Jack Hills Zircons: Earth's Oldest Known Minerals in Australia

The Jack Hills zircons are tiny crystals from a remote range of hills in Western Australia that include the oldest known minerals formed on Earth, about 4.4 billion years old. They crystallised only around 150 million years after the planet formed, in a period from which no intact rocks survive. Their chemistry suggests that the early Earth had cooled enough to form continental-type crust, and probably liquid water, far sooner than scientists once thought.

Background: the missing first chapter

Earth is about 4.54 billion years old, but its first half-billion years, the Hadean Eon, is almost entirely missing from the rock record. Plate tectonics, erosion, melting and meteorite impacts have destroyed or recycled nearly all of the earliest crust.

For much of the 20th century, many scientists pictured the Hadean Earth as a hellish world of molten rock under constant bombardment, as its name, taken from the Greek underworld, suggests. Without rocks from that time, there was little direct evidence either way.

Zircon offered a way around this problem. Zircon, or zirconium silicate, is an exceptionally durable mineral. It resists weathering, survives being eroded and redeposited many times, and can endure high temperatures. A zircon crystal can therefore outlive the rock it originally grew in, ending up as a grain of sand in a much younger sediment.

How a zircon tells time

Zircon is also a natural clock. When a zircon crystal grows, it readily takes uranium into its structure but almost entirely rejects lead. Uranium then decays to lead at precisely known rates: uranium-238 to lead-206 with a half-life of about 4.47 billion years, and uranium-235 to lead-207 with a half-life of about 704 million years.

Measuring the amounts of uranium and lead in a crystal therefore reveals how long ago it formed. Because there are two separate decay chains, geologists can cross-check the two ages. If they agree, the crystal is called “concordant” and the age is considered especially reliable.

Zircons often grow in layers, like tree rings, as conditions in a magma change. Modern instruments can date each layer separately, so a single grain may record several events in its history.

The discovery

The Jack Hills lie in the Narryer Gneiss Terrane, part of the ancient Yilgarn Craton, roughly 800 kilometres north of Perth. A craton is an old, stable core of a continent.

The first clue came in 1983, when researchers working with Bill Compston at the Australian National University reported zircons of about 4.1 to 4.2 billion years from nearby Mount Narryer. They used a newly built instrument called SHRIMP, the Sensitive High Resolution Ion Microprobe, which could date tiny spots within a single crystal.

In 1986 Compston and Robert Pidgeon reported even older grains from the Jack Hills, including one of about 4.28 billion years. The zircons are found in a metamorphosed conglomerate, a rock made of cemented pebbles and sand, deposited roughly 3 billion years ago. The ancient crystals had been eroded from older rocks and carried into this sediment.

The 4.4-billion-year-old grain

In January 2001 a team led by Simon Wilde of Curtin University, with John Valley of the University of Wisconsin–Madison and colleagues, published in Nature the age of a single zircon grain: 4,404 million years, with an uncertainty of about 8 million years. It became the oldest known mineral formed on Earth.

In the same issue, Stephen Mojzsis, Mark Harrison and Robert Pidgeon reported oxygen isotope evidence from Jack Hills zircons about 4.3 billion years old. Both papers argued that the zircons recorded the presence of liquid water.

Because some scientists questioned whether lead might have moved within the crystal and distorted its age, Valley and colleagues re-examined the grain using atom-probe tomography, which maps individual atoms. In 2014 they reported that the crystal’s age was reliable, at about 4.37 billion years for the part they studied.

What the zircons reveal

Crust

Zircons typically crystallise in granite-like magmas, which are characteristic of continental crust. The Jack Hills zircons and the tiny mineral inclusions inside them suggest that at least some granitic or continent-like crust existed by 4.4 to 4.0 billion years ago. How much crust there was, and whether it resembled modern continents, is still debated.

Water

Oxygen has several stable isotopes. Rocks that have been altered by liquid water at low temperatures become enriched in the heavier isotope, oxygen-18. Many Jack Hills zircons are slightly richer in oxygen-18 than zircons from the mantle, suggesting that the magmas they formed from had melted rock that had once been in contact with surface water.

This led to the “cool early Earth” hypothesis, set out by Valley, Wilde and colleagues in 2002. It proposes that Earth’s surface cooled quickly and that oceans existed by about 4.3 to 4.4 billion years ago. The idea is important for understanding the origin of Earth’s oceans.

Temperature

In 2005 Bruce Watson and Mark Harrison developed a “titanium-in-zircon thermometer”. Applied to Hadean Jack Hills zircons, it suggested crystallisation temperatures of around 680 degrees Celsius, consistent with the melting of wet granite-like rocks, although other interpretations exist.

A possible trace of life

In 2015 Elizabeth Bell and colleagues reported a speck of graphite enclosed in a Jack Hills zircon about 4.1 billion years old. Its carbon was enriched in the lighter isotope, carbon-12, a pattern often produced by living organisms. The researchers described it as potentially biogenic, but stressed that non-biological processes could also explain it. The find feeds into wider debates about the origin of life on Earth.

Debates and limitations

The zircons are fragments without their original rocks, so geologists must infer where they came from. Some researchers argue that they could have formed in impact melts or in the reworking of basalt-like crust rather than in true continents. The oxygen isotope signal is small, and its interpretation depends on assumptions about magma chemistry.

Only a small fraction of Jack Hills zircons, typically a few percent, are older than 4 billion years, so tens of thousands of grains have been analysed to build up the record. Even so, they remain the best window into the Hadean.

By the numbers

Timeline

• c. 4,400 million years ago: The oldest known Jack Hills zircon crystallises.
• c. 4,300–4,000 million years ago: Many other Hadean zircons later found at Jack Hills crystallise.
• c. 3,000 million years ago: Eroded zircons are deposited in the sediments that became the Jack Hills conglomerate.
• AD 1983: Zircons of 4.1–4.2 billion years are reported from nearby Mount Narryer.
• AD 1986: Compston and Pidgeon report Jack Hills zircons up to about 4.28 billion years old.
• AD 2001, January: Wilde and colleagues publish the 4.404-billion-year age; Mojzsis and colleagues publish oxygen isotope evidence for water.
• AD 2014: Atom-probe tomography confirms the Hadean age of the oldest grain.
• AD 2015: Possible biogenic carbon is reported in a 4.1-billion-year-old zircon.

Why it matters

The Jack Hills zircons opened a window on the earliest chapter of Earth’s history, after the formation of the Earth and long before the oldest surviving rocks. They suggest that a habitable surface with crust and water may have existed within about 150 million years of the planet’s birth, reshaping ideas about when life could first have appeared.