Comment on "Zircon thermometer reveals minimum melting conditions on earliest Earth" II.
Watson and Harrison (Reports, 6 May 2005, p. 841) interpreted low temperatures (approximately 700 degrees C) for Hadean zircons as evidence of the existence of wet, minimum-melting conditions within 200 million years of solar system formation. However, high-temperature melts (approximately 900 degrees C) are zircon-undersaturated and crystallize zircon only after substantial temperature drop during fractional crystallization. Zircon thermometry cannot distinguish between low- and high-temperature Hadean igneous sources.
Nutman
Allen P
AP
Research School of Earth Sciences, Australian National University, Canberra, ACT 0200, Australia. Allen.Nutman@anu.edu.au