Carbon substitution for oxygen in silicates in planetary interiors

Abstract

Amorphous silicon oxycarbide polymer-derived ceramics (PDCs), synthesized from organometallic precursors, contain carbon-and silica-rich nanodomains, the latter with extensive substitution of carbon for oxygen, linking Si-centered SiO x C 4-x tetrahedra. Calorimetric studies demonstrated these PDCs to be thermodynamically more stable than a mixture of SiO 2 , C, and silicon carbide. Here, we show by multinuclear NMR spectroscopy that substitution of C for O is also attained in PDCs with depolymerized silica-rich domains containing lithium, associated with SiO x C 4-x tetrahedra with nonbridging oxygen. We suggest that significant (several percent) substitution of C for O could occur in more complex geological silicate melts/glasses in contact with graphite at moderate pressure and high temperature and may be thermodynamically far more accessible than C for Si substitution. Carbon incorporation will change the local structure and may affect physical properties, such as viscosity. Analogous carbon substitution at grain boundaries, at defect sites, or as equilibrium states in nominally acarbonaceous crystalline silicates, even if present at levels at 10-100 ppm, might form an extensive and hitherto hidden reservoir of carbon in the lower crust and mantle.T he carbon cycle is one of the most important components in the sustenance of life and the evolution of the environment on our planet. The exchange of carbon between the atmosphere, biosphere, and oceans primarily controls the near-surface carbon cycle in the short term (decades to millennia) (1, 2). However, a large fraction of the carbon in the planet is stored at a greater depth (i.e., in the crust, mantle, core), and it exchanges with the surficial carbon only on much longer time scales (millions of years) through processes involving subduction and volcanic activity (3, 4). The form of stored carbon in these deep reservoirs and the amounts of carbon in various reservoirs, as well as the balance between the inward and the outward fluxes of carbon from them, are poorly constrained (3, 4). In the crust and upper mantle under relatively oxidizing conditions, CO 3 2− provides the familiar trigonal (sp 2 ) coordination environment for carbon in carbonates, mixed CO 2 -H 2 O fluid phases and carbonatiteforming melts. Under more reducing conditions, graphite, diamond, and methane become important and carbonate becomes less important or absent. The substitution of tetrahedral (sp 3 ) carbon for silicon in silicate minerals and melts has often been suggested as a possibility at high pressure (5-13), but there was little experimental evidence for it until recent phase equilibrium studies indicated that CO 2 can undergo transformation from a molecular gas to an extended solid at very high pressure and temperature to form a cristobalite-like structure consisting of a network of corner-sharing CO 4 tetrahedra (13,14). Recent studies have also indicated the possibility of a chemical reaction between SiO 2 and CO 2 to form a silicon carbonate phase under pressure (12) and the...

Carbon-bearing iron phases and the carbon isotope composition of the deep Earth

“…Here, we present a model for understanding carbon isotope distributions within the deep Earth, involving Fe−C phases (Fe carbides and C dissolved in Fe−Ni metal). Our theoretical calculations show that Fe and Si carbides can be significantly depleted in 13 C relative to other C-bearing materials even at mantle temperatures. Thus, the redox freezing and melting cycles of lithosphere via subduction upwelling in the deep Earth that involve the Fe−C phases can readily produce diamond with the observed low δ 13 C values.…”

“…Our theoretical calculations show that Fe and Si carbides can be significantly depleted in 13 C relative to other C-bearing materials even at mantle temperatures. Thus, the redox freezing and melting cycles of lithosphere via subduction upwelling in the deep Earth that involve the Fe−C phases can readily produce diamond with the observed low δ 13 C values. The sharp contrast in the δ 13 C distributions of peridotitic and eclogitic diamonds may reflect differences in their carbon cycles, controlled by the evolution of geodynamical processes around 2.5-3 Ga. Our model also predicts that the core contains C with low δ 13 C values and that an average δ 13 C value of the bulk Earth could be much lower than ∼−5‰, consistent with those of chondrites and other planetary body.…”

“…Diamond-bearing, mantlederived rocks show a very well defined peak at δ 13 C ≈ −5 ± 3‰ with a very broad distribution to lower values (∼−40‰). The processes that have produced the wide δ 13 C distributions to the observed low δ 13 C values in the deep Earth have been extensively debated, but few viable models have been proposed. Here, we present a model for understanding carbon isotope distributions within the deep Earth, involving Fe−C phases (Fe carbides and C dissolved in Fe−Ni metal).…”

Carbon substitution for oxygen in α–cristobalite

“…Some recent studies as to SiOC ceramics, on the other hand, have suggested another possibility that in the crust and mantle under moderately reduced conditions carbon is substituted not for silicon but for oxygen in silicate minerals Mera et al, 2013;Tavakoli et al, 2015). Sen et al (2013) mentioned that significant substitution of carbon for oxygen could occur in silicate melts in contact with graphite at moderate pressure and temperature, which may be thermodynamically far more accessible than carbon substitution for silicon. To date, however, little research has addressed a question of how carbon can be dissolved into silicate minerals under high temperature conditions.…”

Astrobiological Stoichiometry

“…Recently, Sen et al (2013) suggested substitution of C for O in SiC may be far more energetically possible at high pressures than substitution of C for Si. If this indeed happens at high pressure and forms SiO x C 4-x type materials for carbon-rich planets, the different properties of such materials will have profound impact on internal dynamics and evolution of the planets.…”