Intensive hydration of the mantle transition zone beneath China caused by ancient slab stagnation

The mantle transition zone, located at depths of 410-660 km between the lower and upper mantle, is an important water reservoir in the Earth's interior(1-4). However, there are regional-scale heterogeneities in the distribution of water(4,5). The zone beneath northeast China, in particular, is remarkably hydrous(4), but when and how it became hydrous remains uncertain. Here we combine analyses of the geochemistry of late Cenozoic basalts in northeast China with published geochemical analyses. We find a spatial correlation between basalt geochemistry and the distribution of a low-velocity zone in the underlying mantle that is interpreted as a plume upwelling from the mantle transition zone(6). We therefore use the basalt geochemistry to infer the composition of the mantle transition zone. The basalts have high Ba/Th and Pb-207/Pb-206 ratios, which we suggest record an ancient hydration event in the transition zone that occurred more than one billion years ago, probably as a result of dehydration of a subducted slab. We suggest that this ancient hydration event, combined with a more recent hydration event linked to dehydration of the subducted Pacific slab(7), can account for the hydrous nature of the mantle transition zone beneath China. Our results demonstrate that the mantle transition zone can remain as a stable water reservoir in Earth's interior for timescales of more than a billion years.