Primary composition and evolution of kimberlite melts: A case study from the Chigicherla kimberlites with insights from autolith compositions

Abstract

This study reports the first detailed characterization of an autolith embedded within the macrocrystic kimberlite CC-5 from the Chigicherla kimberlite cluster in the Wajrakarur Kimberlite Field (WKF), Southern India. These autoliths occur as large, globular, fine-grained fragments and are interpreted as early-crystallized kimberlite material. Their aphanitic texture is caused by undercooling during emplacement. The autolith phase is depleted in MgO (19.13 wt%) and SiO₂ (21.41 wt%), moderately enriched in Al₂O₃ (3.36 wt%), TiO₂ (3.79 wt%) and K₂O (0.42 wt%), and shows notable concentrations of P₂O₅ (1.34 wt%), CaO (16.99 wt%) and FeO_T (12.12 wt%). It is enriched in Light Rare Earth Elements (LREE) e.g., La, Ce, Nd, Sm and High Field Strength Elements (HFSE) such as Zr, Nb, Th and Ta. An attempt has been made to unravel the formation history of the autolith (aphanitic kimberlite), and to establish a compositional relationship with the host macrocrystic kimberlite. The major-element composition of the autolith suggests a slightly evolved melt relative to the primary kimberlite compositions. The evolutionary trends in primary kimberlite melts are further evaluated using fractionation and assimilation models. Lever-rule based compositional diagrams such as MgO versus SiO₂, FeO_T, and CaO indicate that ~ 30–35 wt% entrainment of bulk garnet lherzolite is required to reproduce the composition of the macrocrystic kimberlites, whereas up to ~ 45 wt% entrainment is needed for the olivine lamproites. These constraints suggest significant mantle wall-rock incorporation during melt ascent and evolution. Forward rare earth element modelling was conducted to estimate source compositions across 0.4–1.5% partial melting, assuming 2–5 vol% residual garnet in the mantle source. The resulting source composition aligns closely with the peridotite xenoliths from the Dharwar Craton, supporting derivation from a metasomatized sub-continental lithospheric mantle.