Arctic rocks reveal Earth's recipe
Volcanic rocks made of 4.5-billion-year-old material from the inner Earth have been found on Baffin Island in Nunavut.
The discovery, published online Wednesday in Nature, suggests previous theories about exactly what the Earth was originally made of aren't quite right.
Dating rocks with isotopes
Isotopes are forms of the same element that have different atomic masses. Some isotopes, like helium-4, are created when radioactive elements such as uranium decay. Others, such as helium-3, are not.
When a volcano erupts and releases a molten mixture from the Earth's mantle, both helium-3 and helium-4 escape into the air. Rocks form when the lava cools. Their helium-4 is replenished by radioactive decay, but their helium-3 is not.
Later, the rock may be pushed back under the Earth's crust, and the process may repeat itself. Rocks that have avoided that kind of process for billions of years will have a higher proportion of helium-3.
Similarly, the ratios of isotopes of other elements can also be used for complementary dating estimates.
Scientists have theorized for decades that the Earth's mantle — the hot, viscous material that makes up most of the inside of the Earth — originally had the same composition as meteorites, said Richard Carlson, a staff scientist at the Carnegie Institution in Washington, D.C., who co-wrote the paper.
The Baffin Island rocks are a remarkable find because their materials were dated to be between 4.55 billion and 4.45 billion years old — only slightly younger than the Earth itself. And yet the proportions of their ingredients differ from those found in meteorites, reported the study led by Matthew Jackson at Boston University.
Carlson said this suggests researchers should change what they're looking for when they search for ancient rocks.
"You shouldn't be looking for something that's exactly like meteorites," he said Wednesday. "You should be looking for something like Baffin Island rock. It sort of changes your target parameters."
The findings suggest there is far less uranium and thorium inside the Earth than expected. The decay of those radioactive materials inside the Earth produces the heat that drives processes like the movement of tectonic plates on the surface. That means scientists might have to adjust their models of such phenomena, Carlson said.
Collected by Montreal researcher
The rocks were originally collected by Don Francis, a researcher at McGill University in Montreal, more than a decade ago in the Canadian Arctic.
Francis shares his rock finds with other researchers, Carlson said.
"Don does a lots of field work, a lot of collecting. He's also very free with his samples."
These rocks were part of the North Atlantic flood basalt — the result of a volcanic eruption 60 million years ago. It spewed material from the Earth's mantle that ended up covering the area from Baffin Island to the coasts of Britain and Iceland, which were close at the time.
Researchers in Scotland noticed the rocks contained the highest proportion ever found on Earth of a special helium isotope, Carlson said. This is the signature of very, very ancient rocks.
Carlson and other researchers measured the ratios of isotopes of other elements, such as lead and uranium, confirming that the material in the rocks was around 4.5 billion years old.
There was only one problem — they didn't have the expected recipe for "primitive mantle" from the ancient Earth, based on comparisons with meteorites. In particular, researchers were puzzled by differences in the ratio of two rare elements call samarium and neodymium.
However, Carlson had discovered a few years ago that that the ratio of those two elements is always different on Earth than in meteorites, helping confirm the Baffin Island rocks were made from the primitive mantle after all.
"It's just our expectation for primitive, based on the meteorite model, is wrong."
Rocks made of primitive mantle are expected to be rare because the contents of the mantle, which are solid but so hot that they behave like a viscous liquid, are constantly moving within the Earth through convection.
Sometimes they partially melt, separating out some elements with different melting points the way distillation separates alcohol from water. The melted material, which floats toward the crust, and the mantle it came from, end up with different compositions. That effect can be seen in most rocks on the Earth's surface.
Carlson said the Baffin Island rocks are unique because the volcanic eruption 60 million years ago was the first opportunity they had to undergo a process like that.
"Once the Earth was thoroughly mixed after formation, this stuff just ran around in the mantle and never made it towards the Earth's surface to be melted."
The Earth's crust, which we live on, has quite a different composition from the mantle, with a lot more silicon, aluminum, calcium and potassium, for example.
The Baffin Island rocks had lower amounts of those types of elements than expected for primitive mantle.
Carlson said it's not clear why, although there are some theories. He favours a theory that there could have been a melting event right at the very beginning of Earth's history that created a different crust, now buried far inside the Earth.