Best of both worlds: combining SHRIMP and CA-TIMS methods in refining geochronological determinations for timescale calibration

Abstract

Accurate and precise calibration of the geological timescale is a fundamental aspect of geoscience. The advent of the 'chemical abrasion' technique (e.g., Mattinson, 2005) for zircon preparation prior to thermal ionisation mass spectrometry (CA-TIMS) has significantly improved concordance and coherence of single-zircon analyses across a range of rock types, enhancing the geological accuracy of the interpreted crystallisation ages. This development affords an excellent opportunity to revisit and re-evaluate the accuracy of SHRIMP 206Pb/238U dating of natural zircon, and particularly the hypothesis that carefully targeting the 'best' areas of the 'best' grains with the ion beam should yield crystallisation ages closely comparable to those determined via CA-TIMS (cf. Kryza et al., 2012). This study examines a series of 10 samples, largely assumed to be airfall ash beds, within the coal-rich Middle to Late Permian successions of the Sydney and Bowen basins of eastern Australia. Recent work focussed on understanding mass extinctions and climate change across the critical Late Permian-Early Triassic boundary in eastern Australia has been hampered by the endemic nature of the local faunal assemblages, which has precluded reliable high-resolution biostratigraphic correlations to global stratotypes in China and elsewhere.