Hygrometric Control on the Lithosphere-Asthenosphere Boundary: A 28 Million Year Record From the Canadian Cordillera

The depth to the lithosphere-asthenopshere boundary (LAB) provides a strong control on the temperature and strength of the lithosphere and its susceptibility to tectonic deformation. We probe the nature of the LAB by deriving the depth and source of mantle-xenolith bearing alkaline lavas aged 28 Ma to 8 ka in the Canadian Cordillera (CC). We document a striking coincidence of the average depth of equilibration of these lavas (65 5 km) with the seismically observed LAB both in the CC and in western USA, and the change in H2O storage capacity at the spinel-garnet transition of fertile peridotite under the geothermal gradient of hot thin back arc regions. The convergence of these factors show the LAB of hot back arcs is hygrometrically controlled at this phase boundary, limiting the H2O to <150 ppm in the mantle lithosphere. Intraplate regions with a shallower LAB require more H2O and/or less fertile lithosphere. Plain Language Summary The surface of the earth is made of tectonic plates that vary in thickness and temperature, and governs how they deform under the forces of plate tectonics. The base of the plate is marked by a change in the speeds of seismic waves in the mantle indicates small amounts of melt below certain depths. We show a narrow depth range at which magmas stall en route to the surface beneath British Columbia and Yukon that correlates with the base of the plate in this region. In this and other continental margins, the depth to the base of the plate is controlled by the combined effects of a change in mineralogy and H2O storage capacity of mantle peridotite that determines whether or not melts are stable under a certain temperature. Key Points Alkaline lavas in the Canadian Cordillera equilibrated at or below a lithosphere-asthenosphere boundary (LAB) of 65 5 km depth The spinel-garnet peridotite transition changes H2O storage capacity in fertile lithosphere to control the LAB depth in hot back arcs Equilibration depths of alkaline lavas constrain a maximum of similar to 150 ppm H2O in mantle lithosphere at the LAB of most intraplate regions

Automated summary

The depth to the lithosphere-asthenopshere boundary (LAB) is crucial for understanding the temperature, strength, and deformation of the lithosphere. This study in the Canadian Cordillera shows a correlation between the equilibration depth of alkaline lavas and the LAB, indicating control of LAB depth in hot back arcs by mineral transitions and H2O storage capacity.