Unveiling mantle heterogeneity in a modern OCT: new insights from the West Iberian margin (ODP Leg 149 and 173)

Magma-poor ocean-continent transition zones (OCTs) encompass extensive regions of tectonically uplifted mantle. While substantial literature has explored fossil analogues (e.g., Picazo et al., 2016), recent studies on mantle sequences exhumed in modern OCTs are limited (e.g., McCarthy et al., 2020), resulting in a fragmented understanding. Key questions regarding the mechanisms, timing, and location of lithospheric breakup, melt production, and the nature of the mantle source remain debated. The West Iberian margin (WIM) is one the best-documented continental margins worldwide, with scientific drilling initiated more than three decades ago (e.g., Boillot et al., 1989). However, peridotites from this region have remained poorly characterized due to their extensive serpentinization. In this study, we revisit the peridotites from the OCT of the WIM, presenting new in situ data for a set of variably serpentinized samples. To capture a comprehensive view of the petrological and geochemical characteristics, we analyzed samples from three boreholes along an E-W transect, from the most proximal to the most oceanward domains: ODP Hole 1068A, 899B, and 1070A. Hole 1068A samples are serpentinized plagioclase lherzolites (cpx ~ 10 vol.%) with Na2O- (0.61-0.87 wt%) and Al2O3-rich (5.83-7.10 wt%) clinopyroxene, high spinel Cr# (0.255-0.322) and TiO2 (0.21-0.30 wt%). These characteristics, along with clinopyroxene convex-upward REE patterns yielding negative Eu anomalies and high YbN= 15-17, resemble those of refertilized domains in the sub-continental lithospheric mantle (SCLM, e.g. Müntener et al., 2010). Geochemical modelling indicates an origin involving MORB-type melt impregnation followed by re-equilibration in the plagioclase stability field. Hole 899B exhibits the higher degree of lithological and chemical heterogeneity. The investigated rock-types are coarse-grained peridotites, varying from spinel to plagioclase harzburgites (cpx ~ 2-8 vol.%). They show evidence of melt-rock interaction, highlighted by olivine-forming, pyroxene-dissolving microstructures. Plagioclase ± secondary orthopyroxene aggregates were also observed in sample 899-1. Mineral compositions (i.e. clinopyroxene and spinel) have moderately depleted to slightly enriched signatures (Cpx: Al2O3= 4.05-6.90 wt%, Na2O= 0.23-0.63 wt%; Sp: Cr#= 0.230-0.463, TiO2= 0.58-0.61 wt% for sample 899-1). Clinopyroxene in spinel harzburgites displays uncommon chondrite-normalized V-shaped REE patterns (LaN/SmN= 2.7-3.4, DyN/YbN= 0.5-0.6) previously recorded in mantle xenoliths from central and eastern Europe (e.g. Downes et al., 2003). In contrast, clinopyroxene in plagioclase-bearing sample shows concave-downward REE patterns, with depleted LREE segments (LaN/SmN= 0.02-0.03) and negative Eu anomalies, suggesting melt depletion followed by re-equilibration in the plagioclase stability field. Hole 1070A peridotites are coarse-grained spinel harzburgites (cpx ~ 2-5 vol.%) displaying pyroxene-dissolving and olivine-precipitating microtextures. Positive correlations among melting indexes in clinopyroxene (i.e. Cr#, Al2O3 and Yb) suggest a residual origin for these lithologies. However, unusual Cr-Na enrichments and hump-shaped clinopyroxene REE patterns indicate open system melting in the spinel stability field, accompanied by percolation of an enriched melt. These features, though rare in modern oceans, were previously attested in some peridotites from slow- to ultraslow-spreading settings (Hellebrand & Snow, 2003; Seyler et al., 2011). Calculated equilibrium temperatures for the WIM peridotites range within those of fossil OCTs (TCa-in-Opx= 921-1029 °C). They reflect a history of melt-rock interaction at high temperature (TREE-Y= 1098-1244 °C), followed by thermal re-equilibration at lower temperatures (TOl-Sp= 770-813 °C). These new data provide compelling evidence of highly heterogeneous mantle domains exposed in the OCT of the WIM. The documented heterogeneity, observed among peridotites from the same borehole and among boreholes, challenges the traditional view of OCTs as regions with simple and fixed distribution of mantle domains with well-defined major and trace element compositions. We posit that melt-rock interaction plays a key role in generating petrological and chemical heterogeneity in OCTs and the upper mantle. References Boillot, G., Féraud, G., Recq, M. & Girardeau, J., 1989. Undercrusting by serpentinite beneath rifted margins. 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