Isotopes
There are many radioactive isotopes in rocks, but some are much more useful than others in applications such as age dating and isotopic tracers.
- Rubidium has a radioactive isotope, 87Rb, which decays to an isotope
of strontium, 87Sr.
In the analysis of rock samples, the abundance of these isotopes is measured relative to that of a stable isotope of strontium, 86Sr.
Through time, radioactive decay of 87Rb results in the ratio of 87Sr to 86Sr (or 87Sr/86Sr) increasing.Similarly, the element samarium has an isotope, 147Sm, that decays to an isotope of neodymium, 143Nd.
These isotopes are measured relative to stable 144Nd.
Again, through time, decay of 147Sm results in increasing 143Nd/144Nd.It is common practice to illustrate the covariation of Sr and Nd isotope compositions on a single diagram, as is done here with the Ascension, St. Helena, and Tristan data.
Ascension and St. Helena volcanic rocks have similar 87Sr/86Sr and slightly, but significantly, different 143Nd/144Nd.
The volcanic rocks of Tristan have substantially higher 87Sr/86Sr and substantially lower 143Nd/144Nd compared to Ascension and St. Helena.
- Radioactive isotopes of uranium and thorium all ultimately decay to isotopes of lead.
238U decays to 206Pb and 232Th decays to 208Pb; both are measured relative to stable 204Pb.
So through time both of the ratios 206Pb/204Pb and 208Pb/204Pb increase.It is common practice to illustrate the covariation Pb isotope compositions on a single diagram, as is done here with the Ascension, St. Helena, and Tristan data.
Ascension and Tristan volcanic rocks have similar 208Pb/204Pb but substantially different 206Pb/204Pb.
The volcanic rocks of St. Helena are distinctive in that they have high 206Pb/204Pb and 208Pb/204Pb; in fact, these lead isotope compositions are exceptionally high for basaltic rocks.
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