Mantle source of back-arc Ecuador volcanoes: insights from B and radiogenic isotopes.

The Plio-Quaternary Ecuadorian volcanic arc belonging to the Northern Volcanic Zone (NVZ) of the Andes consists of the Western Cordillera (fore-arc), the Cordillera Real (main-arc) and scattered volcanoes in the rear-arc zone. The largest volcanoes of the rear arc zone are the Sumaco and El Reventador. These volcanoes, despite being located at the same distance from the trench, exhibit erupted magmas of different compositions. The products of El Reventador are mainly represented by porphyritic lavas, ranging from basalt to rhyolite, with basaltic andesite and andesite being the most abundant. They belong to the medium-K to high-K calc-alkaline series. In contrast, the products of Sumaco are markedly SiO2-undersaturated and vary from basanite to phonolite. Although both volcanoes show Nb-Ta-Ti negative anomalies typical of subduction-related environments, trace element concentrations are markedly different: modest enrichment in incompatible trace elements is typical of El Reventador lavas, whereas the Sumaco products are strongly enriched in all incompatible elements and show quite low LILE/HFSE ratios (i.e. Ba/Nb 35-105). In addition, El Reventador samples exhibit higher 87Sr/86Sr (0.7044-0.7046) and lower 143Nd/144Nd (0.51272-0.51280) with respect to Sumaco (87Sr/86Sr≈0.7041-0.7043, 143Nd/144Nd≈0.51284-0.51297). It is worth noting that the Sr-Nd isotopic fields of the two back-arc volcanoes are well within the Ecuadorian NVZ variation range (0.7035<87Sr/86Sr<0.7047; 0.5126<143Nd/144Nd<0.5130) [1]. Nevertheless, El Reventador Sr isotopic ratios are among the highest values of the Ecuadorian volcanic rocks from literature. The two volcanoes show similar and almost constant 207Pb/204Pb ratios. The other Sumaco Pb isotope ratios (18.738<206Pb/204Pb<18.858; 38.525<208Pb/204Pb<38.594) are comparable with the overall Ecuadorian isotope variations (18.700<206Pb/204Pb<19.150; 38.450<208Pb/204Pb<38.950[1]), whereas El Reventador lavas have the lowest 208Pb/204Pb and 206Pb/204Pb among all the other ecuadorian volcanoes (38.408-38.506 and 18.541-18.682, respectively). Even B isotopes are clearly distinct, with 11B ranging from -1.0 to -2.5 for El Reventador samples and -5.2 to -7.2 for the Sumaco samples. All of this suggests that the erupted magmas of these rear-arc volcanoes represent two different mantle sources. The Sumaco product’s source is consistent with very low degrees of partial melting due to the metasomatization of a supra-slab mantle wedge by small amounts of 11B-depleted fluids released from subducted AOC and sediments. In contrast, under the El Reventador volcano a carbonatic sediment component (with low 206Pb/204Pb and high 11B) is invoked as significant agent of mantle wedge metasomatization. [1] Bourdon et al. (2003) Earth Plan. Sci. Lett. 205, 123-138.