Chlorite Geothermometry: A Review

Abstract

Abstract--Chlorite minerals, found in a great variety of rocks and geological environments, display a wide range of chemical compositions and a variety of polytypes, which reflect the physicochemical conditions under which they formed. Of particular importance for studies dealing with ore deposit genesis, metamorphism, hydrotherrnal alteration or diagenesis is the paleotemperature of chlorite crystallization. However, in order to understand the relationship between chlorite composition and formation temperature and hence use chlorite as a geothermometer, one must determine how other parameters influence chlorite composition. These parameters may include fO2 and pH of the solution and Fe/(Fe + Mg) and bulk mineral composition of the host rock.Four approaches to chlorite geothermometry, one structural and three compositional, have been proposed in the past: 1) a polytype method based on the (largely qualitative) observation that structural changes in chlorite may be partly temperature-dependent (Hayes, 1970); 2) an empirical calibration between the tetrahedral aluminum occupancy in chlorites and measured temperature in geothermal systems (Cathelineau, 1988), which has subsequently been modified by several workers; 3) a six-component chlorite solid solution model based upon equilibrium between chlorite and an aqueous solution, which uses thermodynamic properties calibrated with data from geothermal and hydrothermal systems (Walshe, 1986); and 4) a theoretical method based on the intersection of chlorite-carbonate reactions and the CO2-H20 miscibility surface in temperature-XCO2 space, which requires that the composition of a coexisting carbonate phase (dolomite, ankerite, Fe-calcite or siderite) be known or estimated (Hutcheon, 1990). These four approaches are reviewed and the different calculation methods for the compositional geotherrnometers are applied to a selection of chlorite analyses from the literature. Results of this comparative exercise indicate that no single chlorite geothermometer performs satisfactorily over the whole range of natural conditions (different temperatures, coexisting assemblages, Fe/(Fe + Mg), fO2, etc.).. Therefore, chlorite geothermometry should b e used with caution and only in combination with alternative methods of estimating paleotemperatures.

Exhumation paths of high‐pressure metapelites obtained from local equilibria for chlorite–phengite assemblages

“…the proportion of Si and Fe Mg) increases with pressure. Similar trends are observed in chlorites (Cathelineau and Nieva 1985;Hillier and Velde 1991;De Caritat et al 1993), which correspond to a decrease of the amesite and an increase of the sudoite proportions with decreasing temperature (increase in Si, Al VI and (&, and decrease in Al IV and Fe Mg)). The pressure effect corresponds to a Si, (Fe Mg) decrease and an increase of Al IV , Al VI and & (Leoni et al 1998;Vidal et al 2001).…”

“…In white micas, additional substitutions are the paragonitic (PG: K À1 Na), the Al À1 Fe 3 , and the pyrophyllitic (P: KAlSi À1 & À1 ; Merino and Ransom 1982;Wang and Banno 1987;Leoni et al 1998;Guidotti and Sassi 1998) substitutions. The extent of these substitutions depends on the metamorphic conditions as well as the rock chemistry or, at ®xed P and T conditions, on the mineralogical assemblage (Jenkins and Chernosky 1986;McPhail et al 1990;De Caritat et al 1993;Massonne and Szputka 1997;Leoni et al 1998;Agard et al 2000). These P±T dependencies lead to the formation of Si-rich and (K Na)-de®cient phengites containing a signi®cant proportion of pyrophyllite end-member (i.e.…”

“…Moreover, octahedral contents lower than six are also observed in low-temperature chlorites devoid of signi®cant smectite or mica contamination (Leoni et al, 1998 and references therein). Therefore, chlorite octahedral vacancies calculated from EMPA analyses with the assumption that Fe 2 Fe total should be a real feature, at least at low-T, low-P conditions (Mc Dowell and Elders 1980;Cathelineau and Nieva 1985;Cathelineau and Izquierdio 1988;Hillier and Velde 1991;De Caritat et al 1993;Leoni et al 1998). In contrast, Guidotti et al (1994c) showed that the proportion of Fe 3 substituting for Al in mica is controlled by fO 2 (Fe 3 is minimal in rocks containing graphite).…”

“…For these reasons, chlorite and phengite are good candidates for thermobarometric estimates. Metamorphic pressures are often estimated from the Si content of phengites, and temperatures from the Al IV content of chlorites in diagenetic regimes (`375 C) (Cathelineau and Nieva 1985;Kranidiotis and MacLean 1987;Cathelineau and Izquierdio 1988;Jowett 1991;De Caritat et al 1993). However, systematic P±T estimates based on the compositions of associated chlorites and phengites have not been undertaken, although the compositional evolution of both minerals seems to be correlated with the metamorphic conditions (Worley et al 1997;Leoni et al 1998).…”

Gênese do depósito de ouro Tocantinzinho, província aurífera Tapajós (PA) : evidências a partir de dados de geologia, petrologia e inclusões fluidas

“…Segundo Goldstein & Reynolds (1994), a Te é normalmente difícil de ser distinguida e o nível de dificuldade aumenta muito para inclusões menores que 10 µm e para inclusões pouco salinas (< 5% em peso de NaCl eq.). Porém, estudos de inclusões fluidas no Depósito estrutura da clorita (Cathelineau 1988, Caritat et al 1993.…”