Melt Infiltration and the Thermal-Chemical Corrosion of the Lithosphere-Asthenosphere Boundary

Abstract:

This study investigates the corrosion of the base of the lithosphere by heat and water transport facilitated by infiltrating melt. The origin of the rapid drop in seismic velocity observed near the lithosphere-asthenosphere boundary (LAB) in many locations across the globe is thought to arise from either melt accumulation or a transition from dry lithosphere to hydrated asthenosphere. However, these two hypotheses are difficult to decouple; water lowers the peridotite solidus and melt will transport water due to its incompatibility with the crystalline matrix. To investigate the chemical and thermal evolution of the LAB, we construct forward models of the two-phase system describing 1D mass, momentum and energy conservation in melt and solid phases, including volatile transport and using a water-dependent solidus. In addition to 1D models, 2D models are used to investigate how both geometric and dynamic pressure-driven melt focusing may influence LAB topography. Using relationships between thermodynamic variables and elastic properties we predict seismic velocities and attenuation measurements that are compared directly to EarthScope measurements in the western U.S.. In addition to geophysical observations, the evolution of water content and accumulation zone depth are compared to observed equilibration depth and water contents of magmas from localities in the Basin and Range and the western margin of the Colorado Plateau. Preliminary modeling results in 1D show that in an open system where solid material upwells continuously beneath the LAB, water concentration in the melt increases at the upper boundary of the partially molten zone where crystallization occurs. This leads to a potential feedback wherein the increasing water content locally hydrates the base of the lithosphere, lowering its solidus, and generating melt that can migrate to shallower depths.