A water transport system across the mantle transition zone beneath western North America as imaged by electrical conductivity data

The stability field of hydrous phases carried by subducting slabs regulates water transport along the subduction pathway. The ultimate vertical distribution of this water at different depths in Earth’s mantle is governed by the thermal state of the slab. A warm slab is considered to lose water mainly in the upper mantle. However, whether a warm slab can carry water into the lower mantle (LM) is uncertain because of the scarcity of geophysical observations. We report an electrical conductivity model of the mantle transition zone (MTZ) and uppermost LM beneath North America to confirm the water-transporting ability of a warm slab. A high-conductivity anomaly was identified beneath the western United States. The LM portion of this anomaly is interpreted as a hydrous region containing rehydrated stishovite, whereas the lower MTZ portion of the anomaly is ascribed to the presence of water (~0.8 wt%) released by the hydrous stishovite. We speculate that warm slabs, such as the Farallon slab, can cause the breakdown of large amounts of dense hydrous magnesium silicates, releasing water mainly into the upper mantle. This water rehydrates stishovite when percolating through the slab, enabling the slab to continue to transport water into the LM. The identified high-conductivity anomaly, together with previously recognized electrical features in the upper mantle, demonstrates the existence of a trans-MTZ water transport system associated with the warm subduction of the Farallon slab.