U-Pb LA-ICP-MS GEOCHRONOLOGY OF SYNDEPOSITIONAL BOTRYOIDAL CEMENTS AS STRATIGRAPHIC TOOL

The dating of carbonate sequences commonly relies on index fossils, which provide only relative ages. Such biostratigraphic ages must be related to tie points constrained by absolute, radiometric dates. U-Pb dating of zircons from volcanic ash layers provide such absolute ages. However, ashes are infrequent, random, and difficult to locate in many carbonate sequences, which represents a major limitation for the determination of absolute depositional ages. In the present study, we dated syndepositional botryoidal cements by U-Pb LA-ICP-MS. Botryoidal cements, aragonitic marine cements with radial texture that commonly precipitate syndepositionally at the seafloor in shallow waters, are widespread, especially in Precambrian carbonates, and their direct dating by U-Pb offers great potential for the reconstruction of absolute depositional ages in sediment series devoid of index fossils and suitable ash layers.

To test whether botryoidal cements, typically characterized by high U contents, making them potentially suitable for U-Pb geochronology, record and maintain accurate and precise depositional ages, we apply this approach to a Carboniferous carbonate sequence located in northern Spain. We show that the U-Pb ages of botryoidal cements from four biostratigraphically well-constrained horizons are statistically distinct in their respective depositional ages that closely match zircon ages from interbedded ash layers. Our botryoidal cement-derived depositional ages are also supported by ⁸⁷Sr/⁸⁶Sr data in agreement with the global curve of the Carboniferous. We compare the botryoidal cement-derived depositional ages with U-Pb ages of fossils, micrite, and early diagenetic void-filling calcites measured along and show that botryoidal cements yield better accuracy and precision. Finally, we show that the U-Pb age obtained does not appear to be modified by thermal alteration due to burial, as inferred by clumped isotope thermometry.

Future studies should also explore syndepositional and early diagenetic cements other than botryoids, particularly radiaxal fibrous calcite cements, which often have lower U contents and thus are more challenging for U-Pb dating but are equally widespread in Precambrian carbonates.