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A long-standing goal in high-precision and high-accuracy U–Pb zircon geochronology is the total elimination of discordance caused by Pb loss. In theory, this is fairly straightforward, but in reality it has proven extremely difficult to attain (or even demonstrate attainment of) this goal. However, a new method, “CA (Chemical Abrasion)-TIMS”, is capable of completely removing zircon domains that have lost Pb, and then analyzing residual, perfectly closed-system zircon. CA-TIMS uses high-temperature treatment (in the range of 800–1100 °C for 48 h) to anneal zircon lattice radiation damage from natural alpha, alpha recoil, and spontaneous fission processes; this annealing eliminates elemental and isotopic leaching effects that have limited earlier efforts at chemical “leaching” or multi-step “step-wise dissolution” or “partial dissolution analysis (PDA)”. The annealed zircons are then subjected to a series of partial dissolution steps at progressively higher temperatures. These steps initially remove (usually outer) zircon zones with high U + Th concentrations. These earliest partial dissolution steps are invariably slightly to strongly discordant, due to Pb loss. Later steps sample progressively lower U + Th zircon domains. In zircon samples that lack inheritance, these later steps sample zircon domains that are completely free from Pb loss, defining 206Pb*/238U plateau ages with precisions (and accuracies, subject to decay constant and tracer calibration accuracies) better than 0.1%. |
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