The chemical breakdown of thermodynamically unstable minerals on the surface of the Earth modulates the global atmospheric CO<sub>2</sub> concentrations and transports dissolved elements to the global ocean. Understanding the intensity of these chemical weathering processes through the distribution of these elements in subtropical to tropical basaltic laterite profiles is crucial because basaltic rocks contribute significantly to the global continental weathering flux. The rare earth elements (REE), due to their relative consistency of physicochemical properties along the series, are often used to decipher the basin processes that drive these dissolved loads of natural waters and alter the chemistry of primary rocks. In the present study, the lateritic profiles of Rampurhat, India developed over the basalts of Rajmahal traps have been studied to understand the mobilization and redistribution of REEs during extreme weathering. These lateritic region present an excellent case of subaqueous alteration, which is the uplifted interfluves and parts of paleo-deltas. In this study, four primary lateritic profiles have been systematically sampled and analyzed for their REEs’ concentrations and mineral chemistry through Q-ICPMS and XRD, respectively. Most of the profiles with a few discrepancies showed an increasing CIA index towards the top of the profile, and the REEs data revealed prominent Ce, Eu, and Gd anomalies. The positive Ce anomalies towards the top of the profiles along with a positive correlation with Mn indicate its association with Mn-oxides/hydroxides and clays. Consistent negative Eu anomalies suggest extensive weathering of plagioclase, making the residual products depleted in Eu. The Gd anomalies recorded in the profiles are more or less consistently positive in nature, indicating probable adsorption onto clay particles. Clear fractionation between LREE and HREE are recorded in the lateritic profiles, which are found to be enriched in LREE. The present study will improve our understanding of the fractionation of REEs during the extreme aqueous alteration of basalts.

Mobilization and redistribution of rare earth elements during extreme weathering of basalt

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