Zircon Chronology

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.

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.

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