Recent advances in U-Pb dating in carbonates: characterization of laboratory procedures and examples of applications in South America

U-Pb method via LA-ICP-MS has been gaining great interest to investigate carboatic rocks. Carbonate minerals form in a wide variety of geological environments as a primary and secondary mineral phase, including diagenetic, biogenic, igneous, metamorphic and hydrothermal environments. These minerals can accumulate uranium in their formation, making them a potentially suitable chronometer for U-Pb and U-Th geochronology.

This tecnique is similar to that used for minerals rich in U, such as zircon, however it requires an additional phase of correction for common Pb obtained through the normalization of unknown samples with a standard analyzed in the same run of unknown samples. The sample image is an important tool for evaluating the crystalline growth of the vein. So, the samples were prepared as chips on epoxy resin and polished slabs. At first, images of the samples were taken by optical cathodoluminesce (CL) to asses internal features for vein type classification.

In general, the correction and normalization in the U-Pb method in carbonates is carried out as follows: first, the mass bias correction of the Pb or U-Pb isotopes is carried out using glasse(NIST 612), and subsequently the normalization of 238U/206Pb. A correction factor is calculated from the ratio XRc/XRm, where XRc is the age reported in the literature for the reference material WC1 (254.4 Ma) and XRm the age found for WC1 during the analysis session of the unknown samples. To setup the routine in Multilab UERJ, preliminary tests combined the use of zircon standards and NIST 612 glass as Pb isotope mass bias correction with carbonate matrix reference materials WC1 (254.4 ± 1, 6 Ma), ASH 15 (2.965 ± 0.011 Ma) and JT (13.75±0.11 Ma). The final analytical protocol was setup combining NIST 612 glass and WC1.  

Three application examples were analyzed with results that allow interpretations about the geological processes of carbonate formation. The first corresponds to the carbonate veins filling falts in Potiguar Basin successfully dated and yielded ages between 45 and 30 Ma, suggesting a causal relationship between magmatism and brittle deformation. A second application example used fossiliferous carbonate samples from the Itaboraí Basin known for its content of Paleogene endemic mammals of South America. The results obtained indicate Eocene ages in a jaw fragment with teeth and carbonate rock of chemical or clastic origin containing fossil fragments. The ages vary between 55 ± 19 Ma and 54.89 ± 1.4 Ma.

Another application example was carried out in samples from the Neoproterozoic sedimentary sequence of the Tucavaca Belt, Bolivia, consisting of marbles and intercalations of calc-silicate rocks. Samples of carbonates collected in a 30m continuous vertical profile were polished and analyzes yielding ages ca.580 Ma, coherent with Cryogenian.

In conclusion, the interpretation of the results obtained in preliminary tests makes it possible to conclude that the use of NIST 612 as a standardizer of Pb and U ratios combined with carbonate analyzes can be used by laboratories. The use of optical cathodoluminescence together with LA-ICP-MS is a robust tool for the initial screening of carbonate samples, known for their heterogeneity and low concentration of U. The combination of these techniques was essential for the analytical success of this work, assisting in the evaluation from areas with a higher concentration of U in the samples, in addition to discarding samples where it would not be possible to obtain ages, saving laboratory time and equipment resources. The ages found in this work correlate with events found in literature and regional tectonics showing an importante tool for carbonatic studies.