Subduction-derived fluid/melt percolation in lithospheric mantle revealed by metasomatic orthopyroxene in peridotites from French Massif Central

International audienceSecondary orthopyroxene (OPX2) replacing olivine is known in xenoliths within arc lavas (e.g. [1-3] and references therein), and in some ultramafic complexes [4, 5]. It is interpreted as resulting from interactions between fluids or melts released by the subducting slab and the mantle wedge. The olivine is destabilised according to: olivine + Si-rich fluid/melt OPX2 (± minor clinopyroxene) + less-siliceous fluid/melt. However, metasomatic orthopyroxene has been rarely observed in continental context in xenoliths or ultrahigh-pressure complexes [6, 7]. This contribution reports for the first time the presence of metasomatic orthopyroxene in mantle xenoliths from the French Massif Central (FMC). The samples are fresh spinel lherzolites from the Mont Coupet volcano (Devès, FMC). OPX2 has three modes of occurrences: mode 1, in thin veins (< 30µm) cross-cutting or rimming primary olivine; mode 2, at the contact between primary olivine and clinopyroxene; mode 3, in large (a few hundred µm) reaction pockets that contain secondary olivine, clinopyroxene, small patches of glass with plagioclase laths and numerous vesicles. Small pores (< 2µm) line the contact between OPX2 and the other phases. OPX2 (modes 1, 2) contains rare rounded inclusions of Cl-apatite. The composition of OPX2 is distinct from that of primary orthopyroxene and similar in the three modes of occurrences. The main difference is its lower content in Al2O3: 1.7-2.7 wt. % compared to 2.9-4.4 wt. % in primary orthopyroxene, depending on the sample. It also contains less Cr2O3 (<0.2 wt. %) and TiO2 (