Collaborative Research: Chemical Equilibria Involving Iron and Hydrogen in Metapelites From Western Maine

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9902857GuidottiThe primary goal of this research is to evaluate the chemical equilibriainvolving ferrous and ferric iron and hydrogen in metapelitic rocks fromwestern Maine and related rocks at Black Mountain, New Hampshire. We willstudy samples from graphite, magnetite, and hematite-bearing rocks at threedifferent metamorphic grades: garnet, lower sillimanite, and staurolitezone. This will permit the determination of: (1) in situ partitioncoefficients for Fe3+ and Fe2+ among all the Fe-bearing phases and for Hamong hydrous phases; (2) the temperature and fO2 dependence ofFe3+/Fetotal and effects of Fe3+ substitution on other cation substitutionsplus their implications for geothermometry; and (3) the cationsubstitutions that charge balance variations of H and Fe3+. We will alsocharacterize and seek to understand Fe3+/Fetotal zoning within individualmineral grains as well as variation of Fe3+/Fetotal among grains of thesame mineral in the same rock; ultimately, we will assess the effects ofassemblage on Fe3+/Fetotal in the constituent mineral phases present. These measurements will be made on standard 1" round petrographic thinsections using beam sizes of less than 10 x 15 micrometers and therelatively rapid, non-destructive techniques of synchrotron microXANESspectroscopy (SmX) for Fe3+/Fetotal, ion microprobe (SIMS) for H, and theelectron microprobe (EPMA) for other major elements. With our analyticalresults in hand, we can begin to address the competing effects ofassemblage and crystal chemistry on the incorporation of Fe2+ and Fe3+ inmetapelite minerals. We also hope to effectively and quickly demonstratethe potential importance of SmX as a petrologic tool.