EXPERIMENTS ON THE INTERACTION OF THERMAL-CONVECTION AND COMPOSITIONAL LAYERING AT THE BASE OF THE MANTLE

Accumulation and survival of compositional heterogeneity at the base of the convecting mantle is investigated using laboratory experiments with temperature dependent viscious fluids. Two models of heterogeneity in the D" layer are considered: (1) a reservoir of dense material above the core-mantle boundary and (2) continuous recharge of the D" layer by subduction of cold, negatively buoyant high viscosity slabs of oceanic lithosphere. Survival of a dense reservoir depends on R-rho, the ratio of stabilizing compositional buoyancy to destabilizing thermal buoyancy. Rapid mixing occurs for R-rho less-than-or-equal-to 1; for R-rho greater-than-or-equal-to 1, mixing occurs by slow entrianment only. The reservoir model predicts D" layer heterogeneity is concentrated beneath lower mantle upwellings. Differentiation of composite slabs, consisting of dense crustal and light depleted components, occurs in a hot thermal boundary layer. Composite slabs pile up in the basal thermal boundary layer, and upon heating the light component is ejected by Rayleigh-Taylor instabilities. The dense component accumulates in the boundary layer, arrayed in a pattern of interconnected spokes, and is slowly entrained into thermal plumes. According to these experiments the time required for conversion of slabs to plumes in D" is 1-2 Gyr. The correlation between lateral variations in D" layer thickness and lateral variations in lower mantle seismic velocity suggests that D" may consist of dense material, perhaps former oceanic lithosphere, which has sunk through the lower mantle.