Edmonton scientists study some of Earth's oldest rock

Edmonton scientists hope the discovery of some of world's oldest rock will provide unique insight into how the planet's first continents formed.

Ancient grey slab recovered north of Yellowknife is 4.02 billion years old

This rare rock has been knocking around Earth's surface for more than 4 billion years. (University of Alberta )

Edmonton scientists hope the discovery of some of world's oldest rock will provide unique insight into how the planet's first continents formed. 

At 4.02 billion years old, the ancient rock comes from the Acasta gneiss formation 300 km north of Yellowknife. At 4.02 billion years old, it is not quite the oldest rock out there.

But it is the oldest known rock to still contain the tiny crystals of zircon it had when it was created. And the combination of the rock and the zircon has opened a tiny window into the so-called Hadean eon, the first 500 million years in the life of the Earth after it was created 4.5 billion years ago.

The research, led by scientists at the University of Alberta, suggests the early Earth was largely covered in an oceanic crust-like surface, unlike the continental crust that researchers had expected to find.

"It gives us important information about how the early continents formed," said Jesse Reimink, the study's lead author. "The chemistry of the rock is quite unique."

The Acasta gneiss was once part of a giant plume of molten rock that pushed its way around the young planet before coming to rest and cooling. As it cooled, it interacted with the even older rock around it — an interaction that left its mark inside those crystals of zircon and the rocks in which they are embedded.

Those marks are clues about what was in those early formations, the very earliest crust of the Earth.

"There are some interesting features [on the rock] that are different than the way continents are forming on the modern Earth, and also different from how continents were formed further back in time," Reimink said. "So it kind of points toward something different about early Earth."

There are only three locations worldwide with rocks or minerals older than 4 billion years — in northern Quebec, in Western Australia, and at the rock formation examined in the new study.

"It's extremely rare," said Reimink of the discovery. "We know so little about this time in Earth's history, and we know so little because there are so few samples to work with." 

Rock-hard results 

The rock, an unassuming grey slab, was uncovered  in 2007 roughly 300 kilometres north of Yellowknife, during fieldwork by Tom Chacko, Reimink's PhD supervisor. 

Unlike very ancient rocks discovered before, this one had well-preserved grains of zircon, Reimink said.

"Zircons lock in not only the age but also other geochemical information that we've exploited in this paper," he said. "Rocks and zircon together give us much more information than either on their own. Zircon retains its chemical signature and records age information that doesn't get reset by later geological events. While the rock itself records chemical information that the zircon grains don't."
Jesse Reimink, who recently completed his PhD at the University of Alberta, led the research. (University of Alberta )

Reimink said the chemistry of the rock resembles some that are forming today in modern Iceland, which is transitional between oceanic and continental crust.

"We examined the rock itself to analyze those chemical signatures, to explore the way that the magma intrudes into the surrounding rock," said Reimink. 

"While the magma cooled, it simultaneously heated up and melted the rock around it, and we have evidence for that."

Rare rubble 

Reimink said the lack of evidence of a continental crust in the rock leads to more questions than answers.

But opportunities for further study will be scarce, because ancient rocks are extremely rare and constantly disappearing. 

"Earth is constantly recycling itself, the crust is being deformed or melted, and pre-history is being erased," said Reimink.  

"It's part of what makes this research interesting." 

The article, titled "No evidence for Hadean continental crust within Earth's oldest evolved rock unit," was published in the October issue of Nature Geoscience.