c OPGC, Université Blaise Pascal, CNRS, IRD, Laboratoire Magmas et Volcans, UMR 6524, 63038. Clermont-Ferrand â France d Institut de Recherche pour le ...
Slab input into magma sources of Ecuadorian magmatism constrained by oxygen isotopes data Silvana Hidalgoa,c; Marie C. Gerbeb; Hervé Martinc; Pablo Samaniegoa,d Instituto Geofísico, Escuela Politécnica Nacional, A. P. 17-2759, Quito – Ecuador Université Jean Monnet, Laboratoire Magmas et Volcans, CNRS UMR6524, 42023 Saint Etienne – France c OPGC, Université Blaise Pascal, CNRS, IRD, Laboratoire Magmas et Volcans, UMR 6524, 63038 Clermont-Ferrand – France d Institut de Recherche pour le Développement – Laboratoire Magmas et Volcans, IRD UMR 163, 63038 Clermont-Ferrand – France a b
Recent studies on the Quaternary Ecuadorian volcanic arc reveal the occurrence of both “classic” calc-alkaline and adakitic magmas. Even though a temporal geochemical change from a “classic” to adakitic magmatism is well established, the detail of adakite petrogenesis is still actively debated. Several geochemical studies based on mayor and trace elements and radiogenic isotopes of Sr-Nd and Pb dealt with this issue raising different hypothesis. We propose oxygen isotopes as an innovative tool to study the Ecuadorian magmatisme. A complete study of oxygen isotopes was carried out in different volcanoes of the Ecuadorian arc, distributed along the Volcanic Front and Eastern Cordillera. Our results are quite interesting; most of the Ecuadorian lavas yield high δ 18O values, being these higher in the Volcanic Front (6.0 to 9.0 ‰VSMOW) than in the Eastern Cordillera (6.1 to 7.8‰ VSMOW).
The highest values are attributed to crustal contamination processes
participating during magma differentiation by fractional crystallization. Nevertheless, it is noteworthy that the crustal terranes traversed by Eastern Cordillera magmas have higher δ18O values (9 to 16‰ VSMOW) than the terranes underlying the Volcanic Front (6.0 to 13.7‰ VSMOW). This seems contradictory at first sight, nevertheless, the oxygen isotope compositions from the most primitive rocks, which might not be affected by crustal contamination are already high (6.0 to 7.8‰ VSMOW) with respect to other continental volcanic arc basalts (6.2 ± 0.7‰ VSMOW) and decrease with the distance to the trench
indicating a clear correlation with the subduction. In fact, δ 18O systematics gives also evidence of mantle source metasomatism. In Ecuador, off-shore studies have shown that carbonated and siliceous pelagic sediments are driven into the subduction. This materials typically have δ18O in the 20-25‰ range. The hydrothermally altered basaltic and gabbroic oceanic crust typically ranges from 3 to 12‰, with some occurrences of extreme 18Oenrichments, up to 25‰. The transfer of these oceanic materials downwards in the subduction might contribute to the modification of the mantle source O-isotope signature. Geochemical modeling proposing the addition of both, oceanic basaltic crust and sediments melts indicates that O- isotopic compositions of the most primitive lavas (Mg# > 53) of the Volcanic Front are compatible with a mantle source metasomatized by 3 to 10% of a sedimentary component and 10 to 15% of melts generated by low-degree melting of the upper altered oceanic crust basalts. The compositions of primitive lavas (Mg# > 52) from Eastern Cordillera volcanoes are consistent with a less metasomatized mantle (1 to 3% of sedimentary component and 3 to 5% of oceanic basalts-derived melts).
