Geochemistry of volcanic glass from Mahanadi offshore region, eastern continental margin of India: Constraints on the contribution of latest Toba super-eruption
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Abstract: The tephra layers in multiple sediment cores from the offshore region of the Mahanadi basin in the northern Bay of Bengal were investigated for possible volcanic sources. The glass shards from those tephra layers were studied for size distribution, texture, and elemental geochemistry to establish chronostratigraphic markers for regional and global Quaternary correlation. The textural features of fine-grained (silty) volcanic glasses suggest the distal source of these tephra deposits. Major element composition with elevated SiO2 contents ranging between 75%–76% and dominance of K2O (> 4.5%) over CaO (< 0.9%) suggest ashes have originated from siliceous rhyolitic melts, similar to the petrographic composition of tephra from the Toba volcano. The bulk trace element compositions of the same glass shards were comparable with those reported in the youngest Toba tephra reported elsewhere. Likewise, the LREE-dominated chondrite normalized REE profiles of tephra from the Mahanadi basin closely resemble the characteristic REE patterns in Toba ash from other parts of the Indian Ocean and thus confirmed the contribution of the youngest Toba super-eruption for this ash layers.
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Key words:
- Mahanadi basin /
- Bay of Bengal /
- volcanic glass /
- glass morphology /
- glass-chemistry.
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Figure 1. Occurrences of the Youngest Toba Tuff (YTT) across the Indian Ocean region, southern Asia, and the South China Sea (modified from Lane et al., 2013; Westaway et al., 2011) (a). Bathymetric map of the study area in Mahanadi offshore region, east coast of India (b). Bathymetry data of the studied area was derived from GEBCO Compilation Group (2020) GEBCO 2020 Grid (doi: 10.5285/a29c5465-b138-234d-e053-6c86abc040b9).
Figure 2. Multimodal particle size distribution of volcanic tephra layers in five studied cores from Mahanadi offshore region, Bay of Bengal.
Figure 4. A. The chemical composition of glass shards from the Mahanadi offshore, Bay of Bengal. Total alkali-silica (TAS) diagram showing the rhyolitic composition of glass shards (modified from Le Bas et al., 1986). B. Chemical correlation of glass shards from volcanic ash layer in the Mahanadi basin deposits of the YTT. The data obtained for Krakatau Volcano (Mandeville et al., 1996); Tambora Volcano (Self et al., 1984); Pinatubo, Taal, Ranau and Maninjau Volcano (De Maisonneuve et al., 2020).
Figure 5. The comparative plots of Ba vs. Y (a) and Sr vs. Y between volcanic glass shards from the Mahanadi basin (present study) and Toba tephra deposits in India Occan (b) (Srivastava and Singh, 2021; Pearce et al., 2020; Jumaila et al., 2017; Smith et al., 2011; Pattan, 2002; Song et al., 2000; Chesner, 1998; Westgate et al., 1998). The categorization of YTT among five different populations (I to V) was made by following Pearce et al., 2020.
Figure 6. Chondrite normalized REE patterns of volcanic glass shards from the Mahanadi offshore basin, Bay of Bengal are compared with those from the Indian Ocean (Westgate et al., 1998); South China Sea (Song et al., 2000); Arabian Sea (Pattan et al., 2001) and Toba caldera (Smith et al., 2011). Normalization values are obtained from Sun and McDonough (1989).
Table 1. Locations of sediment cores, water depth, core recovery, occurence depth of tephra layers, grain size distribution and specific gravity of glass shards from Toba ashes at Mahanadi offshore basin
Cores Latitude, Longitude Water
depth/mCore recovery/m Depth of ash
layer/mbsfGrain size/µm Specific gravity/
(gm·cm−3)< 2.0 2–63 63–125 > 125 MD-19 18°59.11’N 85°41.17’E 1480 39.08 9.64–9.72 3.31 81.13 15.6 0.0 2.7 MD-20 18°47.36’N 85°36.44’E 1866 50.08 19.14–19.19 2.01 62.44 31.29 4.26 2.6 MD-23 19°06.25’N 85°50.15’E 1600 38.62 22.2–22.26 1.47 48.42 37.57 12.54 2.6 MD-24 19°03.48’N 85°45.59’E 1578 31.89 18.59–18.69 1.75 62.11 30.77 5.37 2.7 MD-27 18°57.36’N 85°43.44’E 1691 36.06 12.13–12.18 1.6 62.92 31.17 4.31 2.6 
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Table 2. Major element composition (wt%) of glass shards from the Mahanadi offshore region compared with the YTT from other locations. Please note that A = Central Indian Ocean Basin (Pattan et al., 1999); B = South China Sea (Song et al., 2000); C = Arabian Sea (Pattan et al., 2001); D, E, F = Glasses from YTT, MTT and OTT respectively from the Toba caldera (Chesner, 1998)
Major oxides Cores from the Mahanadi basin YTT from other marine environments Glasses from the Toba caldera YTT MTT OTT MD-19 MD-23 MD-24 MD-27 MD-20 A B C D E F SiO2 75.98 76.20 75.64 75.22 75.74 76.81 77.48 77.06 76.8–78.2 75.2–76.9 71.9–76.4 Al2O3 10.86 10.48 10.69 10.92 11.26 12.77 12.42 12.44 11.9–12.9 12.6–13.9 13.3–14.4 Na2O 2.54 2.09 2.33 2.50 3.00 3.41 2.43 3.36 2.6–2.9 2.98–3.5 3.4–3.6 K2O 4.95 5.33 5.18 5.55 4.62 5.08 4.72 5.11 4.8–5.3 4.1–4.8 4.7–4.8 CaO 0.68 0.69 0.79 0.84 0.57 0.79 0.75 0.80 0.62–0.94 0.75–0.82 1.0–1.9 Fe2O3 0.80 1.15 0.95 1.09 0.77 0.92 0.85 0.88 0.84–1.2 1.2–1.4 0.6–2.5 H2O 5.42 5.29 4.40 4.18 5.34 5.23 – 4.94 – – – Na2O + K2O 7.49 7.61 7.42 7.51 8.05 – – – – – –
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Table 3. Trace element concentrations (10-6) in volcanic glasses from the Mahanadi basin. The data are compared with the range of volcanic glass composition from other YTT deposits. Please note that A = YTT from India, Malaysia, Indian Ocean, Toba caldera (Srivastava and Singh, 2019 and references therein); B = YTT glass from ODP-758 site in Bay of Bengal (Jumaila et al., 2017); C = Five populations of YTT glass composition (Pearce et al., 2020); D = YTT glass shards from Central Indian Ocean Basin (CIOB) (Pattan et al., 2002)
Element concentration YTT from the cores of Mahanadi basin YTT from other marine environments MD-19 MD-23 MD-24 MD-27 MD-20 A B C D Sc/10–6 2.7510–6 1.84 2.92 1.99 2.88 1.7–3.02 4.3 – 1.54–1.93 V/10–6 3.64 3.25 4.09 2.89 6.48 – 18.79 – 12.7–17.4 Cu/10–6 3.05 2.14 1.65 1.54 22.5 – 19.08 – 2.43–6.2 Zn/10–6 174 152 226 281 432 – 71.41 – 6.95–10.2 Ga/10–6 14.4 6.84 14.2 10.4 13.9 9.01–11.08 16.55 – 8.3–12.3 Rb/10–6 272 111 239 193 224 199–286 205 200–292 171–252 Sr/10–6 60.3 31.0 52.4 53.5 89.9 25.8–108 87.28 28.9–113 43.7–63.8 Y/10–6 41.6 18.2 36.1 31.2 36.3 28.9–64.7 29.26 36.9–69.8 22–31.7 Zr/10–6 176 71.3 153 135 154 – 119 116–144 57.9–87.6 Nb/10–6 18.4 7.15 16.6 15.1 15.1 10.2–22.5 8.45 17.5–23.9 11–15.8 Cs/10–6 10.4 3.41 10.3 7.37 9.33 5.3–10.8 12.55 5.12–10.8 7.1–10.9 Ba/10–6 453 187 537 338 812 88–1142 527 94.5–1191 350–514 Hf/10–6 5.81 1.59 4.79 4.36 4.89 – 3.94 5.6–6.05 2.8–3.71 Ta/10–6 4.07 0.91 2.99 2.37 2.22 – 1.1 2.15–3.76 1.18–1.74 Pb/10–6 40.9 46.5 59.3 45.4 84.6 25.3–72.4 12.94 57.1–60.9 – Th/10–6 34.1 15.9 32.7 25.1 30.8 25.4–58 21.05 45.8–64.3 23.2–32.9 U/10–6 5.73 2.50 5.92 4.34 5.01 3.99–9.97 5.82 5.28–10.5 6.6–8.1 Sr/Y 1.45 1.70 1.45 1.71 2.48 – 2.98 0.42–3.09 – Ba/Y 10.9 10.3 14.9 10.8 22.4 – 18.01 1.36–32.7 – Zr/Y 4.2 3.9 4.2 4.3 4.2 – 4.07 1.67–3.91 – U/Ce 0.09 0.095 0.083 0.089 0.078 – – – –
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Table 4. REE composition (in 10–6) and systematics in volcanic ash from Mahanadi basin Europium anomaly, Eu/Eu* = Eun/(Smn .Gdn)0.5& Cerium anomaly, Ce/Ce* = 3Cen/(2Lan+Nd). Please note that A = YTT from India, Malaysia, Indian Ocean, Toba caldera (Srivastava and Singh, 2019 and references therein); B = YTT glass from ODP-758 site in Bay of Bengal (Jumaila et al., 2017); C = five populations of YTT glass composition (Pearce et al., 2020)
REEs YTT from the cores of Mahanadi basin YTT from other marine environments MD-19 MD-23 MD-24 MD-27 MD-20 A B C La 33.61 14.47 37.49 25.73 33.44 21.3–60.7 33.81 34.3–68 Ce 63.72 26.18 71.38 48.67 63.86 40.0–99.2 55.99 66–107 Pr 8.14 3.42 8.82 6.09 8.05 4.7–10.1 6.82 8.02–11.1 Nd 24.73 9.14 24.33 18.62 23.49 17.0–34.4 28.78 30.9–37.4 Sm 5.27 1.83 5.04 3.96 4.93 3.3–7.4 5.32 7.09–8.24 Eu 0.48 0.15 0.4 0.36 0.52 0.3–0.74 0.72 0.36–0.75 Gd 4.07 1.37 3.74 3 3.72 2.8–7.95 4.22 6.14–8.89 Tb 0.73 0.23 0.64 0.54 0.65 0.54–1.42 0.73 0.94–1.58 Dy 4.78 1.42 3.98 3.56 4.17 3.2–9.64 3.89 6.3–10.6 Ho 1.09 0.33 0.91 0.82 0.95 0.58–2.17 0.74 1.37–2.37 Er 3.08 0.93 2.59 2.31 2.65 1.91–6.99 2.43 4.2–7.65 Tm 0.55 0.16 0.46 0.42 0.47 0.34–1.18 0.45 0.71–1.29 Yb 4.22 1.28 3.49 3.2 3.57 2.9–7.96 3.01 4.86–8.93 Lu 0.72 0.22 0.6 0.54 0.6 0.48–1.28 0.48 0.77–1.44 ∑REE 155.2 61.1 163.9 117.8 151.1 – – – %LREE 90.2 92.5 92.3 90.3 91.4 – – – Eu/Eu* 0.31 0.29 0.28 0.32 0.37 – – – Ce/Ce* 0.93 0.91 0.95 0.93 0.94 – – – Ndn/Ybn 2.10 2.57 2.51 2.09 2.37 – – –
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