Zircons: How tiny crystals open a window into the early history of Earth
Sandgrain-sized zirconium crystals offer scientists time capsules from more than 4 billion years ago, writes Richard A. Lovett.
Zirconium is the eighteenth most common element in the Earth’s crust – more common than such well-known substances as zinc, copper, nickel, and chromium. But most people have never heard of it, unless in the form of imitation diamonds known as cubic zirconia.
In nature, zirconium forms another type of crystal called zircons. To geophysicists, these are the true gems, because they provide vital time capsules from the Earth’s deepest past.
Chemically, zircons are nothing fancy. They are tiny lumps of zirconium silicate (ZrSiO4) that are ubiquitous in volcanic rocks. But they’re typically only 0.1 mm to 0.3 mm across, making them hard to spot without a magnifying glass. Not exactly the type of thing most of us would notice, let alone care about.
But they have two important traits.
One is that they are incredibly durable. The rocks in which they initially formed may weather away, but the zircons survive as tiny grains of sand that may later be incorporated into the next generation of rocks.
“We have no rocks that are older than 4 billion years,” says John Valley, a geochemist at the University of Wisconsin, Madison. (The Earth itself is 4.543 billion years old.) “[Zircons] are what we study if we want to analyze things that formed that far back.”
Their other trait is that they aren’t pure zirconium silicate. They contain trace amounts of other elements, most importantly uranium, trapped within them as they crystalize. Over the eons, that uranium slowly decays to lead. By comparing the amounts of uranium and lead, scientists can determine the date at which the crystal formed.
Another element, oxygen (the “O” in ZrSiO4), helps tell the conditions under which each zircon formed. That’s because oxygen has two well-known stable isotopes, 16O and 18O, either of which can be incorporated into the crystal as it grows.
Typically, these come from water (H2O), which can contain either 16O or 18O (or a more rare stable isotope called 17O). All these forms of water are chemically identical, but 18O-containing water is about ten percent heavier than 16O-containing water. That causes the two types of water to (very slightly) separate — and to do so by different amounts under different conditions.
Geologists once thought the early Earth was far too hot for its surface to be anything but an ocean of magma, let alone to have liquid water. In fact, the earliest period in the Earth’s history, from its formation to 4 billion years ago, is called the Hadean because it was widely believed to resemble hell, or Hades.That meant zircons from the Hadean period should have oxygen isotope ratios comparable to that of water molecules in the Earth’s mantle. But geologists studying the Jack Hills region of Western Australia, which has yielded the oldest zircons ever found, have been unearthing zircons from as far back as 4.375 billion years ago whose oxygen isotope ratios show they may have formed from magma that incorporated liquid water.
Other zircon research has suggested that life too may date back a lot further than we once thought. This research involves the ratio of non-radioactive carbon isotopes (12C and 13C) in tiny diamonds incorporated in the zircon structure. These diamonds have carbon isotope signatures suggesting the carbon from which they were formed may have included organic material from living organisms.
“This implies that there was life in the Hadean,” says Craig O’Neill, a geodynamicist at Macquarie University. Though, he notes, there are other explanations involving purely geologic processes. “It’s hard to be sure,” he says.
Still more studies have used hyper-sensitive magnets to look for trace magnetic fields carried by magnetic impurities in ancient zircons, in the hope of determining the strength of the Earth’s magnetic field at the time these zircons formed. “The analysis takes about a week,” says John Tarduno, a geophysicist at the University of Rochester, New York. Such studies, he says, indicate that the Earth’s magnetic field might be as old as its zircons.
And that’s just the beginning. In a 2017 study in Science Advances, geophysicists used zircons in Moon rocks brought back by Apollo astronauts to determine that the Moon’s crust solidified 4.51 billion years ago, only 60 million years after the formation of the first protoplanets. And zircons in meteorites blasted off the surface of Mars are being studied to peer nearly as far back into the Red Planet’s early history.
So who cares if copper, zinc, nickel, and chromium are vastly more valuable to the modern economy? Lowly zirconium may be what helps us unravel the greatest of all mysteries: who we are and where we came from.