Chemical and Nd-Hf isotope heterogeneity in depleted mantle domains from the Alpine-Apennine ophiolites.

Here we present geochemical and Nd-Hf isotope data for three depleted mantle bodies from the Northern Apennine (Internal Ligurian, IL), Tuscany (Monti Rognosi, MR) and Western Alps (Monte Civrari, MC) ophiolites. The Internal Liguride and Monti Rognosi peridotites record low to moderate degrees of depletion and display widespread mineralogical and geochemical evidence of melt-rock interaction. Their clinopyroxene compositional variation are mainly related to reactive percolation of residual peridotites by depleted, ortopyroxene-saturated melts in the plagioclase-stability field. Both Sm-Nd and Lu-Hf isotope systems were partially reset by the melt infiltration, but preservation of highly radiogenic ԑNd (up to +15) at the time of the associated MORB-type magmatism (162 Ma) are consistent with derivation of the IL and MR peridotites from ancient mantle reservoirs that experienced long-term depletion. The Monte Civrari mantle body includes domains of residual spinel harzburgites characterized by TiO2 (0.05-0.15 wt. %) and Na2O (< 0.1 wt.%)-poor clinopyroxene, with striking LREE depletion (CeN/SmN = 0.004-0.005), low HREE abundances (YbN ~ 5-6) and fractionated HREE (GdN/YbN = 0.4-0.5). The highly radiogenic Nd-Hf isotope composition of MC peridotites (initial ԑNd and ԑNd up to +29 and + 41, respectively) may reflect incorporation of refractory subcontinental lithospheric mantle (SCLM) in the oceanic lithosphere of the Jurassic Alpine Tethys. However, the thermal evolution of the MC mantle body points to rapid cooling and exhumation from asthenospheric conditions, similar to modern abyssal type peridotites (see also Mc Carthy & Muntener, 2019), which argues against a long residence time in the SCLM after melt extraction. REE and Nd-Hf isotope compositions of the MC peridotites may be reconciled with Jurassic re-melting of an asthenospheric source that underwent a first melting event, starting in the garnet stability field, in Palaeozoic times (> 300 Ma). The Nd-Hf isotopic contrast between Jurassic crustal products (Barry et al., 2015) and associated residual peridotites consolidates the notion that ancient depleted domains may be a significant constituent of the convecting upper mantle. Barry T.L., Davies J.H., Wolstencroft M., Millar I.L., Zhao Z., Jian P., Safonova I. & Price M. 2017. Whole-mantle convection with tectonic plates preserves long-term global patterns of upper mantle geochemistry. Sci. Reports 7, 1870 McCarthy A. & Müntener O. 2019. Evidence for ancient fractional melting, cryptic refertilization and rapid exhumation of Tethyan mantle (Civrari Ophiolite, NW Italy). Contrib. Mineral. Petrol. 174, 69