Kimberlite magma evolution during ascent: an experimental study under different redox conditions

Liu, Z; Chou, I-M; Ta, K; Li, Z; Wang, Y; Klemme, S

Abstract

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. Elucidating the fractional crystallization processes of kimberlite magma after its separation from the source and before emplacement is key to quantifying the degassing of kimberlites, 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 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 (Ni-NiO) buffered oxidizing conditions in a piston-cylinder apparatus to determine the fractionation of Group I kimberlite magma during ascent, and to elucidate the role of redox conditions during the cooling of kimberlite magma. 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 is the first phase to crystallize at all studied temperatures as the pressure decreases from 3.0 to 2.0 GPa. Orthopyroxene is stabilized by the presence of graphite under reducing conditions at 3.0–2.4 GPa. As temperature decreases, rutile, phlogopite, magnesian ilmenite, and apatite crystallize after clinopyroxene under both redox conditions. Carbonates (mainly dolomite) were observed only under oxidizing conditions in the absence of melt and at temperatures not exceeding 1000 °C. The crystallization of magnesian ilmenite occurs mainly under oxidizing conditions, and its presence in kimberlite does not imply diamond-preserving reduced conditions. Our results indicate that, as temperature decreases below 1200 °C, olivine ± pyroxene fractionation becomes inevitable, and low oxygen fugacity promotes a higher extent of mineral fractionation. At each pressure, kimberlite magma becomes progressively lower in SiO2 and evolves towards carbonatitic compositions during cooling, regardless of redox conditions. This suggests that carbonatite derived from kimberlite magma fractionation may stagnate within the lithospheric mantle at ~ 60-100 km depth.