Experimental study on reactions between alkaline basaltic melt and orthopyroxenes: constraints on the evolution of lithospheric mantle in the North China Craton

The experimental results of the reactions between an alkaline basaltic melt and mantle orthopyroxenes under high-temperature and high-pressure conditions of 1300–1400 °C and 2.0–3.0 GPa using a six-anvil apparatus are reported in this paper. The reactions are proposed to simulate the interactions between melts from the asthenospheric mantle and the lithospheric mantle. The starting melt in the experiments was made from the alkaline basalt occurring in Fuxin, Liaoning Province, and the orthopyroxenes were separated from the mantle xenoliths in Damaping, Hebei Province. The results show that clinopyroxenes were formed in all the reactions between the alkaline basaltic melt and orthopyroxenes under the studied P–T conditions. The formation of clinopyroxene in the reaction zone is mainly controlled by dissolution–crystallization, and the chemical compositions of the reacted melt are primarily influenced by the diffusion effect. Temperature is the most important parameter controlling the reactions between the melt and orthopyroxenes, which has a direct impact on the melting of orthopyroxenes and the diffusion of chemical components in the melt. Temperature also directly controls the chemical compositions of the newly formed clinopyroxenes in the reaction zone and the reacted melt. The formation of clinopyroxenes from the reactions between the alkaline basaltic melt and orthopyroxenes can result in an increase of CaO and Al₂O₃ contents in the rocks containing this mineral. Therefore, the reactions between the alkaline basaltic melt from the asthenospheric mantle and orthopyroxenes from the lithospheric mantle can lead to the evolution of lithospheric mantle in the North China Craton from refractory to fertile with relatively high CaO and Al₂O₃ contents. In addition, the reacted melts in some runs were transformed from the starting alkaline basaltic into tholeiitic after reactions, indicating that tholeiitic magma could be generated from alkaline basaltic one via reactions between the latter and orthopyroxene.