OH, F AND CL CONTENTS OF TI-RICH BIOTITE FROM METAPELITIC GRANULITES OF THE KERALA KHONDALITE BELT (SE INDIA): FURTHER INSIGHT INTO TITANIUM SUBSTITUTIONS

By a multi-technique approach including EMP, SIMS, Mössbauer and C-H-N elemental analysis, the composition of biotite from a pegmatite dyke and its host granulite from the Kerala Khondalite Belt of SE India has been determined. Biotite in these rocks formed at T> 800-850°C and P= 5±1 kbar.

The full analyses were normalized on the basis of [O_12-(x+y+z)(OH)_xCl_yF_z]. Biotite megacrystals in the pegmatite are Ti-, F,- and Cl-rich (0.33, 0.46 and 0.17 apfu, respectively), H₂O-poor (OH = 0.87), have an X_Mg of 0.49 and Fe³⁺/Fe_tot ² 3%. The low octahedral vacancies (0.06 pfu) and the large oxygen content in the "hydroxyl" site (OH+F+Cl = 1.50 pfu) confirm the role of the Ti-oxy substitution as a major exchange vector in these high-T biotites.

In the host granulite, fine-grained biotite is Fe³⁺-free, has low Cl (0.03 apfu) and a more variable composition, with Ti, F, and X_Mg in the ranges 0.26-0.36, 0.52-0.67, and 0.67-0.77, respectively. In this sample, the amount of octahedral vacancies is relatively large (0.10-0.18 pfu) and the the sum of volatiles in the hydroxyl site is variable (OH+F+Cl  = 1.71-2.06 pfu). Variations of X_Mg display some systematic patterns as a function of the microstructural position: biotite included in or in contact with garnet has the maximum values, whereas crystals in the matrix have the minima. The concentration of Ti displays systematic negative correlations with F, X_Mg and the sum (OH+F+Cl). On the contrary, Al and octahedral vacancies are virtually constant.

These trends indicate that: a) the Ti-vacancy, along with substitutions involving Al, cannot explain the observed variations; b) conversely, the Ti-oxy exchange was active, and results from combination of two vectors: the more conventional deprotonation Ti⁴⁺ + 2O^2- = Mg²⁺ + 2OH^- and the "defluorination" Ti⁴⁺ + 2O^2- = Mg²⁺ + 2F^-. The latter appears to play a relevant role, probably in relation to the Fe-F avoidance.