IGPP Seminar Series

Hadean Continental Growth: Did Plate Tectonics Begin at 4.5 Ga?

by Dr. Mark Harrison
Australian National University and IGPP, UCLA


The long favored paradigm for continent formation is that initial growth was forestalled until ~4 Ga and then grew slowly until present day. The observation of some early isotopic heterogeneity leaves open the possibility of earlier global fractionations and a minority view has persisted that continental crust was widespread during the Hadean Eon — the period comprising the first 500 Ma of Earth history. Hadean (>4 Ga) detrital zircons from Jack Hills, Western Australia, represent pieces of continental crust that have been sequestered for up to ~4.4 Ga and thus can be used to directly test this hypothesis.Zircons have very low Lu/Hf ratios and thus record near initial 176 Hf/177Hf at the time given by the U-Pb age. 176Hf/177Hf analyses of Jack Hills zircons ranging in age from 3.96 to 4.35 Ga indicate surprisingly large Hf isotope heterogeneities during the early Hadean from which we infer a major differentiation of the silicate Earth at ~4.5 Ga. A consequence of this differentiation is the formation of continental crust with a volume similar in magnitude to the present. Using a new crystallization thermometer, we find that Hadean zircons cluster strongly at ~690 °C — similar to that of granitoid zircon growth today. This substantiates the existence of wet, minimum- melting conditions within 200 m.y. of solar system formation and strongly suggests a regulated mechanism producing zircon-bearing rocks throughout the Hadean. Together with the Hf isotope results, these results further suggest that the Earth had settled into a pattern of crust formation, erosion, and sediment recycling by 4.35 Ga similar in many respects to the known era of plate tectonics. We can begin to speculate on the nature of Hadean plate boundary zones with the goal of assessing whether >4 Ga plate interactions are similar to the contemporary plate tectonic system.
Tuesday, 11 October 2005
3853 Slichter Hall
Refreshments at 3:45 PM
Lecture at 4:00 PM