Heavy boron isotopes in plume-derived magmas trace recycled water in deep-mantle reservoir

The debate on the origin of water in the Earth’s interior focuses on whether it originates from internal primordial sources or externally from the Earth’s surface via subduction-related processes. Here we report boron isotope data for Middle/Late Permian picrites and basalts from the Emeishan Large Igneous Province in South China. The observed heavy δ11B values (up to +27.6‰) fall outside the range of known mantle compositions, but similar to those of slab serpentinites. These data, integrated with radiogenic isotopes and elemental ratios, reveal the recycling of serpentinite-derived water in the source of the Emeishan mantle plume. Our results suggest that subduction of serpentinized oceanic lithospheric mantle can transport considerable amounts of water into the Earth’s interior, leading to localized hydration of the deep mantle, probably at the mantle transition zone. This water is then captured by the upwelling of deeply rooted plume material and recycled back to the Earth’s surface.