Chemical and isotopic evolution of mantle peridotites during plagioclase-facies melt-rock interaction (Tuscany ophiolites, Italy).

Here we present mineralogical, geochemical and Nd-Hf isotope data on mantle peridotites from the Monti Rognosi Jurassic ophiolite in southern Tuscany. The ophiolites are mainly composed of serpentinitized peridotites containing small gabbroic bodies. Relatively fresh mantle assemblages are locally preserved. Selected samples are clinopyroxene-poor lherzolites (Cpx ~5-7 vol %) with a porphyroclastic texture. Clinopyroxene porphyroclasts are generally corroded, most likely because of reaction with a melt that crystallized thin gabbroic (Pl + Opx) veins and patches. The clinopyroxene has relatively high Al2O3 (~7 wt. %), TiO2 (0.5-0.8 wt%) and Na2O (0.2-0.5 wt. %). Accessory Cr-rich spinel with irregular shapes is present and rimmed by calcic plagioclase (An= 81-90 mol%). High Cr# values of the spinel (43-46) are associated with high TiO2 contents (0.5-0.8 wt. %), as typically observed in plagioclase-bearing peridotites that experienced melt/rock reactions. REE-in-two-pyroxene geothermometer based on trivalent REE+Y exchange between Cpx and Opx, yields high T values (TREE) of 1260-1380°C, significantly higher than Ca-in Opx (1090 ± 20 °C), in agreement with the high closure temperature for the REE-in-two-pyroxene geothermometer. The remarkably high TREE may reflect thermal disturbance and/or geochemical disequilibrium due to melt-rock reaction under plagioclase facies conditions. Clinopyroxenes show wide REE heterogeneity, ranging from markedly to slightly LREE-depleted (CeN/SmN = 0.02-0.40) with nearly flat and highly variable HREE (YbN = 10-22) and Lu/Hf ratios (0.22-0.90). Bulk rock REE compositions apparently record small degrees (2-4%) of fractional melting in the spinel stability field, whereas the clinopyroxenes do not match the computed REE compositions of any mantle residual clinopyroxene. The peridotites are therefore considered to not represent simple partial melting residues. We attribute the clinopyroxene compositional variations to reaction with Opx-saturated, MORB-type melts. Nd-Hf isotope compositions of bulk rocks and clinopyroxenes calculated at the time of the gabbro intrusion (ɛNd = +8.4 to +9.9, ɛHf = +9.1 to +13.5) mostly fall in the MORB field. Highly radiogenic Nd preserved by one clinopyroxene separate (ɛNd = +15.6) hint that the peridotites experienced an ancient depletion event and that the reactive melt infiltration reset both Sm-Nd and Lu-Hf isotope systems.