The Archean-Proterozoic boundary: 500 my of tectonic transition in earth history

Changes in the solid Earth at the end of the Archean fall into two categories. First are those related to cooling of the mantle and include a decrease in both komatiite abundance and MgO content, a decrease in Ni/Fe ratio in banded iron formation, and increases in incompatible element ratios (such as Nb/Yb, La/Yb, Zr/Y, La/Sm and Gd/Yb) in non-arc type basalts. A second group of changes is related to the extraction of continental crust from the mantle and stabilization of major cratons at 2.7 to 2.5 Ga. These include an increase in Nb/Th ratio and ε_Nd(T) of non-arc basalts; significant increases in large-ion lithophile and high-field strength elements and a decrease in Sr in continental crust, which reflect a shift in magma types from TTG (tonalite-trondhjemite-granodiorite) to calc-alkaline; a prominent increase in the maximum values of δ¹⁸O of zircons from granitoids after the end of the Archean; a major peak in gold reserves is found at or near 2.7 Ga; and a peak in Re/Os depletion ages from mantle xenoliths at 2.7 Ga consistent with widespread thickening of the continental lithosphere at this time. All of these changes may be related to the widespread propagation of plate tectonics at the end of the Archean. Subduction produces continental crust in numerous arcs, which rapidly collide to form supercratons. Oceanic slabs sinking into the deep mantle could increase the production rate of mantle plumes, as well as increase the heat flux from the core, which warms the newly arrived slabs. The cooling of the deep mantle would begin after 2.5 Ga and continue until about 2.4 Ga when a 200-My slowdown in plate tectonics begins. This may be the reason for the rapid drop in temperature of the mantle recorded by basalts and komatiites. When plate tectonics comes back on track at about 2.2 Ga, Archean supercratons break up and are dispersed.