Diamonds growth in the terrestrial mantle
Diamonds have fascinated humankind for centuries, both for their value and their beauty. Their exceptional properties are exploited not only in jewellery but also in a wide range of technical applications, such as abrasion and machining, as well as technological applications, including semiconductor technology and quantum physics, with potential applications in a new generation of computers. For geologists, diamonds are particularly valuable samples: unique natural witnesses to the deep cycle of volatile elements and the mineralogy of Earth’s interior. To this day, there is still no real consensus on the mechanisms governing the growth of natural diamonds in the upper and lower mantle, apart from the fact that their formation takes place in the presence of fluids.
Experimental petrology plays a key role in understanding the formation of natural diamonds, and recent advances have made it possible to better characterize their growth under conditions representative of the Earth’s mantle. In this presentation, I will show how the laboratory synthesis of lithospheric diamonds under high-pressure and high-temperature conditions can shed light on their deep history, which is sometimes complex due to successive episodes of growth and dissolution. I will also present an analysis of the main impurities (N, H, B) incorporated into the diamond crystal lattice, with a particular focus on hydrogen, and demonstrate how its precise characterization can be achieved by combining different analytical techniques (FTIR, ERDA and NanoSIMS). Hydrogen is the second most abundant major impurity in natural diamonds, after nitrogen and ahead of boron, and can be used as an indicator of their growth environment. Finally, I will show how comparing the hydrogen content of natural diamonds with that of laboratory-grown synthetic diamonds validates the relevance of experimental approaches and provides new insights into the role of diamonds in the deep water cycle.