Mantle-Derived Cargo vs Liquid Line of Descent: Reconstructing the P–T–fO2–X Path of the Udachnaya–East Kimberlite Melts during Ascent in the Siberian Sub-Cratonic Lithosphere

Abstract

Abstract Disentangling the physico-chemical evolution of the melts that give rise to kimberlites during their genesis, ascent through the sub-cratonic lithosphere and emplacement in the crust is challenging. This is because the extensive entrainment and assimilation of, and reaction with, mantle-derived material makes kimberlites mixtures of xenocrystic, magmatic components and alteration minerals, rarely preserving evidence of their original melt composition. Here, a detailed textural and compositional study of coherent and volcaniclastic kimberlite units from the Udachnaya–East pipe (Siberian craton) was performed to reconstruct the pressure–temperature–oxygen fugacity-compositional (P–T–fO2–X) path of kimberlite melts during their ascent through the sub-cratonic lithosphere. Routine and high-precision electron microprobe analyses of olivine, phlogopite and Fe–Ti oxides enabled to discriminate the mantle-derived cargo from the magmatic components, and thus make inferences on the liquid line of descent (LLD) of proto-kimberlite to kimberlite melts. Most of the olivine cores in Udachnaya–East kimberlite are xenocrystic and record conditions of last equilibration in the Siberian sub-cratonic lithospheric mantle at T–P ranging from 812–1227°C at 3.1–5.4 GPa, to 871–1170°C at 4.6–7.0 GPa, depending on the chosen model geotherm (35 mW/m2 vs 40 mW/m2). Based on their Ni, Cr, Mn, Al and P vs Mg/Fe systematics, olivine core populations were associated to the sheared, granular garnet-bearing or spinel-bearing cratonic peridotites. The occurrence of olivine Internal Zones (I) having the same composition as Fe-rich cores, as well as rare mantle-derived xenocrystic cores of phlogopite, bear witness of mantle metasomatism preceding kimberlite ascent. The assimilation of mantle material by initially H2O- and P-rich proto-kimberlitic melts is recorded by the cotectic formation of magmatic olivine Internal Zones (II) and phlogopite Internal Zones around resorbed xenocrystic cores. The LLD then evolved by inducing cotectic precipitation of olivine, phlogopite rims and Cr-spinel at P of 1.5–3.0 GPa, T of 1120–1250°C and fO2 from −2.8 to −1.6 ΔFMQ. Ilmenite–magnetite pairs in the groundmass record the later conditions of crystallization that occurred at P of ~1.0 GPa, T from 1133°C down to 1000°C and fO2 of +0.3 to +0.9 ΔFMQ. These results confirm the progressive oxidation of the melt that finally led to the formation of calcite, apatite and Mg-rich olivine and phlogopite rinds. The F-enrichment of phlogopite rinds reflects a late decrease of the H2O/CO2 activity of the melt that evolved toward alkali-carbonated composition. Our results showed that the crystal cargo of kimberlites can be used to track all processes acting in between the melt-rock reactions in the mantle and the emplacement in the crust.