Trace element systematics constrain the origin of fluids that form gem-quality diamonds

Diamonds crystallise from fluids/melts circulating in the Earth’s mantle. Analysis of these fluids is possible if they remain entrapped in the diamond during growth, but such fluid inclusions are rarely observed in gem-quality stones. We investigated thin films surrounding mineral inclusions, previously described as silicic fluid rims containing Si2O(OH)6 and Si(OH)4, in gem-quality lithospheric diamonds from Siberia. Using micro-Raman spectroscopy and Laser-Ablation Inductively-Coupled-Plasma-Mass-Spectrometry (LA-ICPMS) depth-profiling, we obtained compositional data from silicic fluid rims surrounding both silicate and non-silicate inclusions. Slow LA-ICPMS depth-profiling at the diamond-inclusion interface enabled differentiating the respective diamond/fluid-rim/mineral-inclusion contributions, allowing detection of Sr, Nb, Ba, La, Ce, Nd, and Th from the fluid rim. Here, we compare silicic fluid rims and other mantle-derived fluids/melts based on trace element ratios relative to the primitive mantle. Their (La/Nb)N_(Ba/Nb)N and (Nd/Nb)N_(Th/Nb)N systematics align with primitive mantle-like high-density fluids and group 2 kimberlites, suggesting an origin from kimberlite-like melts.