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.
Highlights
Kimberlites are complex “hybrid” volcanic rocks that contain minerals of different origins, including: (i) xenocrysts from the upper mantle and crust, (ii) primary minerals crystallized from the kimberlite melt, and (iii) secondary minerals formed during post-magmatic alteration
While secondary alteration cannot be ruled out, it is obvious that polymineralic inclusions are different in mineralogy and composition from typical, late-stage groundmass kimberlite, and preserve some characteristics of an early kimberlite melt trapped at mantle depths
Polymineralic inclusions within megacrysts from kimberlites likely represent kimberlite melt trapped at mantle depths which reacted with the host megacrysts during kimberlite ascent and fully crystallized or quenched upon emplacement of the transporting kimberlite
Summary
Kimberlites are complex “hybrid” volcanic rocks that contain minerals of different origins, including: (i) xenocrysts from the upper mantle and crust, (ii) primary minerals crystallized (potentially at different depths) from the kimberlite melt, and (iii) secondary minerals formed during post-magmatic alteration. The findings of previous studies on host megacrysts [43], primary or early kimberlite melt [44,45,46], mantle carbonatite with a subduction polymineralic inclusions in megacrysts from kimberlites are discussed. The aim of this review is to origin [47,48], even diamond-forming [49,50].to Here, the new findings of previous studies on highlight the or potential of polymineralicfluids inclusions provide insights about kimberlite polymineralic inclusions in megacrysts from kimberlites are discussed. They have been reported polygonal to different spheroidal in shape and up to including a few mm olivine, in diameter They have been reported to occur in megacryst minerals, garnet, and 1).
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