New thermometer reveals wet conditions on earliest Earth

Researchers at Rensselaer Polytechnic Institute and Australian National University have found new evidence that environmental conditions on early Earth, within 200 million years of solar system formation, were characterized by liquid-water oceans and continental crust similar to those of the present day. The researchers developed a new thermometer that made the discovery possible.

“Our data support recent theories that Earth began a pattern of crust formation, erosion, and sediment recycling as early in its evolution as 4.35 billion years ago, which contrasts with the hot, violent environment envisioned for our young planet by most researchers and opens up the possibility that life got a very early foothold,” said E. Bruce Watson, Institute Professor of Science and professor of geochemistry at Rensselaer Polytechnic Institute.

According to Watson, the research provides important information and a new technique for making additional discoveries about the first eon of Earth’s history, the Hadean eon, a time period for which still little is known.

The research findings are reported in the May 6 issue of the journal Science in a paper titled “Zircon Thermometer Reveals Minimum Melting Conditions on Earliest Earth.”

Watson collaborated with co-author T. Mark Harrison, director of the Research School of Earth Sciences at Australian National University and professor of geochemistry at UCLA, on the research. The work was supported by the National Science Foundation (NSF), the Australian Research Council, and the NASA Astrobiology Institute.

Watson and Harrison developed a new thermometer that involves the measurement of the titanium content of zircon crystals to determine their crystallization temperature. Zircons are tiny crystals embedded in rock that are the oldest known materials on Earth. Zircons pre-date by 400 million years the oldest known rocks on Earth. These ancient crystals provide researchers with a window into the earliest history of the Earth and have been used to date the assembly and movement of continents and oceans.

“Zircons allow us to go further back in geologic time because they survive processes that rocks do not,” said Watson. “Although they measure only a fraction of a millimeter in size, zircons hold a wealth of information about the very earliest history of Earth.”

In Watson and Harrison’s work, zircons from the Jack Hills area of Western Australia ranging in age from 4.0 to 4.35 billion years were analyzed using the thermometer. The new temperature data supports the existence of wet, minimum-melting conditions within 200 million years of solar system formation, according to the researchers. In the Science paper, the researchers discuss how the thermometer provides clear distinction between zircons crystallized in the mantle, in granites, and during metamorphism, thereby providing consistent information about the conditions on Earth during the crystals’ formation.

Watson describes his research as “materials science of the Earth,” because it involves designing and executing laboratory experiments at the high temperatures and pressures found in the Earth’s deep crust and upper mantle. He teaches undergraduate and graduate geology courses at Rensselaer, including Natural Sciences, Introduction to Geochemistry, and Earth Materials.