Mobilization of rare earth elements during extreme weathering of basalt

Abstract

To investigate the mobility of elements during extreme weathering of basalts, major and trace elements concentrations along with the mineralogical abundance of weathered products, are reported for lateritic profiles developed over the ~118.1 ± 0.3 Ma old Rajmahal Trap basalts in Eastern India. Special emphasis has been given to studying the re-distribution of rare earth elements during extreme chemical weathering of relatively less explored Rajmahal flood basalt, which results in the development of lateritic profiles. Here, we conduct a systematic attempt to decipher the alteration of the composition of laterite developed over the Rajmahal flood basalt.

We demonstrate that the chemical index of alteration (CIA) and the mafic index of alteration (MIA) are less effective in deciphering extreme chemical weathering events, where desilication dominates the weathering process. In the weathered residues, the retention of REEs generally follows the trend of light rare earth elements (LREE) > middle rare earth elements (MREE) > heavy rare earth elements (HREE). Except for La and Ce, all other REEs show a depleting trend with reference to the parent rock composition. The ∑REE did not show any significant relationship with the commonly used weathering indices, which shows that the retention of REEs is not a function of the extent of weathering. The covariation of Ce anomaly with the Fe and Mn distribution in the samples reflects a redox-controlled mechanism. The negative correlation between the Eu anomaly and kaolinite abundance suggests that Eu mobilization from parent rock happens during initial weathering phases when plagioclase weathering dominates. The samples show an interesting positive Gd anomaly, which is explained by enhanced retention of Gd than neighboring Eu and Tb. During the chemical weathering of basalt, the retention of Ho was found to be greater than its geochemical twin Y. The results of this study show that significant differences exist between the REEs' distribution of parent basalt and that of the weathered residue, which has implications for the application of REEs as a provenance indicator and/or weathering proxy. The enrichment of Th and U in the weathered samples is explained through the addition of aeolian dust of upper continental crustal composition.