Dating micrometre-thin rims using a LA-ICP-MS depth profiling technique on zircon from an archaean metasediment: comparison with the SIMS depth profiling method

Laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) was examined as a tool for measuring isotopic variation as a function of ablation depth in unpolished zircon from an Archaean metasediment specimen. This technique was able to identify micrometre-thin (> 3 μm) isotopically distinct mineral domains characterised by ca. 100 Myr younger ²⁰⁷Pb/²⁰⁶Pb ages associated with 2s age uncertainties as low ~ 0.2%, as well as elevated U content relative to grain interiors (up to an order of magnitude). Our calculated drilling rate suggests that each laser pulse excavated depths of ~ 0.06 μm. Ages resolved through the LA-ICP-MS methods overlap the 2s uncertainties of ²⁰⁷Pb/²⁰⁶Pb ages measured using SIMS depth profiling on the same zircon population. The rims were further evinced by the detection of relative enrichment (> 3 orders of magnitude) in REE in the outermost micrometres of the same zircon, measured using a different and independent LA-ICP-MS depth profiling technique. We propose a LA-ICP-MS U-Pb technique capable of quickly identifying and quantifying rims, which are indication of late, yet geologically significant, fluid events that are otherwise undefined.