Preprints
https://doi.org/10.5194/gchron-2022-20
https://doi.org/10.5194/gchron-2022-20
 
04 Aug 2022
04 Aug 2022
Status: this preprint is currently under review for the journal GChron.

Examination of SHRIMP U-Pb zircon geochronology accuracy from samples dated by both SHRIMP and CA-TIMS

Charles Magee Jr.1, Simon Bodorkos1, Christopher Lewis1, James Crowley2, Corey Wall3, and Richard Friedman3 Charles Magee Jr. et al.
  • 1Geoscience Australia, Cnr Jerrabomberra Ave & Hindmarsh Drive, Symonston ACT 2609 Australia
  • 2Department of Geosciences, Boise State University, Boise, Idaho 83725, USA
  • 3Pacific Centre for Isotopic and Geochemical Research, Department of Earth and Ocean Sciences, University of British Columbia, 6339 Stores Road, Vancouver, British Columbia V6T 1Z4, Canada

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.

Charles Magee Jr. et al.

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on gchron-2022-20', Anonymous Referee #1, 23 Aug 2022
    • AC1: 'Author responce to RC 1 and 2', Charles Magee, 17 Oct 2022
  • RC2: 'Comment on gchron-2022-20', Yuri Amelin, 08 Sep 2022
    • AC1: 'Author responce to RC 1 and 2', Charles Magee, 17 Oct 2022

Charles Magee Jr. et al.

Charles Magee Jr. et al.

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Short summary
SHRIMP (Sensitive High Resolution Ion MicroProbe) is a instrument that for decades has used the radioactive decay of uranium into lead to measure geologic time. The accuracy adn precision of this instrument has not been seriously reviewed in almost 20 years. This paper compares several dozen SHRIMP ages in our database with more accurate and precise methods to assess SHRIMP accuracy and precision. Analytical and geologic complications are addressed to try to improve the method.