Thus, a precise measurement of the Sr ratio in a modern volcano can be used to determine age if recycled older crust is present.A ratio for average continental crust of about 0.72 has been determined by measuring strontium from clamshells from the major river systems.Numerical dating, the focus of this exercise, takes advantage of the "clocks in rocks" - radioactive isotopes ("parents") that spontaneously decay to form new isotopes ("daughters") while releasing energy.For example, decay of the parent isotope Rb-87 (Rubidium) produces a stable daughter isotope, Sr-87 (Strontium), while releasing a beta particle (an electron from the nucleus).However, there is still a way to extract a date from the rock.
This amount is a percentage of the original parent amount. Parent Decay and Daughter Growth Curves The half-life of U-235 decaying to Pb-207 is 713 million years.
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.
As rubidium easily substitutes chemically for potassium, it can be found doing so in small quantities in potassium-containing minerals such as biotite, potassium feldspar, and hornblende.
We can expect these differences to be quite pronounced, because rubidium and strontium have different chemical affinities: as we have noted, rubidium substitutes for potassium, and strontium for calcium.
Now consider the distribution of the two strontium isotopes Sr.