Mantle Heterogeneity Generated by Melt-Rock Reaction: New Insights from the West Iberian Margin Peridotites (ODP Legs 149 and 173)

Among continental margins worldwide, the West Iberian Margin (WIM) stands out as one of the best-documented ocean-continent transition zone with oceanic scientific drilling sampling several exhumed mantle outcrops (Boillot et al., 1989). However, due to their high serpentinization, the petrogenesis of the WIM mantle exposures was largely unexplored. Here, we present new petrological and geochemical data obtained on variably serpentinized peridotites from ODP Holes 899B, 1068A, and 1070A. Hole 899B spinel (spl)-harzburgites show pyroxene embayments filled by olivine, anomalously high olivine content (up to ≈ 80 vol.%) and small interstitial orthopyroxene (opx). These textures coupled to depleted signatures (clinopyroxene (cpx)≈ 2-3 vol.%, Yb cpx ≈ 1-6 –N = chondrite normalized) suggest moderate melt extraction followed by reactive porous flow. Holes 1068A and 899B pl-peridotites share notable similarities, including the presence of plagioclase rims mantling deformed spinel, Na O- and Al O -rich cpx, high spl Cr# and TiO . These features, along with cpx convex-upward REE patterns yielding negative Eu anomalies and HREE higher than abyssal peridotites, indicate melt-rock reaction and re-equilibration in the plagioclase field. Hole 1070A peridotites show contrasting geochemical signatures, with compositions ranging from abyssal-like (Al O -poor cpx; high Cr# = 0.246-0.428) to refertilized sub-continental mantle (Na O rich cpx). Opx displays hump-shaped REE patterns with low HREE (Yb ≈ 1-2). Our data reveal widespread heterogeneous mantle domains exposed along a modern OCT. We interpret this heterogeneity as resulting from different stages and styles of melt-rock reaction operating on variably depleted sub-continental mantle lithosphere protoliths. We posit that melt-rock interactions represent among the most effective mechanisms to create petrological and chemical heterogeneity in OCTs and the upper mantle in general.