Thermochemistry of the Mantle Transition Zone Beneath the Western Pacific

The Earth's mantle transition zone has significant control on material flux between upper and lower mantle, thus constraining its properties is imperative to understand dynamic processes and circulation patterns. Global seismic data sets to study the transition zone typically display highly uneven spatial distribution. Therefore, complementary geometries are essential to improve knowledge of physical structures, thermochemistry, and impact on convection. Here, we present a new automated approach utilizing machine learning to analyze large seismic data sets, and derive high‐resolution maps of transition zone discontinuity properties. Seismic measurements from ScSScS precursors are integrated with mineralogical modeling to constrain thermochemistry of the western Pacific subduction zone. Our models map recent subduction patterns through the transition zone, indicating stagnation of slabs and accumulation of basalt at its base, and interaction between stagnant slabs and plumes. These results suggest that the thermochemical properties of upper mantle discontinuities can provide high‐resolution images of mantle circulation patterns. Plain Language Summary Earth's upper mantle displays several discontinuous jumps in its physical properties, which result from changes in its mineral structure as pressure and temperature increase with depth. The major transitions near to 410 and 660 km depth are associated with physical changes that have significant influence on the flow of hot upwelling plumes and cold downgoing slabs. The depth and strength of the discontinuities depend on the local temperature and composition, and therefore constraining these properties can help to track mantle circulation patterns and better understand convection behavior. Global seismic data sets are highly uneven in spatial coverage, and therefore must be supplemented by data sets with different spatial sensitivity. Here, we present an automated approach based on machine learning to analyze seismic phases with such complementary geometry, and apply these techniques to investigate subduction zones beneath the western Pacific. We incorporate high‐resolution observations of the discontinuities with modeling from mineral physics, producing new models of temperature and composition in this region. The models track recent convection patterns through the transition zone, indicating ponding of slabs and plumes. These results suggest that the temperature and composition of the transition zone can be u