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.
The study is titled "Oxidized Hadean magmas and Archean mobile-lid tectonics revealed by Jack Hills zircon." In addition to Houchin and Tissot, co-authors are Mauricio Ibañez-Mejia of the University of Arizona, Elizabeth A. Bell and T. Mark Harrison of UCLA, and Matthew Newville and Antonio Lanzirotti of the University of Chicago. Funding was provided by the National Science Foundation, the Department of Energy, and the Division of Geological and Planetary Sciences at Caltech.
