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

Abstract

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.