“GROUND TRUTHING” BIOTITE IN PERALUMINOUS AND METALUMINOUS METAMORPHIC ROCKS

One of the hallmarks of the RIMG volumes over the last 30 years has been an interplay ranging over mineral physics, crystallography, and petrologic “ground truth” for a wide variety of mineral groups and rock systems, an interplay which, for rock-foming minerals, can be termed "petrologic mineralogy". The mica group minerals have been a common theme in the RIMG series with two volumes devoted specifically to the micas. To extend these themes, we have used large natural biotite data sets to investigate the extent to which Ti substitutes into biotite from peraluminous and metaluminous metamorphic rocks, Ti subsitution mechanisms and petrologic implications of Ti in biotite. We have generated a Ti-saturation surface (TºC - Mg/(Mg+Fe) – Ti surface) for biotites from graphitic, peraluminous metapelites that contain ilmenite or rutile and have equilibrated at roughly 4-6 kbar. The temperatures of the Ti-saturation surface were empirically calibrated using an extensive natural biotite data set (529 samples) from western Maine and south-central Massachusetts in combination with a quantitative petrogenetic grid for metapelites. The surface fit equation was reformulated to produce an empirical Ti-in-biotite geothermometer for peraluminous metamorphic rocks. In addition, the systematics of Ti in biotite provided a means to evaluate the approach to chemical equilibrium. In the peraluminous rocks Ti is incorporated into magnesian biotite primarily via a Ti-tschermak substitution controlled by octahedral-tetrahedral layer misfit. At intermediate X(Mg), Ti-deprotonation subsitution becomes the dominant mechanism for Ti incorporation, particularly at metamomorphic grade above the staurolite zone where there is significant reduction of activity of H2O in the metamorphic fluid from graphite-bearing metapelites. Biotites from metaluminous metamorphic rocks tend to have relatively enhanced Ti content. Analysis of isothermal biotite data from low-Al amphibolite to granulite facies samples indicate that in such low-Al compositions biotite incorporates Ti in accordance with combinations of Ti-deprotonation and a reverse Al-Tshermak’s substitution.