Carbon cycle and deep carbon storage during subduction and magamatic processes

Most of the Earth’s carbon is stored in the deep interior of the Earth, and CO₂ plays a key role over the geologic history. Magmatism is a process, which releases CO₂ and increases the carbon on the Earth’s surface. Plate subduction is a major process that brings Earth’s surface carbon back to its interior since its initiation globally. Therefore, plate subduction and magmatic processes constitute a deep carbon cycle between the Earth’s surface and interior. The cycle will affect the total amount of carbon of the Earth’s surface and makes contributions to the formation to the livable Earth environment and some important mineral resources. However, in contrast to the carbon cycle in the Earth’s surface system, the knowledge on deep carbon cycle is lacking. There are still controversies about the enrichment mechanism of the deep carbon, the location of its occurrence, and the exchanges of carbon among the solid Earth’s spheres. In this study, we made a thorough review on the deep carbon reservoirs, the carbon composition of magmas and its influences on the genesis of magmas, as well as the geochemical behavior of the carbon during plate subduction. It is recognized that, for the mid-ocean ridge basalts and the ocean island basalts, the CO₂ compositions of their mantle sources are highly heterogeneous. Compared to the mid-ocean ridge basalts, the deeper-sourced ocean island basalts have relatively higher concentrations of carbon, indicating that the deep mantle is more enriched in carbon than the shallow upper mantle. The continental lithosphere mantle, transition zone, and even lower mantle may be important reservoirs of carbon. There is a chemical disequilibrium between the carbonated melts and the lithospheric peridotites. The continental lithosphere mantle may be an important carbon reservoir because of the long-term metasomatism of carbonated melts, and the high pressure and strong reducing environment in the mantle transition zone may cause the carbon from the upwelling mantle or subducted slab to be stored in a form of diamond. Carbon in the mantle transition zone or the even deeper sources may be converted to CO₂ by redox melting during mantle upwelling and decompression, which plays a key role in the initiation of mantle melting and genesis of the intraplate volcanic rocks (especially for alkali volcanic rocks). It is concluded that the long-term plate subduction in the Earth’s geologic history is most likely the reason that has caused enrichment of carbon in the deep Earth. However, the geochemical behaviors of carbon and the carbon fluxes estimation related to plate subduction remains a subject of debate. In the future study, it is required to focus more on the CO₂ activities in the magmatic processes, and the geochemical behaviors (i.e., decarbonation) of carbon in the subducting slab.