Abstract

Early Oceanic Crustal Formation and Recycling and its Impact on Mantle Chemistry
Peter van Thienen

Several lines of evidence indicate higher mantle temperatures (by some hundreds of degrees) during the early history of the Earth. Due to the strong effect of t emperature on viscosity as well as on the degree of melting, this enforces a geodynamic regime which is different from the present plate tectonics, and in which smaller scale processes play a more important role. We present results of numerical thermo-chemical convection models including a simple approximate melt segregation mechanism in which we investigate this alternative geodynamic regime, and its effect on the cooling history and chemical evolution of the mantle. Our results show that the crustal production and recycling takes place on two scales. On a small scale, involving the lower boundary of the crust, delaminations and downward transport of eclogite into the upper mantle takes place. On a larger scale, involving the entire crustal column, (parts of) the crust may episodically sink into the mantle and be replaced by a fresh crust. Both are capable of significantly and rapidly cooling a hot upper mantle by driving partial melting and thus the generation of new crust. After some hundreds of millions of years, as the temperature drops, the mechanism shuts itself off, and the cooling rate significantly decreases. In the course of a few hundred million years, this mechanism also generates a chemical stratification in the mantle. Most of the mantle shows a moderate degree of depletion on average (about 6 mass percent melt extracted). The shallow upper mantle is more depleted, and in the deep lower mantle, a layer of eclogite which is enriched in incompatible trace elements and as a result has a significantly higher radiogenic heat production rate is formed. This enriched layer, due to its higher density, is quite stable over the course of 0.5 Gyr at least and only small blobs of eclogite may be entrained in the convection of the rest of the mantle. We therefore speculate that the formation of a deep enriched reservoir in the lower mantle, as suggested by the trace element chemistry of oceanic island basalts, may have already started in the early history of the Earth.