T20 Maximum Depositional Ages (MDAs) from Detrital Mineral Geochronology: Recent Advances in Sampling, Experimental Designs, Interpretive Tacts, and Chronostratigraphic Applications

Sunday, 15 October 2023
8:00 AM - 12:00 PM
320

THE EFFECTS OF CHEMICAL ABRASION (CA) PRIOR TO LA-ICP-MS FOR U-PB DETRITAL ZIRCON GEOCHRONOLOGY AND DEFINITION OF MAXIMUM DEPOSITIONAL AGES

Paper number: T20-10
Program: Technical Sessions
Time: 10:10 AM – 10:30 AM

Authors

Abstract

Detrital zircon (DZ) geochronology by LA-ICP-MS is a common tool for determining maximum depositional ages (MDAs) and sediment provenance. However, Pb-loss can impact zircon age populations by skewing affected zircon toward younger apparent ages, leading to biased results. Chemical abrasion (CA) is a technique that reduces/eliminates Pb-loss in zircon, but it has yet to be applied to large-n DZ analyses. We applied CA to 3 large-n DZ samples (2 Cenozoic; 1 Proterozoic) for LA-ICP-MS and compared treated and untreated aliquots to assess the effects of CA on DZ analyses and estimation of MDAs. We show CA does not negatively impact or systematically bias U-Pb dates and improves accuracy and precision.

We first tested how CA impacts U-Pb dates by comparing treated and untreated zircon from 13 reference materials. The dates of both CA and non-CA aliquots are indistinguishable and within 0.1-4% from the reference ages, indicating that CA does not systematically bias dates. We also analyzed a Precambrian igneous sample that experienced substantial radiation damage to test how CA impacts grains with significant Pb-loss. We show that CA-LA-ICP-MS significantly improved concordance, precision, and accuracy of measurements relative to the non-CA data. Our results for DZ samples show the differences between age distributions in analyzed DZ spectra are slight, indicating that Pb-loss present in untreated samples would not significantly bias the interpretation of sediment sources on a broad scale. However, CA did sharpen several Phanerozoic peak ages and increased concordance in Precambrian zircon populations.

To better assess how CA impacts MDA estimation, the youngest 3-5 zircon in both aliquots from one Cenozoic sample were analyzed using ID-TIMS following LA-ICP-MS screening. All CA-LA-ICP-MS dates are within 0.8-1.5 myr of the CA-ID-TIMS dates and overlap in 2s uncertainty. In contrast, all LA-ICP-MS dates are systematically younger and ~3-9 myr different from their respective CA-ID-TIMS dates. As a result, calculated MDAs from treated and untreated aliquots are substantially different, with the CA-LA-ICP-MS aliquot more closely reflecting CA-ID-TIMS dates. These results suggest that CA prior to LA-ICP-MS may be a useful tool for research that requires higher resolution in DZ age spectra and better constrained MDAs.

Geological Society of America Abstracts with Programs. Vol. 55, No. 6

doi: 10.1130/abs/2023AM-392875

© Copyright 2023 The Geological Society of America (GSA), all rights reserved. Permission is hereby granted to the author(s) of this abstract to reproduce and distribute it freely, for noncommercial purposes. Permission is hereby granted to any individual scientist to download a single copy of this electronic file and reproduce up to 20 paper copies for noncommercial purposes advancing science and education, including classroom use, providing all reproductions include the complete content shown here, including the author information. All other forms of reproduction and/or transmittal are prohibited without written permission from GSA Copyright Permissions.