Nanometric mineral inclusions in fluid-rich diamonds: new opportunities from electron diffractometry

Fluid inclusions in fluid-rich diamonds (i.e., fibrous, cloudy and coated diamonds) represent the only direct means by which the composition and sources of deep-Earth fluids can be directly studied. At the surface, fluid inclusions typically consist of multiphase mineral inclusions (daughter-phases including carbonates and micas) and residual low-density fluids thought to form from parental high-density fluids upon depressurization. However, the sub-micrometric to nanometric size of such mineral inclusions has made rigorous identification and chemical–structural characterization of discrete phases, particularly those in multiphase inclusions, exceedingly difficult. Consequently, traditional chemical (and/or elastic) thermobarometric methods cannot be applied to nanometric inclusions and thus the P/T – depth conditions of fluid-rich diamond formation, and the mantle environments in which they form, remain poorly understood. In previous studies, authors have attempted to address this problem using TEM coupled with EDS and/or EELS to obtain chemical data from mineral inclusions, and SAED (or CBED) to constrain identification based on general crystallographic information (e.g., d -spacings and symmetry). Despite these advancements, difficult and time-consuming FIB-based preparation of diamond films, and the instability of common inclusions (e.g., carbonates) under the TEM electron beam, has prevented statistically meaningful surveys of inclusions in different types of fluid-rich diamonds. Here, the advantages and limitations of single-crystal micro-electron diffraction (MED) are discussed as a method for obtaining detailed crystallographic information (e.g., crystal structure data and structural formulae) from nanoinclusions enabling rigorous phase identification and follow-up work on the P/T/ f O2 stability of inclusions phase-assemblages. This is exemplified in the first rigorous application of MED to fluid-rich diamonds (Wang et al. 2026). These authors identified unique Sr/Ba-carbonates and several species that have never been observed before as inclusions in diamond, including Sr-rich åkermanite and a Ca-rich strontianite phase, which has no naturally occurring analogue. Wang et al. (2026) also completed an anisotropic crystal structure refinement of one nanometric crystal of åkermanite. By combining such MED results with spectroscopic, chemical and isotopic data, the P/T – depth conditions, and the fluid sources associated with diamond-formation, were constrained testifying to the utility of MED applied to sub-micrometric inclusions in diamond.