Disturbed and undisturbed U–Pb systems and the potential implication on calcite dating: preliminary evidence from the Olevano-Antrodoco-Thrust-Fault (Central Italian Apennines)

Riccardo Lanari ¹, Anda Buzenchi ^1,2, Alessandro Bragagni ¹, Bruno Dhuime ², Mauro Brilli ³, Chiara Del Ventisette ¹, Massimo Mattei ⁴, Sandro Conticelli ^1,3 & Riccardo Avanzinelli ^1
1Dipartimento di Scienze della Terra, Università degli Studi di Firenze, Via Giorgio La Pira, 4, 50121, Firenze, Italy., ²Géosciences Montpellier, Université de Montpellier, Campus Triolet cc060, Place Eugène Bataillon, 34095 Montpellier Cedex05, France., ³Istituto di Geologia Ambientale e Geoingegneria, Area della Ricerca Roma 1, Strada Provinciale 35d, 9, 00010, Montelibretti (RM), Italy., ⁴Dipartimento di Scienze, Università degli Studi di RomaTre, Largo San Leonardo Murialdo, 1, 00146, Roma, Italy.

The application of U–Pb dating of synkinematic calcite has exponentially increased in recent years since it may provide a specific timing of fault movement. Despite the evident potentiality, carbonate geochronology still has several open issues. This study provides a further step for the understanding disturbed U–Pb systems, identifying the consequences for calcite dating. We combined U–Pb dating performed on the same calcites, with different methods, at different sample scales (e.g., in situ Laser Ablation and solution ICP–MS+thermal ionization mass spectrometry) with Oxygen (δ¹⁸O)-Carbon (δ¹³C) stable isotope compositions.

High-precision ²⁰⁷Pb/²⁰⁴Pb vs ²⁰⁶Pb/²⁰⁴Pb ratios measured by thermal ionization mass spectrometry on larger samples (in solution), suggest that all our samples experienced a degree of U–Pb reorganization. On the contrary, at the same scale, calcites show a rather clustered distribution of δ¹⁸O. However, the in situ Laser Ablation method provides good isochrons, yielding occasionally U-Pb ages which are geologically inconsistent. Moreover, we report that calcites with a very low spread of δ¹⁸O (<0.7‰) yield similar ages measured with different U-Pb approaches, while calcites with a slightly larger spread in δ¹⁸O (>0.7‰) show U-Pb scattered data points and significantly divergent ages between different methods.

Our findings underscore the potential risks associated with disturbed U-Pb systems, even at the level of laser ablation spots, in yielding analytically robust calcite ages that align with geological context. However, this study also raises important questions regarding the application of calcite U-Pb dating. Addressing these open issues will necessitate further research to challenge existing assumptions and refine the methodology, ultimately enhancing the reliability of our interpretations. Continued exploration in this topic is essential for advancing the field and ensuring accurate age determinations.