Traces of ancient mafic layers in the Tethys oceanic mantle

Abstract

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 187 Os/ 188 Os ratios (0.14-0.20) compared to adjacent host peridotites (187 Os/ 188 Os: 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 mantle peridotite into pyroxenite by melt-rock reaction. The pyroxenite layers are the melting residue of ancient metre-scale basaltic veins in a kilometre-sized marble cake domain in the oceanic mantle that has withstood homogenization on a billionyear time scale.

Mineralogy, geochemistry and geotectonic significance of harzburgites from the southern Dramala upper mantle suite, Pindos ophiolite complex, NW Greece

“…In the southern Dramala mantle layering is missing, whereas it occurs in the northeastern part of the Dramala massif (Sergeev et al, 2014). In Milia, harzburgites are characterized by the limited preservation of a 'diapiric' fabric, which has also been reported (although better developed) from the Vourinos massif.…”

“…Rassios and Dilek (2009) described as 'ductile highways' the mylonite shear zones located in the Dramala complex and presented their significance in the lateral transport of the mantle section during the initial displacement of the Jurassic Pindos oceanic lithosphere. Sergeev et al (2014) examined the lithological and isotopic variations of ancient pyroxenite layers intercalated with harzburgites ('mantle layering') in the northeastern part of the Dramala massif. Kapsiotis (2014) studied Crspinel compositions from the Pindos serpentinized peridotites and supported that these rocks represent MOR-type mantle residues that were later entrapped and modified in the wedge above a SSZ.…”

“…The investigated regions are highly mountainous, with summits up to 2.150 m above sea level (e.g. Such structures are believed to represent the oldest high-T mantle flow fabric in the Dramala complex, known as 'mantle layering' (Sergeev et al, 2014). In both districts voluminous exposures of tectonized and massive harzburgites represent the dominant lithology (~87%), followed by subordinate dunite (~13%) and pyroxenite.…”

“…Kapsiotis (2014) studied Crspinel compositions from the Pindos serpentinized peridotites and supported that these rocks represent MOR-type mantle residues that were later entrapped and modified in the wedge above a SSZ. Sergeev et al (2014) examined the lithological and isotopic variations of ancient pyroxenite layers intercalated with harzburgites ('mantle layering') in the northeastern part of the Dramala massif. They interpreted these layers as the wall rocks of billion-year-old eclogitic layers with high Re/Os ratios, which melted and transformed the adjacent mantle peridotite into pyroxenite via a melt-rock reaction process.…”

Physiognomy and timing of metasomatism in the southern Vourinos ultramafic suite, NW Greece: a chronicle of consecutive episodes of melt extraction and stagnation in the Neotethyan lithospheric mantle

“…Both of these high-T structures are supposed to represent two primordial deformational fabrics inherited by asthenospheric flow. Although they have been documented in the northern sector of the Vourinos and Dramala peridotite complexes (Ross et al 1980;Rassios and Dilek 2009;Sergeev et al 2014), they are generally missing from the southern domains of the perpetual Pindos-Vourinos mantle. At the area of Aetoraches, dunite exhibits sizeable olivine crystals aligned to a planar imprinting ductile fabric similar to that of dunites from the nearby Moho ).…”