Implications for melt transport and source heterogeneity in upwelling mantle from the magnitude of Sp converted phases generated at the onset of melting

Recently detected converted phases in upwelling mantle have been attributed to seismic velocity gradients at the onset of melting. In this study, we investigate conditions required to generate a melting onset phase by combining melt migration models with synthetic receiver functions. We find that increasing upwelling velocity, increasing mantle viscosity, and decreasing water content in the mantle source increases the predicted strength of a melting onset phase. Differences in these parameters between study regions may explain observed variations in converted phase magnitude. For a wet mantle source, the amplitude of synthetic receiver functions, calculated using a standard relationship for dependence of seismic velocity on melt fraction, is lower than observed amplitudes. One possibility is that the observed receiver functions indicate a heterogeneous mantle source in which wet and dry components melt independently. Alternatively, the dependence of seismic velocity on melt fraction at very low melt fractions may be much stronger than that used here, as suggested in recent studies.