Occurrence and petrogenesis of diamondiferous lamprophyres: Insights into mantle metasomatism and diamond formation

Lamprophyres are a rare rock-type that include different lithologies, classified mineralogically (abundance of mica, amphibole, feldspathoids) rather than geochemically (enrichment in alkalis and volatiles). Despite the attention that these rocks have attracted, due to their possible economic interest, their sources, petrogenesis and classification remain a matter of debate. One of the most intriguing aspects is the relationship of some lamprophyre sub-types with diamonds, especially because diamondiferous lamprophyres are the rarest diamond-bearing rock-types. The reason for this association and their potential genetic relationship remain poorly understood. Previous studies on lamprophyres (diamond-bearing and barren) are reviewed to improve our knowledge of peculiar mantle paragenesis, fertilization events, metasomatic agents and the relationship between lamprophyres and diamonds. Calc-alkaline lamprophyres (CALs) are often emplaced in subduction-related settings, where slab dehydration and crustal recycling can stabilize hydrous phases (amphibole/phlogopite). Low degree partial melting of shallow metasomatized sources may explain CALs petrogenesis . To reconcile this shallow formation depth with the presence of diamonds, we propose a lithospheric stacking model, that results in a highly transient geotherm during the initial phases of craton formation. Ultramatic lamprophyres (UMLs) mostly crop out along rifts and are related to a veined lithospheric and/or asthenospheric source. A direct genetic link between diamonds and UMLs can be excluded. Instead, the presence of diamondiferous UMLs relies on spatial and temporal connections (e.g., mantle domains enriched in C-H-phases). Lamprophyric melts are hostile for diamonds, due to their high fO2, H2O and CO2, along with a longer lag time and a lower volume of emplaced magma.