A classification scheme for olivine populations in the Upper Pleistocene to Holocene Igwisi Hills (Tanzania) kimberlite lavas and investigation of grain origins and evolution

Kimberlites are a class of alkaline igneous rock which sometimes contain diamonds and derive from mantle depths typically exceeding 150 km. We investigate the Upper Pleistocene to Holocene 12 ka Igwisi Hills kimberlites to characterise the olivine grains and better understand their origins and evolution. We observe four distinct olivine populations. Macrocrysts are >1500 μm in diameter, rounded, monocrystalline grains with a Mg-rich xenocrystic core (Fo 90.5–92.8) and elevated Ni but depleted Ca and Mn contents, typical of granular mantle peridotite. Nodules are >1500 μm in diameter, rounded, polycrystalline grains with an Mg-rich core (Fo 91.2–92.4), also derived from granular peridotite. Microcryst grains are <1500 μm, subhedral-euhedral and can be subdivided into two types. Microcryst-a grains have a Mg-rich (Fo 90–92.5) xenocrystic core indicative of a mantle peridotite origin and Microcryst-b grains have a Fe-rich (Fo 89–91) core that likely originated from disaggregated neoblasts. Olivine in all four populations displays four magmatic zones: internal zone, rim, rind and outermost rind, typical of kimberlites worldwide. Fe-rich internal zones are likely derived from a primitive kimberlite melt which entrained the cores. Mg-rich rims formed as the kimberlite melt evolved and crystallised around the internal zones. Fe-rich rinds formed as the kimberlite melt continued to ascend and further crystallised around the rims. Mg-rich outermost rinds crystallised from the kimberlite melt during the final stages of ascent. Based on these observations we present a new ascent model for the Igwisi Hills magmas. • Olivine in the lavas from the Igwisi Hills kimberlites is classified into four populations which can be distinguished based on their size, morphology and geochemical zoning patterns. • Olivines from all four populations demonstrate similar Fo zoning patterns to kimberlites globally, showing a xenocrystic core surrounded by up to four magmatic zones. • We use zoning patterns to interpret the evolution and ascent journey of the Igwisi Hills kimberlite magma.