Geochemical processing in a three-dimensional regional spherical shell model of mantle convection

Abstact: A three-dimensional, regional spherical model of processing of oceanic crust, through subduction, stirring, and remelting, confirms the main results from previous two-dimensional models and from threedimensional models with low Rayleigh number and only depth-dependent viscosity. The new model features high Rayleigh number, strongly temperature-dependent viscosity, a set of realistic surface plates, and oceanic crust simulated by passive tracers. It confirms that the processing timescale is the main determinant of the age distributions and mean ages of mid-ocean ridge basalt (MORB) and ocean island basalt (OIB) samples, and also of the present degree of processing of the mantle, which is 99%. In this model the melting depth decreases with time, accounting for its temperature dependence, and so the processing timescale increases with time. An approximate processing theory reveals that MORB and OIB ages depend mainly on the recent processing timescale, whereas the degree of processing depends mainly on the initial processing timescale.