The Water-Saturated Solidus and Second Critical Endpoint of Peridotite: Implications for Magma Genesis Within the Mantle Wedge

The "wet" silicate solidus of mantle peridotite defines the initial melting temperature of Earth's mantle under water-saturated conditions and the second critical endpoint (SCEP) marks the high P-T end of the wet solidus. However, the location of the wet solidus has remained an outstanding issue for over 50 years and the position of the SCEP is hotly debated. Published wet solidi show a difference of 200-600 degrees C at a given pressure while reported SCEPs range from <4 to >6 GPa. Using a large-volume multianvil apparatus, we investigated the water-saturated melting behavior of a fertile peridotite at 3-6 GPa, 950-1200 degrees C, and obtained well-preserved quenched materials. On the basis of textures and compositions of the quenched materials, we bracket the wet solidus to between 950 degrees C and 1000 degrees C at 3 GPa and the SCEP between 3 and 4 GPa. Combining our experimental results with seismologic and petrologic observations, we propose that the lithosphere-asthenosphere boundary in subduction zones should be constrained by the wet solidus and emphasize the role of a deep hydrous partial-melting zone (DHPMZ) on magma genesis within the mantle wedge. We suggest that the DHPMZ is a source of hydrous melts to the primary melting zone in the mantle wedge and that the position of the volcanic front and its magma production rate may largely be controlled by melting and melt segregation processes within the DHPMZ. Our experimental results also suggest that high-magnesian magmas (e.g., boninite, picrite, and komatiite) could be formed at conditions representative of subduction zones.