Abstract
Kimberlite is characterized by explosive eruption powered by excess carbon dioxides (CO2)1 and water2. Given that diamond is the dominant stable phase of carbon in the upper mantle3, it is obscure where does the excess CO2 in kimberlite has come from. Here we show that ferric iron oxidizes diamond at 1900K, 20GPa and 2000K, 25GPa, forming CO2. The lower mantle is dominated by bridgmanite, which is rich in ferric iron4. Bridgmanite decomposes once it is brought to the upper mantle, releasing extra ferric iron. Therefore, the oxidation of diamond may have been popularly occurring at the base of the upper mantle, forming CO2-rich carbonated domains that are the main source of kimberlite. The rising kimberlitic magma reaches the lithosphere mantle of thick cratons before it crosses the solidus line of mantle peridotite, and thus keeps its volatile-rich nature that drives explosive eruptions. When the lithospheric mantle is thinner than ~140 km, kimberlite changes into much less explosive magmas due to partial melting of mantle peridotite, and, consequently, entrained diamond is mostly oxidized during the magma’s slower ascension.
Melting and metasomatism in upper mantle peridotite xenoliths from Labait, North-central Tanzania, and contrasting metasomatic styles in the Tanzanian lithospheric mantle