Carbon in the Core: Its Influence on the Properties of Core and Mantle

Abstract

Earth’s core is known to be metallic, with a density of about 9.90 Mg·m−3 at the core-mantle boundary and as such is substantially denser than the surrounding mantle (5.56 Mg·m−3 at the core-mantle boundary; Dziewonski and Anderson 1981). Comparison with cosmic abundances suggests that the core is predominantly Fe with around 5% Ni (Allegre et al. 1995; McDonough 2003) and 8–12% of one or more light elements (Birch 1952). The latter conclusion comes from the observation that the core is appreciably less dense than pure Fe or Fe-Ni alloys under any plausible core temperature conditions (Stevenson 1981). The nature of the light element (or elements) has been the subject of considerable speculation, because of its bearing on Earth’s overall bulk composition, the conditions under which the core formed, the temperature regime in the core, and possible ongoing interactions between core and mantle. Any element with substantially lower atomic number than iron ( z = 26) would have the required effect on core density, but it must also be of high cosmic abundance and it must be soluble in liquid Fe under both the conditions of core formation and those of the outer core. A review of the likely contributors to the core density deficit (Wood 1993) concluded that S and C were the most likely candidate elements and acknowledged that Si, which is extensively soluble in Fe at low pressures, could also conceivably be present. More recently, arguments have been put forward in favor of H (Okuchi 1997) and O (Rubie et al. 2004) as major “light” elements in the core. Although the presence of any of these other elements would not exclude C from the core, dissolution of most of them in liquid Fe require specific compositions of accreting planetesimals and specific conditions of core formation. In …