Quantifying weathering intensity using chemical proxies: a weathering index AFB

Chemical weathering indices (CWIs) based on bulk chemical rock composition can potentially provide an objective tool for estimation of weathering intensity and classification of weathering products. However, despite their long history and the large number of indices that have now been developed, their applicability still has serious limitations. To overcome most of the limitations, this paper proposes a new weathering index based on the review of geochemical and mineral rock evolution during weathering and analysis of the existing CWIs. The new index, the alumina + ferric oxide to bases ratio (AFB), is expressed as AFB = AFB_w/AFB_p where AFB = (Al₂O₃+Fe₂O₃)/(K₂O + Na₂O + CaO + MgO); w, weathered rock and p, parent rock, with all elements in molecular proportions. The index provides a basis for comparison of weathering intensities between different lithologies by linking the chemical and mineral transformations that characterise the regolith profile. It is sensitive to all stages of weathering, including lateritisation by using the ratio of the stable (relatively immobile) to mobile metals expressed as oxides. An extensive, worldwide chemical database on 40 well-documented regolith profiles developed on felsic, mafic and ultramafic substrate was used for testing the proposed index and its comparison with the most common indices. The rocks were mostly weathered under tropical or subtropical conditions. The results confirmed consistent increase in the AFB value with intensifying weathering. The index is applicable to all silicate rock types including Al-poor ultramafic rocks. Use of parent rock normalisation allows a more accurate comparison of weathering intensities between different lithologies. At the expense of these benefits, the parent normalised index AFB requires data for the unweathered protolith that is commonly difficult to obtain. The index is also sensitive to inhomogeneity of the original rocks. The unreferenced to parent rock AFB_u index has potentially broader applications including provenance and the weathering history of sediments, soil and engineering studies, although collection of more data is required for understanding the index constraints for various conditions and rock types. KEY POINTSA new chemical weathering index, expressed as the alumina+ferric oxide to bases ratio normalised to parent rock provides reasonable results for all major types of silicate lithologies.The index is applicable for all types of weathering including the lateritic environment.On the downside, the index is sensitive to inhomogeneity of the original substrate and to later epigenetic modifications of the residual regolith. A new chemical weathering index, expressed as the alumina+ferric oxide to bases ratio normalised to parent rock provides reasonable results for all major types of silicate lithologies. The index is applicable for all types of weathering including the lateritic environment. On the downside, the index is sensitive to inhomogeneity of the original substrate and to later epigenetic modifications of the residual regolith.化学风化指数（Chemical Weathering Indices, CWIs）基于全岩化学组成构建，可提供一种用于估算风化强度、划分风化产物的客观工具。然而，尽管该类指数发展历史悠久、现有种类繁多，但其实际应用仍存在诸多严重局限。为克服多数局限，本文在梳理风化过程中地球化学与岩石矿物演化规律、分析现有化学风化指数的基础上，提出了一种新型风化指数。该新型指数为氧化铝+三氧化二铁与碱质组分比值（Alumina + Ferric Oxide to Bases Ratio, AFB），其计算公式为：AFB = AFB_w / AFB_p，其中AFB = (Al₂O₃ + Fe₂O₃)/(K₂O + Na₂O + CaO + MgO)；下标w代表风化物，p代表母岩，所有元素均以分子摩尔比例计。该指数通过关联表征风化壳剖面的化学与矿物转化过程，为不同岩性间的风化强度对比提供了依据。该指数对所有风化阶段均敏感，通过以氧化物形式表示的稳定（相对不活动）金属与活动金属的比值，可表征红土化作用等过程。 研究采用了覆盖全球的大型化学数据库，该数据集包含40个记录完整的风化壳剖面，其发育于长英质、镁铁质及超镁铁质母岩之上，用于验证所提出的AFB指数，并与主流化学风化指数进行对比。这些剖面中的岩石多在热带或亚热带环境下发生风化。结果表明，AFB值随风化强度增强呈持续升高趋势。该指数适用于所有硅酸盐岩类型，包括贫铝超镁铁质岩石。 采用母岩归一化处理，可实现不同岩性间风化强度的更精准对比。但该优势亦存在代价：经母岩归一化的AFB指数需要获取未风化原岩的数据，而此类数据通常难以获取。此外，该指数对原始岩石的非均质性较为敏感。未进行母岩归一化的AFB_u指数则具备更广泛的应用潜力，可用于沉积物、土壤的物源分析与风化历史研究，以及工程地质相关研究；不过目前仍需收集更多数据，以明确该指数在不同环境与岩石类型下的适用约束。 核心要点 1. 以归一化至母岩的氧化铝+三氧化二铁与碱质组分比值构建的新型化学风化指数，可对所有主要硅酸盐岩岩性给出可靠的分析结果。 2. 该指数适用于所有类型的风化作用，包括红土化环境。 3. 不过该指数也存在局限：其对原始母质的非均质性以及后续叠加的后生改造作用较为敏感。