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Science 10 February 2006:
Vol. 311. no. 5762, p. 775
DOI: 10.1126/science.311.5762.775a
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Letters
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Technical Comment Abstracts
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Comment on "Zircon Thermometer Reveals Minimum Melting Conditions on Earliest Earth" I
Andrew Glikson
Abstract: Watson and Harrison (Reports, 6 May 2005, p. 841) proposed a model for early Earth magmatism based on crystallization temperatures of Hadean zircons. However, detrital zircon populations are skewed relative to the composition of their source terrains, Archaean isotopic and geochemical mantle signatures preclude reincorporation of Hadean continental crust into the early mantle, and the effects of early impacts should be considered.
Full text at www.sciencemag.org/cgi/content/full/311/5762/779a
Comment on "Zircon Thermometer Reveals Minimum Melting Conditions on Earliest Earth" II
Allen P. Nutman
Abstract: Watson and Harrison (Reports, 6 May 2005, p. 841) interpreted low temperatures (~700°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 (~900°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.
Full text at www.sciencemag.org/cgi/content/full/311/5762/779b
Response to Comments on "Zircon Thermometer Reveals Minimum Melting Conditions on Earliest Earth"
E. B. Watson and T. M. Harrison
Abstract: The mean crystallization temperature of Hadean zircons based on titanium content is ~680°C. This value corresponds to the temperature of wet minimum melting in present-day crust. The low variance of the temperature distribution (±25°C) also points to Hadean zircon growth under conditions that were highly reproducible and thermally regulated. Eutectic-like melting is particularly capable of providing such regulation and is consistent with Hadean zircon growth during wet crustal fusion.
Full text at www.sciencemag.org/cgi/content/full/311/5762/779c
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Science. ISSN 0036-8075 (print), 1095-9203 (online)
