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Quantitative determination of the degree of chemical weathering of rocks is a fundamental task in environmental and engineering geology, and many weathering indices based on whole-rock chemistry have been proposed. However, most classical indices are of limited application to granitoids in a wide area, because these lithotypes generally exhibit wide chemical variation arising from their petrogenesis. The chemical evolution produced during rock weathering, therefore, overprints pre-existing magmatic chemical variation. This problem can cause many classical weathering indices to yield misleading results. This study proposes a method that compensates for the influence of petrogenesis on calculation of the weathering index. The method is based on a bivariate plot of the magmatic chemical variation (MCV) in granitoids, and the degree of chemical weathering (DCW). The MCV axis must be based on an element that reflects magmatic processes and is also relatively immobile during rock weathering. In this study TiO2 contents are utilized for the MCV. The DCW axis is fundamentally defined by the ratios of more-mobile to less-mobile elements during weathering, and hence many classical indices can be applied. The improved value of the degree of chemical weathering (DCWi) for a weathered rock is derived by:where MCV1 is the measured composition (e.g. TiO2 content) of the weathered rock. DCW1 denotes the ratios of more-mobile to less-mobile elements of the weathered rock. The “s” parameter is the slope of the least square linear regression for fresh granitoids in the MCV–DCW relationship. MCVCV is a correction factor which is given by the average point on the MCV axis (e.g. average TiO2) of the fresh rocks. This method is useful for evaluating the degree of weathering of various granitoids, and enhances the practical application of many weathering indices. 相似文献
DCWi=s×(MCVCV-MCV1)+DCW1
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N. Assayag D. Jézéquel M. Ader E. Viollier G. Michard F. Prévot P. Agrinier 《Applied Geochemistry》2008
Lac Pavin (French Massif Central) is a permanently stratified lake: the upper water layers (mixolimnion, from 0 to 60 m depth) are affected by seasonal overturns, whereas the bottom water layers (monimolimnion, from 60 to 90 m depth) remain isolated and are never mixed. Hence, they are capable of storing important quantities of dissolved gases, mainly CO2. With the aim of better constraining the water balance and of gaining new insights into the carbon cycle of Lac Pavin, an isotopic approach is used. The δ18OH2O profiles lead the authors to give a new evaluation of the evaporation flow rate (8 L s−1), and to propose and characterize two sub-surface springs. The sub-surface spring located at the bottom of the lake can be deduced from the 1% isotopic difference between the upper water layers (mean δ18OH2O value: −7.3‰) and the bottom water layers (δ18OH2O=-8.4‰). It is argued that this sub-surface spring has isotopic and chemical characteristics similar to those of the magmatic CO2-rich spring (i.e. Fontaine Goyon, δ18OH2O=-9.4‰), and we calculate its flow rate of 1.6 L s−1. The second sub-surface spring is located around 45 m depth, with a composition close to those of the water surface streams (δ18OH2O<-7.6‰). 相似文献
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《Applied Geochemistry》2005,20(10):1920-1940