Traces of ancient mafic layers in the Tethys oceanic mantle

•We studied ophiolitic mantle rocks from the Tethys ocean (Dramala Complex, Greece).•We report coupled lithological and Os isotopic heterogeneities on a cm-scale.•Pyroxenite layers have 187Os/188Os of 0.14-0.20, compared to 0.12-0.13 in host peridotites.•The layers are melting residues of pyroxenites formed by peridotite-melt reaction.•The reacting melts resulted from melting of ancient (>1 Ga) mafic layers.

Oceanic basalts are formed by melting of a chemically and isotopically heterogeneous mantle source. The oceanic mantle probably resembles a marble cake containing layers of mafic rock - perhaps recycled ocean crust - stored in the mantle for >1 billion years. Many questions about the nature and distribution of these mantle heterogeneities remain. Here we show that lithological and isotopic traces of ancient mafic layers can still be seen in mantle rocks that have melted to form oceanic crust at a spreading centre in the Tethys Ocean. We have found centimetre-scale heterogeneity in initial osmium isotope ratios in mantle rocks from the Pindos Ophiolite. Deformed pyroxenite layers have high 187Os/188Os ratios (0.14-0.20) compared to adjacent host peridotites (187Os/188Os: 0.12-0.13). These layers were formed by a reaction between mantle rock and melt derived from ancient rocks with high Re/Os ratios. We interpret the pyroxenite layers as the wall rocks of billion-year old mafic layers that melted and transformed adjacent mantle peridotite into pyroxenite by melt-rock reaction. The pyroxenite layers are the relics of ancient metre-scale basaltic veins in a kilometre-sized marble cake domain in the oceanic mantle that has withstood homogenization on a billion-year time scale.

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