Mineral chemistry of Ti-rich biotite from pegmatite and metapelitic granulites of the Kerala Khondalite Belt (southeast India): Petrology and further insight into titanium substitutions

Precise chemical composition, including Fe3+ and H, of biotite from a pegmatite dike and its host granulite from the Kerala Khondalite Belt of SE India has been determined using a multi-technique approach involving EMP, SIMS, Mossbauer, and C-H-N elemental analysis. Biotite in these rocks formed at T> 800-850 degrees C and P = 5 +/- 1 kbar. full analyses were normalized on the basis of [O12-(x+y+z)(OH)(x)ClyFz]. Biotite in the pegmatite is Ti-, F-, and Cl-rich (0.33, 0.46, and 0.16 apfu, respectively), H2O-Poor (OH = 0.86 pfu), has X-Mg= 0.49 and Fe3+/Fe-tot <= 3%. The low octahedral vacancies (0.06 pfu) and the high oxygen content in the hydroxyl site (OH + F + Cl = 1.49 pfu) confirm the role of the Ti-oxy substitution as a major exchange vector in these high-T biotites. the host granulite, fine-grained biotite is Fe3+-free, has low Cl (0.03 apfu), and 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. The number of octahedral vacancies is relatively large (0.10-0.18 pfu) and the sum of volatiles (OH + F + Cl) varies from 1.71 to 2.06 pfu. Systematic variations of X-Mg are a function of the microstructural position and are in agreement with retrograde exchange reactions: biotite included in or in contact with garnet has the maximum values, whereas crystals in the matrix have the minima. Titanium has systematic negative correlations with F, X-Mg, and (OH + F + Cl), whereas Al and octahedral vacancies are virtually constant. trends indicate that the Ti-vacancy, along with substitutions involving Al, cannot explain the observed short-scale variations. Conversely, the Ti-oxy exchange appears to be active, resulting from combination of two vectors: the more conventional hydroxylation Ti4+ + 2O(2-) = (Fe,Mg)(2+) + 2OH(-) and the "fluorination" Ti4+ + 2O(2-) = (Fe,Mg)(2+) + 2F(-). The systematic retrograde redistribution involves not only Fe and Mg as commonly observed, but also Ti, F, and H, in a way such to eliminate the primary Ti-oxy component of biotite.