Simultaneous determination of melting phase relations of mantle peridotite and mid-ocean ridge basalt at the uppermost lower mantle conditions

Interpretation of melting phase relationships of mantle peridotite and subducted basaltic crust is important for understanding chemical heterogeneity in the Earth's interior. Although numerous studies have conducted melting experiments on peridotite and mid-ocean ridge basalt (MORB), and suggested that the solidus temperature of MORB is lower than that of peridotite at whole mantle pressure conditions, both solidus temperatures overlap within their uncertainties. In this study, we conducted simultaneous experiments on KLB-1 peridotite and normal MORB (N-MORB) at pressures from 25 GPa to 27 GPa and temperatures from 2398 K to 2673 K, to compare the solidus temperatures and their melting phase relations. The experimental results show that the solidus temperature of the N-MORB is nearly identical to the KLB-1 peridotite at 25 GPa but lower at 27 GPa. In addition, we found that the crossover of melt fractions between KLB-1 peridotite and N-MORB occurs at 25–27 GPa. These changes are likely to be attributed to the majorite-bridgmanite transition of MORB. This indicates that the dominant melting component may change depending on the location of the uppermost lower mantle. Our calculation result on the density of partial melts along the mantle geotherm suggests that partial melts of KLB-1 peridotite are gravitationally stable around the top of the transition zone, whereas partial melts of N-MORB are gravitationally stable even at the top of lower mantle. These results suggest that the distribution of partial melts may be different between KLB-1 peridotite and N-MORB in the deep Earth. Our results may be useful for understanding the fate of partial melts of peridotitic mantle and recycled basaltic crust.