Isotope dating techniques
CHUR was determined by analysing chondrite and achondrite meteorites.The difference in the ratio of the sample relative to CHUR can give information on a model age of extraction from the mantle (for which an assumed evolution has been calculated relative to CHUR) and to whether this was extracted from a granitic source (depleted in radiogenic Nd), the mantle, or an enriched source.Lead has four stable isotopes - Pb with a half-life of ~53,000 years.Lead is created in the Earth via decay of transuranic elements, primarily uranium and thorium.When living things die, they stop taking in carbon-14, and the radioactive clock is "set"!
An example of this application is to the evolution of the Earth's crust and Earth's mantle through geological time.
Since the 1950s, geologists have used radioactive elements as natural "clocks" for determining numerical ages of certain types of rocks. "Forms" means the moment an igneous rock solidifies from magma, a sedimentary rock layer is deposited, or a rock heated by metamorphism cools off.
It's this resetting process that gives us the ability to date rocks that formed at different times in earth history.
Because of their unique properties, it is useful to distinguish them from the conventional radiogenic isotope systems described above. Some He is not affecting the concentration or noble gas ratios of the mantle.
Helium-3 is created by cosmic ray bombardment, and by lithium spallation reactions which generally occur in the crust.