THE UTILITY OF OXYGEN ISOTOPES FOR OUR UNDERSTANDING OF DEEP CRUSTAL PROCESSES – PRELIMINARY RESULTS FROM ZIRCON AND GARNET IN MIGMATITES

Geochemical and isotopic studies of zircon and garnet grown during high-grade metamorphism and anatexis have shown that the compositions of these minerals are sensitive to the timing of mineral growth relative to melt evolution, and to the compositions of source rocks from which melts are derived. Zircon-garnet REE distribution behaviour has successfully been used to pin-point the location and relative timing of zircon growth in migmatites, allowing us not only to date partial melting, but also to assess the effects of melt generation, migration, drainage and recharge in migmatites within the deep crust. Improving our understanding of these processes involves the understanding of oxygen isotope behaviour during mineral-melt interaction.

Recent studies indicate that garnet oxygen isotope compositions are sensitive to changing conditions during metamorphism and the onset of partial melting (e.g. Vielzeuf et al., 2005, Am. Min., 90, 463-472). Integrated oxygen isotope studies of zircon with garnet have shown their utility to trace the sources to leucosomes within migmatites (e.g. Lancaster et al., 2009, J. Met. Geol., 27, 41-54). We present preliminary results of ion microprobe oxygen isotope analysis of zircon and garnet from metapelitic migmatites, in which the REE distribution behaviour have been well established. This ongoing study aims to test interpretations of D_REE^(zrc/grt) equilibrium or disequilibrium by comparison with oxygen isotope data from the same minerals.

Zircon and garnet from four samples from a migmatite terrain were targeted. Results indicate that for samples where zircon and garnet were interpreted to have formed in equilibrium on the basis of REE data, these minerals were near or in equilibrium with respect to their oxygen isotopes. A late-generation pegmatite sample in which zircon and garnet are in clear REE disequilibrium, oxygen isotope disequilibrium was also apparent. In the latter case, results also suggest that the REE-depleted garnet grew from a melt more primitive than zircon in the same rock. Preliminary results indicate good agreement between oxygen isotope and REE data. However, oxygen isotope data alone do not unambiguously determine the relative timing of zircon and garnet growth, but only whether they grew from a melt of the same oxygen isotope composition.