INITIAL ESTIMATE FOR GARNET NUCLEATION RATE AT TOWNSHEND DAM, VT

Because garnet strongly fractionates Mn, spessartine (Sps) 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, we seek to associate an age with a Sps content, and use this association to indirectly date a set of garnet subvolumes 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 (2-sigma); EPMA data for these gives rim X(Sps) 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 (2-sigma). Together with the core X(Sps) of 0.21 and 0.15, the two dated garnets yield growth durations of 7.2±6.0 Ma and 7.2±4.5 Ma, which are consistent with durations calculated by other workers for this area. A linear relationship between garnet age and X(Sps) has a slope of 0.023 X(Sps) / Ma. Core X(Sps) from 8 garnets within a 72 cm³ subvolume of the sample were analyzed, and yield “Mn ages” by reference to the age-X(Sps) relationship ranging from 377.1 ±5.2 to 382.1±4.5 Ma. The estimated nucleation rate for this subvolume is 0.022 ± 0.003 nuclei/cm³/Ma.

These data represent the first attempts of a new method for measuring crystallization kinetics. As Sm-Nd isotopic age analysis for this garnet suite is refined, this will allow a more detailed calibration of the X(Sps)-age relationship, 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.