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 = AFBw/AFBp 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 AFBu 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 POINTS
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.

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.

基于全岩化学成分的化学风化指数（Chemical Weathering Indices, CWIs）可为风化强度估算与风化产物分类提供客观工具。然而，尽管这类指数发展历史悠久、现有种类繁多，其实际应用仍存在显著局限。为克服多数现存局限，本文通过梳理风化过程中的地球化学与岩石矿物演化历程，并对现有CWIs开展系统分析，提出了一种新型风化指数。该新型指数即氧化铝+氧化铁与碱组分之比（AFB），其表达式为AFB = AFBw/AFBp，其中AFB = (Al₂O₃+Fe₂O₃)/(K₂O + Na₂O + CaO + MgO)；w代表风化岩石，p代表母岩，所有元素均以分子比例计。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。其可响应所有风化阶段，包括通过稳定（相对不活动）金属氧化物与活动金属氧化物之比来反映红土化作用。研究依托涵盖全球范围内40个记录完整的、发育于长英质、基性及超基性基质上的风化壳剖面的大型化学数据库，对所提出的AFB指数进行了测试，并与主流常用指数开展对比。测试所用岩石大多形成于热带或亚热带风化环境。结果证实，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石类型，包括贫铝超基性岩。采用母岩归一化方法，可更精准地实现不同岩性间风化强度的对比。但该优势的代价是，母岩归一化AFB指数需要获取未风化原岩的数据，而这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AFBu指数应用场景更广，可用于沉积物、土壤的物源分析与风化历史研究，以及工程相关研究，但仍需收集更多数据以明确该指数在不同条件与岩石类型下的适用限制。

核心要点
1. 一种新型化学风化指数，即归一化至母岩的氧化铝+氧化铁与碱组分之比，可对所有主要类型的硅酸盐岩性给出合理的计算结果。
2. 该指数适用于所有类型的风化作用，包括红土化环境。
3. 但该指数也存在局限性：对原始基质的非均质性以及残余风化壳的后期后生改造作用较为敏感。