Radiogenic Pb and Nd isotopic variability of fine lithic materials in weathering profiles of Deccan Trap, India

Radiogenic isotopes are widely used for sediment provenance. Sedimentary processes are often neglected but may influence the radiogenic Sr isotope-based provenance [1]. This study explores the radiogenic Pb-Nd isotopic systematics in fine lithic materials from the critical zone of Deccan Basalt (Raigad, Maharashtra). An insignificant Pb and Nd isotope variability of fine lithic materials (<20 mm, <5 mm, and <2 mm) is highlighted in the soil (n=5) and saprolite (n=10) samples. The fine soil and saprolite materials also show statistically insignificant differences in CIA (Chemical Index of Alteration) irrespective of the grain size. However, the soil materials (²⁰⁶Pb/²⁰⁴Pb 17.422±0.513, ²⁰⁷Pb/²⁰⁴Pb 15.354±0.104, ²⁰⁸Pb/²⁰⁴Pb 37.843±0.409, and e_Nd –14.4±2.4) are isotopically different as compared to the saprolite materials (²⁰⁶Pb/²⁰⁴Pb 16.823±0.063, ²⁰⁷Pb/²⁰⁴Pb 15.219±0.013, ²⁰⁸Pb/²⁰⁴Pb 37.374±0.056, and e_Nd –15.7±1.0). The less radiogenic Nd isotopes suggest altered basalt as the source rock composition. Further, the mass balance suggests that one topsoil sample has a significant contribution of ~75–80 % dyke materials exposed nearby. Whereas the other soil samples show <10 % and <6 % contributions from the dyke and UCC-type materials, respectively. These data reaffirms the robust use of Pb and Nd isotopes in sediment provenance. 

Reference:

[1] Dasch, E.J., 1969. Strontium isotopes in weathering profiles, deep-sea sediments, and sedimentary rocks. Geochimica et Cosmochimica Acta, 33(12): 1521-1552.