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Abstract

Correct classification of diamondiferous ultrapotassic rocks and differentiation between kimberlite units are essential for evaluating diamond potential and understanding mantle processes. Classification of the Meya dyke, an on-strike extension of the Koidu kimberlite cluster on the Man Craton (Sierra Leone), has long been debated due to its high phlogopite and low carbonate contents. This study integrates petrography, SEM–AMICS modal mineralogy, mineral chemistry, whole-rock geochemistry, and garnet thermobarometry to resolve the classification, petrogenesis, mantle sampling, and diamond potential of three Meya rock units (KIMB 1, KIMB 2, and KIMB 3). Modal mineralogy, diagnostic mineral assemblages (olivine, phlogopite, spinel, perovskite, apatite), whole-rock compositions, and phlogopite and spinel compositional trends demonstrate that Meya rock units are kimberlites and are clearly distinct from carbonate-rich olivine lamproites and olivine lamproites. The three kimberlite units share highly similar bulk-rock compositions and REE patterns, indicating derivation from a common kimberlitic magmatic system. Spatially associated carbonatitic rocks differ in major-element chemistry but show overlapping REE patterns and fluid-mobile element ratios, consistent with late-stage or exsolved volatile-rich melts genetically linked to the kimberlite system. Despite their shared origin, the three units exhibit systematic mineralogical differences, including variations in olivine, phlogopite, perovskite, spinel, xenolith abundance, and deuteric alteration styles. Garnet major- and trace-element systematics record progressive refertilization of the lithospheric mantle from ~110 to 170 km depth, evolving from depleted subcalcic G10 garnets to Ca- and Ti-enriched G9 and G11 garnets. High-temperature metasomatism and deformation near the lithosphere–asthenosphere boundary (~160 km) are indicated by Ti-rich G11 garnets, Fe-rich olivine, and Ti-bearing phlogopite, particularly in KIMB 2. Thermobarometric results show that all units sampled the mantle in the diamond stability field. KIMB 3 exhibits the highest potential for conventional lithospheric diamonds, whereas KIMB 2 shows the strongest indicators for sublithospheric CLIPPIR-type diamonds. Overall, this study identifies Meya dyke as a new kimberlite occurrence, improve the understanding of the Man craton mantle, and have practical implications for local diamond mining.