Discovery of a heavy silicon isotope mantle reservoir

Silicon cycling between Earth's reservoirs provides critical insights into how the Earth operates. While average crust and the bulk silicate Earth (BSE) share similar silicon isotope (δ³⁰Si) compositions, some mantle-derived magmas exhibit lower δ³⁰Si values than the BSE, implying the existence of an unidentified mantle reservoir with complementary higher δ³⁰Si values. We present silicon isotope data from Cenozoic lamproites and their hosted mantle pyroxenite xenoliths from the Himalaya-Tibet orogen. These mantle-derived rocks have higher δ³⁰Si values than the BSE, which resulted from reaction between mantle peridotite and ³⁰Si-rich silicate melts from subducted Indian continental crust. Our results demonstrate that slab melting can produce high-δ³⁰Si melts and complementary low-δ³⁰Si residues. These products are unevenly distributed in the mantle, with high-δ³⁰Si melts stored as metasomatic veins in the lithospheric mantle while low-δ³⁰Si residues are recycled into the deep mantle. This study provides evidence that mantle metasomatism by high-δ³⁰Si slab melts creates heavy silicon reservoirs in the lithospheric mantle above continental slabs or cool mantle wedges above oceanic slabs.