Kimberlite Magma Evolution during Ascent: An Experimental Study under Different Redox Conditions
Kimberlite magmatism provides critical information about the mineralogical and chemical composition of the deep mantle and geodynamic processes within the deep lithosphere and upper asthenosphere. Understanding the fractional crystallization processes of kimberlite magma after separation from its source and before emplacement is essential for quantifying the degassing of kimberlite magmas, which is responsible for their explosive eruption styles. While mineral fractionation has been proposed to explain compositional variations in the evolved aphanitic kimberlites, the mineral assemblages and the extent of crystallization during magma transport remain poorly constrained. In this study, we conducted a series of experiments with the proposed primary kimberlitic magma composition at 3.0–2.0 GPa and 1200–900 °C, under both graphite-saturated reducing conditions and nickel–nickel oxide (NNO)-buffered oxidizing conditions in a piston-cylinder apparatus. This study presents the first experimental investigation into the effects of redox conditions on the fractional crystallization of kimberlite magma. Our results show that olivine crystallizes under all conditions studied, followed by clinopyroxene with decreasing temperature. Orthopyroxene is stabilized under reducing conditions at 3.0–2.4 GPa. Phlogopite, rutile, magnesian ilmenite, and apatite crystallize after clinopyroxene under both redox conditions, while carbonates (mainly dolomite) are observed only under oxidizing conditions in the absence of melt and at temperatures ≤1000 °C. The crystallization of magnesian ilmenite occurs mainly under oxidizing conditions, indicating that its presence in kimberlite does not imply diamond-preserving reducing conditions. Our study shows that, as the temperature of kimberlite magma decreases to and below 1200 °C, olivine ± pyroxene fractionation becomes increasingly significant, and low oxygen fugacity promotes a higher extent of crystallization. This silicate-dominated fractional crystallization could account for the formation of aphanitic kimberlites from Kimberley and Uintjiesberg, South Africa. The residual melt after fractionation is a transitional carbonate–silicate melt and becomes progressively depleted in SiO2 and evolves toward carbonatitic compositions with decreasing temperature, irrespective of redox conditions, which may explain the co-occurrence of carbonatite and kimberlite in the Premier pipe, South Africa. Our experiments confirm that oxygen fugacity strongly controls the stability of carbonates and transitional melts, with transitional melts stable at lower oxygen fugacity than carbonates. Progressive oxidation during magma cooling facilitates promotes the full complete crystallization of kimberlite magma, which may potentially resulting inform the formation of intrusive kimberlite bodies within the lithospheric mantle.
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