DID EARLY ARCHEAN CONTINENTAL CRUST FORM WITHOUT PLATE TECTONICS?

Incompatible trace element ratios that track the source composition of oceanic basaltic magmas in the mantle show a pronounced change at the end of the Archean. During the Archean, Nb/Th, Zr/Nb, Nb/Y, Zr/Y, Th/Yb, and Nb/Yb ratios in these basalts exhibit tight groupings near primitive mantle composition. This contrasts to post-Archean oceanic basalts, which record numerous heterogeneities in mantle sources. The Archean examples also show a relatively constant upper limit of εNd of about +4. It was not until subduction introduced compositional contaminants into the mantle in the late Archean that the numerous sources required for modern oceanic basalts began to appear. This rapid change in greenstone basalt compositions indicates a major change in mantle dynamics and composition at the end of the Archean, probably related to the widespread propagation of plate tectonics. However, Hf model ages of zircons and Nd model ages of granitoids indicate that a significant volume of continental crust formed before 3 Ga, seemingly requiring production of continental crust in a non-plate tectonic setting. The relatively few detrital zircons > 3 Ga may reflect thin continental crust prior to this time, only locally emergent and subject to weathering and erosion. These results, together with those of published thermal models for the mantle (O'Neill et al., 2007), favor a 3-stage thermal/tectonic history for Earth: 1) > 4 Ga, an active stagnant lid regime, perhaps similar to Venus today; 2) 4.0-2.2 Ga, plate tectonics alternating or/and coexisting with stagnant lid tectonics; and 3) < 2.2 Ga, the modern plate tectonic regime. Results also suggest that plate tectonics may not be required for production of continental crust. It may also form in an active stagnant lid regime from hydrous partial melting of thickened oceanic crust or mafic crust in the root zones of oceanic plateaus.