Thursday, July 21, 2011
Diamonds hold secret to plate tectonics' birth
Diamonds hold the secret to when the continents formed locked inside them. A study of the gems suggests that plate tectonics began 3 billion years ago, 1.5 billion years after our planet was born.
We know that plate tectonics has been pushing new bits of crust into existence and engulfing old segments back into the bowels of the mantle for hundreds of millions of years. What we don't know is when it all started.
Stephen Richardson of the University of Cape Town in South Africa now claims to have found the answer locked inside thousands of ancient diamonds collected from around the world.
Diamonds form deep in the mantle before being pushed up towards the surface in volcanic eruptions. Gems considered to be "flawed" by the jewellery industry can contain tiny clumps of minerals from the rocks in which they formed. It is these flawed diamonds that Richardson and Steven Shirey of the Carnegie Institution of Washington in Washington DC, focused on.
Some mineral clumps are made of peridotite, the most common mineral in the upper mantle. Others are made of the rarer eclogite, which is only formed when volcanic rocks from the surface are forced deep into the mantle and crushed by the immense heat and pressure. So to get eclogite, you need plate tectonics.
Diamonds are forever
Richardson and Shirey were able to date the mineral clumps by looking at their isotopic make-up. They found that peridotite clumps ranged from 2 to 3.5 billion years old, but the oldest eclogite was 3 billion years old.
This, say Richardson and Shirey, proves plate tectonics cannot have been active before then.
"It's a very clear argument and I find it compelling," says Catherine McCammon of the University of Bayreuth in Germany. However, she points out that, despite the large number of rocks that Richardson and Shirey studied, there may be older eclogites still to be found. "We're somewhat limited by what nature delivers to us," she says.
So what would Earth have looked like before its planetary conveyor belt got started? Richardson thinks rock would have moved up and down between the mantle and the crust, pushed along by eruptions. Because the crust rocks were less dense than they are today, entire plates did not plunge into the mantle.
It's also possible that a different form of plate tectonics was happening on early Earth, says Kevin Burke of the University of Houston in Texas.
He points out that the Earth was much hotter back then, and that heat needed to escape. "Plate tectonics is an amazingly efficient way of getting heat out of the Earth," he says. So the extra heat may have simply driven up-and-down plate tectonics to run faster. If that was the case, surface rocks would not have sunk deep enough to form eclogite.