INTRAGRAIN OXYGEN ISOTOPE ZONING IN TITANITE BY ION MICROPROBE: CONSTRAINING COOLING RATES AND FLUID INFILTRATION HISTORY ALONG THE CARTHAGE COLTON MYLONITE ZONE, ADIRONDACK MOUNTAINS, NY

Ion microprobe analyses of late Grenville-aged (ca. 1050 Ma) titanite from the Diana metasyenite and crosscutting ultramylonitic shear zones in the Adirondack Mountains, New York, reveal microscale variations in oxygen isotopic ratios that result from combined post-metamorphic mineral growth, diffusive exchange, and fluid infiltration related to post-peak metamorphic shearing along the Carthage Colton Mylonite Zone (CCMZ). Traverses across 400-1000 μm diameter titanite grains generally show a pattern of decreasing δ¹⁸O from grain interior toward grain rims; however, grain interiors commonly display shorter-wavelength variations, some of which represent symmetric growth-related compositional zoning. One relatively simple δ¹⁸O profile across a grain from metasyenite shows a central plateau and a sharp, symmetric, 0.8‰ decrease across the outer 80-90 µm of the grain rim. Numerical modeling of Fast Grain Boundary (closed-system) diffusion (Eiler et al. 1994) during cooling yields a good fit to these grain rims. Modeling the steepness of the rim diffusion profiles, assuming wet diffusion conditions (Morishita et al. 1996; Zhang et al. 2006) and titanite growth at 650-700°C, we calculate cooling rates of ≥20°C/my along the CCMZ during late stage exhumation of the Ottawan orogen. This is significantly higher than the time-averaged, 1-4°C/my cooling rate indicated by regional thermochronologic studies and may reflect episodic exhumation related to movement on the CCMZ. Titanite grains within thin (0.5-20 cm), crosscutting ultramylonitic shear zones show similarly shaped but larger-magnitude (1.5-2.2 ‰) δ¹⁸O zoning profiles, which, when combined with observations of preferential mineralization along the shear zones and whole-rock oxygen isotope disequilibrium with respect to adjacent metasyenite wallrock, indicate open-system behavior. We propose that fluid infiltration along recrystallizing shear zones led to differential isotopic exchange amongst deforming minerals and fluid: grain-size-reduced quartz and feldspar exchanged rapidly relative to titanite, setting up isotopic disequilibrium and promoting the development of large-magnitude δ¹⁸O zoning within titanite.