Great Moments in Earth History: The HadeanPosted: July 17, 2013
No complete rocks have survived to tell of the formative years following Earth’s formation some 4.56 billion years ago. The material that would have existed at that time has been broken apart by the power of wind and water. It has melted and metamorphosed under the immense pressure and heat deep within Earth’s interior. It has been recycled back on to the surface. It has existed at every stage of the rock cycle many, many times over.
Despite the thousands of millions of years Earth’s earliest material has encountered, tiny pieces of some of these rocks still exist. Small microscopic grains of a mineral, once part of rocks that would have witnessed Earth as it existed just a couple of hundred million years after its formation, survive to this day.
The oldest known terrestrial material is a single grain of a mineral called zircon which was found in the Jack Hills formation in western Australia. It is 4.4 billion years old. The grain itself was part of a rock composed of the broken and eroded bits of other ancient material that itself has been subject to billions of years of geologic reworking.
Zircon is an extremely rugged mineral made up of silicon and the obscure element zirconium. Its tenacity in the face of time and its ability to provide scientists with enough information to figure out the age when it was formed are among the many reasons it is exciting to geologists.
Small grains like these are among the only remnants of a geologic eon known as the Hadean. Named after the Greek god of the underworld, the Hadean spans the time from Earth’s formation to 4 billion years ago.
For years, the commonly held view among geologists was that this time was wildly different from the rest of Earth history. The most dramatic pictures painted it as an age with vast magma oceans subject to constant and repeated bombardment from asteroids and comets. These tumultuous conditions, the argument went, would have made abundant liquid water impossible.
But these ancient zircon crystals have more to tell than just their age—they offer a dramatically different story about the Hadean.
Zircon crystals contain a number of impurities. The radioactive element uranium is one of them—its decay allows scientists to determine the age when the zircon crystal was formed. Oxygen and titanium are two other impurities. These two elements, it turns out, provide excellent clues about environmental conditions during the Hadean.
Oxygen exists in a variety of different forms. Most oxygen atoms have a mass of 16. Some have a mass of 18. When a sample has relatively more mass 18 atoms, scientists say that the oxygen is “heavy.” The relative “heaviness” or “lightness” of oxygen can be indicative of different environments. This is because heavy and light oxygen atoms react to different physical conditions in slightly different ways. Heavy oxygen is a signature of cool, wet environments.
In 2001, two teams of scientists studying the oxygen in these ancient minerals published nearly identical results in the same issue of Nature. Both teams found heavy oxygen in the zircon crystals. To both teams, this could mean only one thing. The rocks forming in the mantle at this time came from the melting and recycling of rocks that had been in contact with significant amounts of water. There may have been an ocean during the Hadean, but for these scientists, it was one filled with water, not magma.
In 2005, two scientists—Bruce Watson and Mark Harrison—decided to look for more clues in these ancient crystals. They focused their attention on titanium. After creating synthetic zircon in the lab in different conditions, and after looking at other younger zircon crystals, they realized that the concentration of titanium varied depending on the temperature at which it crystallized.
Using this information, they were able to create a titanium “thermometer”—an equation that relates the concentration of titanium in zircon to the temperature when it formed. This may seem like an unimportant discovery, but knowing the temperature of crystallization is extremely useful. Any rock that is heated in the presence of water will melt, and potentially recrystallize, at a temperature between 650 and 700 degrees Celsius.
Using this “thermometer” Watson and Harrison found that the ancient zircon minerals crystallized at around 680 degrees—right where they would expect if the crystal formed from the recycling of rock that had been in contact with water.
Both the oxygen and the titanium were telling the same story—a story that directly contradicted the traditional view about the Hadean period of Earth history. This new story paints a picture of a world with oceans and tectonics similar to what we know today. While certainly a foreign world compared to our own, it would have been at least recognizable as Earth.
The existence of liquid water has other implications as well. The view of a hostile Hadean period suggested that life could not have evolved anytime before 4 billion years ago. While it may be impossible to know, the existence of a liquid ocean during the Hadean may have provided life with an opportunity to form much earlier than previously assumed.
It is remarkable that anything, even a grain of an obscure mineral, has survived for 4.4 billion years. What is even more remarkable is how much information scientists have been able to glean from just a few of these grains. We are lucky that zircon has so much to say.