Mantle depletion recorded by olivine and plagioclase megacrysts in oceanic basalts

Earth’s oceanic crust is largely formed by melting of the upper mantle where it upwells beneath mid-ocean ridges, providing a geographically widespread elemental and isotopic ‘sample’ of Earth’s interior. Radiogenic isotope variations in oceanic basalts are commonly attributed to compositional heterogeneity in Earth’s upper mantle, albeit reduced by mixing and reaction during melt ascent. Nevertheless, many mid-ocean ridge basalts are biased towards incompatible element enriched radiogenic isotope compositions, and questions remain as to whether their chemistry is indeed representative of the underlying mantle. Here we present Pb isotope data for plagioclase megacrysts (and olivine-megacryst hosted inclusions) from oceanic basalts that crystallised in the lower oceanic crust or mantle, from incompatible element depleted melts. Our data show that the plagioclase megacrysts and olivine grew from melts with substantially less radiogenic Pb compositions than their host lava. High Ca plagioclase megacrysts are common in oceanic basalts, suggesting that depleted melts in the lower crust or mantle may also be widespread, acting to balance the enriched isotope compositions seen in many erupted basalts.