CRYSTALLIZATION TEMPERATURES OF HADEAN ZIRCONS: POSSIBLE PLATE BOUNDARY INTERACTIONS BETWEEN 4.4 AND 4.0 GA

A thermometer based on the concentration of Ti in zircon applied to Hadean detrital zircons from Western Australia yields constraints on the nature of magmatism during the first 500 Ma of Earth history. Jack Hills zircons ranging from 4.0 to 4.35 Ga yield crystallization temperature peaks at 670 and 710°C, suggesting that a limited range of mechanisms produced zircon-bearing rocks during that period. Together with results from other studies (e.g., ?18O, Lu-Hf, inclusion mineralogy) we believe the following conditions may have obtained throughout the Hadean: 1) wet, minimum melting, 2) 700-800°C collisional-type melting, 3) formation of clay minerals under ambient conditions, 4) peraluminous melting of pelitic protoliths, 5) isotopic fractionations requiring a major Hadean continental crust forming event, 6) a continent-mantle recycling time constant varying from 200-50 m.y., 7) global heat flow 3-5 times higher than present, and 8) Mg-rich mantle lithospheres. We argue that constraints 1, 3 and 4 imply the existence of liquid water at or near the Earth's surface, constraints 1, 2, 5 and 6 imply the existence of continental crust throughout the Hadean, and we speculate that constraints 1 and 4 reflect Hadean thrust burial. These three inferences are most easily be explained by the existence of some form of collisional boundary interaction among surface plates, whether rigid or not. We have developed a conceptual model based on the assumption that the continental Moho is defined by the ca. 700°C isotherm (i.e., constraint 1). Assuming torque balance among co-equal proportions of continental and oceanic regions, any assumption regarding the temperature structure beneath divergent zones defines, via mantle fusion systematics, the approximate laminate structure of the continental and oceanic ‘plates'. This family of models can in turn be used to begin to assess the range of possible modes of Hadean plate boundary interactions under the above described scenario.