The Zoning Characteristics And Genesis Of Dendritic Olivine In Datong Basalt

The Datong volcano groups is located in Datong basin,formed in the Pleistocene.Previous studies on the basalt of Datong volcanic groups mainly focused on geochemistry,but few on petrography and genetic mineralogy,the dendritic olivine in Datong basalt has not even been studied or reported.In this study,the dendritic olivine was studied from the aspects of petrology,mineralogy and petrogeochemistry.Genesis and characteristics of chemical zoning preserved in olivine crystals have been used to track the pre-eruptive history and the time scale of the Datong basaltic magmas.P?phosphorus?is incompatible element in olivine.However,high amounts of P can be incorporated when its diffusion rate in the melt is surpassed by the advance of the growth rate.Therefore,the rhythmic enrichments of P can represent the direction of early growth.X-ray electron probe analyzes were performed to a selected dendritic olivine.Using P zone versus false-color image which represent Fo zone,results show that the Fo zone pattern is highly consistent with the P content,which means the zone pattern may formed by crystal growth.Electron probe results show that the compositional zoning of dendritic olivine in Datong basalts is significant different with xenocrysts that caused by diffusion.CaO is higher in dendritic olivine when both Fo are in same vaule.Both dendritic olivine and xenocrysts has same content of NiO,but the trend between NiO and Fo are different,the NiO content of xenocrysts has approximate linear correlation between Fo,and correlation between NiO and Fo of dendritic olivine is different with xenocrysts,the content of Al₂O₃ in dendritic olivine has positive correlation with Fo,while the content of Al₂O₃ in xenocrysts are nearly constant.So we decide the zone pattern in dendritic olivine may formed by growth rather than diffusion.Using PRIMELT3 MEGA software to simulate the composition of olivine crystallized from primary magma through major element of Datong basalts,results show that the compositional zoning of dendritic olivine in Datong basalts is consistent with the simulation,it proved that the compositional zoning of dendritic olivine can be formed by growth.The peridotite xenoliths,xenocrysts and dendritic olivine are found in Datong basalts,and growth contribute the zone pattern of dendritic olivine.We assume that basaltic magmas rises to the surface directly and is not influenced by magmatic activity.