Geochemical Trends and Rare Earth Elements’ Behaviour in the Recently Exposed Weathering Profiles of the Deccan Basalts from Central India

Abstract

The Deccan basalts have been undergoing intense weathering in the tropical conditions since the passage of India through the equator. This study is on the basalts’ weathering from central India’s present-day semiarid climate. Two weathering profiles from the districts of Indore and Dhar, Madhya Pradesh, have been studied using mineralogy and geochemistry. The profiles show incipient weathering with low to medium Chemical Index of Alteration (CIA) and Mafic Index of Alteration-oxidising (MIA_(O)) values. The presence of smectite in weathered zones, mineralogically attests to incipient weathering. The pyroxene and plagioclase-rich zones have produced smectite clays in the basalts. The rare earth elements (REE) and, to a greater extent, the heavy (H) REE show mobility during incipient weathering, which appears to be due to differential retention of REE in the clay minerals. The smectites seem to retain light (L) REE more significantly than HREE. The ancient terrain showing currently incipient weathering indicates recent exposure of the profiles by the erosion under neotectonic activity. In this fashion, the frequent exposures of new weathering surfaces on the basalts may have played a significant role in the uninterrupted consumption of CO₂ by silicate weathering since around 50 Ma.