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Fluorine-controlled composition of biotite in granulites of Madagascar: the effect of fluorine on thermometry of biotite-garnet gneisses

Abstract : 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 TiO2 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.
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Submitted on : Monday, May 16, 2022 - 3:27:56 PM
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Bernard Moine, Philippe de Parseval, Michel Rakotondrazafy, Andrianasolo Ramambazafy. Fluorine-controlled composition of biotite in granulites of Madagascar: the effect of fluorine on thermometry of biotite-garnet gneisses. Contributions to Mineralogy and Petrology, 2019, 174, ⟨10.1007/s00410-019-1607-1⟩. ⟨insu-03669348⟩



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