Layering processes in the evolution of planetary mantle
Final Report Abstract
For the Earth double-diffusive layer formation would provide a reasonable dynamical process that selfconsistently generates layered mantle flow with compositionally distinct reservoirs and has profound implications for the global thermal and compositional evolution of the Earth. It puts tight constraints on the thermal evolution of the core and thus the ability to maintain dynamo action and a global magnetic field. During the early stage of evolution the heat flow out of the core is reduced since the established layers act as an insulator. This would lead to a significantly reduced core cooling rate and would give rise to a late growth of the inner core resulting in a present-day inner core size. The inherent spatial and temporal variability of the system also offers an explanation for the geochemical variety of mantlederived basalts. The recorded episodic growth of continental crust and supercontinents might then not be linked to mantle avalanches at the 660 km discontinuity but induced due to the breakdown of individual layers and the deepening of slab sinking. Assuming that a strict double-diffusive layering is overcome and the present day mantle is in the phase of turning over features like the seismically observed LLSVPs below Africa and the Pacific32 might be remnants and with this an evidence for primordial double-diffusive layering with in the Earth mantle. Besides the specific implications for the Earth, the conceptual consequences of double-diffusive layering in terms of the overall mantle evolution of a planet are far-reaching and should lead to the reappraisal of mantle evolution scenarios: Rather than being at least quasi stationary, the internal structure of a planetary mantle would evolve over time, including periods of slow changes or even virtual stationarity, interrupted by phases of sudden alterations.
