Geochemical and isotopic (Re-Os and Mo) compositions of black shales from the Cumbum Formation, Cuddapah Basin, India
Description
This dataset contains the geochemical (major element, trace element and Fe-speciation) and isotopic (Re-Os and Mo) compositions of black shales from the Cumbum Formation, Cuddapah Supergroup (India). This dataset was used to provide new constrains on the depositional age, setting and the extent of global euxinia during the depositional interval.
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The shale samples were powdered to <100 μm size using an agate mortar and pestle. Major element concentrations were determined using a WD-XRF spectrometer by preparing fusion beads at 1100℃ (Samanta, 2022). For trace element analysis, known amount (~0.1 g) of samples were completely dissolved using HF-HNO₃-HCl acids in a microwave digestion unit (Venugopal et al., 2023). Concentrations of trace elements in these solutions were quantified using a Q-ICPMS instrument. The Total Inorganic Carbon (TIC) content of the samples was determined using a coulometer. Total Carbon (TC) and Sulfur (TS) contents were determined by combustion of bulk sample powders at 1150℃ in an elemental analyzer. The Total Organic Carbon (TOC) content was determined as the difference between total carbon and inorganic carbon (TOC = TC − TIC) concentrations. The iron species associated with carbonates, oxides and magnetite phases were sequentially extracted (Poulton and Canfield, 2005) and the Fe content of the extracted solutions was measured using an AAS. The amount of Fe associated with Pyrite was calculated by gravimetrically quantifying (assuming FeS₂ stoichiometry) the amount of silver sulfide precipitated during a chromous chloride distillation process (Canfield et al., 1986). The Re-Os isotopic compositions were measured using the analytical methodology of Ackerman et al. (2019). For this, known amount of powdered sample, along with ¹⁸⁵Re-¹⁹⁰Os spike solution, was digested in a sealed Carius tube using CrO₃–H₂SO₄ mixture at 240℃. Osmium fraction from the digested solution was extracted following a solvent extraction approach using CH₃Cl and HBr solutions and further purified using a micro-distillation method (Birck et al., 1997). Rhenium was isolated from the digested solution using a combination of acetone–sodium solution extraction followed by conventional anion-exchange chromatography. The Os isotopic signals were measured using Thermal Ionization mass spectrometer in its negative ionization mode and corrected for mass fractionation, isobaric interferences and isotope dilution to obtain the Os concentration and ¹⁸⁷Os/¹⁸⁸Os ratios for the samples. The rhenium concentrations were measured using a sector field ICPMS instrument. Molybdenum isotopic analyses were carried out using a double spike technique (Goswami et al., 2022; Venugopal et al., 2023). A known amount of ⁹⁷Mo-¹⁰⁰Mo double spike was added to the digested solution of the shale powders based on their Mo concentrations, and kept for sample-spike equilibration. Pure Mo fraction was extracted from this solution using ion exchange chromatography (Pearce et al., 2009) and their isotopic compositions were measured using a MC-ICPMS instrument. The measured isotopic ratios were corrected offline for instrumental fractionation and isotope dilution (Rudge et al., 2009; Goswami et al., 2022) and are reported in delta notation (δ⁹⁸Mo) relative to NIST SRM 3134 with a positive offset of +0.25‰ (Nӓgler et al., 2014).
Institutions
- Indian Institute of Science Education Research PuneMaharashtra, Pune