Sunday 11th December 2016

Resource Clips


In the beginning

Baffin Bay’s “birthmarks” date back to Earth’s infancy, geologists say

by Greg Klein

The Book of Genesis somehow overlooks this country but Canada—traces of it, anyway—turns out to be an awful lot older than previously thought. In fact some Baffin Bay rocks contain relics an awful lot older than most of the planet, according to a team of scientists. The wonder of it is that, despite 4.5 billion years of geological turbulence, the Earth still retains these remnants of its 50-million-year babyhood.

Baffin Bay’s “birthmarks” date back to Earth’s infancy, geologists say

These Baffin Bay rocks host 4.5-billion-year-old silicate material
formed when “baby Earth” was less than 50 million years of age.
(Photo: Don Francis)

But don’t expect to see them, handle them or trip over them next time you’re footloose in Nunavut. Their presence can be detected only with an extremely sensitive mass spectrometer.

The findings were reported last week in the academic journal Science under the intimidating title Preservation of Earth-forming events in the tungsten isotopic composition of modern flood basalts. Richard Walker, a co-author and University of Maryland geology professor, took time to explain that to ResourceClips.com in laypeople’s lingo.

He came to this study through his work with high-precision isotopic measurements. That makes tungsten especially interesting. “Its isotopic composition varies primarily as the result of the decay of another element, hafnium, at the other end of solar system history,” Walker explains. “The isotope of hafnium that decays to tungsten, hafnium-182, has a half-life of only about nine million years. So it was present for maybe the first 50 million years of solar system history. Any variations in tungsten isotopic composition that would follow had to have been created within the first 50 million years of solar system history.”

Walker and his colleagues didn’t expect to find such variations in Earth rocks when they began their study of core formation. The hot, metallic centre of the planet seems to have formed in the first 30 million years of the solar system. “By inference it has a very different tungsten isotopic composition from the rest of the planet. So one of the reasons we got into tungsten isotopes is we’re looking for some geochemical evidence for core-mantle interaction.

“Surprisingly, things didn’t turn out at all like we expected. The isotopic composition of the core, by inference, is presumed to be considerably lower than you, me and light bulbs. Almost all the stuff we have measured in early Earth rocks is actually higher. So that requires some process other than extracting the tungsten from the core. That’s what this paper is all about.”

But the rocks that we’re reporting data for in this study are only a few tens of millions of years old. These are not old rocks, they’re what we consider practically modern rocks.—Richard Walker,
professor of geology at
the University of Maryland

His team and another group had previously found similar isotopic compositions in rocks ranging from 2.5 billion to four billion years of age. “But the rocks that we’re reporting data for in this study are only a few tens of millions of years old. These are not old rocks, they’re what we consider practically modern rocks. But they show the isotopic imprint of the process that happened within the Earth—wow!—really, really early in its history while it was still growing.”

Again, the finding isn’t the rocks themselves, as some media reported. It’s the isotopic measurement, imprint, signature or, to use a word concocted by the U of M press office, “birthmark.”

“I kinda like that term,” Walker says. It represents a portion of the Earth’s mantle that was somehow isolated from the rest of the planet’s middle part over 4.5 billion years ago.

Lead author Hanika Rizo of l’Université du Québec found the Canadian examples on Padloping Island off Baffin Island’s southeastern coast. Only a few rocks have been analyzed so far. “The general type of rock that’s being measured extends over thousands of square kilometres,” Walker points out. “We don’t know how much of this rock has that unusual isotopic signature. That’s something we’ll be working on for years to come.”

Similar findings came from the Ontong Java Plateau northeast of Papua New Guinea.

As for the world’s oldest actual rocks, Walker says that’s a matter of debate. “Everybody accepts that there are rocks that are more than 3.9 billion years old.”

He and some colleagues are among those who believe that rocks from the Nuvvuagittuq Belt of arctic Quebec’s Hudson Bay coast date back at least 4.3 billion years.

Zircons from Western Australia’s Jack Hills date back at least 4.4 billion years. “The rocks they’re found in are nowhere near that age but some of the minerals themselves can be dated to even older than 4.4 billion years.”

But as for the “birthmarks” of Nunavut and Micronesia, they convey a sense of drama to the cognoscenti. This planet, “despite having a very exciting and violent birth in the form of probably a sequence of giant impacts building a bigger and bigger Earth, never completely got itself chemically homogenized,” Walker says. “It’s surprising that we have somewhere down there remnants of the Earth that formed more than 4.5 billion years ago. That’s exciting, at least to a geologist—this goes back to the earliest stages of Earth history.”

Along with Walker and Rizo, report authors include Richard Carlson and Mary Horan of the Carnegie Institution for Science, Sujoy Mukhopadhyay, Vicky Manthos and Matthew Jackson of the University of California, and Don Francis of McGill University.


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