Nanometric mineral inclusions from a fluid-rich diamond: identification, structure, and implications for deep Earth

Fluid-rich (cloudy/fibrous) diamonds host millions of micrometric fluid inclusions that can reveal the nature of diamond-forming media. Mineral inclusions in fluid-rich diamonds are nano to micrometric making structural and chemical characterization of the different phases difficult. Consequently, limited work has been done determining the pressures, temperatures and depths at which such inclusions form. The relationship between such conditions and those of fluid-rich diamond formation remains unclear. We report the use of micro-electron diffractometry to identify and anisotropically refine the structure of a nanometric åkermanite inclusion in a fluid-rich diamond from South Africa. Additional nanometric Ba/Sr-carbonate inclusions were detected. FTIR analyses revealed a highly aggregated, gem-quality core surrounded by a rim rich in high-density fluid (HDF) inclusions from which åkermanite crystallized. Åkermanite formed during HDF depressurization due to kimberlite eruption or exhumation. In the latter scenario, åkermanite constrained diamond formation to a minimum temperature of 1000 °C at ∼140 km depth (~4.6 GPa). Analysis of the HDFs reveal low-Mg carbonatitic to silicic compositions. The high BaO and Cl and the δ¹³C values of the diamond (-5.72 to -7.84 ‰) are attributed to formation via penetration of saline fluid into an eclogite, related to subduction of carbonated altered oceanic crust.