Polymineralic Inclusions in Megacrysts as Proxies for Kimberlite Melt Evolution—A Review

Abstract

Polymineralic inclusions in megacrysts have been reported to occur in kimberlites worldwide. The inclusions are likely the products of early kimberlite melt(s) which invaded the pre-existing megacryst minerals at mantle depths (i.e., at pressures ranging from 4 to 6 GPa) and crystallized or quenched upon emplacement of the host kimberlite. The abundance of carbonate minerals (e.g., calcite, dolomite) and hydrous silicate minerals (e.g., phlogopite, serpentine, chlorite) within polymineralic inclusions suggests that the trapped melt was more volatile-rich than the host kimberlite now emplaced in the crust. However, the exact composition of this presumed early kimberlite melt, including the inventory of trace elements and volatiles, remains to be more narrowly constrained. For instance, one major question concerns the role of accessory alkali-halogen-phases in polymineralic inclusions, i.e., whether such phases constitute a common primary feature of kimberlite melt(s), or whether they become enriched in late-stage differentiation processes. Recent studies have shown that polymineralic inclusions react with their host minerals during ascent of the kimberlite, while being largely shielded from processes that affect the host kimberlite, e.g., the assimilation of xenoliths (mantle and crustal), degassing of volatiles, and secondary alteration. Importantly, some polymineralic inclusions within different megacryst minerals were shown to preserve fresh glass. A major conclusion of this review is that the abundance and mineralogy of polymineralic inclusions are directly influenced by the physical and chemical properties of their host minerals. When taking the different interactions with their host minerals into account, polymineralic inclusions in megacrysts can serve as useful proxies for the multi-stage origin and evolution of kimberlite melt/magma, because they can (i) reveal information about primary characteristics of the kimberlite melt, and (ii) trace the evolution of kimberlite magma on its way from the upper mantle to the crust.Here, the focus is on fully-crystallized melt inclusions, so-called "polymineralic inclusions", contained within megacrysts from kimberlites. The term "megacryst" applies to mantle minerals that are >1 cm in size. Common megacryst phases are clinopyroxene, garnet, olivine, ilmenite, phlogopite, and zircon [26][27][28]. Based on their composition, megacrysts can be divided into a Cr-poor and a Cr-rich suite [29][30][31]. The origin of megacrysts themselves has long been debated. Some studies have advocated for a genetic link between a kimberlitic or "proto-kimberlitic" melt [29,[32][33][34][35][36][37][38], whereas others considered the megacryst parental melt to be unrelated to kimberlites [26,39,40]. In the scenario that megacrysts are linked to crystallization from proto-kimberlitic melts in the lithospheric mantle, the spectrum of Cr-poor to Cr-rich megacrysts has been proposed to be a function of variable interaction with the surrounding mantle [30,41,42].The formation of polymineralic ...

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

“…3a). The melt inclusions are similar to so-called 'polymineralic' inclusions commonly observed in kimberlite-borne megacrysts from localities worldwide (Bussweiler, 2019), including megacrystic olivine (Howarth and Büttner 2019;Abersteiner et al 2019). Another important feature of the Igwisi Hills kimberlite lavas is the presence of quenched carbonate-rich melt pockets in the groundmass.…”

Confocal Raman spectroscopic study of melt inclusions in olivine of mantle xenoliths from the Bultfontein kimberlite pipe (Kimberley cluster, South Africa): Evidence for alkali‐rich carbonate melt in the mantle beneath Kaapvaal Craton

“…The water-and silica-poor carbonatitic/alkalicarbonatitic composition of the primary/primitive kimberlite melt was explored from the mineralogy of kimberlites and melt inclusions in kimberlitic minerals in numerous recent publications, [37,65,66,[68][69][70][71]73,74,93,[95][96][97][98][99][100][101][102][103][104][105] and from melt inclusions in kimberlite-hosted mantle xenoliths in rare studies. [5,6,13,47,65,67] High-pressure experiments demonstrated that kimberlite melt originated as a primary alkali-rich carbonatitic liquid (<5-10 wt% of SiO 2 ) and, during ascent, evolved towards more silicarich composition due to dissolution of xenogenic silicates. [106][107][108][109][110] Our study of the melt inclusions in olivine of the sheared peridotites from the Bultfontein pipe further supports the model of the alkali-rich carbonate composition of primary/primitive kimberlite melts.…”

“…Most studies of melt inclusions in mantle-derived minerals from kimberlites were conducted for the multiphase microinclusions in fibrous diamonds and polymineralic inclusions in megacrysts. [7][8][9][10][11][12][13] These types of inclusions were interpreted as entrapped mantle melts or high-density fluids. Melt inclusions studies for peridotite xenoliths from kimberlites are rare and mostly limited to the Udachnaya-East kimberlite pipe (Siberian Craton, Russia).…”

Metasomatic Evolution of Coesite-Bearing Diamondiferous Eclogite from the Udachnaya Kimberlite

“…Thus, carbonatitic, alkali-carbonatitic and saline-carbonatitic melts appear to be important mantle metasomatic agents involved in the formation of fibrous diamonds. Mantle xenoliths rarely enclose carbonate phases [41][42][43][44][45][46][47][48], though crystalline carbonates [26,[49][50][51] and crystallized carbonated melts often occur as inclusions in xenolith minerals [52][53][54][55][56] and in macrocrysts/megacrysts/xenocrysts associated with kimberlites [57][58][59][60][61]. A wealth of implicit evidence indicates metasomatic effects of carbonated melts in mantle xenoliths [62][63][64][65].…”