Abstract
The present sub-permil precision of single zircon chemical abrasion, isotope-dilution, thermal ionisation mass spectrometry (CA-ID-TIMS) UPb dates often reveals age dispersions that are outside of analytical uncertainty. Interpreting these complex age distributions requires the ability to distinguish between protracted crystallization of zircon over a few 100 kyr, age bias due to radiation damage induced Pb-loss, and analytical artefacts. This is a particularly critical issue when a number of these factors occur together. To ensure geologically meaningful results, the complete eradication of Pb-loss is of paramount importance.The impact of Pb-loss can be removed by chemical abrasion (CA) applied prior to the dissolution of zircon. However, CA is an empirical approach that is used without a detailed understanding of how the temperature applied during the annealing step, or the temperature and duration of the partial dissolution step affect the radiation-damaged zones. In addition, the conditions of the CA procedures differ between laboratories making comparisons of age data problematic.This study presents an experimental approach to quantify how chemical abrasion affects the crystal structure and the chemical composition of zircon as well as its UPb age. For this experiment, we have chosen the Plešovice reference zircon, because of its known variation in trace element concentrations and especially the presence of domains rich in actinides. We performed CA experiments under different temperature-time conditions on fragmented Plešovice crystals. These were compared in respect to the changes in trace element concentration, lattice order and UPb date. The most reliable UPb results are obtained by chemically abrading Plešovice fragments at 210 °C for 12 h. Additionally, we demonstrate that the Plešovice zircon cannot be considered homogenous at the current level of precision achieved by CA-ID-TIMS dating due to a natural age variation at the ~900 kyr scale.
Highlights
Isotopic dating of accessory minerals utilizing the radioactive decay of U and Th into Pb is the most precise and accurate technique to date magmatic and metamorphic rocks, or to calibrate the stratigraphic record
Our study contributes to homogenize laboratory procedures to an extent that allows a better comparison of the acquired UePb dates and proposes refined protocols in order to tailor ideal conditions for the chemical pre-treatment
Our experimental study uses the reference zircon from Plešovice, which is confirmed to be homogenous at the 0.5% precision level of its 206Pb/238U age, at the 0.02–0.05% precision level of 206Pb/238U dates, typical of chemical abrasion, isotope dilution, thermal ionisation mass spectrometry (CAID-TIMS) dating, we are able to resolve two growth episodes at 337.201 ± 0.025 Ma (2σ) and 336.465 ± 0.043 Ma (2σ)
Summary
Isotopic dating of accessory minerals utilizing the radioactive decay of U and Th into Pb is the most precise and accurate technique to date magmatic and metamorphic rocks, or to calibrate the stratigraphic record. Despite obvious success in most cases and attempts to investigate the detailed effects of the annealing and partial dissolution step (Huyskens et al, 2016), this technique remains purely empirical and there is no guarantee that Pb-loss will be completely eliminated. It is often modified by different laboratories in terms of duration and temperature of the annealing and the partial dissolution steps Without a full understanding of how the T-t-boundary conditions affect the final UePb date, it becomes problematic to compare quantitatively data from different UePb geochronology laboratories
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