3.0 Ga: the Most Important Watershed in Crustal Evolution

Continental felsic rock complexes are rare from the time in Earth history before 3.0 +/- 0.2 Ga, but widespread afterwards. Generally, pre-3 Ga continent did not become recycled into post-3 Ga continent, as happened to pre-existing continental material in all Proterozoic and Phanerozoic time. Widespread mafic and felsic crust formed 3.0-2.7 Ga is overwhelmingly juvenile in origin. Numerous studies show that Pre-3 Ga felsic crust contains 700-800 °C granitoid melt, zircon with at least moderately elevated oxygen-18 content, and metamorphic facies up to granulite, just like more modern crust. But the sudden increase in preservation around 3 Ga generates three important possibilities for the pre-3 Ga Earth: (1) felsic crust was widespread, but was destroyed in some way around 3 Ga without becoming recycled into newly-forming crust; (2) felsic crust was formed continuously 4.5-3 Ga, but was usually destroyed rapidly until 3 Ga, when it suddenly became possible to preserve large amounts of it; (3) felsic crust was formed only rarely and in small amounts before 3 Ga, after which more was generated. Possibilities 2 and 3 are most consistent with an explanation involving geographical stability of subduction zones. Felsic crust grows best in the recycling environment present in plate convergence zones, and developing this requires subduction to be stabilized in one locale for tens of m.y. at least. In the early Earth the more rapid thermal overturn may have made this difficult or impossible, leading either to rare development of felsic crust, or to its fragmentation and easy destruction. Around 3 Ga, a critical point in cooling was reached where stable continent growth became the norm.