Quantifying Precambrian crustal extraction: the root is the answer

We use two different methods to estimate the total amount of continental cr ust that was extracted by the end of the Archean and the Proterozoic. The f irst method uses the sum of the seismic thickness of the crust, the eroded thickness of the crust, and the trapped melt within the lithospheric root t o estimate the total crustal volume. This summation method yields an averag e equivalent thickness of Archean crust of 49 +/- 6 km and an average equiv alent thickness of Proterozoic crust of 48 +/- 9 km. Between 7 and 9% of th is crust never reached the surface, but remained within the continental roo t as congealed, iron-rich komatiitic melt. The second method uses experimen tal models of melting, mantle xenolith compositions, and corrected lithosph eric thickness to estimate the amount of crust extracted through time, This melt column method reveals that the average equivalent thickness of Archea n crust was 65 +/- 6 km, and the average equivalent thickness of Early Prot erozoic crust was 60+/-7 km. It is likely that some of this crust remained trapped within the lithospheric root. The discrepancy between the two estim ates is attributed to uncertainties in estimates of the amount of trapped, congealed melt, overall crustal erosion, and crustal recycling. Overall, we find that between 29 and 45% of continental crust was extracted by the end of the Archean, most likely by 2.7 Ga. Between 51 and 79% of continental c rust was extracted by the end of the Early Proterozoic, most likely by 1.8- 2.0 Ga. Our results are most consistent with geochemical models that call u pon moderate amounts of recycling of early extracted continental crust coup led with continuing crustal growth (e.g. McLennan, S.M., Taylor, S,R., 1982 . Geochemical constraints on the growth of the continental crust. Journal o f Geology, 90, 347-361; Veizer, J., Jansen, S.L., 1985. Basement and sedime ntary recycling - 2. time dimension to global tectonics. Journal of Geology 93(6), 625-643). Trapped, congealed, iron-rich melt within the lithospheri c root may represent some of the iron that is 'missing' from the lower crus t. The lower crust within Archean cratons may also have an unexpectedly low iron content because it was extracted from more primitive, undepleted mant le, (C) 2000 Elsevier Science B.V, All rights reserved.