Short communication: Resolving the discrepancy between U-Pb age estimates for the "Likhall" bed, a key level in the Ordovician timescale

Abstract

The "Likhall"bed is a rare case of a single-age zircon population from a carbonate rock, which in this case is contextualised with remarkable biotic and environmental changes and with meteorite bombardment of Earth after an asteroid breakup in space. Published high-precision chemical abrasion isotope dilution thermal ionisation mass spectrometry (CA-ID-TIMS) U-Pb age estimates disagree on the typical precision of <0.1ĝ€¯% for a 206Pb/238U date, which has led to discrepancies in the interpretation of the timing of events and their possible cause-effect relationships. Here we evaluate the relative strengths, weaknesses, and discrepancies in the datasets published so far; propose strategies to overcome them; and present a new U-Pb dataset with improved precision and accuracy. Ultimately, we find that domains of residual Pb loss are a significant source of age offset between previously published data, amplified by differences in data evaluation strategies. Our new dataset benefits from an improved chemical abrasion protocol resulting in a more complete mitigation of decay-damage-induced grain portions and points to a weighted mean age estimate of 466.37±0.14/0.18/0.53ĝ€¯Ma for the "Likhall"zircon population. This age is intermediate between previous estimates but outside of analytical uncertainty, and it provides a firm tie point for the Ordovician timescale.