Melting of K-rich carbonated peridotite at 6–10GPa and the stability of K-phases in the upper mantle

Experiments are reported on melting of two peridotite compositions (depleted hazburgitic and fertile lherzolitic) with Mg and K carbonates (1.4 and 6.8wt.% K₂O in the bulk compositions). The experiments were carried out at 6–10GPa using a multianvil apparatus. It was found that the addition of K to carbonated peridotites depresses the solidus by up to 300°C at 10GPa. As a result the solidus of K-rich carbonated peridotite intersects a cratonic geotherm at ~6GPa. In addition to olivine, pyroxene, and garnet, the near-solidus mineral assemblage includes anhydrous phase X, K₂Mg₂Si₂O₇, and K–Mg carbonate, K₂Mg(CO₃)₂. Phase X is stable only directly near the solidus, and the stability field of the K–Mg carbonate is much wider, especially at 10GPa (more than 200°C above the solidus). Near-solidus melts in equilibrium with the K-rich crystalline phases have K–Mg carbonate compositions with up to 30wt.% K₂O and shift with increasing temperature toward dolomitic and, then, silicocarbonate kimberlite-like liquids similar to those obtained in experiments with K-poor carbonated peridotites. Thus, at low H₂O activity possible crystalline hosts for K in the subcratonic upper mantle are anhydrous phase X and K–Mg carbonate. In the presence of these phases, carbonate-dominated melt must be formed in the mantle at >6GPa even in relatively cold regions.