Combined oxygen-isotope and U-Pb zoning studies of titanite: New criteria for age preservation

Titanite is an important U-Pb chronometer for dating geologic events, but its high-temperature applicability depends upon its retention of radiogenic lead (Pb). Experimental data predict similar rates of diffusion for lead (Pb) and oxygen (O) in titanite at granulite-facies metamorphic conditions (T = 650-800 degrees C). This study therefore investigates the utility of O-isotope zoning as an indicator for U-Pb zoning in natural titanite samples from the Carthage-Colton Mylonite Zone of the Adirondack Mountains, New York. Based on previous field, textural, and microanalytical work, there are four generations (types) of titanite in the study area, at least two of which preserve diffusion-related delta O-18 zoning. U-Th-Pb was analyzed by SIMS along traverses across three grains of type-2 titanite, which show well-developed diffusional delta O-18 zoning, and one representative grain from each of the other titanite generations. Type-2 and type-4 titanites show broadly core-to-rim decreasing Pb-206/U-238 zoning, consistent with Pb diffusion at higher temperatures, and uniform or even slightly increasing Pb-206/U-238 near grain rims, indicating subsequent recrystallization and/or new growth below the Pb blocking temperature. Type-2 and type-4 grain cores preserve ca. 1160 Ma ages that correlate with the anorthosite-mangerite-charnockite-granite magmatic phase of the Grenville orogeny, whereas grain rims give ca. 1050 Ma Pb-206/U-238 ages that coincide with the culminating Ottawan phase. The type-3 titanite grain was sampled from a vein and yields Pb-206/U-238 dates older than the syenite into which the vein was emplaced; accordingly, its Pb-206/U-238 dates are interpreted as indicating excess uncorrected common Pb. Type-2 grains with recrystallized or shear-eroded margins show truncated or reversed Pb-206/U-238 zoning but retain symmetrically decreasing delta O-18 zoning, consistent with grain margin modification following arrest of Pb diffusion but before arrest of O diffusion. It is concluded that O diffusion was slightly faster than Pb diffusion in Adirondack titanites at the conditions of (local) peak Ottawan metamorphism, making delta O-18 zoning a useful discriminator of closed-system age domains that did not suffer Pb loss. In addition, the small offset in the O and Pb partial retention zones constrains the timing and temperature of oblique-slip deformation along the Carthage-Colton Mylonite Zone: the details of porphyroclast microstructure and zoning data show that the oblique-slip shear zones were active at ca. 1050 Ma, with deformation initiating near the peak of Ottawan metamorphism at similar to 700 degrees C and continuing through the O blocking temperature at similar to 550 degrees C. Published by Elsevier B.V.