless' helium from hydrothermal fluids and this amount of excess helium is undistinguishable from the radiogenic helium. Both types of inclusions increase the (U-Th)/He age.

Another biasing factor is the zoning of the alpha emitting elements within the dated grain. If the core is enriched in the alpha sources then ejection from the relatively U and Th poor rim is less than from a homogeneous crystal and this leads to an overestimation of the proportion of ejected helium. The age would be overcorrected and erroneously too old. On the other hand U and Th rich rims result in apparently too young ages due to the high proportion of ejected helium. That is why the combination of fission track chronology and (U-Th)/He dating is heavily recommended. On one hand the closure temperature of the FT system is higher than the He system, thus we get a detailed thermal path using both methods. On the other hand the FT method serves also a U-micro mapping. This is like a 'by-product' of the FT method and helps to estimate the zonation of the radioactive elements and allows performing a reliable ejection correction for the He-chronology.

Closure temperature

Helium is an extremely mobile element and it has a high diffusion rate in most solid phases. That is why the closure temperatures in different minerals are low (in zircon and in apatite it is around 170 °C and 60 °C, respectively). The closure temperature depends (i) on the diffusivity of helium in the lattice of the mineral, (ii) on the dimensions of the diffusion domain (= usually the size of the dated crystal) and (iii) on the cooling rate (see Fig. 1). Hence it follows that the closure temperatures of the individual crystals have a slight variation within one sample. Thus, usually individual crystals are dated and from the different apparent ages a more detailed thermal history can be revealed according to the modelling of He diffusion (closure temperature) of the individual crystals.

The closure temperature can be refined iteratively by the estimation of the cooling rate. For this we have to take into consideration all other available geological time constraints beyond the apparent He-age (e.g.: beginning and cessation of sedimentation, age of deformation, and other geochronological data). In an iterative way, by fitting the closure temperature and cooling rate a more reliable thermal history can be achieved than just using fixed values.

Fig. 1: Helium closure temperature (numbers on black lines) as a
function of grain size and cooling rate. The shaded region
indicates ranges typically observed in nature (from Farley, 2000).

Partial retention zone, thermal modelling

Similarly to other methods (e.g. fission track) the closure temperature is not a sharp boundary. The degree of diffusive loss of helium depends on the temperature and the effective heating time. Consequently there is mineral- and method specific 'partial retention temperature zone' (PRZ), where the accumulation of helium is not proportional to time. In apatite, for example in the temperature range between 90 and ca. 40 °C, a part of the continuously forming He gas is diffusing out of the crystal within a few million years. The laboratory experiments revealed the diffusion parameters of the most commonly used minerals. Thus, the partial loss of helium (by other word: the apparent He age) is a valuable parameter for modelling the lowest temperature part of the thermal history.

Applications

The fast diffusivity of helium and the relatively wide occurrence of apatite make the (U-Th)/He dating of apatite the most sensitive thermochronological method. It has the lowest known closure temperature currently (Fig. 2). Thus, the fields of applications are mainly concentrating on the following topics:

Fig. 2: Synopsis of the near-surface geochronological and low-temperature thermochronological methods.

Dating of the final phase of exhumation: The geo- and thermochronological results can be completed by He chronology and the observed low-T cooling ages may give crucial and unexpected results on the final exhumation of basement and sedimentary units (see Fig. 3).
Geomorphology: The depth of apatite He-PRZ is rather shallow (of course, it depends on the local geothermal gradient), hence the low-temperature age signature across a rugged topography can be interpreted as the age of formation of the relief. River and glacier incisions, canyon and scarp developments and their progradation were dated in this way.

Age and offset of faults: In case of proper 3D sampling the vertical age profiles on the different sides of faults can reflect both the displacement relatively to the isotherms and even the age of the faulting.

Dating of detrital grains: The sensitivity of the method allows single grain dating. This can be used as a provenance indicator and even double-dating of individual grains can be performed using the U/Pb age for magma emplacement and (U-Th)/He age for cooling.

Dating of the low-T overprint of sedimentary sequences: The apatite helium PRZ partly overlaps with the time-temperature field of the oil window. Beyond apatite FT dating practically no other geochronological method works in this temperature range. Thus, by combination of the parameters of organic maturation (maximum temperature) and low-T thermochronology the final evolution of the source formations or reservoir rocks can be traced.

Fig. 3: An example on the possible contribution of He thermochronology to the
tectonothermal evolution of structural blocks (Reiners, 2001). The final cooling
below ca. 60 °C has followed the Paleogene post-emplacement cooling by 40 My.