The U-Pb system exploits two independent decay chains -- 238U to 206Pb (half-life 4.47 Gyr) and 235U to 207Pb (half-life 0.704 Gyr) -- providing a built-in cross-check on age reliability. Zircon (ZrSiO4) is the premier U-Pb mineral because it incorporates U but excludes Pb during crystallization, and it is physically and chemically extremely resistant. On a concordia diagram (206Pb*/238U vs 207Pb*/235U, where * denotes radiogenic), undisturbed samples plot on the concordia curve at a position corresponding to their age. Lead loss displaces analyses below concordia along discordia lines whose upper intercept gives the crystallization age and lower intercept dates the Pb-loss event. U-Pb zircon geochronology is the gold standard for determining crystallization ages from the Hadean (>4.0 Ga) to the Cenozoic (~1 Ma), with precisions reaching 0.1%.
U-Pb geochronology is the most precise and widely applied method for determining the age of crystalline rocks. The combination of two decay chains, the ideal geochemical properties of zircon, and modern analytical techniques (SHRIMP ion probe, LA-ICP-MS, CA-TIMS) has made U-Pb zircon dating the cornerstone of Earth history chronology.
The concordia diagram is the interpretive framework. The concordia curve plots all possible concordant ages -- points where 206Pb/238U and 207Pb/235U ages agree. An undisturbed zircon crystallized at time t plots on concordia at the position corresponding to t. The curve is non-linear because the two decay constants differ: 235U decays ~6.3 times faster than 238U, so the 207Pb/235U ratio evolves faster, compressing old ages on the 207Pb/235U axis. This non-linearity is what makes discordia lines informative -- a straight line through discordant analyses intersects concordia at two meaningful ages.
Modern analytical methods achieve extraordinary precision. Chemical Abrasion - Thermal Ionization Mass Spectrometry (CA-TIMS) dissolves individual zircon grains after removing radiation-damaged zones (which are prone to Pb loss), achieving age precisions of 0.05-0.1% (50,000 years on a 100 Ma rock). Laser Ablation ICP-MS analyzes 20-30 micrometer spots in polished grain sections, enabling rapid age surveys of detrital zircon populations (hundreds of grains per day) with 1-2% precision. SHRIMP ion probes provide intermediate precision (~1%) with spatial resolution targeting specific growth zones within individual grains.
Detrital zircon geochronology has revolutionized sedimentary provenance studies. Because zircon survives erosion and transport, the age spectrum of detrital zircons in a sandstone records the ages of source rocks in the drainage basin. Thousands of detrital zircon ages from a single sample can fingerprint sediment sources, reconstruct paleodrainage patterns, and constrain maximum depositional ages. Global compilations of detrital zircon ages reveal episodic crustal production, preservation biases, and the supercontinent cycle through >4 billion years of Earth history.