Paul Mueller with student Stephanie Ingle by a cliff in Stillwater, Montana. Photo courtesy of Paul Mueller.
Darrell Henry overlooking Quad Creek in the eastern Beartooth Mountains, Montana. Photo courtesy of Barbara Dutrau.
Scientists at work in an ion probe lab. The instrument shown is similar to one of those used by Meuller to determine the precise ages of different Earth materials. Photo courtesy Woods Hole Oceanographic Institute.
What are the oldest rocks on Earth, and how did they form? The material that holds the greatest insight into these fundamental questions, because it can contain a record of some of the earliest history of the Earth, is a mineral named zircon. For example, a few grains of zircon found in the early 1990s in a sandstone from western Australia dates back 4.2–4.3 billion years, and we know from meteorites that the Earth is not much older at 4.56 billion years. Geology professors Darrell Henry of Louisiana State University and Paul Mueller of the University of Florida are expert practitioners of several techniques that can extract precise age information from zircons. They’re searching for some of the oldest rocks in the continental crust, for the zircons within them, and for the clues the zircons contain about the formation of the planet.
Originally formed by crystallization from a magma or in metamorphic rocks, zircons are so durable and resistant to chemical attack that they rarely go away. They may survive many geologic events, which can be recorded in rings of additional zircon that grow around the original crystal like tree rings. Like a tiny time capsule, the zircon records these events, each one of which may last hundreds of millions of years. Meanwhile, the core of the zircon itself remains unchanged, and preserves the chemical characteristics of the rock in which it originally crystallized.
Zircon contains the radioactive element uranium, which Dr. Mueller calls “the clock within the zircon” because it converts to the element lead at a specific rate over a long span of time. According to Mueller, this makes zircons “the most reliable natural chronometer that we have when we want to look at the earliest part of Earth history.” He goes on to explain that there are two ways to tell time in geology. “One is a relative time, meaning if there’s a mineral of one kind, and growing around it is a mineral of a second kind, you know the inner mineral formed first, but you don’t know how much time elapsed between the two.” Henry evaluates these kinds of mineral relations in rocks. From the types of minerals and their distributions in the rocks he reconstructs a relative sequence of events that reflects the change over time of parameters like pressure, temperature, and deformation. “If I have a metamorphic rock,” elaborates Dr. Henry, “I can use the types of minerals and their chemistry to determine the conditions that the rock had experienced at some point in its history. For example, a temperature of 700°C and high pressure of several thousand times atmospheric pressure imply that it had been deep in the crust at some time during its geologic history.” He infers what has happened to the rocks, but not how long ago it happened. That’s where the second kind of time comes in: absolute as compared to relative. “We try to supply the when,” explains Mueller. “My job is to look at the chemistry of the rock, including its isotopes, and try to derive the absolute times for events that are recorded in the rock and its zircons.”
How precise are those actual numbers? “Depending on the history of the rock, we can date things nowadays down to something on the order of a few hundredths of a percent of its age,” answers Mueller. That translates, for example, to plus or minus a million years out of three billion. Carbon-14 dating can go no further back than about 70,000 years, because the half-life of carbon-14 is only 5,730 years. (The half-life is the time it takes for half of the original radioactive isotope to change to another element.) In comparison, the half-life of the radioactive uranium 238 isotope is 4.5 billion years, which makes it useful for dating extremely old materials.
Zircon chronology begins in the field. “You go out and look for relative age relationships, see which rock unit was formed first,” says Henry. “For example, there may be a granite which contains pieces of other types of rocks enclosed in the granite. Because of their position, we know that the rocks enclosed in the granite have to be older.” Geologists map an area to identity these relative age relationships. Then they collect samples, which weigh from two to more than one hundred pounds, depending on the rock type. Zircons aren’t rare; in fact, they’re common in granitic rock. But they are tiny grains that make up only a small fraction of any given sample, typically less than a tenth of one percent, and they’re dispersed throughout the rock. This makes separating out the zircons a painstaking process. The rock is ground up to break it into individual mineral grains. Then, “because zircon is more dense than almost any other mineral, we put the ground-up rock in a liquid with very high density so that only the densest minerals fall through to the bottom,” explains Henry. In other words, says Mueller, “zircons sink. We also use the magnetic qualities of the zircons to separate the most pristine ones from the rest.”
Then the detailed geochronology work begins. “I’ll take a fraction of those zircons, make thin sections of them—slices of mineral thirty micrometers thick, roughly as thick as a hair, that are mounted on glass—and get an idea of what they look like in terms of zoning pattern, whether they underwent multiple episodes of growth, how simple or complex they are,” says Henry. He passes this information along to Mueller, along with the sample’s geological context. “I also look at a thin section of the rock to learn something about the framework in which the zircon occurs. Is it in a granite? Or is it in a metamorphic rock that has had a more complex history? Or is it a metamorphosed sedimentary rock? By knowing its history, we can interpret the age of the rock much better.”
“To understand the relative geologic history of a rock, Darrell uses thin sections because he’s interested in the relations among all the minerals, which make up the rock,” explains Mueller. “However, for geochronology, we’re interested in the minerals that make up one tenth of one percent or less.” He looks at the zircon using various techniques—“light reflected off the grains, light transmitted through them, cathodoluminescent light resulting from hitting the zircon with an electron beam”—to establish the scale at which the zircon grains should be analyzed. Quantitative microanalysis of the elements in zircon is done with an electron microprobe. “This allows us to analyze things on a micron (a millionth of a meter) scale using a thin beam of electrons,” explains Henry. “The electrons irradiate the sample, causing atoms within the sample itself to give off X-rays. Each of the atoms of the different elements in the sample gives off X-rays with characteristic wavelengths. You can then compare these to a standard with a known concentration of the element, and come up with an exact composition of that small spot. An individual zircon grain may be composed of many zones of different compositions and ages. Isotopic compositions can be determined with an ion probe. Do we want to look at the whole grain, or should we direct a tiny beam of oxygen ions, 300 micrometers in diameter, on parts of the zircon grain to analyze for U (uranium) and Pb (lead) isotopes so we can date that spot and dissect the zircon’s individual history?” Alternatively, the uranium and lead can be separated chemically when an individual zircon grain is dissolved in hydrofluoric acid. “Then we analyze them on a mass spectrometer, which gives us the ratios of the individual uranium and lead isotopes, and from that we can calculate the time,” explains Mueller.
Ultimately, says Henry, “all of these data are combined into a larger picture of how the Earth worked billions of years of years ago.” In Mueller’s words, “it boils down to the fact that the more we know about the variety of rocks that made up the earliest continents and how these continents evolved, the better our window onto how the Earth formed and the early processes that separated the crust from the mantle and probably even the mantle from the core.” Mueller describes his and Henry’s collaboration as a parallel journey. “Our research marches down the same road, and sometimes we hold hands and sometimes we go our separate ways.” In either case, they’re constantly exchanging information yielded by their different approaches, and there’s always something new to look at. Mueller sums it up: “One rock’s a lot of work.”