Measurement of reduced peridotite-C-O-H solidus and implications for redox melting of the mantle

Abstract

Methane-bearing C-O-H fluids may be important carriers of carbon and hydrogen in the Earth's deeper mantle^1,2. It has been proposed that such fluids have a key function in the genesis of kimberlitic magmas and diamond in the subcontinental mantle^2–6. Because the melting behaviour of mantle peridotite in the presence of C-O-H fluids has not been accurately located as a function of pressure, temperature and oxygen fugacity^1,4, it has not been possible to assess the validity of reduced fluid 'infiltration' models of this kind. We report here advances in high-pressure fluid buffering techniques that have allowed the C-O-H fluid-saturated solidus of a fertile peridotite to be determined to 35 kbar. The results show that initial melting of peridotite saturated with H₂O-CH₄ dominated fluids occurs at temperatures well above stable continental geotherms, even under quite H₂O-rich conditions. This implies that C-O-H fluid-induced melting beneath continents will be restricted to regions where CH₄ can be largely eliminated from the fluid by oxidation (redox melting) or where the geotherm is abnormally high (hot-spot melting), or where there is some combination of these processes.