Meter-Scale Chemical and Isotopic Heterogeneities in the Oceanic Mantle, Leka Ophiolite Complex, Norway

Abstract

AbstractMantle peridotites from three 3 × 3-meter grids sampled at kilometer distances from one another in the ca. 497 Ma Leka Ophiolite Complex (LOC), Norway, are examined to investigate the chemical and isotopic nature of oceanic mantle domains at the centimeter to kilometer scale. The lithology of each grid locality is predominantly harzburgite, but includes layers and lenses of dunite and pyroxenite. Major and lithophile trace element compositions indicate a history of prior melting at pressures at or slightly below the garnet stability field. The common presence of orthopyroxenite veins likely reflects infiltration of silicic melts associated with supra-subduction zone processes. Osmium isotopes and highly siderophile element (HSE) abundance data for centimeter-scale sampling of traverses from the pyroxenites into the harzburgites reveal that the formation of the veins had little effect on Os isotopic compositions, and Os, Ir, Ru and Re abundances in the harzburgites. Adjacent to one of the orthopyroxenite veins studied, however, Pt and Pd abundances appear to have been strongly modified by interactions with vein-forming melts or fluids at distances of as much as 4–6 cm from the pyroxenite-harzburgite contact. Leka harzburgites have initial γOs values (% deviation from a chondritic reference) that range from −4.7 to +2.2 (6.9% variation), with individual uncertainties of ±0.2 units. Averaged initial Os isotopic compositions for harzburgites from the three grid sites separated by as much as 6 km, by contrast, differ by only a maximum of 2.6%. Isotopic heterogeneity on the centimeter to meter scale is, therefore, larger than kilometer-scale heterogeneity, indicating that at least some of the Os isotopic heterogeneity commonly observed globally among mantle peridotites is the result of processes that acted on a local scale. The general uniformity of these isotopic compositions among the three grid sites suggests that the portion of the oceanic mantle sampled by the LOC was homogenous at the kilometer scale with respect to the long-term Re/Os ratio. The long-term projected Re/Os for LOC harzburgites is similar to the average required for modern abyssal peridotites. This observation strengthens previous interpretations, based largely on data for abyssal peridotites, that state the Os isotopic evolution of oceanic mantle is consistent with a long-term 187Re/188Os of ∼0.38. The present ∼3 to 4% difference between the Os isotopic composition of the modern oceanic mantle and estimates for primitive mantle suggests that at least ∼6% of the mass of the oceanic mantle has been removed from it in the form of Re-enriched, mafic oceanic crust. Despite the recycling of this crust back into the mantle, most of it has evidently not been mixed back into accessible portions of the upper oceanic peridotite mantle. Compared to composition estimates for the primitive mantle, the median HSE compositions for the three grid sites are moderately to strongly depleted in Pd and Re, consistent with the corresponding lithophile element evidence for 20–30% melt depletion. As with initial γOs values, most harzburgites from a given grid are characterized by greater variations in absolute and relative HSE abundances than the differences between the median abundances of the three grid sampling locales. This observation indicates that as with Os isotopes, the HSE abundance heterogeneity among the harzburgites most strongly reflects centimeter- to meter-scale melting and remobilization effects. Except for Ru, median HSE abundances for grid harzburgites are similar to median abundances for abyssal peridotites. The ∼30% lower median Ru/Ir in the LOC compared to the median ratio for abyssal peridotites suggests that the abundance of Ru in the oceanic mantle may be more variable than generally thought.