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Because any analytical uncertainty in the age standard propagates into the unknown age, the choice of standard is very important.
In order to qualify as a good U-Th-He age standard, a sample must fulfill the following requirements.
The U-Th-Pb method is used to study igneous and metamorphic processes affecting U-Th-bearing minerals such as zircon and apatite, whereas the U-Th-He method is used to study low temperature processes occurring near the Earth’s surface. (1987), the U-Th-He method has found a large number of applications in tectonics and geomorphology (Reiners and Shuster, 2009).
A fundamental driving force behind these applications have been technological advances in mass spectrometry and micro-analytical technology, which have led to a steady reduction of sample size while increasing sample throughput at the same time.
Because the U-Th-He age equation is scale invariant, it does not matter if a mineral’s U, Th, and He contents are expressed as atomic abundances or concentrations.
They can even be renormalised to unity and plotted on a ternary diagram (Vermeesch, 2010).
First, it dramatically increases sample throughput. Measuring the U and Th content of zircon by isotope dilution requires dissolution in hydrofluoric acid at high temperature and pressure using a Parr bomb for up to 48 hours.
In contrast, measuring the U and Th content by LA-ICP-MS, SIMS, or EMPA can be done in a matter of minutes.
This radioactive decay forms the basis of the U-Th-Pb and U-Th-He methods of geochronology, each of which have different geological significance.In 2000, a method was developed to degas individual mineral grains by means of laser-heating in Pt or Nb ‘micro-furnaces’, followed by acid dissolution and U-Th analysis by isotope dilution in an ICP-MS (House et al., 2000).This method is still the most widely used technique today. developed in-situ U-Th-He geochronology by laser ablation.Section 5 discusses the complications arising from compositional zoning of zircons, and outlines a ‘pseudo depth profiling’ method to semi-quantitatively assess the extent to which a sample suffers from such problems.Finally, Section 7 applies the proposed method to three shards of gem-quality Sri Lanka zircons that had been previously dated with the conventional U-Th-He method.