[meteorite-list] Diamonds and the very early Earth

From: Darren Garrison <cynapse_at_meteoritecentral.com>
Date: Wed, 22 Aug 2007 23:05:45 -0400
Message-ID: <n8upc3h8jk0tmqlbkkojc9grk7dkmlrmlh_at_4ax.com>

http://www.sciam.com/article.cfm?articleID=8F37399C-E7F2-99DF-30361B86A598909B&chanID=sa007

August 22, 2007
 
Nothing Says "Early Earth Was Cool" Like World's Oldest Diamonds
 
The zircon in imitation diamonds proves the best way to preserve more than
four-billion-year-old versions of the real thing
 
Earth is roughly 4.5 billion years old, but her early eons were tempestuous. Not
even rock survives from the first 500 million years of her life?an eon known as
the Hadean?because geologists speculate the planet's surface boiled and bubbled
with molten lava under a steady bombardment of comets and meteorites. But tiny
diamonds discovered in antediluvian zircon crystals sprinkled in
three-billion-year old rocks from Australia hint that the planet's surface fire
might have ceased much earlier than previously believed.

Mineralogy graduate student Martina Menneken of the Westf?lische
Wilhelms?University of M?nster in Germany and her colleagues probed 1,000 of
these ancient zircon crystals for inclusions?tiny outcroppings of other minerals
hidden in the unusually stable lattice. They discovered diamonds of different
shapes and sizes in 45 of the old crystals by using a laser technique called
Raman spectroscopy.

"The biggest [diamond] we found was about 60 microns [(roughly 0.002 inches, or
35 times smaller than the head of a pin)] but some only were about seven
microns," Menneken says. But their sizes or shapes notwithstanding, all of the
diamonds are unique, she adds, because they come from zircon grains that can be
dated (via the decay of uranium impurities into lead) to as long ago as 4.25
billion years, a scant 250 million years after Earth formed.

Zircon crystals can form in a number of ways?on the moon, for example, the
mineral crystallizes in the wake of a meteorite impact. But some geologists
suspect, based on minerals and oxygen isotope levels in the current grains, that
they crystallized in an ancient crust that formed from cooling granite magmas.
Yet diamonds only form when high pressure squeezes graphite into exquisite
clarity?more than 45,000 bars of pressure only found at depths of 100 kilometers
(62 miles) or more below the surface.

"Options are shock, bolide [meteorite] impact or burial" for how the diamonds
formed, says geologist Ian Williams of The Australian National University in
Canberra. "The key question is whether the zircon grew around the diamonds or
the diamonds grew in the zircons."

Because zircon crystals can survive even the dissolution of their host rock by
weathering, it is possible that the grains formed in the surface rock, which
then disappeared below newer surface rock before resurfacing as their latter
hosts' outer layers wore away. The researchers are searching for other signs of
the history of these grains, such as coesite, a dense form of silicon dioxide
created under similar pressure. "We might have had coesite that is totally
transformed to quartz now," which is relatively common in the grains, Menneken
says. Because coesite is "not stable at surface conditions."

"According to our findings, probably the early Earth might have been a quiet,
cool, habitable place," Menneken says. But more research is needed to say for
sure, she notes, such as studies of the carbon isotopes and nitrogen content of
the diamonds to determine how long the little gems experienced extreme pressures
and temperatures.

"A likely explanation is that the diamonds were formed from graphite inclusions
in the zircons during an event when the zircons were buried. If that was so,
then the burial must have postdated the youngest zircon in which a diamond was
found, about 3.1 [billion years ago]," Williams says. But "if the zircons grew
around diamonds as early as 4.2 billion years ago, this implies that there was a
very thick crust on the Earth by that time." And that means the early Earth may
have cooled sufficiently for a host of other adornments, including life.
Received on Wed 22 Aug 2007 11:05:45 PM PDT