Mantle Heterogeneity Generated by Melt Depletion and Melt–Rock Interaction: The West Iberian Margin Peridotites (ODP Leg 149 and 173)
Authors: Secchiari Arianna , Godard Marguerite , Montanini Alessandra 
Published: January 2025

Abstract
Understanding the nature and evolution of mantle sequences exhumed along modern magma-poor ocean–continent transition zones (OCTs) is essential for deciphering the complex interplay of tectonic and magmatic processes from rifting to oceanization. However, the data on these specific tectonic settings remain limited. In this study, we revisit the peridotites from the West Iberian margin (WIM) through a petrological and geochemical investigation of the mantle basement sampled in the Iberia Abyssal Plain (IAP). We focus on three ODP sites constituting an east-to-west transect, from Hole 1068A to Holes 899B and the westernmost Hole 1070A, to capture the full spectrum of mantle variability from the continental to the most oceanward domains. The analyzed samples consist of variably serpentinized, coarse-grained peridotites preserving evidence of melt–rock interactions. In Hole 1068A, strongly serpentinized plagioclase lherzolites are found. Mineral chemistry, i.e. high Cr# and TiO2 in spinel, along with Na2O- and Al2O3-enriched clinopyroxene, indicate that these samples are akin to impregnated peridotites from the sub-continental lithospheric mantle (SCLM). Geochemical modelling based on clinopyroxene REE compositions points to refertilization processes driven by MORB-type melts. In contrast, Hole 899B displays more pronounced variability and apparently contrasting geochemical features. Spinel harzburgites (cpx ~ 2–3 vol %) exhibit evidence of olivine-forming, pyroxene-dissolving microstructures. Their slightly depleted clinopyroxene compositions (Na2O = 0.23–0.60 wt %), combined with atypical chondrite-normalized V-shaped REE patterns, suggest that these rocks experienced partial melting followed by interaction with LREE-enriched melts. These peridotites may represent inherited SCLM, or a refractory peridotite modified by rifting-related melts. One sample however, plagioclase harzburgite 899-1 (cpx ~ 8 vol %), is distinguished by peculiar microtextures—such as opx2 ± pl aggregates replacing olivine porphyroclasts—and spinel compositions (TiO2 up to 0.6 wt %), which indicate late melt–rock impregnation. The concave-downward clinopyroxene REE patterns point to interaction with a depleted melt. Hole 1070A spinel harzburgites (cpx ~ 2–5 vol %) exhibit the most distinctive features. Their low clinopyroxene content, with a positive correlation between Cr#, Al2O3 and Yb, suggests that these peridotites represent a suite of partial melting residues. However, the unusual Cr-Na enrichments and the rare hump-shaped clinopyroxene REE patterns cannot be solely ascribed to melt depletion. These characteristics reflect open-system melting within the spinel stability field, accompanied by the influx of an enriched melt generated in the garnet stability field. This process likely occurred during the deep lithospheric mantle’s transition from rifting to oceanization. Calculated equilibrium temperatures for the WIM peridotites fall within the range of fossil OCTs (TCa-in-Opx = 921–1029°C). As a whole, they indicate a high-temperature history (TREE-Y = 1098–1244°C) related to melt–rock interaction, followed by slow thermal re-equilibration (TOl-Sp = 770–813°C). Our new data provide compelling evidence for mantle heterogeneity in the WIM, observed both among different drill sites and within the same borehole. Our study challenges the classical and simplified perspective of OCTs as regions characterized by a fixed distribution of mantle domains with clearly defined chemical compositions. Instead, we propose that OCTs are composed of a complex mosaic of mantle signatures, reflecting superimposition of recent processes related to rifting and oceanization over an earlier, and possibly ancient, evolutionary history.