Re-Os isotopes and major and trace element geochemistry of carbonaceous shales, Aravalli Supergroup, India: impact of post-depositional processes

Abstract

This study presents Re-Os isotopes and major and trace elemental geochemistry of carbonaceous (black) shales and a weathering profile developed on them from the Paleoproterozoic deposits of the Jhamarkotra Formation, Lower Aravalli Group, India. These measurements provide information on the impact of post-depositional processes, particularly hydrothermal fluid interactions, on the ¹⁸⁷Re-¹⁸⁷Os geochronology and element mobility in shales. In contrast to previously reported compositions for black shales, the Re concentrations of the Jhamarkotra shales are in general lower than their corresponding Os concentrations. The ¹⁸⁷Os/¹⁸⁸Os ratios of the black shales vary widely, from 3.5 to 10.5, without any discernible systematic trend with ¹⁸⁷Re/¹⁸⁸Os (range: 0.4 to 17.7; excluding an outlier); these ¹⁸⁷Re/¹⁸⁸Os ratios are lower by a few orders of magnitude than those generally reported for “fresh” black shales. These observations indicate that the Jhamarkotra shales, though they seem visually well preserved, have undergone post-depositional modification, resulting in “open system” behavior of Re and Os, with Re more mobile than Os. Higher SiO₂/TiO₂ and K₂O/Na₂O ratios of the Jhamarkotra shales compared to their sediment provenance suggest interaction of the shales with hydrothermal (possibly Si and K-rich) fluids. The SiO₂/TiO₂ ratios (an index for hydrothermal alteration) show negative correlation with Re/Corg and positive correlation with Corg/N ratios (42–154), indicating loss of Re and nitrogen during hydrothermal fluid interaction. Earlier studies have shown the presence of Re porphyrins (N-containing organic compounds) in black shales. Thus, the observed co-variations of SiO₂/TiO₂ with Re/Corg and Corg/N ratios suggest mobilization of Re from the porphyrins in the shales. Further, positive correlation between SiO₂/TiO₂ and ¹⁸⁷Re/¹⁸⁸Os ratios indicates that significant amounts of radiogenic Os component were re-distributed throughout the Jhamarkotra shales during the post-depositional processes. In the weathering profile developed on black shales, the Al₂O₃-normalized abundance ratios of mobile major elements are in general lower in the upper layers relative to those in deeper sections attesting to their preferential mobility during chemical weathering. On the contrary, the Al₂O₃-normalized ratios of trace elements mostly showed insignificant depth variations in the weathering profile, suggesting limited mobility over the depth of the profile. The depth profiles of Re/Al₂O₃ and ¹⁸⁸Os/Al₂O₃ and ¹⁸⁷Re/¹⁸⁸Os ratios in the weathering profile by and large reflect compositions of their presumed parent shales, which have gone through significant post-depositional changes.