Distribution of Water in Nominally Anhydrous Minerals during Metamorphic Reactions

Abstract:

Nominally anhydrous minerals are a reservoir for water in otherwise dry rocks. This water may play a role in facilitating metamorphic reactions and enhancing deformation. In this study, we examined orthopyroxene-bearing granites from the Athabasca Granulite terrane in northern Saskatchewan. These rocks intruded the lower crust (pressures of 1 GPa) at circa 2.6 Ga at temperatures of > 900 ºC and were subsequently metamorphosed at granulite facies conditions (700 ºC and 1 GPa) in the Paleoproterozoic (Williams et al., 2000). One of the primary reactions recorded by these rocks is locally known as the “Mary” reaction and involves the anhydrous reaction: orthopyroxene + Ca-plagioclase = clinopyroxene + garnet + Na-plagioclase. Measurements of water concentrations in both product and reactant assemblages were performed using a Bruker Vertex 70 Fourier transform infrared spectrometer and revealed that there is a slight excess of water in product minerals over reactant minerals. There are two possible explanations for this. The first is that water was derived from an external source, possibly hydrous, likely contemporaneous, mafic dikes. This interpretation is supported by higher concentrations of K, which is essentially absent from the reactant minerals, in the Na-rich rims of plagioclase. However, only modest amounts of external fluids could have been introduced, or amphiboles would have been stabilized at the expense of clinopyroxene (Moore & Carmichael, 1998). An alternative interpretation is that slightly more water-rich minerals reacted more readily, releasing water that was then incorporated into their products, whereas the water-poorer minerals failed to react. Support for this interpretation comes from very low water concentrations in orthopyroxene and plagioclase from an unreacted and undeformed sample. This interpretation suggests that water in anhydrous minerals may catalyze metamorphic reactions, and a lack of water may be critical for preserving metastable assemblages.

Moore & Carmichael (1998). Contrib Min Petrol 130, 304-319.

Williams, Melis, Kopf, & Hammer (2000). J. Metamorphic Geol. 18, 41-57.