Small scale heterogeneities in MORB mantle: Insights on pyroxenite-peridotite association from External Liguride ophiolites (Italy)

The occurrence of mafic layers in peridotites constitutes an important compositional heterogeneity in the mantle, and their role in mantle melting and basalt generation is currently debated (e.g. Hirschmann &Stolper, 1996; Salters & Dick, 2002). Pyroxene-rich lithologies are commonly documented in subcontinental lithospheric mantle, from both tectonically-emplaced ultramafic massifs and mantle xenoliths (e.g. Downes, 2007), and rarely sampled in abyssal peridotites (e.g. Warren et al., 2009). They are mostly interpreted as high-pressure magmatic products and rarely as remnants of recycled oceanic crust. However, the origin and composition of such lithologies, and their potential role in creating small-scale heterogeneities in the MORB mantle remain still poorly constrained. In this study, we present field, chemical and isotopic data on pyroxenites and host peridotites from some peridotite massifs of the External Liguride ophiolitic Units (Northern Apennines, Italy) with the aim to widen the knowledge on the origin of pyroxenites in MORB-type settings and discuss the occurrence and extent of chemical and isotopic heterogeneities in the MORB mantle related to the pyroxenitic component. At this latter purpose, pyroxenites and host peridotites have been accurately sampled from single dm-scale profiles, in order to test whether the existence of lithologic heterogeneities in the mantle in turn reflects the occurrence of isotopic contrasts at the small scale. Pyroxenites occur as cm-thick bands (up to 12 cm) parallel to the tectonite mantle foliation and vary from spinel-bearing websterite to clinopyroxenite. As the host peridotites, they are partially recrystallized at plagioclase-facies conditions. The occurrence of orthopyroxene-rich rim between pyroxenite and peridotite and large poikilitic orthopyroxene in the wall-rock peridotites indicate that host peridotites have interacted with melts related to pyroxenite intrusion. These latters display very heterogeneous whole-rock chemistry, almost covering the entire compositional range defined by worldwide lithospheric pyroxenites (XMg = 74-88, Al2O3 = 10-17 wt%, CaO = 7-20 wt%). The pyroxenite chemistry reflects high-pressure magma segregation of tholeiitic melts dominated by clinopyroxene crystallization. Both mineral and bulk-rock compositions from peridotite-pyroxenite traverses reveal that the host peridotites have been significantly modified in terms of major (e.g. XMg, Al2O3, CaO) and trace element (e.g. the LREE) composition by the interaction with pyroxenite component. A multi-step, sequential leaching procedure on clinopyroxene separates enables us to remove partial contamination and provides reliable Sr isotope data. The Sr and Nd compositions of clinopyroxenes from pyroxenites and peridotites fall in the typical range of normal MORB ( 87Sr/86Sr = 0.7023-0.7029; 143Nd/144Nd = 0.5134-0.5128). Internal Sm-Nd isochrons based on plagioclase, clinopyroxene and whole-rock from several pyroxenite samples yield ages ranging from 174 to 183 Ma (with errors between ±11-25) for the low-P mantle exhumation. On a slightly larger spatial scale, isotopic profiles through the pyroxenite-peridotite boundaries indicate cm-scale modification of the wall-rock peridotite, presumably related to emplacement of the pyroxenites. This suggests that deep melt intrusion can locally modify the host peridotites and introduce small scale compositional heterogeneity in a MORB mantle. References: Downes, H. (2007): Lithos, 99, 1-24; Hirschmann, M.M. & Stolper, E.M. (1996): Contrib, Min. Pet., 124, 185-208; Salters, V.J.M. & Dick. H.J.M. (2002): Nature, 418, 68-72; Warren, J., Shimizu, N., Sakaguchi, C., Dick, H.J.M. & Nakamura, E. (2009): J. Geophys. Res., 114, B12203.