Progressive growth of the Earth's continental crust and depleted mantle: Geochemical constraints

Abstract

A three reservoir model, consisting of the continental crust, depleted mantle and a more primitive mantle reservoir is used as a basis to account for both the present-day as well as the evolving isotopic compositions of the Earth's crust and mantle. The rate of growth of the continental crust is used as an input parameter to constrain the concomitant growth and evolution of the depleted mantle reservoir. Recycling of large volumes of bulk continental crust into the mantle is not considered to be an important process, nor is the existence of an additional major enriched reservoir in the early Archean mantle. This relatively simple model of progressively growing continental crust extracted from an increasing volume of depleted mantle can account for the positive ϵ Nd values which characterise the Archean depleted mantle, the evolution of the strontium, neodymium, hafnium and lead isotopic systems as well as the budgets of a wide range of trace elements in the continental crust and depleted mantle; e.g., the nonprimitive Rb/Cs, Nb/U and Th/U observed in MORBs and OIBs as well as the Sm/Nd in the crust and mantle can be reproduced. The Re-Os isotopic system is most sensitive to the formation of basaltic crust in the early Archean and can potentially provide definitive limits on the volumes of stored mafic or ultramafic crust in the mantle. To account for the relatively radiogenic 206Pb 204Pb ratios of modern MORB it is necessary to assume that the overall efficiency of transfer of uranium from the mantle to the crust has decreased markedly since the Archean, a proposed consequence of slab dehydration rather than slab melting. In the post-Archean period, recycling of hydrothermally altered oceanic crust is thus likely to have had a significant influence on the lead isotopic systematics of the mantle. In the model described here it is assumed that the volume of depleted mantle increases in a stepwise manner which is arbitrarily linked to major episodes of rapid crustal formation. From observed crustal age distribution patterns, episodes of rapid crustal formation with high ϵ Nd values occur at ~3600 Ma, ~2700 Ma and 1800 Ma. Thus, in our first order calculations, the crust is modelled as being extracted from ~10% of the mass of the mantle (upper 220 km) from 4500 Ma to 3600 Ma, ~20% (upper 410 km) from 3600 Ma to 2700 Ma, ~30% (upper 660 km) from 2700 Ma to 1800 Ma and 40% to 50% (upper 800–1000 km) of the mantle from 1800 Ma to the present-day. This type of model with growing volumes of both continental crust and depleted mantle has the general effect of buffering the isotopic and trace element composition of the upper mantle through time to an approximately constant, but incompatible element depleted chemical composition.