Ancient zircons provide a new picture of early Earth

Article By:

Harrison, T. Mark Institute of Geophysics and Planetary Physics, University of California, Los Angeles, California.

Last reviewed:2010

DOI:https://doi.org/10.1036/1097-8542.YB100126

The Hadean Eon, around 4.5–4.0 billion years ago (Ga), is the dark age of Earth history. There is no known rock record from this period. As a result, our knowledge of the growth history of continental crust is equally consistent with the planet then hosting a massive early crust or essentially none at all. Without support from a rock record, our understanding of pre-Archean continental crust comes largely from investigating Hadean detrital zircons (mineral grains from broken-down rock). We know that these ancient zircons yield relatively low crystallization temperatures and that some are enriched in heavy oxygen, contain inclusions similar to modern crustal processes, and show hafnium (Hf) isotope evidence of silicate differentiation by 4.51 Ga. These observations are interpreted to reflect an early terrestrial hydrosphere, early felsic crust in which granitoids were produced and later weathered under high water activity conditions, and even the possible existence of plate boundary interactions, in profound contrast to the traditional view of this period.

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