The effect of CO₂ on the water-saturated solidus of K-poor peridotite between 4 and 6 GPa

The effect of various amounts of CO₂ on the solidus of H₂O–CO₂-bearing peridotite was examined by determining the composition of H₂O–CO₂-bearing fluids and melts in equilibrium with garnet peridotite at 4–6 GPa and 900–1100 °C. Two capsules were placed in a rocking multi-anvil apparatus in each experiment. Both capsules contained a fertile peridotite with 10 wt% H₂O, one with 1 (CLZ1) and the other with 5 wt% CO₂ (CLZ5). In both capsules a diamond trap was placed on one end of the capsule as a fluid/melt trap. The H₂O and CO₂ content in the fluid or melt trapped in between the diamonds were measured using the quartz-tube-system technique by releasing the volatiles through infrared gas analyzer. The total dissolved solids in these phases were determined using the cryogenic laser-ablation – inductive couple plasma – mass spectrometry technique. The residual lherzolite consists of olivine, orthopyroxene, ±clinopyroxene, garnet, and ±magnesite as carbonate phase. The solidus of CLZ1 peridotite was located between 900 and 1000 °C at 4 GPa and between 1000 and 1100 °C at 5 and 6 GPa. CLZ5 peridotite melts below 900 °C at 4 GPa and between 900 and 1000 °C at 5 and 6 GPa. The results demonstrate a decrease in melting temperature of hydrous peridotite at pressures between 4 and 6 GPa with increasing amount of CO₂. The H₂O–CO₂-bearing fluids found in this study at 900–1000 °C are similar in composition to low-Mg carbonatitic to silicic high-density fluids found in fluid inclusions in diamonds. With increasing temperature, the melts approach type II kimberlites. We propose that H₂O–CO₂-induced partial melting of metasomatized garnet lherzolite at 4–6 GPa is a possible origin for group II kimberlites.