Ancient Zircon Crystals Provide a Window into Early Earth History

A zircon crystal exhibiting distinct edges, or rims, from a metamorphic event after its initial formation.
Photo Credit: Shane K. Houchin

Scientific Frontline: "At a Glance" Summary: Ancient Zircons and Early Earth History

• Main Discovery: Analysis of ancient zircon grains indicates that early Earth experienced rapid oxidation shortly after its formation and confirms that plate tectonics were active much earlier than previously recognized.
• Methodology: Researchers utilized U XANES oxybarometry at synchrotron facilities to precisely measure trace elements, specifically the oxidation states of uranium, encapsulated within the cores and distinct rims of ancient zircon crystals.
• Key Data: Zircon rims dating to 4.1 billion years ago demonstrated unexpectedly high oxidation levels, indicating crustal oxidation merely 350 million years after Earth's formation, while distinct high-pressure and low-temperature signatures point to subduction zone activity at least 3.35 billion years ago.
• Significance: The results challenge the long-held paradigm that the Hadean eon was a completely dry and highly reduced environment, instead suggesting the early presence of abundant water and the early onset of dynamic geological processes necessary for the evolution of life.
• Future Application: The novel U XANES oxybarometry technique will be applied to analyze hundreds of additional zircon grains spanning various geological periods to construct a more comprehensive record of planetary evolution and shifting environmental conditions.
• Branch of Science: Geochemistry, Geosciences, and Planetary Science.
• Additional Detail: The analyzed zircon crystals, sourced primarily from the Jack Hills region of Western Australia, measure only a quarter of a millimeter in length but feature growth layers analogous to tree rings that preserve exact historical magma chemistry conditions.

A variety of ancient zircon crystals. Formed in the earliest era of Earth's history, these zircons contain a record of what the planet was like at that time.
Photo Credit: Shane K. Houchin

There are many open questions about how our planet formed 4.55 billion years ago: When did plate tectonics start? When did the earth's mantle begin to vigorously circulate in a process called convection? What was Earth like early in its lifetime? Because no rock records from the earliest years of the earth remain, researchers turn to minerals called zircons, which are resilient against physical and chemical alteration over time and therefore preserve a precise chemical record about the moments in which they were formed.

Some of the oldest zircon crystals are 4.4 billion years old. Now, a new Caltech study examines these most ancient zircon grains and discovers evidence for two key findings: first, that the early Earth underwent rapid oxidation sooner than previously believed and, second, that plate tectonics began at least 3.35 billion years ago, providing a key data point in a much-discussed debate among geoscientists.

The work was led by Shane Houchin (MS '22), a graduate student in the laboratory of Francois Tissot, professor of geochemistry and Heritage Medical Research Institute Investigator. A paper describing the research appears in the journal Proceedings of the National Academy of Sciences on March 2.

Zircon crystals are formed within hot magma and crystallize into structures that, like tree-rings, reflect the conditions under which they grew. The most ancient zircon crystals, largely found in the Jack Hills region of Western Australia, provide the best record of the magma chemistry of the early Earth, extending back more than 4 billion years ago. At some point, some of the rocks containing these zircon grains underwent metamorphism, and a new generation of zircon crystallized on the edges of the grains, forming distinct rims. Though the zircon crystals are only a quarter of a millimeter long, advanced analytical techniques can precisely measure trace elements (such as uranium and titanium) encapsulated in the zircon cores and rims, giving clues to the environment at the time of the mineral's formation.

"Short of a time machine, zircon is the only way we study samples of the early Earth," Tissot says.

The earliest millions of years of the earth's lifetime have long been imagined to be a hellish place: a blood-red atmosphere full of smoke and ash from active volcanoes and completely dry and devoid of oxygen. This era, called the Hadean, was thought to be a highly "reduced" environment, as opposed to "oxidized." Oxidation and reduction are two opposite measures of how electrons are available to drive chemical reactions, and measuring the reduction–oxidation—or "redox"—state of a sample can be used to infer the amount of oxygen in the environment. In a geologic context, this is commonly associated with the amount of water that is present during crystallization of magma. A highly reduced environment would be very dry, whereas more oxidation may indicate more water.

The team found that uranium in the rims of the zircons, dating back to 4.1 billion years ago, was much more oxidized than expected. This indicates that if the early Earth did, in fact, start as a highly reduced environment, some event must have taken place to rapidly oxidize the planet within, at most, just a couple hundred million years after its formation. Possible options include the delivery of water through comet collisions, a process of de-gassing, or the initiation of efficient convection of the earth's mantle beginning earlier than expected. Or, perhaps, the planet did not start out as reduced as previously envisioned.

"This new data helps undo the picture of the earth as a very reduced, hellish, dry place at that time," Houchin says. "Instead, the crust appears to be oxidized only 350 million years into its lifetime, indicating that there may already have been a lot of water present at that time."

The team also discovered that the zircon crystals must have experienced both a high-pressure and relatively low-temperature environment after forming. This environment is suggestive of a subduction zone, indicating that a large fragment of crust carried these zircons from the surface deep into the planet where it experienced high pressures. The findings thus suggest that plate tectonics may have been active at least 3.35 billon years ago. Plate tectonics provide the dynamic energetic environment necessary for the evolution of life, and there has been much debate among scientists about when the process began. This new study provides a crucial new data point from early Earth.

The study is the first application of a technique called U XANES oxybarometry (X-ray Absorption Near Edge Structure) to determine the redox state of early Earth, specifically by examining uranium oxidation states in ancient zircon crystals. To do this, the team collaborated with researchers at the Advanced Photon Source at Argonne National Laboratory to utilize their synchrotron facilities. Houchin and the team hope to now apply U XANES to examine hundreds more zircon grains dating from other periods of Earth's history.

Funding: Funding was provided by the National Science Foundation, the Department of Energy, and the Division of Geological and Planetary Sciences at Caltech.

Published in journal: Proceedings of the National Academy of Sciences

Title: Oxidized Hadean magmas and Archean mobile-lid tectonics revealed by Jack Hills zircon

Authors: Shane K. Houchin, François L. H. Tissot, Mauricio Ibañez-Mejia, Elizabeth A. Bell, T. Mark Harrison, Matthew Newville, and Antonio Lanzirotti

Source/Credit: California Institute of Technology | Lori Dajose

Reference Number: es030326_02

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