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Diamonds deformed within the Earth’s mantle

Geoscientists from the ARC Centre of Excellence for Core to Crust Fluid Systems at Macquarie University reveal processes that take place deep in the Earth’s mantle through their study of polycrystalline rocks called diamondites.

The research team comprises PhD student Ekaterina Rubanova, A/Prof. Sandra Piazolo, Prof. William Griffin and Prof. Suzanne O’Reilly. The diamonds in these rocks are like tiny time capsules that preserve structures and encapsulated inclusions that give clues to their past.

Results collected from electron backscatter diffraction (EBSD) analyses in the AMMRF (now Microscopy Australia) at the University of Sydney, showed for the first time that diamondite crystals can undergo significant plastic deformation and recrystallisation within the Earth’s mantle. Despite being the hardest naturally occurring material known to man, diamonds can deform in a similar way to metals and other minerals.

Chemical analyses were carried out on the same rocks, by using electron microprobe analysis of inclusions within the diamonds. They indicated that different chemical environments at different pressures and temperatures had an impact on how the diamonds formed, and were subsequently deformed during separate episodes of their turbulent creation.

Diamonds come to the surface through volcanic action and are consequently found in the remnants of volcanic structures. This research shows that diamonds with vastly different chemical and structural histories are present at a single location indicating that various chemically distinct bodies in the mantle are mixing more vigorously than expected.

Ekaterina Rubanova et al., Geochemistry Geophysics Geosystems,13 (10), 2012.

EBSD orientation map (far left) of typical diamondite with colour-coding showing relative orientations. Internal orientation change (near left) in grain I and grain II showing high and no internal lattice distortion, respectively.