Evidence for large-scale, long-term highly siderophile element heterogeneities in the Atlantic mantle from Leg 153 and 209 peridotites

Abstract

Seafloor abyssal peridotites are products after partial melting and melt-rock interaction, providing constraints on upper mantle processes and heterogeneity. Despite extensive studies of ophiolitic peridotites and a burgeoning dataset for dredged abyssal peridotites, limited studies have been undertaken on abyssal peridotite drill core samples. This omission means a lack of comparison between modern ocean mantle lithosphere and that of ophiolites, where field context is possible in the latter, but not the former. Ocean Drilling Program (ODP) Leg 153 near the Kane fracture zone, and ODP Leg 209, near the Fifteen-Twenty fracture zone, both on the mid-Atlantic Ridge but ∼1000 km distant from one another, recovered serpentinized mantle peridotites. We report whole-rock major-, trace- and highly siderophile-element (HSE: Re, Pd, Pt, Ru, Ir, Os) abundances and 187Os/188Os ratios of peridotites from three cores (maximum depth of 200.8 m) from the two Legs to explore mantle heterogeneity within the Atlantic Ocean basin. Peridotites from ODP Leg 153 are relatively fertile, with Al2O3 contents ranging from 1.1 to 2.7 wt%. They are characterized by relatively radiogenic 187Os/188Os (0.1259 ± 0.0017; 2 S.D.) and uniform HSE abundances. The ODP Leg 209 peridotites are highly refractory, with Al2O3 contents ranging from 0.1 to 0.8 wt%, and imply that they record an extensively depleted mantle. They have 187Os/188Os of 0.1207 ± 0.0023 (2 S.D.) and fractionated HSE abundances, indicative of depletion occurring > 1 billion years ago. Marked heterogeneity both in the extent and timing of depletion are therefore evident along a large segment of the mid-Atlantic ridge that produces zero-age basalts. Measurement of Os isotopes in spatially associated mid-ocean ridge basalts (MORB) is challenging due to their sub ng/g Os contents. Instead, Nd isotopes of MORB are used to show that they are unlikely to have formed directly from abyssal peridotite-like sources. Abyssal peridotites therefore represent long-lived refractory residues during mantle upwelling with the sources of MORB being more enriched. Comparison between drilled abyssal peridotites and ophiolitic mantle reveals larger compositional variations in the latter. The overall higher melting extent in ophiolite mantle is likely reflective of a more complex life cycle than the modern oceanic lithosphere.