GEOCHEMICAL VARIABILITY OF MANTLE ROCKS DRILLED IN AN OCEAN-CONTINENT TRANSITION: NEW INSIGHTS FROM THE IBERIAN MARGIN PERIDOTITES (ODP LEG 149 AND 173)

Mantle rocks exposed along ocean-continent transitions represent an exceptional archive for investigating processes occurring during lithospheric extension and break-up, providing important clues on the chemical heterogeneities in the Earth’s mantle. In this contribution, we revisit the Iberian margin peridotites through a combined petrological and geochemical investigation performed on a new set of drilled samples (ODP Holes 899B, 1068A, 1070A). Hole 899B peridotites are relatively fresh tectonic harzburgites, displaying variable clinopyroxene amounts (≈ 3-8 vol.%) and mineral chemisty (0.898≤Mg#[Ol]≤0.915; Al2O3[Opx]= 2.95-3.46 wt.%; 0.198≤Cr#[Spl]≤0.483). Holes 1068A and 1070A peridotites are characterized by higher serpentinisation degrees and wider compositional ranges (Hole 1068A: Al2O3[Cpx]= 3.77-6.79 wt.%; 0.253≤Cr#[Spl]≤0.376; Hole 1070A: Al2O3[Opx]= 2.65-3.70 wt.%; 0.378≤Cr#[Spl]≤0.452). In-situ trace element analyses revealed a wide spectrum of geochemical signatures. Primary clinopyroxene was found only in Holes 1068A and 899B. The clinopyroxene yields abyssal-type patterns, with gently (LaN/SmN= 0.22-0.27) to steeply plunging (LaN/SmN= 0.02-0.03) LREE for Holes 1068A and 899B, respectively. Orthopyroxene of Hole 899B has low REE concentrations (HREEN≤ 1) and spoon-shaped patterns, excepted for one plagioclase-bearing sample, which exhibits a straight pattern (LaN/YbN =0.001-0.02) and negative Eu anomalies. Hump-shaped REE patterns, yielding variable LREE-MREE fractionation (LaN/SmN= 0.003- 0.16) and flat HREE, were found in Hole 1070A. These patterns are atypical for mantle peridotites and cannot be explained by melt depletion. Our new data shed light on the occurrence of different mantle domains exposed along the ocean-continent transition of the Iberian margin. These features cannot be accounted by margin extension and oceanization, as samples with the most oceanic affinity (Holes 1068A and 899B) are preserved close to the Iberia continental margin, while hump-shaped patterns appear common offshore (Hole 1070A). We interpret these contrasting signatures as related to a complex interplay of depletion and melt-rock interaction, partly inherited from a previous Wilson cycle.