Constraints from uranium and molybdenum isotope ratios on the origin of enriched mid-ocean ridge basalts

Most mid-ocean ridge basalts (MORB) are depleted in highly incompatible elements relative to the primitive mantle and are termed normal (N)-MORB. Some MORB, erupted at ridge segments distal from mantle hot-spots, are enriched in incompatible elements. The origin of these enriched (E)-MORB is debated, although many studies have proposed that recycled oceanic crust shapes 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 δ²³⁸U and low δ^98/95Mo relative to the bulk silicate Earth (BSE). Here, 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, therefore reflecting samples unperturbed by surface processes. Samples have uniform δ²³⁸U and δ^98/95Mo, 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 sources containing recycled oceanic crust or continental derived sediments. Instead, our data fit with a model of low degree partial melting of the uppermost mantle that metasomatises the sub-oceanic lithosphere. 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). Metasomatised portions of oceanic lithospheric mantle preserve these ≥600 Ma U isotopic compositions, which are subducted and stirred back into the convecting upper mantle, ultimately to be sampled at ridges as E-MORB. Molybdenum isotopic compositions of E-MORB are in line with such a model but also reflect isotopic fractionation to higher δ^98/95Mo during low degree partial melting of ≥600 Ma upper mantle, that counter acts the lowering of δ^98/95Mo in the upper mantle by an on-going process of plate recycling.