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

Abstract

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 1000^15008C and the resulting partial melt compositions were compared with those of alkalic ocean island basalts (OIBs). The solidus is estimated between 1000 and 10508C and the liquidus is between 1450 and 14758C. The subsolidus assemblage is cpx þ garnet þ rutile þ calcio-dolomitic solid solution, and the near-solidus melt is carbonatitic (5 5 wt % SiO2, 51· 3w t % TiO2,50· 5w t % Al2O3 ,3 1wt %5CaO525 wt %). At 1245^ 12758C, 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 CO2rich at the temperature of complete mixing of carbonate and silicate melt (i.e. at 1345^13758C). The onset of carbonated silicate melting in our study is � 60^708C 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 highTiO2 ,F eO*, 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 � 15008C. 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.