%0 Journal Article
%T Fluorine-controlled composition of biotite in granulites of Madagascar: the effect of fluorine on thermometry of biotite-garnet gneisses
%+ Géosciences Environnement Toulouse (GET)
%A Moine, Bernard
%A de Parseval, Philippe
%A Rakotondrazafy, Michel
%A Ramambazafy, Andrianasolo
%< avec comité de lecture
%J Contributions to Mineralogy and Petrology
%V 174
%8 2019
%D 2019
%Z 2019CoMP..174...75M
%R 10.1007/s00410-019-1607-1
%K Mg-F affinity
%K Biotite-garnet thermometry
%K Fluorine effect
%K Microprobe F analysis
%K Granulites
%K Madagascar
%Z Sciences of the Universe [physics]Journal articles
%X Because of the strong preference of fluorine (F) for "hydrous" magnesian silicate minerals, the temperature T(0F) given by the reaction (Bi-Gt) of Fe-Mg exchange between biotite and garnet [phlogopite (Phl) + almandine (Alm) = annite (Ann) + pyrope (Py)] can be dramatically underestimated when the reciprocal effects between octahedral and hydroxyl site occupancies in biotite are ignored. In the granulites of southeastern Madagascar, widespread Mg metasomatism was associated with F-rich fluids. It is mainly manifested by the regional occurrence of diopside skarns hosting F-phlogopite and thorianite mineralisation. At first, biotite- and garnet-bearing granites and leucogneisses appear unaffected by this event. However, their biotites are rich in F [up to 6 wt%; X _{F} = X _{F}/(F + OH) = 0.69) and in Mg ( X _{Mg} = Mg/(Mg + Fe) up to 0.8], which shows the tendency of equilibrium with the regional fluids. This equilibration leads to low values of Fe/Mg, which in turn cause strong underestimation of the temperature: T(0F) as low as 500 °C, whereas the temperature T(Ti) given by the TiO_{2} content of biotite is approximately 750 °C. The slightly modified thermodynamic model of Zhu and Sverjensky (Geochim Cosmochim Acta 56:3435-3467, 1992) for reciprocal solid solution is applied to a set of 19 samples to correct this underestimation and to quantify the effect of F on thermometry. The reciprocal activity coefficients for phlogopite ( λ _{(Phl)}) and annite ( λ _{(Ann)}) calculated by the model are added to the winTWQ software to obtain corrected values T(F) of the temperature. Similar values of Δ T = T(F) - T(0F) are obtained by (1) the linear fit of the whole set of the samples and (2) RTln K _{ λ } = RTln λ _{(Ann)} - RTln λ _{(Phl)} = 33.417 X _{F} - 0.1092 ( r ^{2} = 0.9994) equation applied to a median sample: Δ T = 30-40, 60-70, 95-100, 160, and 200 °C for X _{F} = 0.1, 0.2, 0.3, 0.5, and 0.7, respectively. The same calculations based on the Thermocalc thermodynamic data set lead to values of Δ T 1.5 times higher than those given by winTWQ. Thus, the results of these calculations confirm (for the first time from a set of natural samples) the validity of the thermodynamic model of reciprocal solid solution and the necessity of accounting for F in thermometry based on Fe-Mg exchanges involving biotite.
%G English
%L insu-03669348
%U https://insu.hal.science/insu-03669348
%~ INSU
%~ METEO
%~ UNIV-TLSE3
%~ CNRS
%~ CNES
%~ OMP
%~ OMP-LMTG
%~ OMP-GET
%~ UNIV-UT3
%~ UT3-INP
%~ UT3-TOULOUSEINP