A typical frequenly asked question (FAQ): OK, nice, but what does it mean?
A: We have dated some young lavas from the Carpathians of ca. 40 ky age. At the gas extraction from single zircon crystals the He signal was 40 to 260 times above the hot blank.

FAQ: What is the influence of this background on the U and Th signal of a single grain?
A: Zircon crystals contain usually 0.5 to 10 ng U and 0.1 to 2 ng Th. The signal of these elements derived from dissolved crystals is typically 500 to 5000 times higher than the blank. Thus, the background is negligible.

However, in case of He dating of apatite single grains the amount of U & Th sometimes below 10 pg, thus, the blank extraction has some impact on the He age. In our samples until now ca. 4% of the apatite data was influenced more than 10% by blank extraction due to the low U & Th content.

FAQ: Finally, how exact is the He chronology?
A1 (precision): The uncertanity derives from two sources: (i) from the above listed uncertanities of the measurement and (ii) from the non-ideal behaviour and properties of the dated crystal. Usually (i) has much less impact than (ii), which is controlled by the zoning, size, shape, micro-fractions, micro-inclusions, etc. (see 'About the method').

The reproducibility of the dated replicates is the most obvious measure of the reliability of the He age of a sample. The internal scatter determines the acceptable level of geological interpretation of an age. To demonstrate what kind of consistency can you expect from repeated measurements we present here two example tables with multi-aliquot results: one excellent (Sample A) and one khmmm, khmmm, not so good (Sample B):

Both samples are granitoids (from Tibet and from the European Variscides). The first one contains ideal, clear, well developed apatite crystals, while in the second sample practically each grain suffered some imperfections. Thus, in case of the second sample the scatter of the replicates is determined by the mineralogical properties and not the level of detection.

A2 (accuracy): The accuracy of a laboratory can be charactarized by age standard measurements. Age standards are well characterized monomineralic fractions from usually volcanic formations of precisely determined age. The reference ages are measured by other, independent geochronological methods, like U-Pb or Ar/Ar chronology. The following plots show the summary of age standard measurements at the end of 2009.

FAQ: What to do, if the single-crystal ages show wide scatter?
A: We can distuinguis two cases: (i) when the ages have broad distribution, e.g.: 9.8, 21.8 and 71.4 Ma or (ii) when they form a tight cluster and one or two data are anomalously old, e.g.: 4.3, 4.6, 5.3 and 17.3 Ma (both are real examples).
In the first case the wide scatter should come from the partial thermal reset of the sample or from the extremely unlucky conditions in zoning and inclusions. In the second case there is an obvious grouping of the data and there is a potential outlier. Its rejection is doubtless if the potential otlier is much older (rarely younger) than the cluster, but what to do, when the distance it is not very extreme? Only few and rather permissive statistical tools are available for such a low number of observations like the usual numbers of multiplicates at He chronology. The Out?Lier program can be used for the most common tests.

Out?Lier performs the most common outlier tests and indicates the
the probality that the tested data belongs the 'core' population.

FAQ: How the error of a single-grain age is calculated?
A: We apply Gaussian quadratic error propagation for each factors and steps of the laboratory procedure that contributes to the uncertanity. The major sources of the uncertanity are:
- He standard gas shot,
- He signal,
- He drift correction,
- concentration of spike and standard solutions,
- dilution steps of solutions,
- weighting,
- U, Th & Sm signal,
- Drift solution signal,
- drift of ICP,
- ejection correction factor (see Farley et al., 1996).

FAQ: How the results can be presented?
A: We prefer to show the data in details in a complex, rather wide table. The presentation the alpha emitting elements (e.g. U) in mass [ng] indicates how good is the signal/background ratio at the measurement. On the other hand the U concentration [ppm] should reflect on the level radioactive damage (with consideration of the measurent apparent He-age). The degree of metamictization has an impact on the closure temperature and on the modelled thermal path (e.g. Schuster and Farley, 2009).

Example table with the results of He chronology: