2 TERRESTRIAL LEAD-ISOTOPE PARADOXES, FORWARD TRANSPORT MODELING, CORE FORMATION AND THE HISTORY OF THE CONTINENTAL-CRUST

A combined solution for the two major terrestrial lead paradoxes has b een sought, These are the 'future' paradox (upper crustal and upper ma ntle Pb isotope compositions plot in the future field in Pb-207/Pb-204 vs. Pb-206/Pb-204 space) and the Th/U mantle paradox (The Th/U ratio of the upper mantle is ca, 2.6, whereas Pb isotopes indicate a value o f 3.8). Constraints considered other than U-Th-Pb data include siderop hile element concentrations of the mantle (which restrict core formati on scenarios), W isotopes and the oldest lunar ages (bracketing the ac cretion and core formation time scale to 60-100 Ma), and noble gas sys tematics (requiring a two-layered mantle structure dating back to just after accretion). Further, Nd isotopes allow a test of the validity o f crustal growth models used, The transport balance model used include s a continental crust divided into four parts: upper (high U/Pb) and l ower (low U/Pb), as well as older and younger, The latter division is generated by erosion removing proportionally more younger than older c rust, After 2 Ga ago erosion transfers U to the ocean floor in prefere nce to Pb and Th, as a consequence of U solubility in an oxidizing env ironment. Within the constraints imposed on the model, the future para dox cannot be solved by postulating a delayed core formation, An addit ional low U/Pb reservoir required for this can be found in the contine ntal crust. Solving the future paradox requires that particularly the older lower crust reservoir is conservative, which limits the amount o f continental crust that can have been recycled into the mantle over E arth's history. On the other hand, a solution of the mantle Th/U parad ox requires a considerable amount of continent recycling, particularly in the last 1-2 Ga, A restricted family of crustal history scenarios allows a solution to both paradoxes, These are characterized by < 10% of the present amount of continental crust existing just after Earth a ccretion, rapid crustal growth, with relatively insignificant recyclin g into the mantle, during the Archaean, and increasing continent recyc ling in the Proterozoic, reaching ca. 60% of the rate of continent for mation today. Scenarios in which delamination of lower crust accounts for over 5% of continent recycling do not provide solutions. The resul t portrays a non-steady-state Earth in which the net mass of continent al crust is at present still growing at 2 x 10(15) g/a, and the U cont ent of the upper mantle is increasing. (C) 1997 Elsevier Science B.V.