Carbonate-fluxed Melting of Silica-excess, MORB-like Pyroxenite and Genesis of Alkalic Ocean Island Basalts

Trace element and isotopic compositions of ocean island basalts (OIBs) call for chemical heterogeneity of mantle source regions. However, the major element compositions of OIBs that show the strongest isotopic signature of crustal recycling (e.g., HIMU basalts) are not produced by laboratory partial melting experiments of putative mantle lithologies1. Subducted oceanic crust is frequently invoked as a source of mantle heterogeneity and identified as a HIMU component2, but partial melting of MORB-like eclogite, under nominally volatile-free mantle conditions, produces silica-rich, basaltic to dacitic melt3,4 and are not similar to the silica-poor, alkalic HIMU basalts. Here we test whether subducted ocean crust, when carbonated, can generate alkalic OIBs similar to the HIMU basalts. We present partial melting experiments on a nominally anhydrous, silica-excess, MORB-like pyroxenite, to which 5 wt.% CO2 was added as a mixture of Ca-Mg-Fe-Na-K carbonates such that the introduction of CO2 to the bulk composition (G2C) mimics the process of carbonate precipitation into ocean floor basalts. All experiments were performed at a single pressure of 3 GPa at 1050-1500 °C using a piston cylinder apparatus. The solidus is located below 1050°C, and the immediate super-solidus phase assemblage is cpx+garnet+rutile+carbonatitic melt (1050-1200 °C). Rutile disappears between 1200 and 1275 °C, and both cpx and garnet persist to the liquidus (1450-1475 °C). Carbonated silicate melt appears between 1200 and 1275 °C and its composition evolves systematically as a function of temperature, with SiO2 and Al2O3 varying on a CO2-free basis from ~44 to 48 wt.% and ~13.5 to 14.7 wt.% respectively. MgO and CaO concentrations of the partial melt increase (from ~4.4 to 8 wt.% and ~8 to 13 wt.% respectively) and those of FeO*, TiO2 and Na2O decrease (from ~15 to 12 wt.%, ~4 to 2.5 wt.% and ~6.7 to 3.5 wt.% respectively) with increasing temperature from 1275 to 1450 °C. We demonstrate that partial melting of carbonated MORB-like pyroxenite generates nepheline-normative, alkalic basalts. Silicate partial melts generated from G2C resemble silica-poor, alkalic OIBs in SiO2, TiO2, FeO*, CaO, and Na2O content. Although Al2O3 content of the silicate melt reported here is somewhat higher than the concentration of the same in primitive alkalic OIBs, these melt components, in conjunction with peridotite derived melt, provide a good match for silica-poor, alkalic OIBs similar to those produced by the HIMU mantle end member. Carbonated MORB-derived silicate melts can also explain the link between isotopic signatures of recycling and silica-poor, FeO*- and TiO2-rich compositions of alkalic OIBs. 1Jackson M.G. and Dasgupta, R. 2008, EPSL 276, 175-186. 2Hofmann, A.W. 1997, Nature 385, 219-229. 3Pertermann, M. and Hirschmann, M.M. 2003, JPet 44, 2173-2201. 4Spandler C. et al. 2008, JPet 49, 771-795.