Comment on gchron-2022-20

Abstract

Estimations of the reproducibility of U-Pb ages from SHRIMP instruments are based on data from studies nearly two decades old. Since that time, refinement of analytical procedures and manufacturing and installation improvements may have reduced the fundamental uncertainties of SHRIMP U-Pb analysis. This paper investigates 35 SHRIMP-TIMS double-dated “real-world” geologic samples from a variety of igneous rock types to better understand both geological and analytical sources of disagreement between the two dating methods. Geoscience Australia’s (GA) use of high-precision chemical abrasion thermal ionization mass spectrometry (CA-TIMS) for chronostratigraphy in Australian sedimentary basins has produced a substantial selection of precisely dated zircons, which we can use to cross-correlate the SHRIMP and CA-TIMS ages throughout the Phanerozoic. Thirty-two of the 35 ages were reported with external SHRIMP uncertainties less than 1 % (95 % confidence), below previous estimates of the reproducibility limits of the SHRIMP. Six of eight cases where the CA-TIMS age was outside the SHRIMP uncertainty envelope were in samples where the reported SHRIMP age had a sub-0.66 % uncertainty, suggesting that SHRIMP analyses of untreated zircon which report smaller uncertainties are probably overoptimistic. The mean age-offset between SHRIMP and TIMS is 0.097 %, but the distribution is bimodal. Geological explanations for SHRIMP-CA-TIMS age discrepancies are uncovered by considering intrusive and extrusive age results separately. All but one sample where the SHRIMP age is more than 0.25 % older are volcanic. This offset could be explained by the better single-grain age-resolution of TIMS, allowing identification and exclusion of antecrysts from the eruptive population, while SHRIMP does not have a sufficient single-grain precision to deconvolve these populations – leading to an apparent older SHRIMP age. In contrast, SHRIMP ages from plutonic rocks- particularly plutonic rocks from the early Paleozoic- are typically younger than the CA-TIMS ages from the same samples, most likely reflecting Pb loss from non-chemically-abraded SHRIMP zircons, while chemical abrasion of zircons prior to TIMS analysis destroyed or corrected these areas of Pb loss.