Melting mud in Earth's mantle, evidence from sub-Moho S-type granites with extreme oxygen isotope signatures

The role of sediment melting in Earth's mantle and its subsequent incorporation into volcanic arc magmas remains controversial, as direct observation of melt generation in the mantle is not possible. Geochemical fingerprints of sediment provide strong indirect evidence for subduction delivery of surface materials to the mantle, however sediment abundance in mantle-derived melt such as basalt is generally low (0-2%), and difficult to detect. Here, we provide compelling evidence for bulk melting of subducted marine sediment in the mantle through analysis of granite preserved within an exhumed mantle section. Peraluminous granite dykes intruding peridotite below the petrologic Moho in the Oman-UAE ophiolite have unusually high oxygen isotope (δ18O) values for whole rock and quartz, and host magmatic zircon with the highest δ18O values ever reported, up to 27‰ (relative to VSMOW). The extremely high oxygen isotope ratios uniquely restrict the source to high δ18O siliceous marine sediment (i.e. mud; includes shale and chert or siliceous ooze), as no other source rock could produce granite with such anomalously high δ18O. Elemental composition, mineralogy, and zircon Hf identifies the granite suite as S-type, having formed by melting of supracrustal material. Zircon U-Pb ages of 100 Ma confirms a pre-obduction age of the dykes. The formation of high δ18O S-type granite within mantle peridotite has not been documented previously, and requires delivery of high δ18O marine sediment to the asthenosphere by subduction, where it melted and intruded overlying mantle wedge. The S-type granite suite intrusive to the mantle in the Oman-UAE ophiolite contains the most evolved oxygen isotope ratios reported for igneous rocks. Quantifying the extent of sediment melting within the mantle has important consequences for understanding crustal recycling and mantle heterogeneity over time.