Journal of Earth System Science ( IF 1.3 ) Pub Date : 2021-02-03 , DOI: 10.1007/s12040-020-01513-x Ashok K Srivastava , Ajab Singh
Abstract
Geochemistry and 40Ar/39Ar 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 SiO2 (78.4–79.6 wt%), Al2O3 (11.7–13.0 wt%), Na2O (2.0–2.6 wt%) and K2O (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 40Ar/39Ar dates show plateau ages of 0.79 ± 0.02 Ma (790 ± 0.02 ka) and 0.77 ± 0.05 Ma (770 ± 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 40Ar/39Ar dating of ash minerals. Therefore, the dates presently generated through 40Ar/39Ar 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.
Highlights
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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.
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40Ar/39Ar ages of 790 ± 0.02 ka and 770 ± 0.05 ka, on glass shards, are exceptionally high as compared to YTT, therefore, unrealistic and unacceptable, hence, rejected. Literature survey shows that 40Ar/39Ar dating results of volcanic glass shards in particular are influenced by magmatic and post-depositional processes.
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Sedimentological attributes and lithofacies architecture of ash-bearing successions reveal fluvio-lacustrine set-up of deposition for the ash.
中文翻译：
印度中部普尔纳冲积盆地最年轻的多巴凝灰岩玻璃碎片的地球化学和受约束的40 Ar / 39 Ar测年
摘要
对印度中部普尔纳冲积盆地的甘地格拉姆（G-2）和哈奇（H-1）这两个地方的最年轻的多巴凝灰岩灰的玻璃碎片进行了地球化学和40 Ar / 39 Ar测年。由EPMA测定的主要氧化物表现为SiO 2（78.4–79.6 wt％），Al 2 O 3（11.7–13.0 wt％），Na 2 O（2.0–2.6 wt％）和K 2通过ICP-MS分析，O（3.4–4.4 wt％）和微量和微量元素显示出较高的LREE含量，即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）和较低的HREE值，即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）以及显着的Eu浸出。微量元素显示出高含量的Rb（188.6–200.6 ppm），Sr（37.79–48.14 ppm），Ba（347.9–375.6 ppm）和低Ga（8.90–9.67 ppm），Sc（2.33–3.09 ppm），Nb（10.24） –11.6 ppm）。玻璃碎片的化学成分类似于印度半岛和马来西亚（包括鸟羽破火山口）报道的最年轻的鸟羽凝灰（YTT）灰分。的40的Ar / 39对于甘地格拉姆和胡奇地区，灰烬的Ar日期显示出0.79±0.02 Ma（790±0.02 ka）和0.77±0.05 Ma（770±0.05 ka）的高原年龄。这些日期似乎在地质上是没有意义的，因此不可接受，因为YTT灰通常被认为是大约10年。基于地球化学，裂变径迹和灰烬矿物的40 Ar / 39 Ar测年，年龄为75 ka 。因此，考虑到由于同位素分馏和沉积后过程引起的Ar同位素，熔体中的碱和玻璃碎片的移动行为，目前通过40 Ar / 39 Ar技术在玻璃碎片上生成的日期已被丢弃。
强调
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来自印度Purna盆地的多巴灰的玻璃碎片地球化学表明，其成分与印度，马来西亚，海洋盆地和印度尼西亚多巴火山口的源区的YTT灰组成相似。
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与YTT相比，玻璃碎片上的40 Ar / 39 Ar年龄分别为790±0.02 ka和770±0.05 ka，因此不切实际和不可接受，因此被拒绝。文献调查表明，火山岩玻璃碎片的40 Ar / 39 Ar测年结果尤其受岩浆和沉积后过程的影响。
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含灰演替的沉积学特征和岩相结构揭示了灰烬的河流湖相构造。
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