Quantifying chemical weathering intensity and trace element release from two contrasting basalt profiles, Deccan Traps, India

Babechuk, M. G.; Widdowson, M. and Kamber, B. S. (2014). Quantifying chemical weathering intensity and trace element release from two contrasting basalt profiles, Deccan Traps, India. Chemical Geology, 363 pp. 56–75.

DOI: https://doi.org/10.1016/j.chemgeo.2013.10.027

Abstract

Weathering profiles developed on basalt substrate contain information relevant to climate, atmospheric composition and evolution, nutrient release into the hydrosphere, and understanding Martian regolith. In this study, the chemical compositions of two profiles developed on Deccan Trap basalt are examined. One is sub-Recent and has only progressed to a moderate degree of alteration (Chhindwara profile), whereas the other is ancient (Paleocene) and the degree of alteration is extreme (Bidar laterite). In an attempt to better quantify the chemical changes during incipient to intermediate weathering of mafic substrates, we propose a new index: the mafic index of alteration (MIA). Similar to the chemical index of alteration (CIA), the MIA quantifies the net loss of the soluble major element cations (Ca, Mg, Na, K ± Fe) relative to the insoluble major elements (Al ± Fe). The redox-dependent weathering behaviour of Fe is factored into two separate arrangements of the MIA that apply to oxidative [MIA_(O)] or reduced [MIA_(R)] weathering. The MIA can be visualised in a variety of ternary diagrams in the Al–Fe–Mg–Ca–Na–K system. To chemically quantify the stages of advanced to extreme weathering, at which the MIA and CIA are ineffective, the SiO₂ to (Al₂O₃+Fe₂O₃) mass ratio, based on the established Si–Al–Fe (SAF) ‘laterite’ ternary diagram, is used; we propose that this ratio be referred to as the ‘index of lateritisation’ (IOL). Major element chemical variations, as expressed by weathering indices, were used to relate the extent of weathering with the behavior of trace elements (alkali, alkaline earth, rare earth, and Nb) in the profiles. During the early stages of basalt weathering, the mobile trace elements (Sr, Be, Li) are anti-correlated with the chemical weathering indices and thus released during these stages. By contrast, the monovalent elements (K, Rb, Cs, Tl), excluding Na and Li, appear to be associated with the pedogenetic clay minerals and the elements with the closest ionic radii are most closely related. Fractionation of the REE (Sm/Nd, Eu/Eu*, Ce/Ce*) is evident during weathering of the basalt. The loss of Eu is linked with Sr, Ca, and Na and thus plagioclase dissolution during the stages of incipient to intermediate weathering. The fractionation of Sm/Nd suggests that basaltic weathering products may not always preserve their parent rock ratio and, consequently, their Nd isotope composition over time. Finally, weathering in the sub-Recent profile is shown to have progressed across two lava flows, whose morphology initially controlled the extent of weathering. Certain compositional variations in the original flows (e.g., immobile element ratios) are preserved through the effects of chemical weathering and have the potential to influence mass balance calculations across the entire profile.

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