16 September 2011

Diamonds’ deep origins unearthed

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The chemical make-up of diamonds formed inside the Earth's lower mantle suggests the carbon cycle reaches much deeper than previously thought, new research suggests.
'Super deep' diamond

A raw diamond from Juina, Brazil, with a small window polished into it to see if any inclusions are inside. Scientists estimate it may have formed as deep as 1,000 km beneath the Earth's surface. Credit: Image © Science/AAAS

BRISTOL: The chemical make-up of diamonds formed inside the Earth’s lower mantle suggests the carbon cycle reaches much deeper than previously thought, new research suggests.

A study, published today in the journal Science, describes how ‘super deep’ diamonds formed in the Earth’s lower mantle – a depth of more than 660 km – from elements originating on its surface.

The findings are helping scientists trace the movement of carbon material between layers in the Earth’s structure, offering new insight into the depths of the carbon cycle, upon which most living things depend.

“We looked at a suite of diamonds from Brazil and discovered minute mineral inclusions with chemical compositions indicating that they formed in the Earth’s lower mantle, but from materials that originated in the oceanic crust at the Earth’s surface,” said geochemist and lead author Michael Walter from the University of Bristol in the UK.

Precious layers of Earth

Diamonds formed in the lower mantle have been discovered before, but this is the first time scientists have been able to prove that key mineral ingredients originated at the Earth’s surface.

Diamonds are formed over billions of years from sooty carbon deposits, which become buried deep in the Earth’s mantle – a consequence of plate tectonics and subduction at deep-ocean trenches.

The weight of the over-laying rock causes high temperature and pressure forcing the carbon molecules to rearrange themselves, creating crystal diamonds. As a diamond forms it traps minute minerals of the surrounding rock in its structure.

Transportation cycle

Led by Walter, the international team of researchers examined minerals trapped within the crystal structures of diamonds from the the Juina-5 kimberlite pipe in Brazil.

Walter analysed these encased minerals using Raman Spectroscopy – a laser analysis technique – and found that the inclusions trapped in the diamonds have the same mineral properties as rocks formed in the lower mantle.

“In terms of transportation, we know from experiment and theory that the minerals that make up the most common mantle rocks change dramatically at around 660 km to assemblages of denser minerals,” explained Walter.

“So if we find mineral inclusions in diamond of a particular composition that matches one of these lower mantle phases, as we did here, we can infer a depth of origin.”

“When we analysed the carbon in the diamonds using a mass spectrometer, we found that most of the diamonds had an extremely light isotopic composition,” said Walter. “So the story of subducted oceanic crust into the lower mantle and returned to the surface was really coming together.”

The team estimates the Juina-5 kimberlite diamonds may have formed more than 1,000 km under the Earth’s surface.

The precious stones would have eventually been transported to the surface in the magma of a deep volcanic eruption. As the magma cools it forms igneous rocks such as kimberlite which entomb the diamonds.

Exciting evidence of large scale recycling

Richard Arculus, an Earth scientist at the Australian National University in Canberra, said the finding was very “exciting”.

“We’ve known for sometime that we can get sediments that have been deposited at the Earth’s
surface inserted 100 km deep into the interior and then returned to the surface, but we had no direct evidence of material returned from dramatically greater depths,” he commented.

“The authors have found evidence that organic carbon – something produced on the Earth’s surface by photosynthesis – has been taken down to the mantle at least 700 km and probably more, and then returned to the surface in an explosive eruption” he said.

The minerals are transformed physically by extreme heat and pressure, he explained, and then encapsulated in a diamond container before making the ascent back to the surface.

“It shows that there is large-scale, global recycling of carbon – and presumably other materials and compounds – from the surface of the Earth to the interior and back again.”

Structure holds hidden secrets

The Earth’s crust is made up of a mosaic of landmasses, or tectonic plates, which are constantly being destroyed through subduction and formed through spreading.

The extreme conditions found in the mantle layer, causes the rocks to melt, creating a viscous layer which the earth’s crust sits on. Convection currents move material within the mantle mixing the layer; it is this movement that gives rise to tectonic plate motion at the surface.

When the layers comprising plates subduct into the Earth’s mantle at deep-sea trenches, something must rise to the surface, a consequence stemming from the fact the Earth has a fixed volume. But it’s difficult to measure the extent of this cycle.

Arculus said these findings are a step toward being able to quantify how deep surface materials are going and how long they can stay deep in the Earth before returning to the surface.

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