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Chemical Heterogeneities in the Mantle

Ancient chemical heterogeneities

Geochemists have demonstrated that there are detectable variations within the mantle of trace element concentrations and isotopic compositions. A surprising thing about these heterogeneities is that they seem to be quite ancient (about 1.8 Ga on average), even though mantle convection overturns the mantle within about 100 Ma. It is not clear how these chemical heterogeneities can survive the continuous stirring due to mantle convection. Some people propose instead that there must be distinct layers in the deep mantle which do not mix readily with that part of the mantle stirred by the surface plates.

Stirring tracers in convection

Some of my recent (2001) numerical models have yielded mean ages of heterogeneities of over 2 Ga, thus easily accounting for the observed ages. These models simulate chemical components using tracer points which are carried with the fluid. The models feature fairly realistic plates as an integral part of the convection system. The passive stirring movie illustrates the stirring of passive tracers in the plate model shown in the plate movie.

Gravitational settling

These models can also readily explain the more “depleted” character of mid-ocean ridge basalts compared with oceanic island (or hotspot) basalts. The tracers simulate the presence of subducted oceanic crust within the mantle, and this material is estimated to be slightly denser than average mantle. When the tracers are assigned a slight excess mass to take account of this excess density, they tend to segregate towards the bottom, leaving the topmost fluid “depleted” and the deeper fluid “enriched”. The stirring and segregation movie illustrates this process.

Notes: Last modified: Wed, 19 Dec 2001