Decoding Metamorphic Nanodiamonds Formation Through Their Internal Structures

ABSTRACT Metamorphic diamonds offer insights into Earth's evolution, modulating the global carbon cycle through subduction into the mantle and exhumation. However, the formation and internal structure of minute diamonds remain poorly understood. Here, we study the internal structures of diamond‐bearing inclusions from the Eastern Alps using state‐of‐the‐art analytical techniques. Atomically resolved images revealed intricate structures, including well‐faceted monocrystalline diamonds, complex nanocrystalline clusters of intergrown diamond and graphite and amorphous matter. Associated fluids contain CO 2 , CH 4 and H 2 O, indicating formation from C–O–H fluids. The proposed model of C–O–H fluid behaviour suggests monocrystalline diamond precipitation at the onset of retrograde metamorphism. Amorphous layers formed through interaction between the trapped fluid and the host garnet during the further exhumation. When the fluid reached the P–T conditions of the transition zone between the diamond and graphite stability fields, polycrystalline clusters formed. For the first time, we can see a detailed record of metamorphic fluids evolution, preserved in the internal structures of microsized diamond inclusions.