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.
