Comparative study of sample preparation methods and leaching for strontium isotope analysis in carbonate rocks.

The ⁸⁷Sr/⁸⁶Sr ratio of Sr dissolved in the world’s oceans has varied through time, which allows dating and correlating sedimentary rocks (Denison et al., 1994). Phanerozoic Sr isotope stratigraphy investigations usually use well-preserved, low-Mg calcite fossils. However, fine-grained carbonate components such as micrite are also used (Bailey et al., 2000). This study aims to assess different procedures for the Sr analysis using limestone samples from the Itaituba Formation of the Amazonas Basin to compare the results with the expected values of ⁸⁷Sr/⁸⁶Sr isotope ratios. Six samples were selected and two types of subsamples were prepared. One subsample consisted of whole rock powder ground by hand using an agate pestle, while the other was obtained by micro-drilling. The micro-drilling sampling method, based on petrographic analyses, is used to extract fine-grained carbonate components. It aids in avoiding visible portions like veins and dissolution features. Subsequently, four scenarios were evaluated: the leaching method using hydrochloric acid (HCl) in whole rock and in micro-drilled samples and the leaching method using acetic acid (HAc) in whole rock and micro-drilled samples. The typical method used in most labs, including the Isotopic Geology Laboratory (LGI) at the Federal University of Rio Grande do Sul (UFRGS), involves dissolution using HCl. However, using a weak acid like HAc, which is gentler compared to a strong acid like HCl, can be a way to help prevent reactions with clay minerals in the rock matrix that can lead to the release of radiogenic Sr. Samples were weighted (∼0.2g) treated in 10ml of 0.1N HCl and placed in an ultrasonic bath for 30mins and centrifuged for 10mins. The supernatant was discarded. Then, 10mL of 1N HCl was added to the remaining precipitate, and the same process was repeated. Finally, 500 μl of the resulting supernatant from each sample was pipetted and left on a hot plate at 100°C until dry. The second dissolution method utilizes HAc and follows the methodology described by Hohl et al. (2022). Samples were weighted (∼0.3g for micro-drilled and 1g for whole rock samples) and treated with 50 ml of 1M ultra-pure glacial HAc per gram of sample used. The samples were placed in an ultrasonic bath for 15mins, then left at room temperature for another 6hrs, ultrasonicated again, and centrifuged at 4500 rpm for 5mins. Around 30% of the HAc leachate stock solution was dried, and then 1ml of concentrated and double-distilled HNO₃ was added in three steps on a hotplate at 110ºC. Afterward, all dry samples were redissolved in HNO₃ before being deposited in the column. Then, Sr was separated via ion exchange chromatography in a column using Eichrom resin (50–100µm). For element elution and Sr collection, 1ml of 5N HNO₃ and 1ml of Milli-Q H₂O were used, respectively. The samples were prepared and analyzed at the LGI-UFRGS. Isotopic ratios were measured in static mode using a Thermal Ionization Mass Spectrometer-Triton™ Multicollector. The samples were loaded onto a previously degassed rhenium filament with 2μl of 0.25N H₃PO₄. Sr ratios were normalized to ⁸⁶Sr/⁸⁸Sr=0.1194. The ⁸⁷Sr/⁸⁶Sr ratios that closely matched the expected values in the studied samples were obtained using HAc in the leaching procedure. In most cases, these ratios corresponded to those of the micro-drilled samples. The results obtained in the HCl leaching procedure were not so close to the expected values. The HAc leaching procedure has a notable impact on the final isotope results, especially when paired with the micro-drilled sampling method. This combination emerges as the most effective approach for recovering original signatures. Utilizing HAc dissolution and micro-drilled samples significantly enhances the probability of obtaining primary seawater ⁸⁷Sr/⁸⁶Sr isotope ratios from the studied carbonates. The study is ongoing, and we will expand the number of samples and apply the method in other case studies.