Sorbadere F., Medard E., Laporte D., Schiano Pierre. (2013). Experimental melting of hydrous peridotite-pyroxenite mixed sources : constraints on the genesis of silica-undersaturated magmas beneath volcanic arcs. Earth and Planetary Science Letters, 384, p. 42-56. ISSN 0012-821X.
Titre du document
Experimental melting of hydrous peridotite-pyroxenite mixed sources : constraints on the genesis of silica-undersaturated magmas beneath volcanic arcs
Sorbadere F., Medard E., Laporte D., Schiano Pierre
Source
Earth and Planetary Science Letters, 2013,
384, p. 42-56 ISSN 0012-821X
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.
