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Fluorine-controlled composition of biotite in granulites of Madagascar: the effect of fluorine on thermometry of biotite-garnet gneisses
Moine, Bernard
de Parseval, Philippe
Rakotondrazafy, Michel
Ramambazafy, Andrianasolo
Géosciences Environnement Toulouse (GET) ; Institut de Recherche pour le Développement (IRD)-Université Toulouse III - Paul Sabatier (UT3) ; Université de Toulouse (UT)-Université de Toulouse (UT)-Institut national des sciences de l'Univers (INSU - CNRS)-Observatoire Midi-Pyrénées (OMP) ; Institut de Recherche pour le Développement (IRD)-Université Toulouse III - Paul Sabatier (UT3) ; Université de Toulouse (UT)-Université de Toulouse (UT)-Institut national des sciences de l'Univers (INSU - CNRS)-Centre National d'Études Spatiales [Toulouse] (CNES)-Centre National de la Recherche Scientifique (CNRS)-Météo-France-Institut de Recherche pour le Développement (IRD)-Institut national des sciences de l'Univers (INSU - CNRS)-Centre National d'Études Spatiales [Toulouse] (CNES)-Centre National de la Recherche Scientifique (CNRS)-Météo-France-Centre National de la Recherche Scientifique (CNRS)
International audience
Contributions to Mineralogy and Petrology
insu-03669348
https://insu.hal.science/insu-03669348
https://insu.hal.science/insu-03669348
Contributions to Mineralogy and Petrology, 2019, 174, ⟨10.1007/s00410-019-1607-1⟩
BIBCODE: 2019CoMP..174...75M
DOI: 10.1007/s00410-019-1607-1
info:eu-repo/semantics/altIdentifier/doi/10.1007/s00410-019-1607-1
en
Mg-F affinity
Biotite-garnet thermometry
Fluorine effect
Microprobe F analysis
Granulites
Madagascar
[SDU]Sciences of the Universe [physics]
info:eu-repo/semantics/article
Journal articles
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 <SUB>F</SUB> = X <SUB>F</SUB>/(F + OH) = 0.69) and in Mg ( X <SUB>Mg</SUB> = 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<SUB>2</SUB> 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 <SUB>(Phl)</SUB>) and annite ( λ <SUB>(Ann)</SUB>) 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 <SUB> λ </SUB> = RTln λ <SUB>(Ann)</SUB> - RTln λ <SUB>(Phl)</SUB> = 33.417 X <SUB>F</SUB> - 0.1092 ( r <SUP>2</SUP> = 0.9994) equation applied to a median sample: Δ T = 30-40, 60-70, 95-100, 160, and 200 °C for X <SUB>F</SUB> = 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.
2019