Immiscible Transition from Carbonate-rich to Silicate-rich Melts in the 3 GPa Melting Interval of Eclogite + CO₂ and Genesis of Silica-undersaturated Ocean Island Lavas

摘要

We explore the partial melting behavior of a carbonated silica-deficient eclogite (SLEC1; 5 wt % CO₂) from experiments at 3 GPa and compare the compositions of partial melts with those of alkalic and highly alkalic oceanic island basalts (OIBs). The solidus is located at 1050–1075 °C and the liquidus at ∼1415 °C. The sub-solidus assemblage consists of clinopyroxene, garnet, ilmenite, and calcio-dolomitic solid solution and the near solidus melt is carbonatitic (<2 wt % SiO₂, <1 wt % Al₂O₃, and <0·1 wt % TiO₂). Beginning at 1225 °C, a strongly silica-undersaturated silicate melt (∼34–43 wt % SiO₂) with high TiO₂ (up to 19 wt %) coexists with carbonate-rich melt (<5 wt % SiO₂). The first appearance of carbonated silicate melt is ∼100 °C cooler than the expected solidus of CO₂-free eclogite. In contrast to the continuous transition from carbonate to silicate melts observed experimentally in peridotite + CO₂ systems, carbonate and silicate melt coexist over a wide temperature interval for partial melting of SLEC1 carbonated eclogite at 3 GPa. Silicate melts generated from SLEC1, especially at high melt fraction (>20 wt %), may be plausible sources or contributing components to melilitites and melilititic nephelinites from oceanic provinces, as they have strong compositional similarities including their SiO₂, FeO*, MgO, CaO, TiO₂ and Na₂O contents, and CaO/Al₂O₃ ratios. Carbonated silicate partial melts from eclogite may also contribute to less extreme alkalic OIB, as these lavas have a number of compositional attributes, such as high TiO₂ and FeO* and low Al₂O₃, that have not been observed from partial melting of peridotite ± CO₂. In upwelling mantle, formation of carbonatite and silicate melts from eclogite and peridotite source lithologies occurs over a wide range of depths, producing significant opportunities for metasomatic transfer and implantation of melts.