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

¹ Laboratoire Magmas et Volcans-OPGC, Université Blaise Pascal and CNRS-5 rue Kessler, 63038 Clermont-Ferrand, France
² Laboratoire de Géologie, Ecole Normale Supérieure, 24 rue Lhomond, 75231 Paris, France

Unusually large ¹⁸O heterogeneities (4) 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 ¹⁸O values (12–14) are relatively poor in Ca (7–9 mol% Grs) and rich in P (400–900 ppm P₂O₅). In contrast, garnet rims with lower ¹⁸O values (7–12 ) 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 ¹⁸O intra-crystalline contrasts result from mineral growth in an open magmatic system involving the interaction of partial melts with distinct ¹⁸O 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 ¹⁸O 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 m²/s (at 850 °C) are retrieved from the measured profiles. Like Ca, O diffusion in garnet at magmatic temperatures (850–900 °C) is both slow enough to preserve large ¹⁸O heterogeneities and fast enough to generate relaxation profiles.

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