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International audience; In the Balmuccia massif (NW Italy), a pseudotachylyte vein network (N068 trending) in a spinel lherzolite is interpreted as the product of frictional melting during a single M-w>6 earthquake. The subvertical fault underwent a metric dextral coseismic displacement, raking 60 degrees SW. The average width of the main slip surface is similar to 5mm. A dense network of thin (20-200m) injection and ultramylonite-like veins decorates the fault walls. In the injection veins, Raman microspectrometry mapping reveals pockets of still preserved amorphous silicate, containing approximate to 1% of structurally bound H2O. In the ultramylonite-like veins, electron backscattered diffraction mapping reveals that ultrafine (0.2-2m) olivine grains exhibit a strong fabric with (010) planes parallel to shearing, consistent with temperatures above 1250 degrees C during deformation and suggesting fast recrystallization from the frictional melt. The veins also exhibit pyroxene and recrystallized spinel, which proves that the earthquake occurred at a minimum depth of 40km. The energy balance demonstrates that complete fault lubrication must have occurred during coseismic sliding (i.e., dynamic friction coefficient 40km) earthquake of 6

A 50 m thick and 150 m long dunite body occurs as a subconcordant, tabular structure in the Balmuccia Massif, an Alpine peridotite thought to represent part of the subcontinental mantle. The contacts with the host spinel-facies depleted lherzolite are sharp. The dunite body is composed of spinel-rich dunite containing centimetre-size lenses of relict Cr-diopside websterite, spinel-poor granoblastic dunite and virtually monomineralic Cr-spinel layers exhibiting flow structures. Orthopyroxene is a minor, relict phase in all the lithologies; clinopyroxene is intergranular and amphibole is a minor accessory phase. Overall the dunite body is fairly refractory (Fo in olivine: 90·7-93·8). Strontium and neodymium isotope ratios of clinopyroxene separates from the dunitic body resemble those of a Cr-diopside websterite suite that forms a series of dykes cutting the main peridotite host. It is proposed that the dunites were generated in a part of the mantle veined by early Cr-diopside websterites by a three-stage process involving partial melting of pyroxenite, infiltration of the pyroxenite-derived melt into the depleted lherzolite and its consequent open-system partial melting and focused flow of the resultant partial melts leading to the production of reactive dunite channels through both peridotite and pyroxenite. This process has been simulated using pMELTS assuming that the pyroxenite partially melts at 1·5 GPa and focused melt transport occurs at pressures greater than 0·7 GPa. The results show that, depending on the focusing factor assumed, dunite can form from peridotite at P < 1·2 GPa and from pyroxenite at P < 1·1 GPa, in both cases over a large pressure range. The model accounts for specific characteristics of the dunite, such as its refractory composition, the presence of orthopyroxene relics, the occurrence of relict websterite lenses in the spinel-rich dunites and the flow structures in the Cr-spinel layers. The proposed mechanism allows dunite formation to occur well within the spinel stability field, and therefore at greater depth than dunites in ophiolites, which generally formed within the plagioclase stability field. The aggregated model melts extracted from the segments where dunite forms are high-Mg alkali basalts resembling, after olivine fractionation, the compositions of enriched-type mid-ocean ridge basalt from slow- and ultraslow-spreading ocean ridges.