The fluid record of diamond growth from dehydration fluids during slab warming

The serpentinized oceanic mantle plays a central role in deep volatile cycling. Yet, our understanding of how subducting slabs remain hydrated and retain chemical signatures typically associated with crustal rocks after final serpentine breakdown is still incomplete due to the scarcity of direct samples of deep-seated rocks and fluids. Here, we report on fluids preserved in a diamond grown within a section of deserpentinized oceanic mantle, which provides critical insights into the role played by hydrated ultramafic rocks within subducting slabs in transporting water and other volatiles to the depths of the diamond stability field. Infrared spectroscopy of the heavily included core zone of this Brazilian diamond reveals absorption bands characteristic of liquid water, indicating the presence of trapped fluids. X-ray micro-computed tomography resolves the three-dimensional distribution of mineral and fluid inclusions. Cracking the diamond to recover solid inclusions allowed the release of trapped fluid, precipitating minerals enriched in K, Mg, CO₂, and H₂O, with minor F, Na, Si, S, Cl, Fe, Ni and Ba. The fluid elemental makeup is similar to that of fluids hosted in fibrous diamonds. When compared to deserpentinization fluids trapped in olivine from ophiolite complexes, the residual diamond-forming fluid shows a stronger sediment-derived component (e.g., higher K). The diamond exhibits a progressive decrease in N aggregation state from core (50%B) to rim (5%B), whereas C- and N-isotope compositions remain largely uniform throughout growth. The close association with a deserpentinization assemblage further suggests that this diamond formed during prolonged release of fluids during slab warming.