A thermodynamic model for di-trioctahedral chlorite from experimental and natural data in the system MgO–FeO–Al2O3–SiO2–H2O: applications to P–T sections and geothermometry

Abstract

We present a new thermodynamic activitycomposition model for di-trioctahedral chlorite in the system FeO-MgO-Al 2 O 3-SiO 2-H 2 O that is based on the Holland-Powell internally consistent thermodynamic data set. The model is formulated in terms of four linearly independent end-members, which are amesite, clinochlore, daphnite and sudoite. These account for the most important crystal-chemical substitutions in chlorite, the Fe-Mg, Tschermak and di-trioctahedral substitution. The ideal part of end-member activities is modeled with a mixing-on-site formalism, and non-ideality is described by a macroscopic symmetric (regular) formalism. The symmetric interaction parameters were calibrated using a set of 271 published chlorite analyses for which robust independent temperature estimates are available. In addition, adjustment of the standard state thermodynamic properties of sudoite was required to accurately reproduce experimental brackets involving sudoite. This new model was tested by calculating representative P-T sections for metasediments at low temperatures (\400°C), in particular sudoite and chlorite bearing metapelites from Crete. Comparison between the calculated mineral assemblages and field data shows that the new model is able to predict the coexistence of chlorite and sudoite at low metamorphic temperatures. The predicted lower limit of the chloritoid stability field is also in better agreement with petrological observations. For practical applications to metamorphic and hydrothermal environments, two new semi-empirical chlorite geothermometers named Chl(1) and Chl(2) were calibrated based on the chlorite ? quartz ? water equilibrium (2 clinochlore ? 3 sudoite = 4 amesite ? 4 H 2 O ? 7 quartz). The Chl(1) thermometer requires knowledge of the (Fe 3? /RFe) ratio in chlorite and predicts correct temperatures for a range of redox conditions. The Chl(2) geothermometer which assumes that all iron in chlorite is ferrous has been applied to partially recrystallized detrital chlorite from the Zone houillère in the French Western Alps.

Hydrothermal Alteration in the Main Sulfide Zone at Unki Mine, Shurugwi Subchamber of the Great Dyke, Zimbabwe: Evidence from Petrography and Silicates Mineral Chemistry

“…However, use of other chlorite thermometry equations suitable for low temperatures [64] indicates that the MSZ chlorites may have been formed at very low to medium temperatures except in a few samples (Figure 20a). This may have happened long after cooling of the intrusion.…”

“…Brey and Kohler (1986) [65] and Putirka (2008) [66]. Lower temperatures for chlorites calculated using the equations of Lanari et al [64] and relatively higher temperatures for chlorites were calculated using the equation of Cathelineau [62]. (a) Plot of pressures obtained from two-pyroxene thermometry of versus height across the MSZ.…”

“…For low temperature chlorites, use was made of a thermodynamic-activity composition model of Lanari et al (2014) [64]. The occurrence of both orthopyroxene and clinopyroxene, as in rocks under investigation, can permit one to calculate both pyroxene thermometry and pyroxene barometry [65,66].…”

Rare Earth Elements in Mineral Deposits: Speciation in Hydrothermal Fluids and Partitioning in Calcite

“…Gibbs energy minimization (GEM) codes such as GEM-Selektor [45] are more ideal to solve complex multicomponent and multiphase systems, which can involve any number of solid solution endmembers and mixing on different crystallographic sites. The GEM-Selektor code package includes several types of nonideal mixing models [44,[50][51][52]. This can be partly related to the larger thermodynamic input data (i.e., Gibbs energy , enthalpy , entropy , heat capacity functions , and volume ) of their thermodynamic databases and to their numerical solver [44,45] in comparison to LMA codes.…”

Outlining the thermal posdepositional evolution of the Ordovician successions of northwestern Argentina by clay mineral analysis, chlorite geothermometry and Kübler index.

“…Moreover this method was severely questioned by different authors (Hillier and Velde, 1991;De Caritat et al, 1993;López Munguira et al, 2002). More recently, several thermodynamicbased chlorite geothermometers for di-trioctahedral chlorite have been proposed (Bourdelle et al, 2013;Lanari et al, 2014;Bourdelle and Cathelineau, 2015).…”