Experimental melting of hydrous peridotite–pyroxenite mixed sources: Constraints on the genesis of silica-undersaturated magmas beneath volcanic arcs

Abstract

The most primitive arc magmas expressed as melt inclusions in forsteritic olivine display silica-undersaturated, nepheline normative compositions, sometimes associated with high CaO contents. Involvement of a heterogeneous mantle source containing an amphibole-bearing clinopyroxene-rich component is often mentioned to account for the origin of these magmas. However, the proportions and the role of such mantle source component in arc magma genesis remains a matter of debate. To better understand the generation of silica-undersaturated magmas in arcs and the role of clinopyroxenites during arc magma genesis, we have performed melting experiments on a heterogeneous hydrous mantle (hydrous lherzolite mixed with variable amounts of amphibole-bearing clinopyroxenite) at 1 GPa under oxidizing conditions (close to the fayalite–magnetite–quartz: FMQ-buffer). Unlike under anhydrous conditions, pyroxenites and peridotites have similar solidus temperatures under hydrous conditions, but the melt productivity still increases with increasing fraction of pyroxenite in the source. For peridotite-rich sources (up to 50% clinopyroxenite) the presence of orthopyroxene buffers the partial melts to compositions identical in terms of major elements to regular peridotite melts. When orthopyroxene leaves the residue (above 50% of pyroxenite), melts become nepheline-normative, with CaO/Al2O3 ratios >1. Comparison between experimental melts produced by melting homogeneous mixed sources (homogeneous melting) and aggregated melts from heterogeneous sources (heterogeneous melting) shows that the later are more silica-undersaturated and richer in CaO. Our experiments confirm that nepheline-normative melt inclusions have sampled pure or poorly-mixed clinopyroxenite melts from a heterogeneous mantle source, while hypersthene-normative lavas are likely to result from a more advanced stage of magma mixing. Amphibole–clinopyroxenite heterogeneities in the mantle wedge could originate by density-driven delamination of lower crustal cumulates consisting of clinopyroxene + amphibole ± olivine. Partial melts of these cumulates can be efficiently extracted and suffer only little interaction with surrounding peridotites before their entrapment as melt inclusions; alternatively, they can react with peridotite to form secondary orthopyroxene-free metasomatic veins, which may also contribute to the source of silica-undersaturated arc magmas.