Selection of apatite crystals

Usually single-crystal aliquots are dated. The apatite crystals are selected very carefully;
the following crieria are crucial to get reliable AHe ages

- The crystals must be free of inclusions. A tiny zircon inclusion can carry as much uranium as the entire apatite grain, because their typical uranium contents differ significantly (apatite: 1 to 20 ppm, zircon 200 to 1000 ppm). During gas extraction we detect the total amount of helium and the distinction of apatite- and zircon-generated He is not possible. However, at the standard dissolution process of apatite grains (diluted HNO3) the zircon inclusions remain undissolved and for the age calculation their U & Th amount is not available. Such a contribution to the total helium without the detection of parent isotopes would shift the AHe age towards old numbers. Vermeesch et al. (2007) suggested to use HF digestion for apatites to dissolve also the refractory inclusions. This method yield more consistent AHe ages incase of problematic, inclusion rich samples, but people usually avoid the usage of HF: and still the best procedure is the careful selection.

Cross-polarized image of more apatite grains, in the middle there is
one horizontal species in extinction. Its zircon inclusions are bright,
due to their random orientation and huge birefringence. Grains even
with one tiny inclusion are not considered for He geochronology.

- The detection of fluid inclusions in an apatite crystal is more difficult than the recognition of zircon inclusions, because they have less contrast to apatite in refractive index and they are usually isotropic. Fluid inclusions carry probably negligible amount of actinide elements, however the hydrothermal fluids are typically rich in noble gases, thus the so called 'motherless' He can be trapped in such inclusions. This can also enhance the AHe age.

- The apatite crystals should be euhedral and intact. During the alpha-decay the emitted alpha particle gets significant kinetic energy and travels in the lattice of apatite ca. 20 micrometer. Thus, decaying actinides along the margin of the crystal are ejectiong a part of the alpha particles outside of the crystal. That is why the single crystals of typical size of accessory minerals (~100 micrometer) can never quantitatively keep the decay product. For the correction of this partial loss Farley et al. (1996) introduced the Tf factor (fraction of total) that depends on the surface / volume ratio of the dated grain. This correction factor (called also as ejection correction) can be estimeted properly only if the crystal is intact and terminated by both ends by an assemblage of pyramidal faces.

The effects of alpha-ejection on He retention in an apatite hexagonal prism. FT is the
total fraction of alphas retained within the crystal (from Farley et al., 1996).

- The size of the crystal is also crucial. If the depleted zone along the rim of grain is to much within the entire grain than the correction factor can be unrealistic high (see figure above). Thus, grains having less than 75 mictrometer in width are usually not considered for He geochronology.

An ideal apatite crystal for He geochronology. (i) euhedral, (ii) both ends
are terminated by pyramides, (iii) no visible inclusion, (iv) proper width.

- The zoning of the alpha-emitting elements in a crystal has also a significant impact on the apparent AHe age. If the actinides and Sm are enriched in the rim of the crystal then the ejected proportion of alpha particles is more than in case of homogeneous distribution of these elements. U, Th and Sm rich core within has an opposite effect; and the ejection correction results in overcorrection. See more detailed description and modelling possibilities in Meesters and Dunai (2002a, 2002b) and in the manual of HeFTy program of Richard Ketcham.

Back-scattered electron images and U & Th concentrations of zircons showing
very contrasting zonation. Such a strong zonation can introduce significant age
scatter, via inaccurate alpha-ejection corrections (from Reiners et al., 2004).

The software DECOMP offers a qick method to estimate the effect
of zoning on the apparent AHe age (Meesters and Dunai, 2002a, 2002b)

The combined application of fission track and He dating of a given mineral phase results in not only a complex description of the thermal history, but the mapping of the uranium within the dated crystals is a 'by-product' of the FT chronology. These images give some hint on typical style of the within-grain distribution of U and the magnitude of variation.

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The different factors mentioned above may contribute to scatter of intra-sample apatite (U–Th)/He ages. Their impact was exellently summarized by Fitzgerald et al., 2006 (show table in pdf).

The selection of the grain is performed by two optical microscope. First a specially equipped binocular is used (with cross-polarized transmitted light) for pre-selection. In the second step a common petrographical microscope is used for the 'hunting' of tiny inclusions. This second one has much better optical resolution. The selection of crystals can be done in air or under alcohol. Using immersion the procedure is more elaborate, but of course the detection of internal part of a grain is much better. We use the petrographical microscope for the measurement of the dimensions of the grains and for the documentation of it by digital photographs.

After proper documentation the selected crystals are placed in a pre-cleaned platinum capsule or in a small envelope made from niobium foil and closed gently.

Placing the selected crystal in a Pt capsle of ca. 1 mm in diameter.	Closing the Pt capsule (needs 'cool', fine-motory movements!)

Acknowledgement: At the learning the first steps of the tricks of crystal selection István get frienly help from Sorcha Diskin and from Fin Stuart. Sorcha told always: Rubbish in - rubbish out! That's the truth of it! Thanks again!