Experimental study of fluorine and chlorine contents in mica (biotite) and their partitioning between mica, phonolite melt, and fluid

Abstract

This paper reports the results of a study of the composition of mica (biotite) crystallizing in the system of phonolite melt-Cl- and F-bearing aqueous fluid at T ∼ 850°C, P = 200 MPa, and $$ f_{O_2 } $$ = Ni-NiO, as well as data on F and Cl partitioning between coexisting phases. It was established that Cl content in mica is significantly lower than in phonolite melt and, especially, in fluid. Fluorine shows a different behavior in this system: its content in mica is always higher than in phonolite melt but lower than in fluid. The mica-melt partition coefficients of Cl and F also behave differently. The Cl partition coefficient gradually increases from 0.17 to 0.33 with increasing Cl content in the system, whereas the partition coefficient of F sharply decreases from 3.0 to 1.0 with increasing total F content. The apparent partition coefficients of F between biotite and groundmass (melt) in various magmatic rocks are usually significantly higher than the experimental values. It was supposed that the higher Bt/glassDF values in natural samples could be related to the influence of later oxidation reactions, reequilibration of biotite at continuously decreasing $$ f_{H_2 O} $$ /f HF ratio, and an increase in this coefficients with decreasing total F content in the system.