Formation of compositional structures by sedimentation in vigorous convection

Abstract

Sedimentation is a mechanism in a convecting fluid that leads to differentiation and to the formation of compositional structures. The formation of layers is an important mechanism for both local features like magma chambers and for global features like the Earth’s mantle. Besides layering, sedimentation can lead to the formation of compositional heterogeneities at the bottom of a convective system. This may be potentially important for the dynamics of the lower mantle. We present a novel algorithm that enables us to numerically study sedimentation of finite-sized particles in non-dilute convective suspensions. Our approach considers a consistent settling velocity and the density contribution due to particle mass. We implement the settling algorithm into a 2D convection model and vary the buoyancy ratio B for five different Rayleigh numbers, covering a range of four orders of magnitude. We find B to be a critical parameter and its critical value to depend on the Rayleigh number. For subcritical values we observe that the presence of a crystal phase reduces convective vigor and most crystals stay suspended. Episodic layering can be seen for the critical value of B. A supercritical buoyancy ratio forces layering on the system, which we find to be applicable to convective magma chambers. Layer formation in all observed layering cases occurs on a time scale that is short compared to the time that magma chambers take to solidify. We find that dynamical layer formation that is connected to crystal settling and the crystals’ density contribution is a likely mechanism for creating layered structures within the convective lifetime of a magma chamber. Additionally, we observe that when the forces are appropriately balanced most of the crystals remain suspended while compositional heterogeneities accumulate at the lower boundary.