Historical Geology/Other isochron methods

In this article I shall point out some other dating methods which work the same way as the Rb-Sr method. The reader who has not read the article on the Rb-Sr method will find this present article almost completely incomprehensible, and should go back and read it.

The isochron method generalized
I have introduced the isochron method in the context of rubidium and strontium. But is there anything particularly special about those two elements? Not really. For the isochron method to work, what we need are three isotopes with the following properties.

(1) An unstable isotope. This should have a fairly long half-life if it is to be of any use in dating rocks, but not too long, or it will hardly undergo any decay at all. A figure expressible in billions of years is ideal. In the Rb-Sr method, we used 87Rb.

(2) A stable daughter isotope of isotope (1). In the Rb-Sr method, we used 87Sr.

(3) An isotope which is the same element as isotope (2) and which is neither unstable nor radiogenic, so that in a closed system it remains constant in quantity. In the Rb-Sr method, we used 86Sr.

Given a set of three such isotopes, we can apply exactly the same reasoning as we did for 87Rb, 87Sr and 86Sr, and it will be equally valid.

The isotopes
The table below shows some sets of three isotopes which can be treated like rubidium and strontium for the purposes of dating; the table also shows the half-life of the parent and its decay mode. The numbers (1) (2) and (3) are as in the section above.