U th dating wiki
The parent isotope will be present dissolved in the ocean or in lakes, but when decay takes place the insoluble daughter isotope will precipitate out as sediment and will form part of the upper layer of marine or lacustrine sediment.It will subsequently be buried in its turn by further sediment, and being radioactive will undergo decay.This damage is most concentrated around the parent isotope (U and Th), expelling the daughter isotope (Pb) from its original position in the zircon lattice.In areas with a high concentration of the parent isotope, damage to the crystal lattice is quite extensive, and will often interconnect to form a network of radiation damaged areas.Therefore, we can assume that the entire lead content of the zircon is radiogenic.
Zircon incorporates uranium and thorium atoms into its crystalline structure, but strongly rejects lead.The methods discussed in this article each require two isotopes: a parent isotope which is soluble (or the commonly occurring compounds of which are soluble) and a radioactive daughter isotope which is not soluble.The table below shows three such systems together with the half-life of the daughter isotope, since this is the crucial figure.This fact, combined with the practical difficulty of measuring whether the level of Th has reached 99.9%, 99.99%, or 99.999% of secular equilibrium, limits the useful range of the method to about 500,000 years.Because this method can be applied to organic materials, it can be correlated with the radiocarbon method, and the dates produced by both methods can be shown to be concordant.
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This is particularly true of Pb; since it has a half-life of only 22 years, this makes it useless for most geological purposes.