Carbonate melt interaction with natural eclogite at 6 GPa and 1100–1200 °C: Implications for metasomatic melt composition in subcontinental lithospheric mantle

Compositional ranges of carbonate melts stable under P-T conditions corresponding to the base of subcontinental lithospheric mantle (SCLM) are limited by alkali-rich near-eutectic compositions. In the present work, we investigated the interaction of such melts with the natural eclogite of Group A. It was found that the interaction is accompanied by decreasing Ca# in the melt (L) and increasing Ca# in garnet (Grt) according to the reaction: 3CaCO₃ (L) + Mg₃Al₂Si₃O₁₂ (Grt) = 3MgCO₃ (Mgs and/or L) + Ca₃Al₂Si₃O₁₂ (Grt), where Mgs is magnesite. The interaction with the Na-Ca-Mg-Fe carbonate melt increases amount of jadeite component in clinopyroxene (Cpx) consuming Al₂O₃ from garnet and Na₂O from the melt according to the reaction: Na₂CO₃ (L) + CaCO₃ (L) + 2Mg₃Al₂Si₃O₁₂ (Grt) + 2CaMgSi₂O₆ (Cpx) = 2NaAlSi₂O₆ (Cpx) + Ca₃Al₂Si₃O₁₂ (Grt) + 2MgCO₃ (Mgs, L) + 3Mg₂SiO₄ (Ol). As a result, garnet and omphacite compositions evolve from eclogite Group A to eclogite Group B. A byproduct of the reaction is olivine (Ol), which may explain the formation of inclusions of “mixed” eclogite (garnet + omphacite) and peridotite (olivine) paragenesis in lithospheric diamonds. The interaction with the K-Ca-Mg-Fe carbonate melt increases the K₂O content in clinopyroxene to 0.5–1.2 wt%, while the Na₂O content lowers to 0.3 wt%. The following range of carbonatite melt compositions can be in equilibrium with eclogite at the base of SCLM (1100–1200 °C and 6 GPa): 18(Na_0.97K_0.03)₂CO₃∙82(Ca_0.63Mg_0.30Fe_0.07)CO₂–42(Na_0.97K_0.03)₂CO₃∙58(Ca_0.46Mg_0.45Fe_0.09)CO₂. Our results also suggest that the partial melting of ‘dry’ carbonated eclogite, if any, at 1100 °C and 6 GPa yields the formation of a carbonate melt with the following composition (mol%) 25(Na_0.96K_0.04)₂CO₃∙75(Ca_0.64Mg_0.31Fe_0.05)CO₂, corresponding to 18–27 wt% Na₂O in the melt on a volatile-free basis.