U th dating on calcite
Pristine aragonite skeletons of reef building corals can be rapidly recrystallized to calcite by the interaction of the corals with freshwater in coastal aquifers.
The aragonite/calcite transformation is accompanied by opening the coral's U-Th isotope system in which uranium is partly lost while Th remains adsorbed and reincorporates into the newly formed calcite.
The main assumptions were that all subsamples from the same coral have identical deposition age and the sub-samples Th (and hence 230Th) was fully preserved during recrystallization to calcite (increasing the 230Th/238U isotope ratio).
Diagenesis to calcite occurred several thousand years after the initial precipitation of the aragonitic skeleton.
We analyzed several subsamples from selected reef coral skeletons in an attempt to follow the recrystallization scheme of the corals and find a reliable method to estimate the age of these heavily altered corals.
In contrast, uranium is soluble to some extent in all natural waters, so any material that precipitates or is grown from such waters also contains trace uranium, typically at levels of between a few parts per billion and few parts per million by weight.
As time passes after the formation of such a material, uranium-234 in the sample, with a half-life of 245,000 years, decays to thorium-230.
Instead, the uranium-thorium technique calculates an age from the degree to which secular equilibrium has been restored between the radioactive isotope thorium-230 and its radioactive parent uranium-234 within a sample.
Thorium is not soluble in natural waters under conditions found at or near the surface of the earth, so materials grown in or from these waters do not usually contain thorium.