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

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...