Thanks to elastic recoil detection analysis (ERDA), the hydrogen content of natural diamonds is known to ranges from <5 ppm to a few tens of ppm wt. (Vangu et al. 2023, DRL 136, 110007) , which makes hydrogen the second impurity in diamonds after nitrogen and before boron. This confirms that natural diamonds grow in CHO fluids under lithospheric conditions, as demonstrated experimentally (e.g. Luth et al., 2022, RIMG, vol 88). This also shows that hydrogen is preserved throughout the history of at least the upper mantle diamonds and can be used to trace their formation conditions.

We combined diamond growth experiments in hydrated fluids at 7GPa, 1200-1400°C in multi-anvil presses with ERDA and nanoSIMS analysis of hydrogen in the resulting diamond growth areas. We used the protocol developed in previous studies, which reproduces diamonds similar to those from the upper mantle (Bureau et al., 2024, GCA, 368). The latter required specific preparations using cryogenic methods intended to avoid any loss of hydrogen.

Results show that diamonds grown in supercritical fluid/melts containing up to 30 % of water contain hydrogen in comparable amount than in some natural diamonds. This suggests that the presence of water is necessary for the growth of diamonds in the lithosphere and places new constrains on the deep-water cycle.