Lavas erupted in subduction zones are dominated by calc-alkaline, hypersthene-normative compositions. However, melt inclusions trapped in magnesian olivines (Fo ≥ 88) from arc basalts often show alkaline, nepheline-normative compositions. Although their trace element compositions are comparable to those of their host lavas, these melt inclusions cannot be simply explained by the typical models for arc lava genesis. The study of melt inclusions from several island arcs from all over the world has emphasized their compositional variability, in particular in calcium and Sc, two elements compatible in clinopyroxene. The coupling of this study with thermodynamic modelling has led to the development of a source model involving the contribution of amphibole-bearing clinopyroxenites at about 1 GPa. Nepheline-normative arc melt inclusions thus underline the source heterogeneity at the origin of arc magmas. Partial melting experiments of amphibole-clinopyroxenite – hydrous peridotite homogeneous mixtures performed at 1 GPa have provided additional information on the origin of silica-undersaturated arc magmas, and also a better understanding of the melting behaviour of the two types of lithologies. For proportions of pyroxenite lower than 50 % in the mixed source, the derived melts show hypersthene-normative compositions that are comparable to peridotite-derived arc lavas. For higher proportions of pyroxenite in the source, the derived magmas become nepheline-normative, calcium-rich, and reproduce the compositional range of the melt inclusions. Fifty percent of mixing corresponds to the limit of orthopyroxene stability. As long as this phase is present in the residue, the generated melts are hypersthene-normative. In addition, the presence of amphibole in the source increases the silica under-saturation of liquids and thus accounts for the extreme enrichment in normative nepheline of some inclusions. Our melting experiments of a homogeneous source have allowed to determine the compositions of magmas derived from a mode realistic melting process, involving a heterogeneous source. As pyroxenites have higher melt productivity than peridotites, most melts produced from a pyroxenite-peridotite mixed source are comparable to those produced by a dominantly pyroxenitic source, i.e., they display normative nepheline compositions. Thus, in the case of mixing of magmas derived from heterogeneous source, the transition between hypersthene- and nepheline-normative compositions occurs for 30-40 % of pyroxenite melts, generated from 20-25 % of pyroxenite in the source. The origin of these lithologies in the subarc mantle is explained by the delamination of lower crustal cumulates consisting of clinopyroxene + amphibole. Nepheline-normative arc melt inclusions would record multi-stage mixing between clinopyroxenite and peridotite melts. In contrast, the compositional homogeneity of hypersthene-normative lavas would reflect a more advanced stage of mixing and differentiation, thereby reducing the pyroxenite signature in derived magmas.
