Solidus of carbonated peridotite from 10 to 20 GPa and origin of magnesiocarbonatite melt in the Earth's deep mantle

We have experimentally determined the solidus of an alkali-bearing carbonated peridotite (with 5 wt.% CO₂) between 10 and 20 GPa. Based on K-deficit in all low-temperature runs we assumed that some melt could be present in the low temperature runs and the true solidus of an alkali-bearing carbonated peridotite is placed below 1200 °C. However, based on the disappearance of magnesite and the appearance of the visible quenched melt coexisting with silicate phases, the 'apparent' solidus, which may be applicable for peridotite with low alkali contents, was identified. The 'apparent' solidus temperature increases from ~1380 °C at 10 GPa to ~1525 °C at 15 GPa and the 'apparent' solidus curve becomes almost flat from 15 GPa to 20 GPa, where it is located near 1550 °C. At 10 GPa, the 'apparent' solidus of carbonated peridotite is ~550 °C lower than the solidus of CO₂-free natural anhydrous peridotite. The solidus of the present study was also ~120 °C lower than the solidus determined by Dasgupta and Hirschmann [Dasgupta, R., Hirschmann, M.M., 2006. Melting in the Earth's deep upper mantle caused by carbon dioxide. Nature, 440, 659-662.] for natural carbonated peridotite. The drop in the solidus temperature is mainly due to the effect of alkalis (Na₂O, K₂O). The melt near the 'apparent' solidus has high CO₂ (>40 wt.%) and contains b6.0 wt.% SiO₂, <0.30 wt.% Al₂O₃ and <0.25 wt.% TiO₂. The composition of near-solidus partial melt is close to that observed at 6-10 GPa in the CMS-CO₂ and CMAS-CO₂ systems, and natural carbonated peridotite, with some variations in Ca/Mg-ratio. High alkali contents in measured and calculated partial melts are consistent with the compositions of deepseated fluids observed as inclusions in diamonds and may be consistent with the compositions of parental melt, reconstructed for natural magnesiocarbonatite. We have demonstrated that magnesiocarbonatite-like melt can be generated by partial melting of carbonated peridotite at pressure up to at least 20 GPa. The generation of calciocarbonatite and ferrocarbonatite is unlikely to be possible during melting of carbonated peridotite in the deep mantle.