TO MELT OR NOT TO MELT: THE ROLE OF VOLATILES IN DETERMINING STABLE ASSEMBLAGES IN DEEP CRUSTAL BASALTS

Tonalites, trondhjemites, and granodiorites (TTGs) are the dominant assemblage in Archean cratons and form the basis for continental crust in modern arc settings. These rock types likely form by melting of basaltic protoliths in the hornblende ± garnet stability field. However, the specific reactions and conditions that generate TTG melts cannot be observed, although they have been derived experimentally, and require the application of deep crustal analog materials and thermodynamic modeling. One place where in-situ partial melts of deep-crustal basalts can be observed is the Athabasca Granulite terrane in northern Saskatchewan. This terrane contains a suite of mafic dikes (the Chipman dikes) that intruded the lower crust. These dikes contain a variety of metamorphic textures, with end-members of coronitic garnet + clinopyroxene (without leucosome) and peritectic garnet surrounded by roughly tonalitic partial melt. Dikes containing these distinct textures are chemically indistinguishable, and P-T calculations for the two populations overlap, so the underlying cause for these different textures is poorly understood. Preliminary modeling using the Melts and Theriak-Domino programs indicates that a potentially important source for the variation in assemblages is the volatile content of the basalts. All of the Chipman dikes are hydrous, containing abundant hornblende, which is involved in melting (hornblende + plagioclase = garnet + melt). However, the activity of water in the dikes has not been established, resulting in poorly constrained thermodynamic models. Although several whole-rock methods for measuring volatile content exist, they cannot be used to distinguish primary versus late volatiles. However, in-situ methods, like Fourier transform infrared spectroscopy, can be used to measure concentrations of volatiles in different phases and different textural settings, which can be used to distinguish original from later fluids. We will present results incorporating in-situ measurements of volatiles with thermodynamic modeling to develop a clearer understanding of the conditions under which some Chipman dikes partially melted, in order to produce a model that might be extended to TTG-type melts in general.