Monomineralic millimeter-sized olivine nodules are common in kimberlites worldwide. It is generally thought that such ‘dunitic nodules’ originate from the base of the cratonic lithosphere and that their formation marks the onset of deep-rooted kimberlite magmatic plumbing systems. However, thermobarometric constraints to support such a model have been lacking thus far. This study focuses on the petrography and textures, as well as on pressure–temperature estimations, of well-preserved dunitic nodules from the Quaternary Igwisi Hills kimberlite lavas on the Tanzania craton, with the ultimate goal to constrain their origins. We utilize EBSD-determined textural information in combination with olivine geochemistry data determined by EPMA and LA-ICP-MS methods. We find that host olivine grains in these nodules are compositionally similar to olivine in garnet-facies cratonic mantle peridotites, and such an association is supported by garnet inclusions within olivine. Projection of Al-in-olivine temperatures onto a regional geotherm suggests that the host olivine grains equilibrated at ~ 100–145 km depth, which points to origins from mid-lithospheric levels down to the lower cratonic mantle if a depth of 160–180 km is considered for the lithosphere–asthenosphere transition beneath the Tanzania craton. These first pressure–temperature estimates for dunitic nodules in kimberlites suggest that their formation also occurs at much shallower depths than previously assumed. Recrystallized olivine grains (i.e., neoblasts) show random crystallographic orientations and are enriched in minor and trace elements (e.g., Ca, Al, Zn, Sc, V) compared to the host olivine grains. These features link neoblast formation to melt-assisted recrystallization of cratonic mantle peridotite, a process that persisted during kimberlite magma ascent through the lower half of thick continental lithosphere. Partial recrystallization of olivine-rich mantle xenoliths makes these materials texturally weaker and subsequent liberation of mineral grains promotes the assimilation of compositionally ‘unstable’ orthopyroxene in rising carbonate-rich melts, which is considered to be an important process in the evolution of kimberlite magmas. Dunitic nodules in kimberlites and related rocks may form as melt–rock equilibration zones along magmatic conduits within the lower half of the cratonic mantle column all the way up to mid-lithospheric depth. Such an origin potentially links dunitic nodules to olivine megacrysts, which are equally considered as melt/fluid-assisted recrystallization products of peridotitic mantle lithosphere along the ascent pathways of deep-sourced CO₂–H₂O-rich ultramafic melts.

单矿物毫米大小的橄榄石结核在全球金伯利岩中很常见。人们普遍认为，这种“均质结核”起源于克拉通岩石圈的底部，它们的形成标志着根深蒂固的金伯利岩浆管道系统的开始。然而，迄今为止，缺乏支持这种模型的热气压约束。本研究侧重于来自坦桑尼亚克拉通上第四纪伊格维西山金伯利岩熔岩的保存完好的纯质结核的岩相学和纹理以及压力-温度估计，最终目标是限制它们的起源。我们将 EBSD 确定的纹理信息与 EPMA 和 LA-ICP-MS 方法确定的橄榄石地球化学数据相结合。我们发现这些结核中的寄主橄榄石颗粒在成分上与石榴石相克拉通地幔橄榄岩中的橄榄石相似，并且橄榄石中的石榴石包裹体支持了这种关联。将橄榄石中的铝温度投影到区域地温上表明，寄主橄榄石颗粒在约 100-145 公里的深度平衡，如果深度为 160-180 公里，这表明起源于岩石圈中部水平到下克拉通地幔被认为是坦桑尼亚克拉通下方的岩石圈-软流圈过渡。这些对金伯利岩中纯晶结核的首次压力-温度估计表明，它们的形成也发生在比以前假设的更浅的深度。重结晶橄榄石颗粒（即新生细胞）显示出随机的晶体取向，并且富含微量元素和微量元素（例如，Ca、Al、Zn、Sc、V）与主体橄榄石颗粒相比。这些特征将新生细胞的形成与克拉通地幔橄榄岩的熔融辅助重结晶联系起来，这一过程在金伯利岩岩浆上升通过厚大陆岩石圈的下半部分期间持续存在。富含橄榄石的地幔捕虏体的部分重结晶使这些材料的结构变弱，随后矿物颗粒的释放促进了成分“不稳定”的斜方辉石在上升的富含碳酸盐的熔体中的同化，这被认为是金伯利岩岩浆演化的一个重要过程。金伯利岩和相关岩石中的单质结核可能形成为熔岩平衡带，沿着克拉通地幔柱下半部的岩浆管道，一直到岩石圈中层深度。富含₂ –H ₂ O 的超镁铁质熔体。