The weathering intensity scale (WIS): An alternative approach of the chemical index of alteration (CIA)

Abstract

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: 4Si% = [(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 4Si% parameter and the R3/(R3++R2++M) ratio. The effects of the diagenetic illitization on the composition of sediments (socalled 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(Al1.78Fe30.05Mg0.17)(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 4Si% 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 4Si% 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).