A harder look at the geology of diamonds

My recent post about diamonds was a rapid romp through some of the most marvellous things earth scientists have discovered about them. In the interests of keeping the casual reader engaged I left out many things. If this left you with some nagging questions, I hope they’ll be answered here.

How in earth do they know that?

Much of the information we gain from diamonds comes from inclusions within them. The minerals that are included must be at least as old as the diamond – how else could they get there? This means that either they are older and were swallowed up by a growing diamond, or they formed at the same time as the diamond. Some inclusions have flat sides that are oriented parallel to the the crystal structure of the diamond around them, suggesting they grew at the same time. Evidence like this justifies talking about the age of the diamond when in fact we can only directly date the age of the inclusion.

The most dramatic claim for diamonds is that some of them contain carbon that was once part of a living organism. Remarkable claims require remarkable evidence – how can we say such a thing?

Diamonds from life

“Our bodies are startdust; our lives are sunlight”1. All life on earth2 depends on photosynthesis for energy. Photosynthesis is a process that captures energy from sunlight, storing it in the form of carbohydrates. This involves capturing carbon from carbon dioxide (releasing the oxygen into the atmosphere). A key enzyme called rubisco, working deep within the photosynthetic machinery, converts carbon dioxide containing carbon-12 in preference to that containing carbon-13. The carbon that ends up as carbohydrate is then richer in carbon-12. This ‘light carbon’ signature is found in living things and their non-living remains. Organic carbon in sediments has 3 percent more carbon-12 than carbonates (limestones) do.

A set of diamonds, called the eclogite-suite has unusually light carbon isotopes. Showing that this is derived from organic carbon requires us to consider other possibilities. There is a well understood carbon cycle near the earth’s surface – carbon is regularly exchanged between atmosphere, crust and the oceans. This means carbon isotope ratios give a consistent value against which the photosynthetic fractionation can clearly be seen. In the earth’s mantle, the carbon cycle is less well understood. Other processes exist that change isotopic ratios. Also there is no reason to assume that ‘primordial’ carbon, that has always been in the mantle, has a consistent isotopic ratio. Maybe portions of the mantle have always contained extremely light carbon?

A recent study in Geology (see reference below) provides further evidence that light carbon in eclogite-suite diamonds is indeed organic carbon. Looking at diamonds and their inclusions, the paper shows an anti-correlation between low carbon isotope ratios (‘light carbon’) and anomalously high  oxygen isotope ratios. The oxygen isotope pattern is interpreted as being caused by alteration of hot oceanic crust (basalt) by sea-water circulating through it. Just as with the carbon, there are other possible explanations for the oxygen signal. Showing a strong association between the two isotopic signals is important as it is exactly what you would expect if the material came from subducted oceanic crust. Other explanations for the isotope patterns wouldn’t predict the correlation between them. That some diamonds made are from subducted critters is not just a beautiful idea: it’s probably true as well.

Ancient sulphur

Another interesting isotopic signature affects sulphur isotopes and indicates they were affected by UV radiation in an oxygen poor atmosphere – conditions that only occurred on the surface of the early earth. This pattern of isotopes is different from ‘light carbon’ as it isn’t related to the mass of the isotopes – it’s know as mass-independent fraction (MIF). As a very recent paper in Nature reveals traces of MIF are found in other material brought up from the deep earth – sulphide grains in lavas produced from a mantle plume. This backs up the diamond evidence in that it shows that ancient crustal material was subducted into the deep earth.  It goes further however, as the lava is only 20 million years old, suggesting that some ancient subducted crust is still down there.

Another recent study involving MIF in sulphur adds a twist. The MIF signal has been used to date the ‘great oxygenation event’, an important milestone in earth history when photosynthesising critters finally managed to increase oxygen levels in the atmosphere. It turns out that as well as persisting in diamonds, the MIF signal can survive a sedimentary cycle – sediments formed in an oxygen atmosphere may still contain a MIF signal derived from older eroded rocks. This is important as sediments containing a MIF signal are the best way to date the onset of Oxygen in the atmosphere. Its now clear that such signals need to be used with care.

There’s more great research on diamonds, tracking their movements and relating them to plate tectonics, but I’ll save some for another day.

REFERENCES
Schulze, D., Harte, B., , ., Page, F., Valley, J., Channer, D., & Jaques, A. (2013). Anticorrelation between low 13C of eclogitic diamonds and high 18O of their coesite and garnet inclusions requires a subduction origin Geology, 41 (4), 455-458 DOI: 10.1130/G33839.1