Bushveld symplectic and sieve-textured chromite is a result of coupled dissolution-reprecipitation: a comparison with xenocrystic chromite reactions in arc basalt

Textures of Bushveld chromite from thin seams and accessory disseminations in the Platreef and the northernmost Waterberg Project area were compared with textures of xenocrystic chromite from mantle xenoliths found in Neogene basalt in the Kurile Island Arc. The sieve-textured to symplectic rims around the resorbed chromite in the Kurile samples resulted from the reaction between chromite and chromite-undersaturated basaltic melt, with the inclusions in chromite being entrapped during episodes of chromite primary growth, chemical dissolution, and reprecipitation or secondary growth. The relics of the lattice-oriented etch tunnels suggest that the dissolution preferentially developed along the crystallographic planes and defects. The Bushveld chromites exhibiting similar textures are interpreted as reaction-textured chromites, by analogy with the Kurile samples. The Bushveld sieve-textured, fish-hook to symplectic and amoeboidal to atoll-like chromites, are believed to have been formed due to coupled dissolution-reprecipitation of the earlier cumulus or xenocrystic chromite during interaction with chromite-undersaturated evolved melt. The electron backscattered diffraction data confirm the same single-crystal crystallographic orientation of all domains of the reaction-textured chromites as well as their clustered semi-dissolved relics. Therefore, Bushveld inclusion-rich chromite might have captured different populations of melt inclusions during its discontinuous out-of-equilibrium growth with fast episodic resorption and regeneration. The occurrence of reaction-textured chromites indicates a zone of interaction between dynamic magmatic influxes where chemical equilibrium was not achieved whereas a complete re-equilibration between chromite and the stagnant and sequestered interstitial liquid was attained during the formation of the massive chromitites.