The method is applicable to very old rocks because the transformation is extremely slow: the half-life, or time required for half the initial quantity of rubidium-87 to disappear, is approximately 50 billion years.
Sr generated by Rb decay occupies unstable lattice sites in Rb-rich minerals and tends to migrate out of the crystal if subjected to a thermal pulse, even of a magnitude well below the melting temperature.
These minerals develop such high Sr ratio of 0.712 could be assumed in all dating studies without introducing significant errors.
Hence, the whole-rock system may remain closed, even though mineral systems are open.
This paper attempts to accomplish two objectives: First, to explain what isochron dating is and how it is done, and second, to provide an analysis of how reliable it is.
In this kind of evaluation, it is important to avoid both over- and underestimates of its reliability.
(1961) on a plot of isotope ratio against time (Fig. Therefore, a mineral isochron yields the age of cooling from the thermal event, when each mineral again became a closed system.
However, a whole-rock domain of a certain minimum size remains as an effectively closed system during the thermal event, and can be used to date the initial crystallisation of the rock. Isotopic evolution then occurs along near-vertical parallel paths (due to the extreme amplification of the y axis).