Generation of mid-ocean ridge basalts at pressures from 1 to 7 GPa

We propose a model for the generation of average MORBs based on phase relations in the CaO-MgO-Al2O3-SiO2-CO2 system at pressures from 3 to 7 GPa and in the CaO-MgO-Al2O3-SiO2-Na2O-FeO (CMASNF) system at pressures from similar to0.9 to 1.5 GPa. The MELT seismic tomography (Forsyth et al., 2000) across the East Pacific Rise shows the largest amount of melt centered at similar to30-km depth and lesser amounts at greater depths. An average mantle adiabat with a model-system potential temperature (T-P) of 1310degreesC is used that is consistent with this result. In the mantle, additional minor components would lower solidus temperatures similar to50degreesC, which would lower T-P of the adiabat for average MORBs to similar to1260degreesC. The model involves generation of carbonatitic melts and melts that are transitional between carbonatite and kimberlite at very small melt fractions (<0.2%) in the low-velocity zone at pressures of similar to2.6 to 7 GPa in the CMAS-CO2 system, roughly the pressure range of the PREM low-velocity zone. These small-volume, low-viscosity melts are mixed with much larger volumes of basaltic melt generated at the plagioclase-spinel lherzolite transition in the pressure range of similar to0.9 to 1.5 GPa.