NIOBIUM URANIUM EVIDENCE FOR EARLY FORMATION OF THE CONTINENTAL-CRUST/

Niobium/uranium ratios in greenstone-belt basalts and gabbros indicate that parts of the Late Archean mantle beneath Western Australia under went a level of melt extraction, resulting in formation of the contine ntal crust, comparable to that seen in the present mantle. The implica tion is either that (i) the amount of continental crust that formed be fore 2.7 x 10(9) years ago was much greater than generally thought or (ii) crustal growth occurred by severe depletion of small volumes of t he mantle rather than by moderate depletion of a large volume of mantl e.