Joel Rodney , Morten Andersen, Bramley Murton, Tim Elliott

Most mid-ocean ridge basalts (MORB) are depleted in highly incompatible elements relative to the primitive mantle, termed normal (N)-MORB. Some MORB, erupted at ridge segments distal from mantle upwellings, are enriched in incompatible elements. The origin of these enriched (E)-MORB is debated, although many studies have argued for recycled oceanic crust shaping their compositions. Uranium (U) and molybdenum (Mo) isotope ratios have been argued to trace the contribution of recycled oceanic crust in the source of N-MORB, which has high and low δ238U and δ98/95Mo relative to the bulk silicate Earth (BSE) respectively. We provide U and Mo isotopic data on E-MORB samples from the northern mid-Atlantic ridge (13° & 45° N). We analysed hand-picked, leached MORB glass, yielding 234U/238U near secular equilibrium and therefore reflecting unperturbed by surface processes compositions. Samples have uniform δ238U and δ98/95Mo, with means of −0.307 ± 0.032 ‰, 2sd, and −0.14 ± 0.04 ‰, 2sd, respectively, both within uncertainty of BSE, and distinct from N-MORB. These data, as well as unremarkable Ce/Pb and radiogenic Pb isotopic compositions in E-MORB globally, are incompatible with their source containing recycled oceanic crust or continental derived sediments. Instead, our data fit with a model of oceanic lithosphere metasomatism by low degree partial melting of the uppermost mantle. Given BSE like U isotopic compositions of E-MORB that are isotopically unfractionated during low degree partial melting, we suggest that the initial melting event must have occurred prior to the recycling of isotopically distinct in U oceanic crust into the upper mantle, i.e., prior to ca. 600 Ma, the estimated time of deep ocean oxygenation. Molybdenum isotopic compositions of E-MORB are in line with such a model, but also reflect isotopic fractionation to higher δ98/95Mo during low degree partial melting of ≥600 Ma upper mantle, that counter acts the lowering of δ98/95Mo in the upper mantle by an on-going process of plate recycling. Metasomatised oceanic lithospheric mantle portions freeze in these ≥600Ma U and Mo isotopic compositions, which are subducted and stirred back into the convecting upper mantle, ultimately to be sampled at ridges as E-MORB.