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Diamonds belched up by magma
By Alexandra Witze
Drop a Mentos candy in a bottle of Diet
Coke, and carbon dioxide will bubble
violently out of the soda. Similar chemical reactions may send certain kinds of
magma frothing up from deep within the
Earth, carrying diamonds along the way.
The discovery, reported in the Jan. 19
Nature, solves several mysteries about
why and how diamond-bearing rocks
called kimberlites, which contain many
kinds of crystals that formed 150 kilometers or more deep, get to the surface.
Since magma ought to get denser the
more crystals it picks up, in theory rising kimberlites should run out of steam
deep underground.
Kelly Russell, a volcanologist at the
A thin slice of kimberlite rock from
Canada, seen through a microscope
and in polarized light, shows minerals
that rose from deep within the Earth.
University of British Columbia, and his
colleagues realized that gas could fuel
kimberlites if the magma starts out relatively poor in silicon dioxide (aka silica),
a major component of the Earth’s crust.
As magma rises it begins to dissolve
the surrounding rock — especially that
containing lots of orthopyroxene, a min-
eral rich in magnesium, iron and silica.
The orthopyroxene releases its silica
into the magma, and as the silica con-
tent rises, the magma’s ability to hold
dissolved carbon dioxide drops. The gas
bubbles out and pushes the kimberlite to
the surface at supersonic speeds.
Russell confirmed the idea’s plausi-
bility with tests in a high-temperature
laboratory at the University of Munich.
Lionel Wilson, an earth scientist at
Lancaster University in England, says
the study fits with other ideas about how
kimberlites rise. In 2007, he and James
Head of Brown University proposed that
diamond-bearing magma moves upward
by shattering rocks above it. But their
calculations showed it slowing down in
shallower depths. The chemistry proposed by Russell’s team would give the
magma enough oomph to continue all
the way to the surface.
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February 25, 2012 | SCIENCE NEWS | 11
