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.
