ANNEALING AND CHEMICAL ABRASION CREATES NEW OPPORTUNITIES FOR U-Pb GEOCHRONOLOGY OF MAFIC AND SILICA-UNDERSATURATED ROCKS

The annealing and chemical abrasion technique developed by Dr. James Mattinson has transformed high-resolution geochronology of Earth systems dynamics. This transformation owes much to the improved accuracy and precision of the resulting U-Pb dates on zircon, but is also an outcome of a more subtle but equally profound ability to integrate textural, geochemical, and both low- and high-resolution geochronological information from the same crystals. Tandem in situ and isotope dilution analysis on the same zircon crystal was only made practicable with the advent of annealing and chemical abrasion of targeted crystal fragments.

In this presentation we highlight another opportunity created by annealing and chemical abrasion, namely the extraction of the uncommon and often unfamiliar zircon crystals found in mafic and silica-undersaturated rocks. Localized zircon growth can occur in nominally zircon-undersaturated magmas in micro-environments around oxide minerals, in the most evolved residual melts of internal differentiation, or associated with crustal contamination of the host magmas. To overcome the low yield of traditional separation methods for these rare crystals often associated with more magnetic minerals, we have described and applied a technique of bulk annealing of water shaking table heavy mineral lags, followed by aggressive bulk chemical abrasion in aqua regia and hydrofluoric acid. This technique can consistently recover as little as a few zircon crystals per gram of sample, and preserves small and delicate crystals that would normally be trapped in ferromagnesian phases or otherwise lost in physical processing.

Carbonatites and associated alkaline rocks are another source of unusual zircon of unfamiliar morphology. We document the successful application of the bulk annealing and chemical abrasion method to extract zircon from carbonatitic tuffs of the Rukwa Rift, East Africa. This application also illustrates the power of the tandem in situ – isotope dilution U-Pb geochronology approach made possible by chemical abrasion. We document the extreme geochemical variability (including intermediate daughter product disequilibrium) in the zircon crystal load of carbonatites, and leverage this geochemical information for successful high-resolution dating of these tuffs.