The Role of Plate Boundaries in Degassing: Dynamic Mantle Models

In recent years, the geodynamical community has put forth a number of models which examine the chemical evolution of the mantle with respect to noble gases. These models are able to satisfy general constraints such as heat flow and overall degassing rate, but have difficulty in reproducing the noble gas isotopic heterogeneity observed in oceanic basalts. Such models generally employ a free slip surface boundary condition and thus lack the formation of sharp and long-lived zones of convergence and divergence that are associated with subduction zones and mid-oceanic ridges. This makes it difficult in the models to distinguish mid-oceanic ridge volcanism from ocean island volcanism. Model approximations of plate tectonics include the prescription of a kinematic surface boundary condition, or by using an advanced rheological description with imposed weak zones or a finite yield strength criterion. Here, we use another approach, which is based on the force balance method by Gable (JGR, 1991). This is essentially an advanced kinematic boundary condition in which segments of constant boundary velocity are prescribed in a way that is dynamically consistent with the overall convective flow. By doing this, we explicitly link degassing to zones of plate divergence in our model, thereby consistently following the degassing associated with mid ocean ridge volcanism. Our model simulates mantle convection by the numerical solution of the time dependent Boussinesq equations on a two dimensional cylindrical finite element mesh, with mantle noble gas inventories discritized to a large number of passive tracers. We present the results from a suite of model runs with temperature- and pressure-dependent viscosity, and with a variable number of plates.