Radiometric dating

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The family of techniques that convert a measured ratio of parent to daughter isotope in a mineral into an absolute age, using a decay constant determined in the laboratory. The workhorse of deep time is uranium–lead in zircon, and its power comes from redundancy rather than from any single measurement: uranium-238 decays to lead-206 with a half-life near 4.47 billion years while uranium-235 decays to lead-207 with a half-life near 0.70 billion years, so one crystal carries two independent clocks that must agree. Where they disagree, the crystal has leaked, and the disagreement is itself the diagnostic. Argon-40/argon-39 on sanidine gives a second, chemically unrelated system; reconciling the two required an explicit intercalibration of decay constants and standard ages, which is why boundary ages shifted slightly in the early 2010s without any new rock being collected. The critical point for a reader is that a radiometric age dates the crystallisation of a mineral, not the death of an organism — a fossil is dated by bracketing it between datable volcanic ash beds above and below.

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