Diamonds log their travel to the depth

One of the investigated diamond from Juina/Brazil (picture: O. Navon, Hebrew University).

Cloudy zones of the diamond due to millions of tiny inclusions (picture: O. Navon, Hebrew University).

Microscopic inclusions have tremendous internal pressure – and witnessed an eventful past

18.04.2017: Diamonds with brown opaque cloudy zones from the Juina area (Brazil) are worthless for gemstone traders but highly valuable for geoscientists. “Their appearance is due to millions of tiny inclusions in the diamond crystal”, says Christian Schmidt from the GFZ section Chemistry and Physics of Earth Materials. Recently, a team from the Hebrew University Jerusalem and from the GFZ studied these nanometer-sized inclusions. Like a logbook, they record the travel of the diamonds to the base of the transition zone above the lower Earth´s mantle over millions of years. Now, this story is told by the team around the first author Oded Navon, Hebrew University Jerusalem, in the journal Earth and Planetary Science Letters.

Bent diamond

Oded Navon obtained the rare samples from Jeff Harris (University of Glasgow) and from DeBeers and brought them to Potsdam to Richard Wirth to study the inclusions by transmission electron microscopy. “The individual inclusions were way too small for Raman spectroscopic analyses”, says Christian Schmidt. “But we were hoping to get lucky.” The scientists hoped that the great number of inclusions in a small volume would give them a signal. They succeded. The analyses clearly showed that the inclusions contained solid nitrogen. Moreover, these nitrogen crystals were under an enormous pressure of 10.9 GPa, which is about 110,000 times of the atmospheric pressure at sea level. The high internal pressure can be seen on the polished surface of the diamond sample, which bulges above the inclusions by up to 2 nanometers.

To the best of our knowledge, this is the highest pressure ever measured in a mineral inclusion”, writes the team. The reason why the diamond around the inclusions did not crack and that the nitrogen did not evaporate is the high strength of the diamond and the very small inclusion size of 20 to 300 nanometers maximum.

Dragged to an enormous depth

Based on their measurements, the scientists were able to reconstruct the pressure under which the inclusions formed. That pressure was about 22 GPa, implying that these diamonds must have been at greater depths than most natural diamonds, which originate in the upper mantle. According to the team, the most plausible scenario is that the studied diamonds formed in the transition zone to the lower mantle and incorporated nitrogen in their lattice during crystallization.  They were then dragged down by a convection current in the shallow upper mantle to a depth of about 630 km, where the temperature is above 1600 °C. At these conditions, the nitrogen exsolved rapidly and formed millions of tiny inclusions, perhaps within 10,000 years according to calculations. This process was interrupted by a mantle plume or by a melt, which transported the diamond upward. Then, they remained below the continental lithosphere until the “waiting” diamonds were carried to the surface by a kimberlitic eruption. 

In this scenario, it is quite unlikely that such a diamond reaches the Earth´s surface”, says Christian Schmidt. “Indeed, there are only around ten locations worldwide where such diamonds with signatures from great depths have been found.” Extremely rare and still of little value for the diamond industry, these gems are priceless for geoscientists. (rn)

Original study: Navon, O., Wirth, R., Schmidt, C., Brooke, M.J., Schreiber, A., Emmanuel, S., 2017. Solid molecular nitrogen inclusions in Juina diamonds: Exsolution at the base of the transition zone. Earth and Planetary Science Letters. DOI: 10.1016/j.epsl.2017.01.035