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.