The story of 13 diamonds and their inclusions
Diamond is a very special mineral: not only is it the hardest natural crystal, and one of the most beautiful ones, but it also is the deepest mineral found on Earth’s surface. While forming in Earth’s mantle, diamond can encapsulate small pieces of surrounding material, little crystals of mantle minerals, which it can then safely carry hundreds of kilometres up to the surface, when diamond is collected by ascending magma. Inclusions in diamond (Fig. 1) are usually very small (tens to hundredth of a millimetre), but they are the only available samples of Earth’s mantle from depths >250 km and, thus, have a very interesting story to tell.
I would like to share with you a story of thirteen garnet inclusions in diamonds from the Jagersfontein kimberlite in South Africa. Found by Dr Jeff Harris (University of Glasgow), and later studied by Professor Thomas Stachel (University of Alberta), these inclusions were small, orange in colour and resembled most other garnets encapsulated by diamonds. However, when these garnets were exposed to the surface and analysed for their composition, the researchers found that they contained an elevated amount of silica, indicating that these particular crystals came from much greater depths than their common peers, ranging from 250-550 km. This was a great discovery, since to date no more than 200 garnet inclusions from these depths have been found, most of which were unavailable for future studies.
The first detailed study of these inclusions was conducted in the mid-2000s, when Tappert et al. (2005) investigated the origins of the carbon in the diamonds enclosing these garnets. To their surprise, it emerged that this carbon is largely derived from organic components that could only have originated in oceanic crust, subducted into the deep mantle, perhaps, billions of years ago (Fig. 2).
If the carbon constituting the diamonds was from the surface, what about the garnet inclusions? It took another 10 years until Ickert et al. (2015) analysed oxygen isotopes of the garnets and concluded that this material, too, was ultimately derived from oceanic crust residing at Earth’s surface before being returned into the deep mantle. How fascinating it is to realise that the material from which our garnets are formed, made at least two full cycles (Fig. 3): initially being part of the mantle, then erupted to the surface to become a mid-ocean ridge basalt, then transported back into the mantle via subduction and ultimately brought up to the surface as inclusions in diamond!
But the story does not end here. In 2018, Kiseeva et al. (2018) studied the oxidation state of iron in these garnets, reporting the first finding of highly oxidised iron in natural samples derived from the mantle transition zone (the mantle layer between 410 and 660 km depth). The researchers assumed that through reaction with surface carbonate, the Fe2+ iron in these garnets could have transformed with pressure into more oxidised Fe3+ iron, resulting in a very high proportion (up to 30%) of highly oxidised iron. This was a truly surprising discovery.
If indeed, there is a lot of oxidised material in the Earth’s deep mantle and transition zone, would it be possible to detect it with seismic measurements? That was the next question addressed by the scientists. In 2019, the researchers studied sound velocities of these garnets (unpublished). The results were unambiguous: this different form of iron would be “invisible” to seismic measurements, meaning that there could be a large proportion of oxidised material in the Earth’s mantle undetectable by geophysical measurements.
To date, the thirteen garnet inclusions from Jagersfontein have already given us a plethora of information about the composition of Earth’s mantle and the operation of deep plate tectonics, and I believe they still have a lot to share.
- Ickert, R.B., Stachel, T., Stern, R.A., Harris, J.W., 2015. Extreme 18O-enrichment in majorite constrains a crustal origin of transition zone diamonds. Geochemical Perspectives Letters 1, 65-74.
- Kiseeva, E.S., Vasiukov, D.M., Wood, B.J., McCammon, C., Stachel, T., Bykov, M., Bykova, E., Chumakov, A., Cerantola, V., Harris, J.W., Dubrovinsky, L., 2018. Oxidized iron in garnets from the mantle transition zone. Nature Geoscience 11, 144-150.
- Tappert, R., Stachel, T., Harris, J.W., Muehlenbachs, K., Ludwig, T., Brey, G.P., 2005. Diamonds from Jagersfontein (South Africa): messengers from the sublithospheric mantle. Contributions to Mineralogy and Petrology 150, 505-522.