RECOGNIZING THE ONSET OF PLATE TECTONICS ON EARTH: THE APPEARANCE OF WIDESPREAD ENRICHED MANTLE BETWEEN 2 AND 3 GA?

Although ε¹⁴²Nd anomalies in some Archean basalts require preservation of small volumes of pre-3.5-Ga enriched mantle (EM) in the lithosphere or deep mantle, incompatible element ratios in greenstone oceanic basalts suggest that significant volumes of EM did not appear in Earth until 2-3 Ga. No evidence exists for widespread MORB-like DM or OIB-like EM sources before 2 Ga. Zr/Nb, Nb/Th and La/Sm ratios and ε¹⁴³Nd(t) show that most Archean basalts come from depleted mantle (DM), in many cases hydrated (subduction-like mantle) (HM). The uniformity of these ratios in pre-2.5 Ga basalts requires vigorous mixing of most of the mantle between magma ocean crystallization and about 3 Ga. A relative depletion of Nb in DM sources throughout time supports sequestration of Nb into the core during its formation at high pressures. Incompatible element ratios in pre-2.5 Ga DM sources are similar to basaltic sources on stagnant-lid planetary bodies like the Moon and Mars. However, by 2 Ga 50% of terrestrial non-HM greenstone basalts appear to come from EM sources, the appearance of which between 2 and 3 Ga may reflect the onset and propagation of plate tectonics around the globe. Prior to 3 Ga, Earth may have been in a stagnant-lid regime with most basaltic magmas coming from a rather uniform, variably depleted mantle source, in many cases hydrated. Between 2 and 3 Ga, possible episodes of subduction were superimposed on the stagnant-lid regime, but not until about 2 Ga did plate tectonics become widespread. It was not until extraction of continental crust and recycling of oceanic and continental crust into the mantle that “modern type” EM and DM reservoirs developed beginning about 2.5 Ga.