How can we chemically analyse fluids in situ around mineral inclusions in gem-quality diamonds?

Diamonds crystallize from fluids or melts circulating in the Earth's mantle. Direct analysis of these fluids is possible when they remain entrapped during diamond growth, a rare feature in gem-quality stones. We present a three-step approach to analyse thin silicic fluid films (≤5 μm; cf. Nimis et al., 2016) surrounding mineral inclusions in gem-quality diamonds using slow LA-ICPMS depth profiling.

1. Non-destructive characterization: FTIR and Raman spectroscopy are used to classify diamonds and identify mineral and fluid inclusions. Confocal micro-Raman mapping reveals a heterogeneous 3D distribution of fluid films containing Si₂O(OH)₆ and Si(OH)₄ around mineral inclusions.
2. Laser ablation time estimation: LA drilling experiments on a diamond fragment helped to determine the ablation rate. Combined with the inclusion depth, this allows estimation of the time needed to reach a mineral inclusion at a constant LA repetition rate.
3. Targeted LA-ICPMS depth profiling: A photo of the marked diamond, showing both the diamond surface and the inclusion in focus, is overlaid on the live image, enabling accurate targeting of the mineral inclusion during LA-ICPMS analysis. Slow ablation (~2 Hz) resolves distinct chemical signals. Fluid-mobile (e.g., Ba, Sr) and mineral-specific elements (e.g., Mg, Fe for olivine) are used to differentiate between fluid, host diamond, and mineral inclusion in a single depth profile.

Despite limitations from pit tailing, graphitization, depth-dependent ablation rates, and biased inclusion depth estimates (diamond’s high refractive index), we successfully analysed two samples. These results allow comparison of entrapped fluids with previously studied High-Density-Fluids (HDFs) from fibrous diamonds using trace element ratios.