Behaviour and Cycling of Neodymium, Strontium, Lithium and their Isotopes during Basalt Weathering

Sharma, Anup Kumar (2025) Behaviour and Cycling of Neodymium, Strontium, Lithium and their Isotopes during Basalt Weathering. PhD thesis, Indian Institute of Science Education and Research Kolkata.

Text (PhD thesis of Anup Kumar Sharma (18RS032)) 18RS032.pdf - Submitted Version Restricted to Repository staff only Download (16MB)

Abstract

Over the past decades, several studies have focused on developing reliable elemental and isotopic proxies to investigate modern and past weathering records. Assessing the robustness of isotopic composition of any element as a tracer of weathering processes necessitates a detailed understanding of the elemental behaviour and isotopic distribution among various phases during weathering and transport. In this thesis, we develop a comprehensive framework to evaluate the impact of weathering and post-weathering processes on the geochemical cycling of Nd, Sr, Li and their isotopes. This thesis presents a comprehensive study on Nd, Sr, Li and their isotope through the analysis of chemical composition of bulk phases, as well as chemically extracted exchangeable and oxyhydroxide phases, from two weathering profiles (the Dalahi and the Pakuria) developed on basalts of the Rajmahal Volcanic Province (RVP) in India, after a thorough evaluation of these weathering profiles for their preservation state and potential external contamination. The major element concentrations in the Dalahi and Pakuria profiles clearly document progressive chemical weathering. Based on variations in [Sm], [Nd] and εNd as a function of chemical index of alteration (CIA), we establish that preferential release of radiogenic neodymium (¹⁴³Nd) results from differential weathering of minerals. We estimate that up to 40% of Nd is lost from the parent basalts during weathering. The εNd values of the weathering solutions, calculated based on mass balance considerations, are ~5.0 units higher than that of the parent basalts. Our results also unequivocally demonstrate that the degree of radiogenic Nd release is positively correlated with CIA. Additionally, our study shows that the εNd value of oxyhydroxide phases are higher compared to the saprolites, however, they do not consistently record the εNd released to the weathering solution due to complex mechanisms of Nd adsorption that differ across the profiles. Combining these observations, we hypothesize that riverine particulate εNd is sensitive to adsorption of dissolved Nd and would be regulated by the duration of water-particle interaction during transport. Therefore, we predict that particulate εNd would be different between the high and low discharge periods in a river catchment. Utilising published data from fourteen rivers worldwide, spanning diverse lithologies and climatic regimes, we demonstrate that differential release of Nd isotopes due to incongruent weathering and adsorption of dissolved Nd, collectively modulate particulate εNd through the length and duration of river transport on a global scale. An important implication of this study is that the εNd offset between the leachates and detrital phases of marine sediments can serve as a robust tracer for investigating weathering records of glacial-interglacial periods. The upward decrease in Sr/Al ratios and its significant negative correlation with CIA suggest loss of Sr from the profiles as a result of progressive chemical weathering. Additionally, the upward increasing trends of Rb/Al and Rb/Sr ratios, along with positive correlations of Rb/Sr with CIA, suggest a preferential loss of Sr over Rb. Mass balance calculations show that up to 70% and 85% Sr is lost from the Dalahi and Pakuria profile, respectively. Our results on Sr and its isotopic composition in the exchangeable and oxyhydroxides phases suggest preferential incongruent release of radiogenic Sr isotope during weathering. Additionally, we show that the ⁸⁷Sr/⁸⁶Sr ratio of residual phases are lower by up to 0.0012 compared to the bulk ⁸⁷Sr/⁸⁶Sr ratio. Furthermore, the radiogenic Sr released to the weathering solution is efficiently captured by secondary weathering products such as clay minerals and oxyhydroxides within the weathering profiles. Our results unambiguously demonstrate that the degree of preferential incongruent release of radiogenic Sr isotope scales with weathering intensity. The increasing release of more radiogenic Sr isotope at greater weathering intensity is supported by significant positive correlations between ⁸⁷Sr/⁸⁶Sr ratios of exchangeable and oxyhydroxide phases with CIA. These observations, together with published data on soils and weathering profiles, allow us to suggest that the ⁸⁷Sr/⁸⁶Sr differences between bulk phases and parent rock can be used as a proxy of silicate weathering, except in cases where the saprolites are influenced by external inputs such as marine aerosol and dust. The upward increasing Li/Al ratios in the profiles suggest Li enrichment through its sorption. Significant positive correlations of Li/Al with Mn/Al, Cs/Al, Fe/Al and Al/Si provide further support to the notion of Li sorption onto clay minerals and Fe-Mn oxyhydroxides. The δ⁷Li values exhibit a decreasing upward variation in the profiles, suggestive of preferential adsorption and incorporation of ⁶Li into these secondary phases, further corroborated by its inverse correlations with Al/Si, Cs/Al, and [Mn] in adsorbed phases. In the Dalahi profile, δ⁷Li, adsorbed Li (%), and pH decrease in the top section but increase in the underlying layer, presumably due to preferential ⁷Li desorption from interlayer sites of clay minerals and its subsequent re-adsorption onto clay minerals and Fe-Mn oxyhydroxides in the underlying layer as ⁷Li-rich porewater migrates downward. The pH- dependent desorption of Li from the top section is further supported by negative correlations of [Li] with [Ba] in adsorbed phases. The fractionation factors (α) modeled for Li sorption onto secondary phases is consistent with a batch fractionation model with apparent fractionation factors ranging from 0.971 to 0.998. The δ⁷Li values of the weathering solutions, calculated based on two independent approaches, are 8.1‰-12.6‰ higher than that of parent basalts and are similar to those reported for rivers and groundwaters draining the basaltic rocks. Our results suggests that mobility of Li relative to Ca and Mg decreases as the weathering intensity increases and that the relative mobility of Li can be predicted by using the CIA values during basaltic weathering. This suggests that δ⁷Li values can serve as a tracer for Ca and Mg mobility during basaltic weathering, which have important implications for CO₂ drawdown during chemical weathering. The results of this study and their evaluation provide important information on behaviour of Sr, Nd, Li and their isotopes during weathering and post-weathering processes. Furthermore, these results provide ways to utilize the isotope compositions (of Nd, Sr) to trace weathering and transport of rocks and weathered products and to apply Li isotope composition as a proxy to determine relative mobility of Li, Ca and Mg during weathering and transport as a function of weathering intensity.

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