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Summer Research Fellowship Programme of India's Science Academies

A comparitive geochemical and strontium isotopic study of Precambrian and Cenozoic glauconites

Debosmit Banerjee

Department of Petroleum Engineering, Indian Institute of Technology (Indian School of Mines), Dhanbad 826004, India

Professor Ramananda Chakrabarti

Centre For Earth Sciences, Indian Institute of Science, Bangalore 560012, India

Abstract

Glauconite is an iron potassium phyllosilicate belonging to the mica group of minerals. It crystallizes in the monoclinic system and has a characteristic green colour. It is an authigenic product formed near the sediment water interface in shallow marine environments. Authigenic minerals are formed in situ during diagenesis. It is because of its authigenic nature that glauconite is extensively used by geochemists as an efficient tool for radiometric dating of sedimentary rocks. For this purpose, the initial 87Sr/86Sr ratio is taken to be equal to the seawater Sr isotope ratio at the time of formation of the mineral. This project investigates the validity of such an assumption and whether Sr isotopic composition of glauconites of varying depositional ages can be used to infer the seawater Sr isotopic composition through time. A comparative study based on elemental ratios of the Precambrian and relatively younger Cenozoic glauconitic samples will also be done. Relatively younger glauconitic samples of Late Palaeocene- Early Eocene were collected from the Naredi formation of Kutch along with Mesoproterozoic glauconites from Semri Group and Bhander Group of the Vindhyan Supergroup as well as from Raipur group of Chhattisgarh basin. The elemental study of the glauconite in the samples was done by analysing the samples with the Inductively Coupled Plasma Mass Spectrometer (ICP-MS) and then the present 87Sr/86Sr ratio was determined using Thermal Ionization Mass Spectrometer (TIMS) based on which the (87Sr/86Sr)initial ratio was calculated and fitted on the seawater Sr evolution curve. The marine origin of the samples was proven and the Sr evolution curve for each of the samples was plotted.

Keywords: isotope geochemistry, sedimentary geochemistry, ancient seawater, strontium isotope, glauconite

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