DETERMINING GARNET CRYSTALLIZATION KINETICS FROM GROWTH ZONING AND MN-CALIBRATED SM-ND AGES

Essential to an understanding of metamorphic rocks are the rates at which metamorphic reactions occur—kinetics. Rates of crystallization give insight into the details of metamorphic rock formation. This project uses Sm-Nd ages as well as Mn growth zoning to date and calculate the nucleation and growth rates of 78 garnets from Townshend Dam, VT.

Cores, rims, and intermediate zones of twelve garnets with normal Mn growth zoning were analyzed using electron probe microanalysis (EPMA) and Sm-Nd isotopic methods. The Sm-Nd data reveal a heterogeneous matrix at the cm scale, complicating age interpretations. Therefore, Sm-Nd ages were calculated using two strategies: a more precise (but perhaps less accurate) two-point isochron strategy pairing each garnet with its local matrix, and a more conservative strategy pairing each garnet zone with all matrix analyses to account for uncertainty in the initial equilibration volume for Nd. A linear fit to each Mn-age dataset allows crystallization kinetics to be calculated.

A subvolume from the sample measuring 3940 cm³ was extracted and disaggregated. Each garnet was cut through the morphological center and analyzed for Mn via EPMA. Garnets show smooth Mn zoning with high Mn cores tapering toward the rim, excepting those with upticks at the outermost rim. By referring each Mn datum to the Mn-age relation, 5,500 points were indirectly dated. The two-point isochron strategy yields a growth duration of 4.9 ± 2.7 Myr, a nucleation duration of 4.7 ± 2.6, and a nucleation rate of 0.005 ± 0.002 nuclei/cm³/Myr. The multi-point isochron strategy yields a 3.3 ± 9.1 Myr growth duration with a 3.2 ± 9.0 Myr nucleation duration. The nucleation rate is 0.007 ± 0.001 nuclei/cm³/Myr.

Incremental growth rates were calculated for each subvolume garnet, and these rates vary within each garnet as well as throughout the subvolume. Due to EPMA noise and closely spaced analysis points, some calculated incremental growth rates are unreasonable, and such data points were culled.

Isolated garnets grew larger and faster than garnets that nucleated near others, indicating competition for nutrients during growth. Though some retrogression textures were observed in the subvolume samples, this project provides a novel and robust natural kinetic dataset from which future research may be conducted.