Carbon speciation and solubility in silicate melts

Natalia V. Solomatova (1) , Razvan Caracas (1) , Ronald L Cohen

1 LGL-TPE - Laboratoire de Géologie de Lyon - Terre, Planètes, Environnement

To improve our understanding of the Earth's global carbon cycle, it is critical to characterize the distribution and storage mechanisms of carbon in silicate melts. Presently, the carbon budget of the deep Earth is not well constrained and is highly model-dependent. In silicate melts of the uppermost mantle, carbon exists predomi nantly as molecular carbon dioxide and carbonate, whereas at greater depths, carbon forms complex polymer ized species. The concentration and speciation of carbon in silicate melts is intimately linked to the melt's composition and affects its physical and dynamic properties. Here we review the results of experiments and calculations on the solubility and speciation of carbon in silicate melts as a function of pressure, temperature, composition, polymerization, water concentration, and oxygen fugacity. 16 16.1. INTRODUCTION Evidence of carbon-bearing phases in the Earth's mantle includes the release of CO 2 in volcanic eruptions, dissolved CO 2 in magmatic glasses and glass inclusions (Mörner & Etiope, 2002), diamonds and carbonate minerals in mantle xenoliths (Eggler, 1987; Sobolev & Shatsky, 1990), and the existence of carbonatite and kimberlite magmas (Wyllie et al., 1990). There are two possible sources of carbon: primordial carbon and the carbon delivered by later come tary and asteroid bombardment. Primordial carbon existed in the proto-Earth and subsequently survived the moon-forming impact, and its amount is currently unknown. From all existent carbon, part of it might be locked in the core, part in some deep mantle reservoir, another fraction lies at the surface, and the remaining is resurfaced after surviving subduction. To determine how much carbon may have remained in the Earth after the giant impact, the chemistry and thermodynamics of carbon in silicate (particularly, with the bulk silicate Earth composition) must be determined as a function of pressure and temper ature. Estimates of the amount of carbon exchanged between the surface and the mantle range between 30 and 130 megatons per year, and estimates of the carbon concentration stored within the core range between 0.2 and 4 wt.% (McDonough, 2003; Mookherjee et al., 2011; Wood, 1993). Carbon is mainly subducted into the Earth in the form of carbonates within metasomatically calcium-enriched basaltic rock, calcified serpentinites, and sedi mentary carbonaceous ooze of the seafloor (Brenker et al.,

Domaines: Minéralogie

Natalia V. Solomatova, Razvan Caracas, Ronald L Cohen. Carbon speciation and solubility in silicate melts. Carbon in Earth’s Interior, 2020, ⟨10.1002/9781119508229.ch16⟩. ⟨hal-03002077⟩