Radioactive dating metamorphic rocks
This sinking is driven by the temperature difference between the subducting oceanic lithosphere and the surrounding mantle asthenosphere, as the colder oceanic lithosphere is, on average, denser.
At a depth of approximately 80–120 kilometers, the basalt of the oceanic crust is converted to a metamorphic rock called eclogite.
Consider some molten rock in which isotopes and elements are distributed in a reasonably homogeneous manner.
Its composition would be represented as a single point on the isochron plot: Note that the above is somewhat simplified.
Isochron dating requires a fourth measurement to be taken, which is the amount of a different isotope of the same element as the daughter product of radioactive decay.
The simplest form of isotopic age computation involves substituting three measurements into an equation of four variables, and solving for the fourth.