• Geochemistry and constrained $^{40}$Ar/$^{39}$Ar dating of Youngest Toba Tuff glass shards, Purna alluvial basin, Central India

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    • Keywords

       

      Geochemistry; Youngest Toba Tuff; $^{40}$Ar/$^{39}$Ar dating; glass shards; Quaternary.

    • Abstract

       

      Geochemistry and $^{40}$Ar/$^{39}$Ar dating of glass shards of Youngest Toba Tuff ash from two localities, i.e., Gandhigram (G-2) and Hudki (H-1) of the Purna alluvial basin, Central India have been carried out. Major oxides, determined by EPMA, exhibit SiO$_{2}$ (78.4–79.6 wt%), Al$_{2}$O$_{3}$ (11.7–13.0 wt%), Na$_{2}$O (2.0–2.6 wt%) and K$_{2}$ O (3.4–4.4 wt%) and minor and trace elements, analyzed by ICP-MS, reveal higher contents of LREEs, i.e., La (26.52–27.15 ppm), Ce (50.25–51.26 ppm), Pr (5.02–5.24 ppm), Nd (20.1–21.31 ppm), Sm (3.1–3.36 ppm) and lower values of HREEs, i.e., Tb (0.5–0.57 ppm), Ho (0.5–0.6 ppm), Tm (0.28–0.35 ppm), Yb (2.68–2.94 ppm), Lu (0.45–0.48 ppm) along with prominent dip of Eu. Trace elements show high content of Rb (188.6–200.6 ppm), Sr (37.79–48.14 ppm), Ba (347.9–375.6 ppm) and low Ga (8.90–9.67 ppm), Sc (2.33–3.09 ppm), Nb (10.24–11.6 ppm). The chemical composition of glass shards is similar to the Youngest Toba Tuff (YTT) ash reported from Indian peninsula and Malaysia including Toba Caldera. The $^{40}$Ar/$^{39}$Ar dates show plateau ages of 0.79 $\pm$ 0.02 Ma (790 $\pm$  0.02 ka) and 0.77 $\pm$  0.05 Ma (770 $\pm$  0.05 ka) for ash of Gandhigram and Hudki localities. These dates seem to be geologically meaningless and are not acceptable as the YTT ash is universally considered to be of ca. 75 ka age based on geochemistry, fission track and $^{40}$Ar/$^{39}$Ar dating of ash minerals. Therefore, the dates presently generated through $^{40}$Ar/$^{39}$Ar technique on glass shards are being discarded considering mobile behaviour of Ar isotopes, alkalis in the melts and glass shards due to isotopic fractionation and post-depositional processes.

      $\bf{Highlights}$

      $\bullet$ Glass shards geochemistry of Toba ash from Purna basin, India suggest its compositional similarities with the YTT ash of several river basins of India, Malaysia, ocean basins and source area of Toba Caldera, Indonesia.

      $\bullet$ $^{40}$Ar/$^{39}$Ar ages of 790 $\pm$  0.02 ka and 770 $\pm$  0.05 ka, on glass shards, are exceptionally high as compared to YTT, therefore, unrealistic and unacceptable, hence, rejected. Literature survey shows that $^{40}$Ar/$^{39}$Ar dating results of volcanic glass shards in particular are influenced by magmatic and post-depositional processes.

      $\bullet$ Sedimentological attributes and lithofacies architecture of ash-bearing successions reveal fluvio-lacustrine set-up of deposition for the ash.

    • Author Affiliations

       

      ASHOK K SRIVASTAVA1 AJAB SINGH1

      1. Department of Geology, SGB Amravati University, Amravati 444 602, India.
    • Dates

       
  • Journal of Earth System Science | News

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