Isotopic measurements estimate an age only with a suitable geological model.
Key knowledge
Radiometric dating combines isotope measurements with a decay model to estimate elapsed time. Interpretation requires accounting for initial daughter material and subsequent isotope gain or loss. A mineral's isotopic age can record formation or later resetting, rather than every event in the rock's history.
Conditions and scope
Choose an appropriate isotope system and assess retention, initial composition and alteration.
A clastic grain's age can predate sediment deposition; the geological event dated must be stated.
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References
Sources retain their recorded scope. A source link alone does not establish verification of every statement.
Radiometric dating combines isotope measurements with a decay model to estimate elapsed time. Interpretation requires accounting for initial daughter material and subsequent isotope gain or loss. A mineral's isotopic age can record formation or later resetting, rather than every event in the rock's history.
A stated isotope system and sample-retention history underlie an age interpretation.
Radiometric dating combines isotope measurements with a decay model to estimate elapsed time. Interpretation requires accounting for initial daughter material and subsequent isotope gain or loss. A mineral's isotopic age can record formation or later resetting, rather than every event in the rock's history.
Geological dating uses the decay model but additionally requires a sample-history model.
Geological history is organized into nested eons, eras, periods, epochs and ages. Rock successions and fossil correlation established relative ordering; numerical dating supplies additional age constraints. The International Commission on Stratigraphy maintains a revised global framework whose boundary definitions and numerical estimates must be distinguished.
Isotopic dates constrain numerical ages within conditions.
Superposition constrains age order in an undisturbed depositional sequence; inclusions predate their host, and cross-cutting features postdate what they cut. Unconformities record missing time through nondeposition and/or erosion. These relations can order events without assigning a numerical age.
Fossil assemblages can constrain relative age and depositional environment through observed succession and known ecological context. Numerical calibration requires independently dated associated material. Preservation, reworking and unequal sampling affect which organisms enter the geological record.
Natural archives such as ice, sediments, corals and tree rings record properties related to past environments. A reconstruction requires a proxy–environment relation, chronology and uncertainty assessment. Different archives have different spatial, seasonal and temporal sensitivity, so multiple lines of evidence are compared rather than read as identical thermometers.
Some archive chronologies depend on bounded isotopic dating.
Conjugate continental geology and fossils, ocean-floor magnetic reversal sequences, spreading geometry and paleomagnetic directions provide complementary constraints on past plate positions. Reconstructions combine these observations with age control and a reference frame. Subduction removes oceanic records, while deformation and remagnetization can change preserved signals; alternative older reconstructions may fit incomplete evidence.
For parent P decaying into daughter D at constant λ, normalization to a stable isotope S gives D/S=(D/S)₀+(P/S)(e^(λt)−1). Coeval samples with a common initial daughter ratio, differing parent ratios and closed subsequent evolution define an isochron with slope m=e^(λt)−1 and age t=ln(1+m)/λ. A fitted straight line by itself does not prove those geological assumptions.
A specified parent-decay model is the prerequisite.
For a closed system with corrected common lead and retained decay-chain products, radiogenic ratios satisfy ²⁰⁶Pb*/²³⁸U=e^(λ₂₃₈t)−1 and ²⁰⁷Pb*/²³⁵U=e^(λ₂₃₅t)−1. Varying t traces concordia. Discordant ratios can reveal disturbance, inheritance or disequilibrium, but a discordia intersection is an age interpretation conditional on a specified geological loss/mixing history.