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Project Progress

Harrison, with colleague E. B. Watson (RPI), published a paper this year in which they show that ancient zircons from Western Australia’s Jack Hills preserve a record of conditions that prevailed on Earth during the Hadeon Eon (4.5 to 4.0 billion years before present). They applied their newly-developed geothermometer based on the concentration of titanium in zircon and found that these most ancient of Earth minerals record temperatures of about 700°C (Figure 1). Such temperatures are low for igneous rocks and require that the host rocks in which the zircons grew formed by partial melting in the presence of water (so-called “wet minimum temperature melting”). The implication is that the Earth had settled into a pattern of crust formation, involving a water cycle, just 200 million years after the formation of the solar system.