Oxygen isotope heterogeneities and diffusion profile in composite metamorphic-magmatic garnets from the Pyrenees

Unusually large 8110 heterogeneities ( greater than or equal to4parts per thousand) within single crystals are reported in garnets from dioritic migmatites in the Pyrenees. These heterogeneities, together with contrasting Ca and P zoning, allow the identification of different growth zones. Garnet cores with high PO values (12-14parts per thousand) are relatively poor in Ca (7-9 mol% Grs) and rich in P (400-900 ppm P2O5). In contrast, garnet rims with lower 5110 values (7-12parts per thousand) are richer in Ca (12-14% Grs) and poorer in P (100-200 ppm). These growth zones can be ascribed to a metamorphic event followed by crustal partial melting and contamination by magmas from the mantle. High 5110 intra-crystalline contrasts result from mineral growth in an open magmatic system involving the interaction of partial melts with distinct delta(18)O signatures. At the garnet core-rim interface, compositional profiles in major divalent cations are consistent with the relaxation of an initial sharp step in Ca, Fe, and Mg by Ca <----> (Fe, Mg) interdiffusion. At the same interface, an O-isotope profile is documented. The analogy of Ca and O isotope profiles suggests that the delta(18)O distribution may also result from a diffusion process. In this particular case (temperature, garnet composition, oxygen fugacity), O diffusion appears to be of the same order of magnitude as Ca <----> (Fe, Mg) interdiffusion. Considering a duration of 10 Ma for the plutono-metamorphic event in the Pyrenees, Ca and O diffusivities in the range 10(-22) m(2)/s (at 850 degreesC) are retrieved from the measured profiles. Like Ca, O diffusion in garnet at magmatic temperatures (850-900 degreesC) is both slow enough to preserve large delta(18)O heterogeneities and fast enough to generate relaxation profiles.