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What is the significance of the Jack Hills zircons?

Submitted by Joseph Reese, Edinboro University of Pennsylvania

Why is this important?

Up to 4.4 billion years old, zircon grains are the oldest earth materials discovered.

What we know...

The Jack Hills zircons are the oldest terrestrial materials found so far. These detrital grains from a quartzite / metaconglomerate unit in western Australia clock in at between 4.0-4.4 Ga with a single grain dating at 4.4 Ga in age.

Oxygen isotopic studies yielding high isotopic ratios indicate that the magma from which these zircons originated was derived from recycled rock that had interacted with surface waters and not from a mantle source. As Mark Harrison commented These zircons tell us that they melted from an earlier rock that had been to the Earth's surface and interacted with cold water. The presence of quartz inclusions as well as results from neodymium and hafnium isotopic studies support a felsic source suggesting that continental crust may have been forming very early in Earth's history and that tectonic processes like subduction were operating as well. Implications are profound including the presence of continents oceans and perhaps life very early on in Earth's history.

References and other Resources

Here are some resources that can be used to teach about the implications of these mineral grains.

Images from Aaron Cavosie at the University of Puerto Rico, useful for teaching about zircons:
A PowerPoint file of images from the Jack Hills zircons (PowerPoint 17.1MB Apr23 07)
A low resolution version of the same file (PowerPoint 1.1MB Apr23 07)
Zircons are Forever, by John W. Valley, William H. Peck, Elizabeth M. King (1999). This is a reprint of a 1999 article from the University of Wisconsin-Madison Geology Alumni Newsletter, with links to extensive references and images.
Images from a Cool Early Earth, by John W. Valley, William H. Peck, Elizabeth M.King, and Simon A. Wilde, (2002) Geology. 30: 351-354.
A Cool Early Earth? Scientific American article A comprehensive webpage: Zircons Are Forever

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