Minerals Send Mixed Signals About Early Earth

Aug. 4, 2006 — The most durable minerals on the planet are sending mixed signals about what sort of place Earth was 4.4 billion years ago.

Some geologists have recently claimed that tiny zircons crystals reveal a surprisingly cool and hospitable planet very early on. But new research counters that the old "Hadean" zircons don’t look very different from those made today by submarine volcanoes — and so don’t necessarily rule out a Hades-like young Earth.

Whichever theory is right carries implications for life — how early it began on our planet and how likely it is that it could ever exist on Mars. But scientists aren't even close to reaching a consensus.

"Those (cool early Earth) interpretations that have been put forward may be right, but the evidence isn’t there," says zircon researcher Laurence Coogan of the University of Victoria in Victoria, British Columbia.

Coogan has coauthored a paper with Richard Hinton in the August Geological Society of America Bulletin questioning the meaning of various measurements of isotopes of oxygen, hafnium and titanium and other more rare elements in the ancient crystals.

Some researchers have claimed that the elements found in the oldest zircons, recycled and trapped in some of the world’s oldest rocks found at Jack Hills, Western Australia, contain isotopic evidence that they originally crystallized in rocks similar to today’s continental crust.

If so, they would support the Cool Early Earth hypothesis. This theory includes the idea that Earth's gravity and the release of initial heat from the planet forming was able to more quickly “differentiate” the denser rocks from less dense rocks to create today’s layered planet with active plate tectonics.

Today's Earth has a dense mantle, over which floats the lighter-weight rafts of continental crust – sort of like the head on a glass of beer — which are in a perpetual tectonic shoving match with denser and thinner plates of oceanic crust.

To test the claims that that the oldest Jack Hills zircons were originally formed in continental-type crust, Coogan and Hinton compared them to very young zircons formed in volcanic rocks called basalt, erupted onto the ocean floor at the Mid-Atlantic Ridge.

"We found very little compositional difference between Hadean zircons and zircons that just crystallized in magma," Coogan told Discovery News.

But that doesn’t mean the matter is settled.

"What we call it is ‘proto continental crust,’" said zircon researcher John Valley of the University of Wisconsin at Madison, who is regarded as a member of the Cool Early Earth camp. The rocks in which the zircons formed may not have been quite the same as continental crust today, says Valley, but they were heading in that direction.

On the other hand, Valley agrees with Coogan that there is still a lot of work to be done to settle the matter and that "It’s a mistake to think that any of the answers are final yet."

The Hadean zircon question is important and getting a lot of attention from scientists for several reasons, explained geoscientist Paul Hoskin of Central Washington University in Ellensburg, Wash.

"This is extremely interesting because it may mean that life was able to evolve in the early oceans much sooner — by about 1 billion years — than previously thought," said Hoskin. Right now the earliest evidence for life is at about 3.4 billion years ago.

There are also implications for other planets, like Mars, says Martin Whitehouse, a zircon researcher at the Swedish Museum of Natural History.

If, for instance, the ancient zircons are from an early Earth that did not have plate tectonics or oceans, but still had a proto-crust of some sort, it might have been a lot like the four billion-year-old surface of Mars seen today.

If that’s the case, it could suggest that water is the key difference between the planets, says Whitehouse. If Mars had had more of it to help flux hot rocks into magma and lubricate crustal plates, perhaps it might have evolved into a planet a lot more like Earth, he explained.

"The more we understand the early crust on Venus and Mars the more we are going to understand Earth," said Whitehouse.

In other words, the planetary science goes both ways. And since scientists have found no zircons in Martian meteorites, he said, the Hadean zircon research could help select where to land future missions to the Red Planet.

For now, however, about the only thing getting clearer about Earth’s early crust is how much we don’t know.

"The paper of Coogan & Hinton highlights the fact that there is much work to be done yet to understand Earth’s earliest history and it reminds us that alternative, more mainstream explanations, may turn out to be more satisfactory," said Hoskin.