The weathering intensity scale (WIS): An alternative approach of the Chemical Index of Alteration (CIA)
The weathering intensity scale (WIS): An alternative approach of the Chemical Index of Alteration (CIA)
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DOI：
10.2475/02.2013.03
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发表时间：
2013-02
影响因子：
2.9
通讯作者：
A. Meunier;L. Caner;F. Hubert;A. El Albani;D. Prêt
中科院分区：
文献类型：
--
作者：
A. Meunier;L. Caner;F. Hubert;A. El Albani;D. Prêt
化学蚀变指数（CIA）是计算风化强度的一种工具（Neslaud和Young，1982年）。它已被广泛用于重建地球上过去不同时期的气候，并从页岩和杂砂岩地层系列中确定沉积物源岩。然而，由于它会引起一些不确定性，我们提出了一种新的方法，基于M+-4Si-R2+系统（M+ = Na+ + K+ +2Ca 2 +; 4Si = Si/4; R2+ = Fe 2 + + Mg 2+），考虑到二氧化硅。在这些坐标中，风化花岗岩、镁铁质和超镁铁质岩石的化学成分确定了明显分离的趋势，这些趋势都向4Si极会聚，即高岭石成分（绿泥石图靠近R2+极）。因此，给定母岩的蚀变强度可以通过其化学成分向高岭石极的迁移来测量：Δ 4Si %= [（4Si蚀变样品-4Si未蚀变母岩）× 100]/（100 - 4Si未蚀变母岩）。风化的最终阶段导致不溶的R3+组分（R3+ = Al 3 + + Fe 3+）的逐渐积累。它是在铝土矿中获得的，其中高岭石被三水铝石取代。该比值随风化程度的增加而变化，即可溶组分（M+和R ~（2+））的淋溶、Fe ~（2+）的氧化和残余组分（Al ~（3+）、Fe ~（3+））的富集。由于氢氧化物不能在M+-4Si-R2+系统中表示，因此包括最终铝土矿阶段的风化强度等级（WIS）必须基于Δ 4Si %参数和R3+/（R3+ + R2+ + M+）比率的共变。成岩伊利石化作用对沉积物组成的影响（所谓的钾交代作用）很难测量，因为沉积物中的K2 O含量可能因碎屑钾矿物的继承而变化很大。尽管存在这种变化，但如果给定系列中页岩的成分在M+-4Si-R2+坐标中朝向成岩伊利石极（[Si3.30Al0.70] O 10（Al1.78Fe3+0.05Mg0.17）（OH）2K0.87），则需要进行校正。假设原始沉积物（K2 O沉积物）的K2 O百分比等于K2 O百分比最低的页岩样品的K2 O百分比，计算了墨西哥湾沿岸页岩系列的伊利石含量：（K2 O页岩-K2 O沉积物）× 100/9.9。校正修改了所有元素的量，而不仅仅是钾。因此，用于校正的组成的M+、4Si和R2+参数的归一化值被改变，并且Δ 4Si %参数被修改。WIS已测试新元古代页岩杂砂岩系列（米尔巴特组，阿曼），其中沉积物已沉积在对比的气候条件（冰期和间冰期）。Δ 4 Si %参数沿地层桩沿着的变化表现出大幅度和低幅度的振荡。前者与CIA给出的数据一致，可以对应于全球条件（温度、纬度）的变化，而后者可以指示更多局部条件（降水、海拔、剥蚀、径流）的变化。
The chemical index of alteration (CIA) is a tool to calculate the weathering intensity (Nesbitt and Young, 1982). It has been largely used to reconstitute the past climates on Earth at different epochs and to determine the sediment source rocks from shale and graywacke stratigraphical series. However, because it induces some uncertainties, we propose a new approach based on the M+-4Si-R2+ system (M+ = Na+ + K+ + 2Ca2+; 4Si = Si/4; R2+ = Fe2+ + Mg2+) that takes silica into account. In these coordinates, the chemical compositions of the weathered granitic, mafic and ultramafic rocks determine clearly separated trends which all converge toward the 4Si pole, namely the kaolinite composition (chlorites plot near the R2+ pole). Consequently, the alteration intensity for a given parent rock can be measured by the migration of its chemical composition toward the kaolinite pole: Δ4Si% = [(4Sialtered sample − 4Siunaltered parent rock) × 100]/(100 − 4Siunaltered parent rock). The ultimate stage of weathering leads to the progressive accumulation of the insoluble R3+ components (R3+ = Al3+ + Fe3+). It is attained in bauxite deposits where kaolinite is replaced by gibbsite. This ratio varies from 0 to 1 with increasing weathering degree, namely the leaching of soluble components (M+ and R2+), the oxidation of Fe2+ and the concentration of the residual ones (Al3+, Fe3+). Because hydroxides cannot be represented in the M+-4Si-R2+ system, the Weathering Intensity Scale (WIS) including the ultimate bauxite stage must be based on the co-variation of the Δ4Si% parameter and the R3+/(R3+ + R2+ + M+) ratio. The effects of the diagenetic illitization on the composition of sediments (so-called K-metasomatism) are difficult to measure since the K2O amount of sediments may be highly variable due to the inheritance of detrital potassic minerals. In spite of this variability, if the compositions of the shales in a given series are aligned toward the diagenetic illite pole ([Si3.30Al0.70]O10(Al1.78Fe3+0.05Mg0.17)(OH)2K0.87) in the M+-4Si-R2+ coordinates, a correction is required. The illite amount has been calculated for the Gulf Coast shale series assuming that the K2O percent of the original sediment (K2Osediment) is equal to that of the shale sample having the lowest K2O%: (K2Oshale − K2Osediment) × 100/9.9. The correction modifies the amounts of all the elements and not only that of potassium. Consequently, the normalized values of the M+, 4Si, and R2+ parameters for the corrected composition are changed and the Δ4Si% parameter modified. The WIS has been tested on a Neoproterozoic shale-graywacke series (Mirbat Group, Oman) in which the sediments have been deposited in contrasted climatic conditions (glacial and interglacial). The variation of the Δ4Si% parameter along the stratigraphic pile exhibits large and low magnitude oscillations. The former are coherent with that given by the CIA and could correspond to changes in global conditions (temperature, latitude) while the second could indicate changes in more local ones (precipitation, elevation, denudation, runoff).