Carbonate-fluxed Melting of MORB-like Pyroxenite at 2{middle dot}9 GPa and Genesis of HIMU Ocean Island Basalts

We present partial melting experiments on a carbonate-added, mid-ocean ridge basalt (MORB)-like pyroxenite composition (G2C; 5 wt % CO2). Experiments were conducted at 2·9 GPa and 1000–1500°C and the resulting partial melt compositions were compared with those of alkalic ocean island basalts (OIBs). The solidus is estimated between 1000 and 1050°C and the liquidus is between 1450 and 1475°C. The subsolidus assemblage is cpx + garnet + rutile + calcio-dolomitic solid solution, and the near-solidus melt is carbonatitic (<5 wt % SiO2, <1·3 wt % TiO2, <0·5 wt % Al2O3, 31 wt % < CaO < 25 wt %). At 1245–1275°C, with the disappearance of rutile, a carbonated basaltic melt is found to coexist with carbonatitic melt, cpx, and garnet. The silicate melts are alkalic basalts with SiO2 of ∼44–47 wt % on a volatile-free basis, and the melt becomes most silica-poor and CO2-rich at the temperature of complete mixing of carbonate and silicate melt (i.e. at 1345–1375°C). The onset of carbonated silicate melting in our study is ∼60–70°C cooler than the solidus of the carbonate-free MORB-pyroxenite at a similar pressure. G2C-derived carbonated silicate partial melts are similar to nephelinitic to basanitic ocean island basalts in general, and those derived from the HIMU mantle end-member in particular. The key similarities in major and minor element signatures include low SiO2 and high TiO2, FeO*, CaO, and Na2O. The main discrepancies between G2C partial melts and natural alkalic OIBs are higher Al2O3, lower CaO/Al2O3, and lower MgO of the former. We hypothesize that such discrepancies might be resolved if carbonated MORB-like pyroxenite produces partial melts at somewhat higher pressures and if a hybrid peridotite–carbonated pyroxenite source is considered. Geodynamic consideration of carbonated silicate melting of pyroxenite bodies beneath ocean islands suggests that volatile-enriched alkalic OIBs with the HIMU signature are probably generated from subducted, carbonated crust over a depth range, with the onset of melting as deep as 180–200 km, for a potential temperature of ∼1500°C. However, rather than a direct decompression of carbonated ocean crust, the key processes involved in the generation of a carbonated silicate melt in equilibrium with MORB-pyroxenite may involve melt–rock reaction and melt–melt mixing.