Extending the metasomatic record in fibrous diamonds: insights from surface etch features

Abstract

Resorption features formed on diamonds provide key insights into diamond-destructive mantle metasomatism and also offer a novel proxy for constraining kimberlitic fluid composition. However, the surface features of fibrous diamonds remain poorly documented, limiting our understanding of their post-growth history. Here, the etch features, impurity characteristics, and compositions of high-density fluids (HDFs) in the outermost fibrous growth layers of coated diamonds from the Mbuji-Mayi kimberlite, the Democratic Republic of the Congo (DRC) are investigated. These diamonds containing silicic–low-Mg carbonatitic HDFs exhibit negative stepped point-bottomed (p/b), stepped curved-bottomed (c/b), and ‘pure’ c/b trigons without stepped walls, together with positive and truncated trigons on {111} faces. The correlations among different types of trigons, along with the co-occurrence of other surface features, reveal three distinct resorption events–one in the mantle and two during the kimberlite ascent. Comparisons with previous experimental and natural etch features suggest that carbonate-rich melts in the mantle were responsible for stepped p/b trigons on coated diamonds, whereas ‘pure’ c/b trigons and positive trigons both resulted from interaction with CO₂-rich fluids, but at different depths during kimberlite ascent. This study presents the first systematic dataset of resorption features on fibrous diamonds, advancing our understanding of the formation mechanism of etch features on diamonds under natural settings and extending metasomatic records of fibrous diamonds beyond their crystallization stage. These results suggest a carbonate-rich melt environment after the growth of fibrous diamonds in the lithospheric mantle beneath the Congo craton, and confirm the CO₂-rich nature of fluids exsolved from the Mbuji-Mayi kimberlite magmas.