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"As long as they're hot, you want them to stay at
high pressure," said Lionel Wilson, a professor of volcanology at the
University of Lancaster and co-author of the Nature paper, "and then
if you're going to decompress them, you've got to get them cold really
fast."
Wilson and James Head, a professor of planetary
geology at Brown University, developed the theory while searching for
a solution to the problem of how round glass droplets might be formed
on the moon where there is no atmosphere and little gravity to shape
them. As the scien-tists talked over one proposed mechanism, recalls
Head, they each thought: "hmm, that could explain how kimberlites
form."
"We had tried to understand how you get this glass
that comes out of eruptions on the moon," said Head, "The chemistry we
saw seemed to require low pressures at great depth, which is kind of
an oxymoron. We realized the molten rock comes up in a crack and when
the crack opens, there is a small zone of very low pressure."
A handful of nagging questions have challenged
geologists trying to explain the genesis of car-rot-shaped kimberlite
formations. The diamonds are one problem. The shape is another. The
in-verted cone of a kimberlite is filled with cracked and broken
chunks of solidified magma, mixed with glass globules and some
diamonds, all confined in a narrow, underground space, not scat-tered
across the landscape as they would be in a normal volcanic explosion.
Geologists find little evidence for flowing lava
around the formations, suggesting that the erup-tion is suddenly
capped or stoppered. Also, glass globules, like those moon artifacts
that Head and Wilson were trying to explain, appear throughout the
formation and it's not at all clear how they could be formed
underground.
In the mechanism that Wilson and Head suggest, a
wedge of liquid carbon dioxide forms above a source of carbon
dioxide-rich magma at a depth of about 250 km. As the wedge drives
upward, it fractures the rock it passes through, dropping fragments
into the tube of magma beneath it.
Streaming upward at speeds of about 108-180
kilometers per hour, the underlying magma resup-plies the carbon
dioxide in the tip chamber. The balance between the overlying carbon
dioxide and the excess dissolved in the magma maintains an equilibrium
pressure at the tip of about 70 MPa during ascent, driving this
rock-busting locomotive upward. As carbon dioxide bubbles out of the
magma, a layer of foam forms on top – ultimately the source of the
unexplained glass spheres.
When the tip of the carbon dioxide chamber breaks
the surface, the fluid rapidly expands to be-come a gas, sending a jet
of carbon dioxide, magma foam, and rock fragments shooting into the
air at speeds of up to 5000 km per hour, typical of a booster rocket
or a jet engine. A cavity three kilometers deep might empty in only
about ten seconds. This rapid expansion flash-freezes the magma near
the surface and sends shock waves through the cavity, imploding the
cavity walls and filling the chamber with rubble.
Pressure builds once again in the mostly-sealed
chamber and waves of expansion and compres-sion bounce up and down the
chamber, cooling and shattering magma, breaking up the surround-ing
rock, spattering magma droplets into glassy spheres and sorting and
rearranging the resulting fragments. The rapid chilling soon seals off
the magma supply completely, ending the process. The whole progression
likely takes less than an hour.
One implication of the new theory is that surface
geologic conditions have little to do with where kimberlites form –
and thus where diamonds are deposited. Unlike some recent theories of
kim-berlite formation, no source of underground water is needed to
drive the explosive power of the formation. If the theory proves true,
it would suggest that diamonds have been found in South Africa or on
the Canadian Shield mainly because that's where people have looked for
them and that a concerted search could turn up kimberlites – and the
associated diamonds - anywhere. |
