PERSPECTIVES FROM GLOBAL MODELING OF TERRESTRIAL PB AND ND ISOTOPES ON THE HISTORY OF THE CONTINENTAL-CRUST

Transport balance modeling has been carried out to reconstruct present day Pb and Nd isotope data of the main accessible terrestrial reservo irs (upper mantle and continental crust). Models start from solar syst em initial ratios given by meteorite data and assume a start of Earth accretion at 4.55 Ga. The growth of the continental crust, as well as its partial recycling into the mantle, through geological time are sen sitive parameter sets, which can therefore be constrained. The best fi t solution shows the crust to grow from zero at 4.4 Ga to 75% of its p resent mass by 1.6 Ga, with slower net growth thereafter. A marked inc rease in the rate of crust-mantle recycling, from c. 10% of the crust production rate to c. 50%, is indicated to have occurred between 2 and 1.5 Ga. It is speculated that this results from increased erosion aft er sufficient atmospheric oxygen made an ozone layer, and therefore la nd life, possible.