The effect of shearing on the formation and interactions of dunite channels

Abstract:

Dunites produced by orthopyroxene (opx) dissolution in high porosity channels could provide pathways for opx-undersaturated melt to reach shallow levels [1]. Previous studies have considered these compacting-dissolution channels in the presence of a uniform upwelling mantle flow. In this study, the analysis of dunite channel formation and orthopyroxene dissolution was extended by introducing shearing on horizontal planes, as could be present in flowing mantle beneath spreading centers.

A numerical experiment was formulated using the finite element library deal.II[3] applying a high order Discontinuous Galerkin (DG) method to examine melt flow in a deforming, porous matrix. The conditions are similar to those in [2] with an additional prescribed horizontal shear rate component in the solid matrix. A Gaussian perturbation in the porosity at the base extends vertically through the domain, defining a melt channel. By varying the upwelling and shear rate, opx dissolution was examined to determine the behavior of dunite formation and interaction.

Models of dissolution channels in upwelling mantle show that shearing deformation introduces several important effects for dunite formation. Shearing tilts the developing dunite, introducing an asymmetry in the opx gradient between the dunite channels and the surrounding harzburgite. The downwind gradient is sharp, nearly discontinuous; whereas the upwind gradient is gradual. Shearing limits the formation of dunite channels because increased shearing broadens the region of lower opx dissolution.

The dunite tilting causes buoyant, nearly vertically flowing melt to move across rather than along the dunite, preventing the dunite from providing an opx-free pathway. Our numerical experiments are continuing to determine conditions that may allow melt to move along a tilted dunite channel. Alternatively, minimal shearing in upwelling mantle may be required for dunite to act as a channel for vertical melt transport. This could imply that the deformed dunites observed in ophiolites may have undergone shearing subsequent to transporting melt at greater depth. [1]. Suhr et al. (2003) Geochem. Geophys. Geosyst., 4, 8605. [2]. Schiemenz et al. (2011) Geophys. J. Int. 186, 641-664. [3]. Bangerth et al. (2007) ACM Trans. Math. Software 33, doi: 10.1145/1268776.1268779.