The chemical composition of the Archaean crust

Summary Three separate observations, from sedimentary rock compositions, subduction zone igneous rocks and heat flow constraints combine to suggest that the composition of the Archaean crust was distinct from that of the present-day continental crust. The sedimentary REE patterns are consistent with derivation of the upper crust from a 1:1 mixture of the Archaean bimodal basic-felsic suite. The andesite model, while adequate to account for present-day continental growth does not provide enough Cr and Ni for the Archaean crust. Average crustal component heat flow values of 14 mW m −2 for Archaean terrains are significantly lower than the post-Archaean average of 23 mW m −2 for crustally produced heat. Correlations from the sedimentary trace element data provide Archaean upper crustal values of 1.5% K, 5.7 ppm Th and 1.5 ppm U. Heat flow constraints restrict such compositions to the upper layers. The total crustal values are consistent with a 2:1 basic/felsic composition, and give values for the total crust of 0.75% K, 2.9 ppm Th and 0.75 ppm U, which provide crustal heat flow components of 14 mW m −2 for a 30 km thick crust and 19 mW m −2 for a 40 km crust.