No abstract

Pyroxenite layers embedded within peridotite represent widespread lithological mantle 2 heterogeneities and are potential components in the mantle source of many oceanic basalts. 3 Pyroxenites can be generated by several magmatic and metamorphic processes. However, in 4 most natural samples (especially in ultramafic massifs), their primary characteristics are 5 partially or completely erased by later petrologic evolution (e.g. metamorphism, metasomatism or partial melting). Here we investigate a suite of pyroxenites from the 7 External Liguride Jurassic ophiolites (Northern Apennines, Italy). They are spinel-bearing 8 websterites and clinopyroxenites, partially recrystallized under plagioclase-facies conditions, 9 and occur as cm-scale layers parallel to the tectonite foliation of their host peridotites. 10 Pyroxenites have bulk Mg-numbers from 74 to 88 and display rather constant LREE depletion 11 over the MREE (La N /Sm N = 0.15-0.35), but variable MREE-HREE fractionation, with some 12 of them having markedly positive HREE slopes (Sm N /Yb N =0.30-0.96). The HREE 13 enrichment coupled with high Zr and Sc contents in clinopyroxene porphyroclasts from 14 spinel-bearing domains provides strong evidence that garnet was present in the precursor 15 mineral associations. Mass balance calculations suggest that the pyroxenites originally 16 contained up to about 40 vol% of garnet, indicating they originated by segregation of melts at 17 rather high pressure (P > 1.5 GPa). Parental melts of pyroxenites have reacted to some extent 18 with the host peridotite during mantle infiltration. Lack of olivine in the primary mineral 19 assemblage and orthopyroxene-rich rims along the contact with wall-rock peridotites indicate 20 that pyroxenites have crystallized from silica-rich melts. These, likely, had REE patterns and 21 Sr-Nd isotope compositions similar to enriched MORB. We propose that the pyroxenites 22 originated from melts derived from a hybrid eclogite-bearing peridotite source, and then 23 reacted with the host peridotite to form "secondary pyroxenites". Their existence has been 24 invoked in current models of basalts petrogenesis. During later decompression, they 25 experienced an intermediate recrystallization at spinel-facies conditions, at 1.2-1.5 GPa and 26 minimum temperature of 950-1000°C, and partial re-equilibration at low-pressure 27 plagioclase-facies. The latter is dated by internal Sm-Nd isochrons at 178 (±8) Ma and is 28 associated with Mesozoic exhumation, during extension of the Tethys lithosphere.

Pyroxenites embedded in peridotite are often invoked as a major cause of short-length\ud scale isotopic heterogeneities in the upper mantle, but there has been little direct evidence.\ud We report spatially controlled chemical and Sr-Nd isotopic compositions of pyroxenites and\ud their host peridotites from an ophiolitic mantle sequence in the Northern Apennines, Italy,\ud with depleted mantle compositions, representing a surface exposure of veined upper mantle, a\ud potential source for mid-oceanic-ridge basalts (MORB). Interaction between pyroxenites and\ud adjacent mantle rocks results in centimeter-scale chemical modifi cations in the host peridotites,\ud systematically lowering their Sm/Nd ratios. Over time, this interaction causes the host\ud peridotite at >0.1 m scale to acquire an isotopic heterogeneity larger than the range defi ned\ud by the peridotite and pyroxenite end-members. Moreover, the 143Nd/144Nd variation of a single\ud outcrop covers most of the global Nd isotopic variability documented in abyssal peridotites.\ud Such pyroxenite-peridotite veined mantle domains may represent the enriched component\ud rarely found in abyssal peridotites, but often invoked to account for the low end of 143Nd/144Nd\ud variations in MORB