Zircons: Time Capsules from the Early Earth
Zircons are minerals that typically exist as tiny crystals in rocks. They have, however, a big story to tell. A few zircon crystals have been found that are the oldest Earth materials ever discovered, and they reveal clues about periods of geological time for which there is no direct evidence. Almost entirely as a result of studies of zircons, scientists are developing new hypotheses of what Earth's first 500 million years may have been like. Travel to a remote island off Greenland's coast and a zircon-making lab in New York State to learn how geologists are recovering and understanding these time capsules.
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Zircons Recast Earth's Earliest Era
For geologists, the first 500 million years of Earth's history is particularly mysterious. How did the planet evolve after it formed 4.5 billion years ago? Until recently, the impressions geologists had of this early period-the Hadean eon-evoked its name: hell on Earth. Scientists believed early Earth was exceptionally hot, with a simmering sea of magma that covered much of the planet. Meteorites frequently battered the surface. Their perception was that early Earth was not a place where continents could form or life could survive.
Ten years ago, there was little evidence to challenge this impression of the early Earth. Indeed, no direct evidence of that time exists. Rocks older than 4 billion years are not available to study, because they have long since eroded away, have been transformed by geologic processes, or are too deep underground to access.
But tiny survivors of Earth's early era do persist: zircon crystals. A common mineral made of the elements zirconium, silicon, and oxygen, zircon has formed throughout geologic history and is exceedingly tough. "It's very, very difficult to destroy zircon," says Martin Whitehouse, a leading zircon researcher. "It is the oldest preserved material that we've got."
Cutting-edge techniques are allowing geologists to extract information from ancient zircons about the conditions in which they grew. These time capsules have brought the hellish scenario of early Earth into question. Zircons are also enabling scientists to formulate intriguing new hypotheses about when major planetary features, such as continents and oceans, formed. "If we didn't have zircon," says Whitehouse, "we'd understand the Earth a whole lot less."
To the Ends of the Earth
The tiny, treeless island of Akilia, located off Greenland's west coast, is a prime spot to find ancient zircons. Akilia is made of some of the oldest surviving rocks on Earth, and the lack of vegetation makes them easy to detect and collect. A fist-sized sample of rock from the island may contain dozens of zircon crystals. Whitehouse extracts the zircons in his lab at the Swedish Museum of Natural History by pulverizing and sifting rock samples with a variety of devices, including one called a jaw-crusher.
During deep burial, rocks are exposed to high temperatures and pressures, which may cause their constituent minerals to break down. Zircon crystals, however, usually survive and may in fact grow larger. These minerals not only survive geological processes such as melting, weathering, and chemical attack, which can destroy rocks and other minerals, but they record each event as a ring of new growth. Consequently, zircons can form a series of growth rings over time, much like tree rings.
In recent years, geologists have figured out how to determine the age of each microscopic growth ring in a zircon. To date a zircon, scientists take advantage of the fact that zircon usually incorporates a small amount of uranium as it grows. This uranium slowly decays to lead. Because zircons incorporate very little lead as they grow and because scientists know uranium's decay rate, the ratio of uranium to lead in the zircon can be used to calculate age.
Whitehouse and his colleagues use a high-resolution device called a secondary ion mass spectrometer, or ion probe, to analyze zircons. The probe emits a beam of ions (charged particles) that strikes the zircon, causing it to release "secondary" ions such as uranium ions and lead ions. The beam can be focused on a very small spot, enough for the analysis of individual growth rings only 10 to 15 microns wide (1 micron is one-thousandth of a millimeter). The device then measures the concentrations of uranium and lead ions from the sample to yield an age.
Decoding Zircon's Dates
The most complex single grains of zircon from Akilia preserve several phases of growth over a billion years of history. It's not uncommon for Whitehouse to analyze specimens with cores aged 3.85 billion years. "One way to interpret these grains is that the rock [that contained the zircon cores] formed 3.85 billion years ago," he says. The zircon cores likely crystallized as the rock solidified from cooling magma.
Darker growth rings around the ancient cores are 3.65 billion years old, and hint at another significant event. The host rock, says Whitehouse, may have melted, or it may have experienced metamorphism: a change in its mineral structure from high-temperature, high-pressure conditions.
What Earth processes can bury rocks at great depths, where high pressures and temperatures can cause such changes and form new rings on zircons? Tectonic processes, those that result from the movement of individual plates of Earth's solid outer shell, called the lithosphere. One example is when colliding tectonic plates build mountain ranges. "In effect, you can look at a complicated zircon like this, and see a record of large-scale events on the planet, . . . the sort of things that nowadays are building the Himalaya or the Alps," says Whitehouse. "These sorts of events were happening when these rings were growing. In 3.7 billion years' time, a geologist will be able to sit here and look at a Himalayan zircon and see this event happening, also."
Hot Times on Early Earth?
So far, the oldest zircons come from rocks in the Jack Hills of central Australia. Using the uranium-lead decay system, scientists have determined that these zircons are 4.375 billion years old, which means that they formed during the Hadean eon. E. Bruce Watson, a professor at the Rensselaer Polytechnic Institute (RPI) in Troy, New York, leads a team that is developing new techniques to extract even more information from zircons to illuminate conditions during the earliest era of Earth.
Watson supposed that another zircon impurity, titanium, could be used as a thermometer to gauge ancient Earth temperatures. When a crystal grows, the amount of impurities it incorporates often depends upon temperature, explains Watson. "But we have no advanced knowledge of what that dependence on temperature is."
To determine this dependence for zircon, the RPI team remakes the conditions of early Earth in their laboratory. They manufacture zircons in the presence of titanium dioxide at various temperatures and pressures. Watson then uses an ion probe to measure the amount of titanium that entered the zircons' crystal structures and studies the relationship of temperature and pressure to titanium concentration. Watson has found that pressure does not affect the titanium content-but temperature does.
"The higher the temperature of crystallization, the more titanium you get in the structure," says Watson. "We can then go to an unknown zircon, analyze the titanium concentration, and thereby determine the temperature at which it formed."
Watson's lab has used this "titanium thermometer" technique to determine the temperatures of the 4.375-million-year-old Hadean zircons from Australia. Most of them, his team has found, crystallized at temperatures around 680 degrees Celsius (1,256 degrees Fahrenheit). For the early Earth, that is actually quite cool.
The results are painting a very different picture of Earth's first 500 million years. "The picture coming out from the zircons is that the early Earth really wasn't a bubbling, boiling magma ocean," says Martin Whitehouse. About 4.4 billion years ago, says RPI's Watson, Earth was cool enough that it "had continents that were above sea level, that erosion of those continents was occurring, and sediments were forming. That necessitates the presence of oceans, so that means liquid water on the surface of the Earth. It was cool enough so that oceans didn't boil-potentially cool enough that living organisms could get a foothold."
"In some sense the physical conditions at the surface of the early Earth, as seen through the eyes of these time capsules from that period, was not that different from today," Watson says. "That is what is revolutionary about this idea."