Application of an OH-in-rutile Oxybarometer to the Roseland Anorthosite, Virginia

The Roseland Anorthosite, located in Nelson and Amherst Counties, VA, was emplaced immediately prior to or during the Grenville orogeny (1045±44 Ma). The anorthosite contains large (>0.5 mm) magmatic rutile crystals within a contaminated zone <100 meters wide along a contact with older granulite-facies gneisses. An OH-in-rutile oxybarometer (Colasanti 2007 GCA 71(15s) A181) was applied to rutile crystals from Roseland in order to evaluate the use of this experimentally-calibrated technique for natural samples. Doubly-polished sections of single rutile crystals were analyzed for hydroxyl (OH) concentrations using polarized FTIR spectroscopy. The average OH concentration is 53±36 ppm H2O by wt. in rutile from the Roseland Quarry near the Tye River and 138±57 ppm for rutile from a roadcut near the Piney River. OH concentrations vary by up to 15 ppm from core to rim within a single crystal, with some crystals showing higher OH concentrations in the rim and others containing higher concentrations within the core. No zoning of trace chemical components is observed within single rutile grains, and the composition of the rutile from both locations is nearly uniform (0.23-0.33 wt% FeO; 0.10-0.13 wt% Nb2O5; 0.14-0.17 wt% ZrO2; 0.02-0.09 wt% Al2O3; 0.03-0.05 wt% Cr2O3). Rutile OH concentration is not correlated to the concentration of Fe, Al, Cr, or charge deficiency (total trivalent cations). Assuming magmatic temperatures of ~850-950°C (Herz 1987 USGS Prof. Paper 1371), the OH-in-rutile oxybarometer predicts that the Roseland Anorthosite magma had an fO2 close to the MH buffer (5-6 log units above FMQ). An alternative explanation is that, regardless of the initial fO2 of the magma, slow cooling after peak Grenville metamorphism resulted in the low OH concentrations preserved in the rutile, since equilibrium OH concentration decreases exponentially with decreasing temperature. Although the OH- in-rutile oxybarometer is not strongly dependent upon water activity, low magmatic water content could also affect the OH concentration in the rutile. Good constraints on peak metamorphic conditions and cooling history must exist to successfully apply this technique to rocks with a complex geologic history.