No abstract

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°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 δ 18 O zoning. U-Th-Pb was analyzed by SIMS along traverses across three grains of type-2 titanite, which show well-developed diffusional δ 18 O zoning, and one representative grain from each of the other titanite generations. Type-2 and type-4 titanites show broadly core-to-rim decreasing 206 Pb/ 238 U zoning, consistent with Pb diffusion at higher temperatures, and uniform or even slightly increasing 206 Pb/ 238 U 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 anorthositemangerite-charnockite-granite magmatic phase of the Grenville orogeny, whereas grain rims give ca. 1050 Ma 206 Pb/ 238 U ages that coincide with the culminating Ottawan phase. The type-3 titanite grain was sampled from a vein and yields 206 Pb/ 238 U dates older than the syenite into which the vein was emplaced; accordingly, its 206 Pb/ 238 U dates are interpreted as indicating excess uncorrected common Pb. Type-2 grains with recrystallized or shear-eroded margins show truncated or reversed 206 Pb/ 238 U zoning but retain symmetrically decreasing δ 18 O 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 δ 18 O 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~700°C and continuing through the O blocking temperature at~550°C.Published by Elsevier B.V.

Oxygen isotopes are an attractive target for zoning studies because of the ubiquity of oxygenbearing minerals and the dependence of mineral 18 O/ 16 O ratios on temperature and fluid composition. In this study, subtle intragrain oxygen isotope zoning in titanite is resolved at the 10-lm scale by secondary ion mass spectrometry. The patterns of d 18 O zoning differ depending on microstructural context of individual grains and reflect multiple processes, including diffusive oxygen exchange, partial recrystallization, grain-size reduction, and grain growth. Using the chronological framework provided by structural relations, these processes can be related to specific events during the Grenville orogeny. Titanite was sampled from two outcrops within the Carthage-Colton Mylonite Zone (CCMZ), a long-lived shear zone that ultimately accommodated exhumation of the Adirondack Highlands from beneath the Adirondack Lowlands during the Ottawan phase (1090-1020 Ma) of the Grenville orogeny. Titanite is hosted in the Diana metasyenite complex, which preserves three sequentially developed fabrics: an early NW-dipping protomylonitic fabric (S 1 ) is crosscut by near-vertical ultramylonitic shear zones (S 2 ), which are locally reoriented by a NNW-dipping mylonitic fabric (S 3 ). Texturally early titanite (pre-S 2 ) shows diffusion-dominated d 18 O zoning that records cooling from peak Ottawan, granulite-facies conditions. Numerical diffusion models in the program Fast Grain Boundary yield good fits to observed d 18 O profiles for cooling rates of 50 AE 20°C Ma À1 , which are considerably faster than the 1-5°C Ma À1 cooling rates previously inferred for the Adirondack Highlands from regional thermochronology. High cooling rates are consistent with an episode of rapid shearing and exhumation along the CCMZ during gravitational collapse of the Ottawan orogen at c. 1050 Ma. Texturally later titanite (syn-S 2 ) has higher overall d 18 O and shows a transition from diffusion-dominated to recrystallization-dominated d 18 O zoning, indicating infiltration of elevated-d 18 O fluids along S 2 shear zones and continued shearing below the blocking temperature for oxygen (~≤500°C for grain sizes at the study site). The texturally latest titanite (post-S 3 ) has growth-dominated d 18 O zoning, consistent with porphyroblastic grain growth following cessation of shearing along the Harrisville segment of the CCMZ.

We characterize oxygen isotope zoning within single titanite crystals from the Carthage-Colton mylonite zone (CCMZ), Adirondack Mountains (New York State, United States), by ion microprobe. Smooth gradients of δ 18 O, up to 0.6‰ over 90 µm, resulted from diffusive exchange of oxygen during cooling from peak metamorphic temperatures of 650-700 °C. Modeling of the observed profi le indicates punctuated cooling rates of 30-60 °C/m.y. along the CCMZ, set within long periods of much slower cooling. These results indicate a previously unrecognized period of rapid cooling along the CCMZ that is interpreted to result from the post-Ottawan collapse of the Grenville mountain belt and exhumation of the central Adirondack Highlands at ca