Concordia diagram

The diagram tests whether two uranium–lead isotope systems record the same geological age. Its horizontal axis commonly represents the present-day ratio ²⁰⁶Pb/²³⁸U, while the vertical axis represents ²⁰⁷Pb/²³⁵U. A mineral that remained closed to uranium and lead since crystallization plots on a curved line called concordia, because both decay schemes yield the same age at every point along it. The curve is calculated from radioactive decay constants and the measured isotopic ratios, rather than fitted freely to the data.

Zircon is especially valuable because its crystal structure accepts uranium but generally excludes lead when it forms, and because it resists diffusion and alteration better than many minerals. Analyses may be made on whole grains, growth zones, or small domains identified by imaging and microbeam methods.

Concordant analyses lie on, or within analytical uncertainty of, the concordia curve and normally provide a direct crystallization age. A point displaced from the curve is discordant: its two isotope systems imply different apparent ages because lead was lost, inherited lead was incorporated, or the grain experienced more complex geological history.

In the classic discordia model, several analyses affected by a shared episode of lead loss define a straight line that intersects concordia at two positions. The upper intercept commonly approximates the original crystallization age, whereas the lower intercept may indicate the age of disturbance or lead-loss mobilization. These interpretations are model-dependent: a line can also reflect mixing of zircon domains or multiple alteration events, so petrographic context and uncertainty modeling are essential.

A concordia plot is only as reliable as the isotope measurements and the geological selection of the analyzed material. Laboratories measure uranium, radiogenic lead, common lead, and isotope ratios using techniques such as isotope-dilution thermal ionization mass spectrometry, secondary-ion mass spectrometry, or laser-ablation inductively coupled plasma mass spectrometry.

Common lead must be estimated or corrected, and decay constants, tracer calibration, instrumental fractionation, and correlated analytical errors must be incorporated. Software such as Isoplot has traditionally displayed error ellipses, regression lines, intercepts, and weighted statistical tests, but a visually attractive line is not by itself proof of a single lead-loss event. Concordia age estimates are therefore interpreted alongside cathodoluminescence images, trace-element data, crystal chemistry, and field relationships.

The lower-intercept idea is an interpretation rather than a universal rule, and some real zircon populations do not follow a simple two-intercept discordia. Continuous lead loss, several disturbance episodes, inherited cores, metamictization, or mixtures of age domains can produce curved arrays, scattered points, or misleading regressions.

The method also has applications beyond dating the crystallization of igneous rocks. Concordia–discordia relationships can constrain metamorphic overgrowths, reheating, impact events, hydrothermal alteration, and sediment provenance when detrital zircons are analyzed individually. An important edge case is nearly concordant young zircon: small amounts of common lead or analytical uncertainty can substantially affect its apparent age. Conversely, very old grains may preserve multiple microscopic growth episodes that are invisible in a bulk analysis, making spatially resolved measurements decisive.

Concordia

The theoretical curve on a uranium–lead isotope-ratio diagram representing closed-system evolution and equal ages from both decay schemes.

Discordia

A regression line through discordant analyses, commonly interpreted as intersecting concordia at crystallization and disturbance ages.

Common lead

Lead present in a sample that is not produced by radioactive decay and must be estimated or corrected.

Intercept age

An age corresponding to where a discordia line intersects the concordia curve.

Radiogenic lead

Lead generated by the radioactive decay of uranium.