Mineral
impurities in a diamond may decrease the gem’s value for jewelers, but
for geologists they can prove to be priceless: South African diamonds containing
garnet have recently provided evidence that very deep diamonds can form from
surface materials, including carbon from dead organisms buried on the seafloor,
rather than solely from materials located deep within the planet.
Diamonds with garnet inclusions (shown
here) containing a mineral called majorite can form at depths up to 500 kilometers
below the surface, and scientists now say that the diamonds may have an organic
source from the surface of Earth.
Courtesy of Jeff W. Harris, University of Glasgow.
Diamonds provide one of the few means geologists have to obtain direct information
about the chemical and mineralogical composition of Earth’s inaccessible
mantle. Normally, diamonds form in a “stability zone” within the upper
mantle at depths of 150 to 200 kilometers, where the temperature and pressure
allow carbon from mantle rocks to stably transition to diamond. The diamonds
are then brought to the surface via vertical magmatic columns called kimberlite
pipes.
However, in recent decades, some rare diamonds have been found with garnet inclusions
containing a silica-rich mineral called majorite that only forms below 250 kilometers.
Majoritic garnets, acting as depth gauges, revealed that some diamonds can form
at depths up to 500 kilometers. But how the carbon got to such depths remained
unknown.
To answer the question, diamond geologist Ralf Tappert of the University of
Alberta in Edmonton, Canada, and his colleagues analyzed the composition of
deep diamonds collected from the Jagersfontein diamond mine in South Africa
by the DeBeers mining company, which partially funded the research and owns
the mine. Tappert’s team reports in the July Geology that the garnet
inclusions exhibit anomalous values of the element europium — values that
are more consistent with shallow crustal rock rather than deep mantle rock.
Additionally, they found the diamonds themselves to have unusually “light”
carbon isotope signatures. Because plants and animals preferentially use the
lighter carbon-12 isotope, a light carbon isotope ratio could indicate a potential
organic source of carbon.
The team therefore concluded that the diamonds formed very deep within Earth,
but from surface materials, including the detritus of ocean organisms, brought
down by subduction of basaltic oceanic crust. “The thing to keep in mind
is that the europium anomalies are from the garnet and the carbon isotope [data]
are from the host diamond, but they match,” Tappert says. “This is
the surprising thing — that they both give a crustal signature.”
The finding has implications for the study of plate tectonics, mantle geochemistry
and the carbon cycle, as it provides evidence for a pathway by which surface
carbon could be conveyed through the deep mantle and back to the surface. “I
think it’s really important because it shows that such deep recycling is
a possible process,” says Steven Shirey, a geochemist at the Carnegie Institution
of Washington. “Tracing that return flow is a very exciting aspect of it.”
Tappert says that further research is needed to rule out whether any inorganic
processes could produce isotopic signatures that could be mistaken for organic
ones — an ongoing and pervasive question within the geological and biological
sciences.
However, Shirey says that so far, the inorganic processes suggested have been
“a little more speculative.” But Tappert’s argument “focuses
on a recognition that the signature you see in the diamond is one that is typical
of what happens in shallow levels in the oceanic crust,” Shirey adds. “And
that’s a fair argument.”
Additionally, he says that if the combination of signatures could eventually
be established as either organic or inorganic, the finding might help, for example,
astrobiologists determine whether the origin of any isotopically light carbon
ratio found on other planets was organic or primordial. “It gives you some
hope that you might be able to see a tracer of ancient life whose signature
would have been stored in the deep planet by some earlier process,” Shirey
says, adding that such a find would also be evidence that the planet was once
tectonically active.
Tappert says the next step would be to see if diamonds from other ultradeep
deposits exhibit the same pattern of signatures. However, such ultradeep diamonds
are extremely rare — only 100 diamonds containing majoritic inclusions
have been found worldwide.
Sara Pratt
Geotimes contributing writer
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