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Date Permissions Signed
Date of Award
Master of Science (MS)
Hirsch, David M., 1969-
Stelling, Peter L.
By working under the assumption that garnet strongly fractionates Mn, the change in measured XMn from the core to rim of a garnet can be used to represent the progression of time since nucleation. Core, mantle, and rim zones from nine snowball garnets collected at the Pinney Hollow Formation, Townshend Dam, VT were analyzed using electron probe microanalysis (EPMA) as well as Sm/Nd isotopic methods. Two-point garnet-matrix Sm/Nd ages are complex, with some showing apparent reverse age zoning and unrealistically young ages. Therefore, some garnet ages were culled, including those with poor 147Sm/144Nd ratios (< 0.75), low Nd concentrations (> 0.2 ppm), and rim ages that pre-date core ages. The remaining nineteen Sm/Nd ages show a 10.0 ± 3.5 Myr garnet growth duration with the oldest garnet core age being 383.3 ± 3.3 Ma, and the youngest rim age being 373.3 ± 1.3 Ma. Using the isotopic ages along with their corresponding Mn data, an association between Mn and time was developed for the entire sample. This relationship was calculated to be Age = 2 x 10^1 (+16/-6) Ma (XMn) + 379.3 ± 0.1 Ma. Because the nine dated garnets came from a very large sample (TD0914a), a subvolume was extracted for kinetics work. The kinetics subvolume measured 3940 cm3, and was disaggregated to collect every garnet porphyroblast from within. Each subvolume garnet was cut through the morphological center and analyzed for Mn content using EPMA. By referring each EPMA Mn datum to the Mn-age equation, 5,500 points from 78 garnets were indirectly dated. These garnets grew over 4.9 ± 2.7 Myr and nucleated over a 4.7 ± 2.6 Myr duration. The nucleation rate for the sample is 0.005 ± 0.002 nuclei/cm3/Myr or approximately one nucleation event per cubic centimeter per 200 Myr. Incremental growth rates were calculated for each of the 78 subvolume garnets, and are provided as growth rate (mm/Myr) versus distance from the core (mm). Much growth rate variation occurs throughout the entirety of the subvolume as well as within each garnet. However, due to EPMA noise in Mn content coupled with closely spaced analysis points, some incremental growth rates are unreasonably large or small (-500 to 500 mm/Myr). EPMA data that produce such extreme values were culled, leaving more reasonable rates. Garnets in the sample show competition for nutrients during growth. Garnets that nucleated near other nuclei early in the crystallization episode showed stunted growth compared to those that nucleated in an isolated location. 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.
Western Washington University
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Bloom, Rose, "Determining garnet crystallization kinetics from growth zoning and Mn-calibrated Sm-Nd ages" (2013). WWU Graduate School Collection. 309.