The chemical index

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Palaeogeography, Palaeoclimatology, Palaeoecology 293 (2010) 175–183

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The applicability of the Chemical Index of Alteration as a paleoclimatic indicator: An example from the Permian ofthe Paraná Basin, Brazil
Karin Goldberg a,⁎, Munir Humayun b
a b

Universidade Federal do Rio Grande do Sul (UFRGS), Programa de Geologia do Petróleo, Av. Bento Gonçalves, 9500 Prédio 43137 Sala 203 Porto Alegre, RS 91509-900, Brazil Florida State University, National High Magnetic Field Laboratory & Department of Geological Sciences, 1800 E. Paul Dirac Drive, Tallahassee, FL 32310, USA

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The Chemical Index of Alteration (CIA) was expressed as CIA (molar) to become a more sensitive measure of the degree of chemical weathering. The CIA (molar) has the value of 1 for fresh feldspars and for unweathered, non-peraluminous igneous rocks, but increases towards infinity as chemical weathering progresses. The utility of this index as a humidityindicator for both modern and ancient muds was tested. A database comprising 281 entries with chemical data for modern and ancient sediments deposited under arid, semi-arid, semi-humid and humid climate was assembled from the literature, and used to calibrate the relationship between humidity and CIA (molar) values. For both modern and ancient sediments, the histograms display a shift towards highervalues with increasing geological indicators of humidity, whereas sediments deposited under conditions inferred to be arid from other geological indicators show values tightly clustered around 1. The Permian of the Paraná Basin was then used as an example of the applicability of the CIA (molar) for paleoclimatic studies. Major element analyses (Al2O3, K2O, Na2O, CaO) were performed on 55 claystonesand shales from three drill cores in a stratigraphic succession spanning the Early to Late Permian. The CIA (molar) for these rocks accorded with expectations based on sedimentologic and paleontologic evidence, discriminating well between arid and humid conditions in the basin independently determined from sedimentological and paleontological indicators. The use of the CIA (molar) as apaleohumidity indicator, however, can be limited by the presence of carbonate-rich sediments, the occurrence of post-depositional K+ addition (metasomatism, metamorphism, diagenetic illitization), as well as by inheritance of clays from sedimentary rocks in the source area. Nevertheless, with appropriate caution, the CIA (molar) is a useful tool for the assessment of the humidity conditions in the rockrecord. © 2010 Elsevier B.V. All rights reserved.

Article history: Received 22 May 2009 Received in revised form 27 April 2010 Accepted 25 May 2010 Available online 1 June 2010 Keywords: Paleoclimatology Weathering Permian Paraná Basin Chemical index of alteration

1. Introduction The Chemical Index of Alteration (CIA) was proposed by Nesbitt and Young (1982) as a measure of the role played bychemical weathering in the production of clastic sediments. The ratio CIA = (Al2O3/Al2O3 + CaO* + Na2O + K2O) × 100 (where CaO* is the calcium content of silicates) is based on the assumption that the dominant process during chemical weathering is the degradation of feldspars and the formation of clay minerals. The importance of chemical weathering increases proportionately under humid conditions,with leaching of the alkalis (Na+, K+) and Ca+ 2, and concentration of Al and Si in the residue. Conversely, in an environment dominated by physical weathering, abrasion is the primary producer of sediments by mechanical breakdown into smaller grain sizes. The chemical alteration of feldspars to form clay minerals is thus negligible under physical weathering.

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