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Abstract

Ilmenite megacrysts from the Monastery kimberlite formed near the lower boundary of the subcontinental lithospheric mantle (SCLM) at about 5 GPa pressure in proto-kimberlitic carbonated silicate melt. A first stage of melt differentiation in the kimberlite source created Ti-oxide- and carbonate-richer domains separated from Si-Mg-richer ones, promoting the nucleation and growth of ilmenite and olivine megacrysts. Further differentiation of melt contained in ilmenite megacryst-hosted inclusions led either to the separation of immiscible CO2- and H2O-bearing Ca-Ti-Fe-Al oxide melt portions, or to the crystallisation of MUM-type spinel, ferric phlogopite and, less commonly, perovskite and hydrous morimotoite-andradite garnet. In a second stage the residual, Ti-, Al- and K-depleted carbonated hydrous silicate melt formed patterns indicative of increasing carbonate–silicate melt immiscibility. Binodal and spinodal melt decomposition is indicated by emulsion textures and interconnected networks, separating Ca-carbonate and Mg-Fe-silicate melts that mature from <10 μm droplets or veins networks to >1 mm-sized domains. The preserved textures of progressing melt separation are consistent with a continuum of melt unmixing from the stage preceding, and likely initiating, megacryst growth in the SCLM to the quenching of residual variably evolved melt after kimberlite emplacement in the shallow crust. The most evolved separated liquids were carbonate melts (XCa 0.96–1.00) and hydrous high-Mg silicate melts with variable Fe contents (∼4–13 wt% FeOtot). Where quenched evolved silicate melt is preserved as glass, SIMS analysis indicates average volatile contents of 3.3–8.9 wt% CO2 and 6.5–7.9 wt% H2O. Average H2O/CO2 ratios in the residual silicate melt vary between 0.8 and 2.0. Our observations provide textural and compositional evidence in support of previous work that proposed close chronological and causative links of carbonated silicate melt immiscibility, the growth of megacrysts, and the ascent of kimberlite magma.
Graphical abstract