Abstract

Whether and where recycled oceanic crusts melt in the deep mantle are fundamentally important questions for understanding the evolution and dynamics of the Earth's mantle, and they currently remain unclear. Here, we find compelling evidence for the wide distribution of eclogite melting around a depth of 300 km by investigating the origins of the X-discontinuity. We show that both the transformation of orthopyroxene into high-pressure clinopyroxene and the coesite-stishovite transition are dominant mechanisms. The degree of partial melting of oceanic crust is crucial for the X-discontinuity mechanisms since melting promotes the enrichment of orthopyroxene by consuming solid silica. The silica phase transition dominates in the relatively low-temperature region, while the orthopyroxene phase transition in the high-temperature region results in the indistinguishable seismological Clapeyron slope of the X-discontinuity, with both transitions presenting a large positive Clapeyron slope. The X-discontinuity provides a key method for identifying partial melting of recycled oceanic crust.