Partial melting of stagnant oceanic lithosphere in the mantle transition zone and its geophysical implications

Widespread low velocity anomalies have been observed in the upper mantle around many oceanic subduction zones. Fluid or melt released from a stagnant slab may have contributed to the formation of these anomalies. Furthermore, slab partial melting or dehydration is also thought to be closely related to the origin of intraplate volcanoes (i.e., Changbaishan). However, experimental evidence on the process of slab partial melting is very limited. Here, our experimental results show that partial melting of stagnant oceanic lithosphere may occurs for temperatures above ~ 1300-1400 °C, with residual phases composed of wadsleyite/ringwoodite + garnet + clinopyroxene/stishovite/akimotite. The density of melt was approximately 1.0-1.5 g/cm3 less dense than the surrounding mantle, which provided a buoyancy force for ascent to the upper mantle across the 410-km seismic discontinuity. The ascending melt may react with mantle peridotite, leading to the formation of a variably metasomatized mantle, which may contribute to the formation of the observed low velocity anomalies above stagnant slab. Re-melting of the metasomatized mantle may have contributed to the origin of the intraplate volcanoes, e.g., Changbaishan volcanoes. We suggest that partial melting of stagnant oceanic lithosphere in the MTZ may have close relations with the origin of the big mantle wedge beneath eastern China.