Equilibration Depth and Temperature of Neogene Alkaline Lavas in the Cordillera of Alaska and Canada as a Constraint on the Lithosphere-Asthenosphere Boundary

Abstract

We have estimated the geochemical equilibration depth and temperature of the widespread Neogene alkaline basalts in the Cordillera of Alaska, NW Canada and in Mexico using geobarometry on bulk compositions that have been minimally differentiated in upward transit. The method has uncertainties of about ±10 km and < 70 oC. The regional averages of geochemical equilibration depth for 12 sites in Alaska vary from 50±10 to 84±2 km, somewhat broader than those from the Cordillera of western Canada, western USA, and Mexico. There are no associations of depth with terranes or geological provinces. The final equilibration depth of lavas with the surrounding mantle is concluded to be where partial melt percolating from greater depths ponds at the lithosphere-asthenosphere boundary (LAB) until it becomes gravitationally unstable and moves upward in conduits. The top of the low velocity zone from seismic receiver functions, taken to be the LAB in regions of Alaska where Neogene volcanism occurs, varies from 60 - 85 km, covering the range of geochemical equilibration depths of the alkaline lavas. A mean lava equilibration depth of 65 ±10 km occurs in 24 of 36 alkaline volcanic centers from Alaska to Mexico, and several other global locations, suggesting the LAB may be controlled to a first order by the change in H2O storage capacity and viscosity across the garnet-spinel peridotite phase change at this depth. The scatter and variation in equilibration depths and temperatures are a factor of two greater than the recognized uncertainties, and are not yet explained.