Oxygen-isotope evidence for altered oceanic crust in the Atlantic MORB source

A long-wavelength geochemical gradient in Mid-Atlantic Ridge (MAR) samples collected between 33 and 41°N has previously been attributed to mixing of material from depleted and enriched (EMII-type) mantle components. Enriched mantle domains are principally identified using isotopes of incompatible trace elements, which can be dominated by small mass fractions of enriched material. Oxygen, by contrast, occurs in roughly constant abundance in most mantle and crustal minerals and deviations from mantle δ^(18)O reflect near-surface processes; thus variations in δ^(18)O can be used to constrain the percentage of recycled near-surface materials within a given source region. We present here high-precision oxygen-isotope analyses of MAR glasses and discuss the implications for the nature and percentage of the regional enriched component