Mineral mapping and in-situ Rb-Sr dating of Proterozoic glauconites

Abstract

Precise and accurate dating of glauconites in marine sediments and sedimentary rocks is of high interest to both basin exploration purposes as well as earth system evolution studies.

This project tests the new in-situ Rb/Sr dating method of glauconites using LA-ICP MS/MS system, and analyses samples of glauconites from selected Indian Basins, including the Vindhyan Basin, Chhattisgarh Basin and Kutch Basin. Previous studies report the ages of glauconite samples collected from these basins using different and more established methods such as K-Ar and TIMS-based Rb-Sr dating. These popular methods have however the problem of possible contamination of bulk glauconite samples (typically consisting of tens to hundreds of separated glauconite grains) with older detrital minerals and/or younger clay minerals. The new in-situ Rb/Sr dating method used in this project in combination with detail mineral mapping of samples (Nanomin technique) has potential to overcome the above problems and analytical issues. As to results, the samples analysed by the in-situ Rb/Sr method including samples: G2 (Vindhyan Basin Semri Group) , G6 (Vindhyan Basin Semri Group Chitrakoot Fm), G8 (Chhattisgarh Basin Raipur Group) and G10 (Kutch Basin) yielded, respectively, the following Rb/Sr ages of 1454 14 Ma, 1386 25 Ma, 920 12 Ma and 127 36 Ma. These in-situ Rb/Sr ages for most part agree with the published ages for equivalent glauconite samples from these basins analysed by K/Ar and/or TIMS-based Rb/Sr techniques.