Dating sedimentary rocks using in situ U-Pb geochronology of syneruptive zircon in ash-fall tuffs <1 mm thick

Abstract

Absolute ages for sedimentary rocks are required to construct a temporal framework in which to decipher Earth's history. The most reliable method for dating Precambrian sedimentary rocks is U-Pb geochronology of zircon in intercalated volcanic rocks. However, extracting sufficient zircons involves destructive mineral separation procedures, often requiring several kilograms of sample. We have dated felsic tuffs <1 mm thick using single polished thin sections from diamond drill core of Neoarchean shale from the Pilbara craton, Australia. The tuffs contain abundant euhedral zircon crystals, commonly 10–30 μm long. In situ SHRIMP (sensitive high-resolution ion microprobe) geochronology of the zircon in 8 tuffs gives 207Pb/206Pb dates between ca. 2600 Ma and ca. 2680 Ma, all consistent with their stratigraphic position and available age constraints.The dates represent the timing of syndepositional volcanism and provide reliable estimates for the age of deposition. Zircons in a 0.5 mm tuff band 15 mm above the Jeerinah impact layer provide an age of 2632 ± 7 Ma, which represents a close approximation for the timing of a major asteroid impact, the strewn field of which is distributed across two Archean cratons. In situ geochronology of zircon in felsic tuffs requires only small volumes of sample and is especially useful for drill core samples where material is limited. It avoids the destruction of samples associated with mineral separation, retaining textural information and avoiding potential contamination. This approach adds significantly to the number of sedimentary rocks that can be dated accurately.