Plesovice zircon : A new natural reference material for U-Pb and Hf isotopic microanalysis

An Astronomically Tuned Neogene Time Scale (ATNTS2012) is presented, as an update of ATNTS2004 in GTS2004. The new scale is not fundamentally different from its predecessor and the numerical ages are identical or almost so. Astronomical tuning has in principle the potential of generating a stable Neogene time scale as a function of the accuracy of the La2004 astronomical solution used for both scales. Minor problems remain in the tuning of the Lower Miocene. In GTS2012 we will summarize what has been modified or added since the publication of ATNTS2004 for incorporation in its successor, ATNTS2012. Mammal biostratigraphy and its chronology are elaborated, and the regional Neogene stages of the Paratethys and New Zealand are briefy discussed. To keep changes to ATNTS2004 transparent we maintain its subdivision into headings as much as possible.

The Neogene Period

This paper reviews the basic principles of radiometric geochronology as implemented in a new software package called IsoplotR, which was designed to be free, flexible and future-proof. IsoplotR is free because it is written in non-proprietary languages (R, Javascript and HTML) and is released under the GPL license. The program is flexible because its graphical user interface (GUI) is separated from the command line functionality, and because its code is completely open for inspection and modification. To increase future-proofness, the software is built on free and platform-independent foundations that adhere to international standards, have existed for several decades, and continue to grow in popularity. IsoplotR currently includes functions for U-Pb, Pb-Pb, 40 Ar/ 39 Ar, Rb-Sr, Sm-Nd, Lu-Hf, Re-Os, U-Th-He, fission track and U-series disequilibrium dating. It implements isochron regression in two and three dimensions, visualises multi-aliquot datasets as cumulative age distributions, kernel density estimates and radial plots, and calculates weighted mean ages using a modified Chauvenet outlier detection criterion that accounts for the analytical uncertainties in heteroscedastic datasets. Overdispersion of geochronological data with respect to these analytical uncertainties can be attributed to either a proportional underestimation of the analytical uncertainties, or to an additive geological scatter term. IsoplotR keeps track of error correlations of the isotopic ratio measurements within aliquots of the same samples. It uses a statistical framework that will allow it to handle error correlations between aliquots in the future. Other ongoing developments include the implementation of alternative user interfaces and the integration of IsoplotR with other data reduction software.

IsoplotR: A free and open toolbox for geochronology

Calibration of the geological time scale is achieved by independent radioisotopic and astronomical dating, but these techniques yield discrepancies of ∼1.0% or more, limiting our ability to reconstruct Earth history. To overcome this fundamental setback, we compared astronomical and 40 Ar/ 39 Ar ages of tephras in marine deposits in Morocco to calibrate the age of Fish Canyon sanidine, the most widely used standard in 40 Ar/ 39 Ar geochronology. This calibration results in a more precise older age of 28.201 ± 0.046 million years ago (Ma) and reduces the 40 Ar/ 39 Ar method9s absolute uncertainty from ∼2.5 to 0.25%. In addition, this calibration provides tight constraints for the astronomical tuning of pre-Neogene successions, resulting in a mutually consistent age of ∼65.95 Ma for the Cretaceous/Tertiary boundary.

https://science.sciencemag.org/content/sci/320/5875/500.full.pdf

Synchronizing Rock Clocks of Earth History

Age constraint on Burmese amber based on U–Pb dating of zircons

Genesis of iron-oxide-apatite deposits in a collisional orogen: a petrologic and geochronologic study of magnetite ores in the new jersey highlands

11 Chemical abrasion in hydrofluoric acid (HF) is routinely applied to zircon grains prior to U-Pb 12 dating by isotope dilution thermal ionization mass spectrometry (ID-TIMS) to remove radiation- 13 damaged portions of grains affected by Pb loss. Still, many chemically abraded datasets exhibit 14 evidence of residual Pb loss. Here we test how the temperature and duration of chemical 15 abrasion affects zircon U-Pb and trace element systematics in a series of 4-hour, single-crystal 16 stepwise dissolution experiments at 180 °C and 210 °C. Microtextural data for the zircon 17 samples studied is presented in a complementary paper by McKanna et al. (2023). We find that 18 stepwise dissolution at 210 °C is more effective at eliminating U, common Pb (Pb c ), and light 19 rare earth element (LREE) enriched material affected by open system behavior; reduces the 20 presence of leaching-induced artefacts that manifest as reverse discordance; and produces 21 more consistent and concordant results in zircon from the three rocks studied. We estimate 22 that stepwise dissolution in three 4 h steps is roughly equivalent to a single ~8 h leaching step 23 due to the insulating properties of the PTFE sleeve in the Parr pressure dissolution vessel, 24 whereas traditionally labs utilize a single 12-hour leaching step. To better understand the 25 causes of Pb-loss in zircon, we calculate time-integrated alpha dose estimates for leachates and 26 residues from measured radionuclide concentrations to determine: 1) the alpha dose of the 27 material dissolved at the two leaching conditions, and 2) the apparent minimum alpha dose 28 required for Pb loss to occur: ≥ 6 × 10 17 α/g. We conclude that a single 8 h leaching step at 210 29 ˚C should yield crystallization ages in the majority of zircon and that this can be used as an 30 effective approach for routine analysis. However, Ultimately, the effectiveness of any chemical 31 abrasion protocol will be sample-dependent. By framing Pb loss and zircon solubility in terms of 32 alpha dose, however, workers can better tailor the chemical abrasion process to specific zircon 33 samples to improve the accuracy and precision of U-Pb results

Blair Schoene

Papers

Genesis of iron-oxide-apatite deposits in a collisional orogen: a petrologic and geochronologic study of magnetite ores in the new jersey highlands

Geochronological and Geochemical Effects of Zircon Chemical Abrasion: Insights from Single- 1 Crystal Stepwise Dissolution Experiments 2

Paleogeography and high-precision geochronology of the neoarchean fortescue group, pilbara, western australia