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

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 mobile major elements (Ca, Mg, Na, K ± Fe) relative to the immobile 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 anticorrelated 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. Of these elements, those 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. CHEMGE6928 highlights New chemical weathering index is introduced: the mafic index of alteration (MIA)  Si:(Si+Fe+Al) ratio to quantify advanced weathering (the index of lateritisation)  Separate lava flows in a weathering...

87Sr/86Sr and trace element mapping of geosphere-hydrosphere-biosphere interactions: A case study in Ireland

“…Independently, the REE have emerged as a powerful tracer of water source, water mixing, and aqueous processes (e.g. Noack et al, 2014;Pourret et al, 2010;Tweed et al, 2006), in addition to chemical weathering process and soil formation (Babechuk et al, 2014;Condie, 1991;Hu et al, 2006;Laveuf and Cornu, 2009;Nesbitt, 1979), and studies of plant material (Brioschi et al, 2013;Miao et al, 2011;Squadrone et al, 2017;Thomas et al, 2014;Tyler, 2004).…”

REE geochemical characteristics of titanium-rich bauxites: the Permian Kanigorgeh horizon, NW Iran

“…There are also many available publications concerning distribution and behavior of major elements, trace elements, and REEs in various profiles of bauxite deposits and/or prospects (e.g., Temur et al, 2005;Temur, 2006;Calagari and Abedini, 2007;Laskou andEconomouEliopoulos, 2007, 2013;Ma et al, 2007;Mameli et al, 2007;Karadağ et al, 2009;Calagari et al, 2010;Hatipoğlu et al, 2010;Gamaletsos et al, 2011;Hatipoğlu, 2011;Wang et al, 2012;Abedini and Calagari, 2013a, 2013b, 2013cGu et al, 2013;Hanilçi, 2013;Liu et al, 2013;Wang et al, 2013;Wei et al, 2013;Zhang et al, 2013;Babechuk et al, 2014;Giorgis et al, 2014;Mongelli et al, 2014). The results of these investigations show that consideration of factors controlling the distribution and behavior of REEs during bauxitization processes is one of more interesting geochemical subjects for further research works.…”

Copper Isotope Compositions of Cenozoic Mafic-Intermediate Rocks of the Northern Great Basin and Snake River Plain (Usa)

“…Most igneous rocks will have a CIA value between 35 and 50, mafic rocks tending to be on the lower end of this spectrum (Babechuk et al, 2014). During chemical weathering processes the values of the weathering index will increase due to the loss of mobile elements relative to immobile elements (Babechuk et al, 2014).…”

“…The procedure used to calculate both the chemical index of alteration (CIA) and mafic index of alteration (MIA) parallels that used by Babechuk et al (2014 where CaO* represents only the silicate bound Ca and is therefor corrected for the presence of carbonate and apatite (Fedo et al, 1995). …”

“…The MIA is a chemical weathering index derived from the CIA and expanded to include the mafic elements Mg and Fe (Babechuk et al, 2014). There are two different equations for the MIA: 1) when the alteration environment is oxidizing and 2) when the alteration environment is reducing.…”