Comparative Assessment of Paleodepositional Conditions in Carbonates from the Irecê Basin and Vazante Group Using REY and Nd Isotope Signatures

Abstract

Rare earth elements and yttrium (REY) plus Nd isotopes are used as powerful tracers to reconstruct ancient depositional environments (Fanton et al., 2002) . Meanwhile the REY normalized pattern elucidates the influence of associated minerals and fluid ligands, the εNd can distinguish the continental-mantle contribution for the ancient oceans. For this reason, carbonate rocks formed in different periods of time should have different isotopic signatures of the crust evolution.

Rare earth elements; Nd isotopes; carbonate rocks

Introduction

The Irecê and Vazante basins, both located within the Congo–São Francisco Craton, had partially coeval deposition but show distinct depositional histories and environments. Irecê, an intracratonic basin in central Bahia, records Meso–Neoproterozoic deposition, while Vazante, considered a passive margin basin, was mainly deposited in the Neoproterozoic. To investigate the oceanic conditions during the formation of these carbonate rocks, geochemical proxies were used. The chemical signatures of ancient waters are preserved in the carbonates. This study focuses on REY (Rare Earth Elements and Yttrium) and neodymium isotopic signatures as tracers. These parameters allow discussion of paleoclimatic and paleogeographic conditions and enable comparison between the basins to establish correlations or point out divergences in their evolutionary histories.

Experimental

The samples CR were collected at the outcrop Gruta Cristal, located at municipality of Bonito, BA. The samples IPA were collected at Irecê Basin, in America Dourada municipality, BA. The samples VZ and CARB were collected at Vazante municipality, northeast of MG. The samples were pulverized with a disc mill (Herzog). 100 mg (for REY determination) and 500 mg (for Nd isotope) of sample powder were weighted. The procedure was performed with 50 mL (for REY determination) and 100 mL (for Nd isotope) acetic acid (HAc)1 mol/L, and filtered. The samples were then fully evaporated (120 0C) in hot plates and then solubilized again with 5 mL HCl 1.25 mol/L. The matrix was removed with 15 mL HCl 1,25 mol/L and the REY were recovered with 12 mL HCl 6 mol/L. The samples destined for REY determination, were then evaporated and dissolved again with HNO3 3% with an internal standard of In, followed by analysis with Q-ICP-MS (Agilent). For the Nd isotope determination, a second separation was performed after the evaporation of HCl 6 mol/L step. The samples were dissolved again and Nd and Sm were separated with chromatography column as performed by Gioia and Pimentel (2000). The different fractions of Nd and Sm were then determined by MC-ICP-MS Neptune (ThermoScientific). The reference material JDo-1 was used to evaluate method accuracy.

Results and Discussion

The reference material analyzed wielded results of ¹⁴³Nd/¹⁴⁴Nd with 0.512242±14 (2σ). This is in accordance with reported values of previous studies, with 0.512251±12 (Li et al., 2021), 0.512248±5 (Kraemer et al., 2019) and 0.512245±6 (Viehman et al., 2020). Figure 1 presents the results for the sample analyzed.

The graphic presented in Figure 1 shows difference in the carbonates relative to its provenance. εNd values < 0 indicates that these carbonates were formed without a mantle contribution, excluding the influence of mid-ocean ridge and hydrothermal environments. The CR samples appear to be under more crustal influence than CARB and IPA samples, due to lower εNd values. The absence of Eu anomalies corroborates the εNd values, since no significant positive Eu anomaly is observed. Y/Ho anomalies indicate different types of sources from Fe and Al bearing minerals, as observed for VZ, CARB and CR/IPA samples. Major element analysis could provide detailed information about the composition of these minerals. Fo the graphic in Figure 1 c) there’s a pattern of LREE enrichment that starts with CR samples and goes decreasing (thus, HREE enrichment is increasing) with CARB samples and then IPA samples. At the same time, the HREE enrichment have a correlation with the increase of εNd values, indicating that the environment of the carbonate deposition changed slowly. Meanwhile, for the carbonates of VZ, the environment could be disturbed by different events.

Conclusions

The work successfully validated a method for the determination of Nd isotopes in carbonates. The normalized REY profiles of the sample in the combination of the Nd isotopes consisted in a useful tool to discriminate different basins formed, especially regarding the εNd and Y/Ho anomalies.

Acknowledgements

The authors thank the CNPq, CAPES and FINATEC for the fellowship and the Federal Police to provide the ICP-MS.