Origins of olivine in Earth’s youngest kimberlite: Igwisi Hills volcanoes, Tanzania craton

Abstract

Nearly monomineralic several millimeter large rounded olivine nodules are common in kimberlites from localities worldwide. These polycrystalline nodules comprise either single or multiple anhedral olivine grains that are accompanied by smaller olivine neoblasts. 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, but thermobarometric constraints to support such a model have been lacking thus far.In this study, we focus on the petrography and textural characteristics, as well as on pressure-temperature estimations, of exceptionally 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 major and trace element data determined by EPMA and LA-ICP-MS methods to achieve this goal. We find that host olivine grains in these nodules are compositionally similar to olivine in garnet-facies cratonic mantle peridotites, and such a petrogenetic association is supported by rare garnet inclusions within olivine.Projection of Al-in-olivine temperatures onto a regional cratonic 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 range of 160-180 km is considered for the present-day lithosphere-asthenosphere transition beneath the central Tanzania craton. These first pressure-temperature estimates for dunitic nodules in kimberlites suggest that their formation may also occur at significantly shallower depths than previously assumed for other occurrences worldwide.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 characteristics may link neoblast formation to melt-assisted in-situ recrystallization of cratonic mantle peridotite, a process that evidently persisted during kimberlite magma ascent through the lower half of thick continental lithosphere. Partial recrystallization of olivine-rich mantle xenoliths en route to surface increases the length of grain boundaries and also leads to the formation of abundant fractures within host olivine grains, which facilitates melt and fluid percolation that makes the xenoliths texturally weaker. Subsequent liberation of mineral grains from recrystallized peridotite xenoliths 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.We show that 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 levels. The dunitic nodul...

Melt stripping and agglutination of pyroclasts during the explosive eruption of low viscosity magmas

“…The Igwisi Hills volcanoes (IHV) comprise three small, closely-spaced monogenetic kimberlitic volcanoes on the western margin of the Tanzanian craton 4 , 45 – 47 (Fig. 1a, b ).…”

“…b Geological map of the Igwisi Hills volcanoes with our sample site marked by the asterisk (−4.887884° latitude, 31.933439° longitude). The map is adapted from Shaikh et al 47 , with permission. c Field photograph taken from Brown et al 4 of the bedded pyroclastic fallout deposits sampled as part of this study that make up part of the pyroclastic cone on the west of the Central volcano.…”