Development of a new method for measuring metamorphic kinetics

Because garnet strongly fractionates Mn, spessartine (XSps) content can be treated as a rock-wide proxy for time. By using Sm-Nd isotopic dating to discretely date cores and rims of garnet crystals, I associated an age with a XSps content, and used this association to indirectly date a set of garnets in a subvolume from the specimen, a Grt-Chl-Pg-Bt-Ms schist from the Pinney Hollow Formation collected at Townshend Dam, VT. Using high-resolution X-ray computed tomographic data, two garnets were selected based on size and accessibility for Sm-Nd dating. These garnets yielded rim ages of 372.8±5.7 Ma and 372.8±4.0 Ma (2σ); EPMA data for these gives rim XSps values of 0.02 and 0.03 respectively. Core ages were unsuccessful due to low Sm/Nd ratios in garnet indicating incomplete cleansing of REE-rich inclusions. Using the average of four published core ages from other Pinney Hollow garnets at this locality, the core age is estimated at 380 ± 3.5 Ma (2σ) paired with the core XSps values for the two garnets of 0.21 and 0.15. A linear relationship between garnet age and XSps has the equation: Age = (43.58 Ma)(XSps) + 376.96 Ma. Core XSps from 8 garnets within a 72 cm3 subvolume of the sample were analyzed, and yield 'Mn ages' by reference to the age-XSps relationship ranging from 377.2 ±3.0 to 382.2±6.0 Ma. The estimated nucleation rate for this subvolume is 0.022 +0.032/-0.008 nuclei/cm3/Ma. The two dated garnets yield growth durations of 9.8±7.3 Ma and 8.0±6.6 Ma, consistent with durations calculated by other workers for this area. Incremental growth rates from dated and subvolume garnets exhibit sharp transitions and negative growth rates, but generally are higher in the core and lower in the rim. Attempts to fit predicted growth rate laws to the incremental growth rates are inconclusive in determining a kinetic control. Crude nearest neighbors analysis suggests diffusion controlled growth. These data represent the first application of a new method for measuring crystallization kinetics. As Sm-Nd isotopic age analysis for this garnet suite is refined, more detailed calibration of the XSps-age relationship will be possible, which will in turn result in more precise Mn ages for garnet subvolumes, and better estimates of instantaneous and time-averaged nucleation and growth rates. Nevertheless, as the first quantitative estimate of nucleation rate, these results represent a significant step toward a new technique for measuring crystallization kinetics.