Petrogenesis of arc lavas from the Rucu Pichincha and Pan de Azucar volcanoes (Ecuadorian arc): Major, trace element, and boron isotope evidences from olivine‐hosted melt inclusions

Abstract

Primary melt inclusions in olivine phenocrysts (Fo74–89) of basic lavas from Pichincha and Pan de Azucar volcanoes (in the front and rear arcs of the Ecuadorian Andes, respectively) were analyzed by electron microprobe for major elements and by ion microprobe for trace element and boron isotope compositions. Although melt inclusions in the most magnesium‐rich olivines contain relatively primitive magmas, their compositions are not directly linked to those of the whole rocks through a differentiation scheme. They are characterized by nepheline‐normative compositions with low SiO2 contents (39.8–47.9 wt%) and unusually high CaO contents (up to 15.4 wt%), which cannot be derived from melting of a simple peridotitic mantle. We explain their formation by the presence of amphibole‐bearing olivine‐clinopyroxenites in the source of these melts. The trace elements patterns of the melt inclusions show the typical trace element features of arc magmas, such as enrichment in LILE and LREE, and negative anomalies in Nb and Ti. Across‐arc variations of mobile versus less mobile incompatible element ratios indicate a decreasing input of a mobile phase from the slab to the mantle wedge with the distance to the trench, along with a decrease in the degree of melting. Boron isotope compositions are highly variable within each volcano (δ11B from −9.5 ± 1.3‰ to +3.5 ± 1.4‰ for the Pichincha melt inclusions and from −17.9 ± 0.8‰ to −1.9 ± 1.4‰ for the Pan de Azucar melt inclusions) and suggest trapping of isotopically heterogeneous melts. Modeling of both dehydration and fusion of the slab indicates that the Pichincha melt inclusions were formed by melting a source enriched by the addition of 1% of a heterogeneous aqueous fluid derived from the dehydration of both the sediments and the altered oceanic crust (after 74 and 76% of B loss, respectively). The phase that metasomatizes the source of the Pan de Azucar melt inclusions can be either an input of 0.1% of a heterogeneous aqueous fluid or more likely 0.5–1% of a heterogeneous silicate melt.