Dehydrating a granulite: The behavior of H2O and CO2 during high-pressure metamorphism

The Athabasca Granulite Terrane, in northern Saskatchewan, consists of isobaric assemblages of high-pressure, high-temperature metamorphic rocks. Within the terrane, is a suite of 2.6 Ga granitoids that were probably intruded into the deep crust and subsequently metamorphosed at granulite facies (emplacement at >900° C and 1.0 GPa and metamorphism at 750° C and 1.0 GPa; Williams et al., 2000). The original mineral assemblage within these granites includes alkali feldspar + plagioclase + quartz × orthopyroxene. Several granulite facies metamorphic reactions, including what is locally termed the 'Mary' reaction (orthopyroxene + Ca-plagioclase = garnet + clinopyroxene (× hornblende in more hydrous areas) + Na-plagioclase + quartz), are commonly preserved. Because the Mary reaction typically involves anhydrous mineral phases, studying the distribution of structural H₂O and CO₂ in the minerals involved in this reaction should reveal the partitioning behavior of these volatile phases during high-grade metamorphism. We used a Bruker Vertex 70 Fourier transform infrared spectrometer with a focal plane array detector to map volatile concentrations in several samples preserving the Mary reaction. Most of our samples contain pyroxene crystals that were partially hydrated to amphibole, but the other mineral phases involved in the reaction are generally pristine. Preliminary results show that CO₂ and H₂O behave differently during metamorphism. CO₂ concentrations are generally low and homogenous within single mineral grains. However, maps of CO₂ concentrations indicate that there is slightly more structural CO₂ in product minerals (mostly in garnet, but also, in some samples, in quartz and Na-plagioclase mantles on Ca-rich plagioclase) than in reactant minerals. Water, however, is preferentially concentrated along grain boundaries and is zoned in primary, Ca-rich plagioclase and alkali feldspar, both of which commonly contain H₂O-poor cores and H₂O-rich rims. This zoning could arise from water migrating into or out of feldspars in response to deformation or could be a relict of igneous zoning. Although structural water is mostly absent from garnet crystals and Na-rich overgrowths on plagioclase, it appears to be concentrated in neoblastic quartz associated with garnet. In general, it appears that CO₂ can partition into the products of granulite-facies metamorphic reactions (garnet, Na-rich plagioclase, and quartz), while structural water is either lost from reactant mineral phases or relegated to metamorphic quartz. Williams, M. L., Melis, E. A., Kopf, C. F. & Hammer, S. (2000). Microstructural tectonometamorphic processes and the development of gneissic layering: a mechanism for metamorphic segregation. J. Metamorphic Geol. 18, 41-57.