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isochroninitialSr/Sr

Potassium‐40 (⁴⁰K) is a relatively rare radioactive isotope of potassium. Approximately 9.1% of⁴⁰K atoms decay to the daughter isotope Argon‐40 (⁴⁰Ar), almost entirely by electron capture that converts a proton into a neutron. This lowers the atomic number by one without changing the atomic mass number. Argon‐40 is produced only from the decay of potassium‐40, with a half‐life of 1.248 Ga. The other 88.9% of the radioactive⁴⁰K atoms decay into the daughter isotope Calcium‐40 (⁴⁰Ca) by β particle emission which converts a neutron into a proton. This increases the atomic number by one without changing the atomic mass number. Because calcium‐40 from other sources is abundant in variable amounts in most rocks its ratio with⁴⁰K cannot be used to obtain rock ages.

ssss1 Rubidium–strontium systematics, showing evolution in the composition of four representative minerals (1–4) from initial composition (blue line) to current composition (red line) as⁸⁷Rb decays into⁸⁷Sr over time. Whole rock compositions would lie somewhere between minerals 1 and 4 depending on the specifics of mineral composition and their proportions in the rock. Slope of red line yields provisional age of rock.