Did mantle chemistry change after the late heavy bombardment?

During the Hadean the Inner Solar System was subject to a high flux of asteroidal debris, culminating in a dramatic increase in the rate of cratering on planetary surfaces between 4.0 and 3.8 Ga known as the late heavy bombardment. It left a subtle mark in tungsten isotopes of the Earth’s continental crust that formed during and shortly after the cataclysm (see Tungsten and Archaean heavy bombardment, August 2002 EPN). It has also been suggested that it enriched the mantle in elements, such as those of the platinum group, that have an affinity for metallic iron, a major constituent of many meteorites. The most likely rocks of the Archaean crust to show hints of such enrichment are ultramafic lavas known as komatiites, though to have formed by high degrees of partial melting of plumes rising from deep in the Archaean mantle. Komatiites from their type locality in South Africa and from the Pilbara area of Western Australia do indeed suggest that there was significant effects (Maier, W. D. et al. 2009. Progressive mixing of meteoritic veneer into the early Earth’s deep mantle. Nature, v. 460, p. 620-623). The Finnish-Australian-Canadian team found that the older komatiites (3.2-3.4 Ga) contain less platinum-group elements (PGE) than do those from the later Archaean and early Proterozoic (2.0-2.9 Ga). This they ascribe to a surface layer of meteoritic debris gradually being mixed into the mantle by convection. In their discussion they suggest that once the Earth’s core formed (almost certainly very soon after the Moon-forming event at 4.45 Ga) it effectively leached all PGE from the lower mantle, and could only have achieved higher concentrations by mixing of later meteoritic debris. Their results suggest that this went on through the Hadean, but reached its acme and then stabilised in the late Archaean once the earlier Archaean alien debris had been churned in.


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