Mantle heterogeneity generated by melt depletion and melt-rock interaction: the West Iberian margin peridotites (ODP Legs 149 and 173).

Understanding the nature and evolution of mantle sequences exhumed along modern magma-poor ocean-continent transition zones (OCTs) is crucial for deciphering the complex interplay of tectonic and magmatic processes during rifting, lithospheric breakup, and the genesis of oceanic lithosphere. However, the existing data pertaining to these specific tectonic settings remain limited. In this study, we reassessed peridotites from the West Iberian margin (WIM) by performing a petrological and geochemical investigation of variably serpentinized mantle exposures sampled in the Iberia Abyssal Plain (ODP Holes 899B, 1068A and 1070A). The investigated samples exhibit a diverse spectrum of mineral modes, textures and chemistry. Hole 899B spinel harzburgites are coarse-grained granular peridotites (cpx content ~ 2-3 vol.%), displaying evidence of melt-rock interaction, evidenced by olivine-forming, pyroxene-dissolving microstructures. Mineral chemical variations, along with in-situ trace element analysis, reveal that these peridotites experienced moderate amounts of melt extraction, occurring partly within the garnet stability field, followed by reactive percolation of LREE-enriched melts. Microtextural analysis combined with mineral major element compositions, including high Cr# and TiO2 in spinel, as well as Na2O- and Al2O3-enriched clinopyroxene, supports a derivation from melt impregnation processes for plagioclase peridotites of Holes 899B and 1068A. Geochemical modelling based on clinopyroxene REE compositions indicates that these peridotites originated through melt-rock reactions driven by chemically heterogeneous melts, ranging from depleted to MORB-like compositions. Among the examined samples, Hole 1070A spinel harzburgites (cpx ~ 2-5 vol.%) exhibit the most distinctive features. The low modal clinopyroxene abundances, preserving a positive correlation between Cr#, Al2O3 and Yb, indicate that Hole 1070A peridotites represent a suite of residual mantle rocks recording variable degrees of melt extraction. However, the unusual Cr-Na enrichments, coupled to hump-shaped clinopyroxene REE patterns, cannot be solely ascribed to melt depletion. The counterintuitive features of these samples can be explained by a process of open-system melting occurring in the spinel stability field associated to contemporaneous percolation of an enriched melt. Calculated equilibrium temperatures for the WIM peridotites are within the range of fossil OCTs (TCa-in-Opx= 921-1029 °C). They record a history of melt-rock interaction at high temperature (TREE-Y= 1098-1244 °C), followed by thermal re-equilibration at relatively low rates (TOl-Sp= 770-813 °C). Our data provide new compelling evidence for a sub-continental lithospheric origin of the peridotite samples from ODP Holes 899B and 1068A. We show that the petrological and geochemical signature of the WIM peridotites reflect a polyphase history, which was achieved through the superimposition of rifting-related processes to melt extraction events, related to, or older than, the last Wilson cycle.