Clays and Clay Minerals, Vol. 57, No. 3, 371–382, 2009.
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Authors
Inoue, Atsuyuki
Meunier, Alain
Patrier-mas, Patricia
Rigault, Cecile
Beaufort, Daniel
Vieillard, Philippe
Issue Date
2009
Type
Article
Language
Keywords
Periodicals , Geology , Chlorite , Diagenesis , Ferric Iron , Geothermometry , Hydrothermal Alteration , Lowgrade Metamorphism , Mossbauer Spectroscopy , Solid Solution
Alternative Title
Application Of Chemical Geothermometry To Low-temperature Trioctahedral Chlorites
Abstract
Low-temperature chlorites formed in diagenetic to low-grade metamorphic environments generally have greater Si contents and larger numbers of octahedral vacancies, and smaller Fe+Mg contents than higher-grademe tamorphic chlorites. Theco mpositional variations arech aracterized approximately by four end-member components: Al-free trioctahedral chlorite, chamosite, corundophilite, and sudoite. The solid solution is considered to be a random mix of cations and vacancies in the octahedral sites. Using the compositions of chlorites from Niger, Rouez, and Saint Martin diagenetic-hydrothermal series, a new, more convenient geothermometer, applicable to low-T chlorites is proposed and comparison made with geothermometers proposed previously. The chlorites studied contain appreciable amounts of Fe(III) (>14% of the total Fe), determined by Mo¨ ssbauer spectroscopy. The calculations under which all Fe was regarded as ferrous gave considerable overestimates for the formation temperature, irrespective of the geothermometer used. This problem was reduced by taking into account the presence of Fe(III) in the octahedral sites. The geothermometer from this study gave more reasonable estimates than the geothermometers proposed by Walshe (1986) and Vidal et al. (2001), particularly in theca se of the Niger chlorites which crystallized in the lowest-temperature conditions. The ordered-site substitution model of solid solution developed by Vidal et al. (2001) predicted satisfactorily the formation temperature of the Rouez chlorites and of someo f theSaint Martin chlorites, suggesting that thec hloritec ompositions are controlle d by the Si&R2+ -2 exchange at low-T conditions while they are controlled by Tschermak exchange at higher temperatures. The decreasing number of vacancies with temperature are poorer in Fe-rich than in Fe-poor chlorites. Furthermore, the ordered-site occupation of cations and vacancies in trioctahedral chlorite occurs concomitantly with the compositional changes ruled by increasing temperature conditions.
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Citation
Clays and Clay Minerals, Vol. 57, No. 3, 371–382, 2009.
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