Geochemical and rheological constraints on the dynamics of the oceanic upper mantle

Abstract

I provide constraints on mantle convection through observations of the rheology and composition of the oceanic upper mantle. Convection cannot be directly observed, yet is a fundamental part of the plate tectonic cycle. Relative motion among plates is accommodated by localized deformation at their boundaries. I demonstrate that in the ductile regime, strain localization occurs when different mineral phases are mixed together, limiting grain annealing. Upper mantle flow is by dislocation creep, resulting in seismic anisotropy due to mineral alignment. I use a shear zone in the Josephine Peridotite to quantify the relationship between mineral orientation and shear strain, providing an improved framework for the interpretation of seismic anisotropy. The upper mantle is generally assumed to be homogeneous in composition. From detailed isotopic and chemical analyses of abyssal peridotites from the Southwest Indian Ridge, I show that the mantle is heterogeneous at a range of length-scales. Abyssal peridotites recovered at ocean ridges are generally interpreted as the depleted residues of melt extraction. I find that melt-rock reaction is a significant part of the melt extraction process, modifying the composition of the lithospheric mantle. The generation of heterogeneous lithosphere provides a source for asthenospheric heterogeneity, via subduction and mantle convection.