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An event like metamorphism could heat the crystal to the point where Pb will become mobile.
Another possible scenario involves U leakage, again possibly as a result of a metamorphic event.
All organic matter, while it lives, absorbs this isotope from its environment.
Libby theorised that the amount of both non-radioactive and radioactive isotopes of carbon in an organism remain a constant throughout its life, and after this life ends, the absorption stops and carbon-14 starts to decay.
We can also construct a Concordia diagram, which shows the values of Pb isotopes that would give concordant dates.
The Concordia curve can be calculated by defining the following: ).
Nd ratios on several minerals with a mass spectrometer and then from the slope determine the age of the rock. If a magma cools quickly on the surface of the Earth, some of the Ar may be trapped.
The initial ratio has particular importance for studying the chemical evolution of the Earth's mantle and crust, as we discussed in the section on igneous rocks. If this happens, then the date obtained will be older than the date at which the magma erupted.
Zircon has a high hardness (7.5) which makes it resistant to mechanical weathering, and it is also very resistant to chemical weathering. Chemically, zircon usually contains high amounts of U and low amounts of Pb, so that large amounts of radiogenic Pb are produced.
Other minerals that also show these properties, but are less commonly used in radiometric dating are Apatite and sphene.
The paper is published in Earth and Planetary Science Letters.
Radiocarbon dating, which was discovered for science as a method of dating ancient specimens of organic matter by US chemist Willard Libby, consists in measuring the levels of carbon-14, a radioactive isotope of the element that has a halflife of 5,700 years.
To see how we actually use this information to date rocks, consider the following: Usually, we know the amount, N, of an isotope present today, and the amount of a daughter element produced by decay, D*.