High precision multi-collector 40Ar/39Ar dating of moldavites (Central European tektites) reconciles geochrological and paleomagnetic data
Gianfranco Di Vincenzo
Chemical Geology
In Press, Journal Pre-proof, Available online 22 July 2022
“Highlights
• Moldavites are excellent material to be dated by the 40Ar/39Ar method.
• 40Ar/39Ar data demonstrate an excellent intra- and inter-sample reproducibility.
• Results yield R(Mold/ACs) = 12.4952 ± 0.0038 and R(Mold/FCs) = 0.52106 ± 0.00014 (±2σ)
• Relative to the most recent calibration for FCs, data yield 14.735 ± 0.013 Ma (±2σ)
• Results places definitively the Ries impact in the reverse polarity chron C5ADr.”
“Moldavites (Central European tektites) are genetically related to the meteorite impact event that produced the 24-km diameter Ries crater (Germany) during the Langhian, and representing one of the youngest large impact structures on Earth. Despite the numerous geochronological studies over the last decades and the potential implications for stratigraphic, paleontological and paleoclimatic studies, the age of the Ries impact is still debated. In this study, I investigate in detail moldavite samples by multicollector 40Ar/39Ar laser dating in order to address the age of the Ries impact. Data were obtained relative to the key Fish Canyon sanidine (FCs) and Alder Creek sanidine (ACs) reference materials, over a period of nearly two years, in four irradiations of different duration. Results, completed through the step-heating and the total fusion techniques, demonstrate an excellent intrasample and intersample reproducibility of moldavites and prove that analytical performances of moldavites in terms of uncertainties on the 40Ar*/39ArK ratios are in line with those achievable by the most widely used FCs and ACs reference minerals. Results from total fusion analyses yield a mean R Moldavite ACs = 12.4952 ± 0.0038 (±2σ) and a mean R Moldavite ACs = 0.52106 ± 0.00014 (±2σ). Data also allow to define a direct mean R ACs/FCs = 0.041703 ± 0.000032 (±2σ) and an indirect (using moldavites as intermediary), and more precise, R ACs/FCs = 0.041700 ± 0.000019 (±2σ). Using the internally consistent astronomically-calibrated ages for the ACs (1.1848 ± 0.0012 Ma) and the FCs (28.201 ± 0.046 Ma) reference minerals from the literature, the R-values yield indistinguishable ages for moldavites of 14.7495 ± 0.0045 (± 0.016 Ma, including all known source of errors) and 14.7486 ± 0.0039 Ma (± 0.025 Ma), respectively. The most recent astronomically calibrated age for FCs (28.176 ± 0.023 Ma) yields a slightly younger, but more precise, age of 14.7355 ± 0.0039 Ma (± 0.013 Ma). Results place definitively the age of the Ries impact in the reverse polarity chron C5ADr of the ATNTS2022, and suggest that the Ries impact preceded the switch from reversed to normal chron by at least 127 ± 15 ka.”
