Application of chemical geothermometry to low-temperature trioctahedral chlorites
Inoue, Atsuyuki (Chiba University. Department of Earth Sciences)
Meunier, Alain (Université de Poitiers)
Patrier-Mas, Patricia (Université de Poitiers)
Rigault, Cecile (Université de Poitiers)
Beaufort, Daniel (Université de Poitiers)
Vieillard, Philippe (Université de Poitiers)

Data:	2009
Resum:	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-grademetamorphic chlorites. Thecompositional variations arecharacterized 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 Mössbauer 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 the case 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 s, suggesting that thechloritecompositions are controlle d by the Si& R 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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Llengua:	Anglès
Document:	Article ; recerca ; Versió publicada
Matèria:	Chlorite ; Diagenesis ; Ferric Iron ; Geothermometry ; Hydrothermal Alteration ; LowgradeMetamorphism ; Mössbauer Spectroscopy ; Solid Solution
Publicat a:	Clays and Clay Minerals, Vol. 57, Núm. 3 (2009) , p. 371-382, ISSN 1552-8367

DOI: 10.1346/CCMN.2009.0570309

12 p, 1.1 MB  Accés restringit a la UAB

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