The evolution of calcite-bearing kimberlites by melt-rock reaction: evidence from polymineralic inclusions within clinopyroxene and garnet megacrysts from Lac de Gras kimberlites, Canada

Abstract

inclusion trails surround the inclusions. In Cr-pyropes, the inclusions additionally contain Al-spinel, clinopyroxene, and dolomite. The major and trace element compositions of the inclusion phases are generally consistent with the early stages of kimberlite differentiation trends. Extensive chemical exchange between the host phases and the inclusions is indicated by enrichment of the inclusions in major components of the host crystals, such as Cr 2 O 3 and Al 2 O 3 . This chemical evidence, along with phase equilibria constraints, supports the proposal that the inclusions within Cr-diopside record the decarbonation reaction: dolomitic melt + diopside → forsterite + calcite + CO 2 , yielding the observed inclusion mineralogy and producing associated (CO 2 -rich) fluid inclusions. Our study of polymineralic inclusions in megacrysts provides clear mineralogical and chemical evidence for an origin of kimberlite that involves the reaction of high-pressure dolomitic melt with diopside-bearing mantle assemblages producing a lower-pressure melt that crystallizes a calcite-dominated assemblage in the crust.

Orthopyroxene survival in deep carbonatite melts: implications for kimberlites

“…A reaction similar to (4) but involving the CaCO 3 component of carbonate melt was used by Harmer and Gittins (1997) to explain dolomitic to calcitic carbonatite evolution, but no experimental constraints exist on either variant of this reaction. A recent study looking at clinopyroxene megacrysts (>1 cm in size) in kimberlites found large reaction rims attesting to the instability of clinopyroxene during the ascent of the kimberlite, and crystallized melt inclusions trapped in these megacrysts consist largely of olivine and calcite showing possible evidence of this reaction occurring in nature (Bussweiler et al 2016). …”

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

“…The formation of polymineralic inclusions in megacrysts has been attributed to a variety of metasomatic agents in the upper mantle, including the megacryst magma, which also crystallized the host megacrysts [43], primary or early kimberlite melt [44][45][46], mantle carbonatite with a subduction origin [47,48], or even diamond-forming fluids [49,50]. Here, the findings of previous studies on polymineralic inclusions in megacrysts from kimberlites are discussed.…”

“…More recently, Bussweiler et al [44] revisited the Diavik megacryst samples and extended the investigation to include both clinopyroxene and garnet megacrysts from the Lac de Gras area (with samples from the Diavik and Ekati diamond mines). They concluded that polymineralic inclusions represent early kimberlite melt trapped at mantle depths which reacted extensively with the respective host minerals.…”

“…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 inclusions in megacrysts has been attributed to a variety of metasomatic agents in the upper mantle, including the megacryst magma, which also crystallized the host megacrysts [43], primary or early kimberlite melt [44][45][46], mantle carbonatite with a subduction origin [47,48], or even diamond-forming fluids [49,50]. Here, the findings of previous studies on polymineralic inclusions in megacrysts from kimberlites are discussed.…”

“…Polymineralic inclusions within clinopyroxene megacrysts are typically surrounded by "spongy rims" (i.e., regions where the clinopyroxene is altered and contains abundant fluid/melt inclusions), whereas polymineralic inclusions within garnets are typically lined by kelyphite ( Figure 2). These features have been interpreted as textural evidence for extensive reactions between the polymineralic inclusions and their host minerals [44]. In contrast, polymineralic inclusions within olivine megacrysts were interpreted to show little evidence of this chemical interaction [45].…”