Carbon-Bearing Phases throughout Earth’s Interior

Abstract

Carbon (C) occurs in the mantle in its elemental state in the form of graphite and diamond, but also as oxidized compounds that include carbonate minerals and carbonated magmas, as reduced components such as methane and carbide, and as gaseous phases in the C–O–H chemical system. The occurrence of C-bearing phases characterized by different oxidation states reflects magmatic processes occurring in Earth’s interior that link to its oxygenation through space and time. Improving our understanding of the physical and chemical behavior of carbon at extreme conditions sheds light on the type and depth of possible reactions taking place in the interior of Earth and other planets over time and allows the identification of deep carbon reservoirs and mechanisms that move carbon among different reservoirs from the surface to the atmosphere, thereby affecting the total terrestrial budget of carbon ingassing and outgassing. Carbon occurs in diverse forms depending on surrounding conditions such as pressure, temperature, oxygen fugacity (fO2), and the availability of chemical elements that are particularly reactive with carbon to form minerals and fluids. Despite the low abundance of carbon within Earth, the stability of C-rich phases in equilibrium with surrounding minerals provides an important geochemical tracer of redox evolution in Earth and other planets, as well as an important economic resource in the form of diamonds. Knowledge of carbon cycling through the mantle requires an understanding of the stable forms of carbon-bearing phases and their abundance at pressures, temperatures, and fO2 values that are representative of Earth’s interior. Such information is necessary to identify potential carbon reservoirs and the petrogenetic processes by which carbon may be (re) cycled through the mantle over time, eventually being brought to the surface by magmas and to the atmosphere as dissolved gaseous species. Accurate estimates of carbon abundance in Earth’s interior are challenging for many reasons, such as the unknown primordial budget of carbon, the low solubility of carbon in the dominant silicate minerals of the upper and lower mantle, the low modal abundance of accessory carbon-bearing minerals and graphite/diamond in mantle xenoliths, and because magmas occurring at shallow depths are the product of igneous differentiation, magma chamber processes, and degassing. Experimental studies conducted at high