湖南宁远早侏罗世玄武岩中橄榄岩包体的含水性和元素地球化学特征

Water content and element geochemistry of peridotite xenoliths hosted by Early-Jurassic basalt in Ningyuan,Hunan Province

运用电子探针(EMP)和激光熔蚀等离子体质谱(LA-ICPMS)对湖南宁远早侏罗世玄武岩中的橄榄岩包体矿物进行了主要元素和微量元素的系统分析,结果表明这些橄榄岩是经历了小程度部分熔融的原始地幔残留,并经历了后期交代作用的影响,硅酸盐熔体可能是重要的交代介质。运用显微傅立叶变换红外光谱技术(Micro-FTIR)对宁远橄榄岩中的单斜辉石和斜方辉石进行了详细的观察,结果显示两种辉石均含有以OH缺陷形式存在的结构水,其含量(H2O的质量分数,下同)分别为147×10-6～461×10-6和40×10-6～126×10-6。根据矿物百分含量计算的全岩水含量为34×10-6～108×10-6,除1个样品外,其余样品的水含量均>50×10-6。结合文献中的资料看来,由橄榄岩包体所代表的宁远中生代岩石圈地幔的含水量要明显高于华北克拉通新生代岩石圈地幔的含水量(多<30×10-6)。宁远中生代岩石圈地幔和华北新生代岩石圈地幔之间的差异反映的可能是中国东部岩石圈地幔含水性的时代演化,即伴随着岩石圈减薄的进行,上涌软流圈的热烘烤使得岩石圈地幔的水含量不断降低。

Abstract： Even a small amount of water can have a disproportionately large effect on physical and chemical properties of minerals or rocks, and hence knowledge of its speciation, concentration and distribution is crucial for understanding the composition, structure, physical and chemical characteristics and geodynamics of the lithospheric mantle. Peridotite xenoliths hosted by alkali magma are representative samples from the lithospheric mantle, and their water content may provide information about the distribution of water in the lithospheric mantle. Peridotite xenoliths are hosted by both Mesozoic and Cenozoic basalts in eastern China. Although some research work has been done for peridotite xenoliths hosted by Cenozoic basalts, investigations on peridotites hosted by Mesozoic basalts remain insufficient. This paper presents major and trace element concentrations of minerals from peridotite xenoliths hosted by Early Jurassic basalts in Ningyuan, Hunan Province, which were obtained by EMP and LA-ICPMS respectively. Covariations between major and trace elements suggest that the peridotite xenoliths are relicts of primary mantle after low-degree partial melting. Elevated content of highly incompatible elements （Th, U, La, Ce etc. ） in some clinopyroxenes demonstrates that the Ningyuan lithospheric mantle has undergone mantle metasomatism. Based on low La/Yb and high Ti/Eu ratios of clinopyroxenes, the authors hold that the metasomatic agent was silicate melt rather than carbonatite melt. Micro-FTIR analysis demonstrates that both clinopyroxene and orthopyroxene display prominent absorption bands in 3 000-3 700 cm^-1 region, which resulted from hydroxyl defect in their structure. The calculated water content （H20 wt. ） of clinopyroxene and orthopyroxene ranges from 147×10^-6 to 461×10^-6 and 40 ×10^-6 to 126×10^-6 respectively. Based on the water content and mineral model, it is estimated that the whole-rock water content of the Ninyuan peridotites should vary in the range of 34×10^-6-108×10^-6. In combination with the previously reported data, it is held that the water content of the Mesozoic lithospheric mantle of Ningyuan is much higher than that of the Cenozoic lithospheric mantle of the North China Craton （mostly 〈30×10^-6）. The low water content of the Cenozoic lithospheric mantle in the North China Craton is attributed to the heating action by an upwelling asthenosphere flow in consistent with NCC lithospheric thinning during the Late Mesozoic to Early Cenozoic period. Therefore, the difference between the Ningyuan Mesozoic lithospheric mantle and the Cenozoic NCC lithospheric mantle probably reflects the temporal evolution of water content in the lithospheric mantle of eastern China.