Shrimp dating technique
The only exceptions are nuclides that decay by the process of electron capture, such as beryllium-7, strontium-85, and zirconium-89, whose decay rate may be affected by local electron density.
For all other nuclides, the proportion of the original nuclide to its decay products changes in a predictable way as the original nuclide decays over time.
The procedures used to isolate and analyze the parent and daughter nuclides must be precise and accurate.
This normally involves isotope-ratio mass spectrometry. The precision of a dating method depends in part on the half-life of the radioactive isotope involved.
For most radioactive nuclides, the half-life depends solely on nuclear properties and is essentially a constant.
It is not affected by external factors such as temperature, pressure, chemical environment, or presence of a magnetic or electric field.
This drift, which is difficult to correct for, remains the main factor currently limiting the precision and accuracy of the U-Pb SHRIMP opal ages.
Nevertheless, an assumption of similar behaviour of standard and unknown opals under similar analytical conditions allowed successful determination of ages with precisions of ??? SHRIMP-based U-series and U-Pb ages are consistent with TIMS dating results of the same materials and known geological timeframes.
The major advantages of the technique, compared to the conventional thermal ionisation mass spectrometry (TIMS), are the high spatial resolution (??? m), the ability to analyse in situ all isotopes required to determine both U-Pb and U-series ages, and a relatively short analysis time which allows obtaining a growth rate of opal as a result of a single SHRIMP session.
This predictability allows the relative abundances of related nuclides to be used as a clock to measure the time from the incorporation of the original nuclides into a material to the present.
The basic equation of radiometric dating requires that neither the parent nuclide nor the daughter product can enter or leave the material after its formation.
Among the best-known techniques are radiocarbon dating, potassium–argon dating and uranium–lead dating.
By allowing the establishment of geological timescales, it provides a significant source of information about the ages of fossils and the deduced rates of evolutionary change.