A CENTURY OF U-PB GEOCHRONOLOGY: THE LONG QUEST TOWARDS CONCORDANCE

The U-Pb system is well suited for dating rocks because of the occurrence of two chemical identical decay systems (²³⁸U → ²⁰⁶Pb and ²³⁵U → ²⁰⁷Pb), which decay at different rates, therefore permitting a direct verification of closed or open system behavior and validation of the obtained dates. The second favorable factor is the availability of minerals incorporating trace amounts of U and having different stabilities and retention abilities. The main geochronometer is the mineral zircon, which is widespread in many rock types, is robust and not easily destroyed by geological processes, has very low diffusivity for Pb, and incorporates only U but essentially no Pb during crystallization. The main weakness of zircon is the fact that its structure is gradually destroyed by radiation damage, increasing diffusivity, inducing fracturing, and permitting access to fluids which affect the chemical balance of the mineral and remove Pb. These processes results in discordance of the U-Pb system and invalid ages. Other causes of discordance are high-temperature processes causing recrystallization and partial Pb loss, or mixing of different zircon (titanite, monazite) generations. The history of the development of U-Pb geochronology is that of a long struggle to overcome discordance. It was carried on by systematic studies of the mineralogical properties of zircon (and other accessories), identifying the nature of discordant and concordant domains and developing techniques to isolate and date concordant domains. Much of this progress was driven by the geniality and persistence of George Tilton, Leon Silver, Tom Krogh and Jim Mattinson, helped along by the steady improvement in instrumentation and the ability to analyze increasingly smaller domains of minerals. The accumulated knowledge and the array of modern techniques have made it possible for U-Pb dating to overcome much of the historic difficulties caused by discordance. Yet, the problem persists at more detailed levels, affecting high-precision dating by ID-TIMS, but also SIMS and LA-ICP dating when unrecognized discordance ends up biasing interpretations. Although greatly diminished the problem has not disappeared and the geochronological community would be well advised to maintain an awareness of discordance and keep refining the techniques.