滇东南马关木厂新生代玄武岩中橄榄岩包体的含水性

对来自滇东南马关木厂的尖晶石二辉橄榄岩中名义上无水矿物(NAMs)进行显微傅里叶变换红外光谱(Micro-FTIR)分析,结果显示,单斜辉石、斜方辉石和橄榄石中均含有以羟基形式存在的结构水,单斜辉石的水含量为160×10~(-6)~557×10~(-6)(质量分数,下同),斜方辉石的水含量为85×10~(-6)~207×10~(-6),橄榄石的水含量为5×10~(-6)~12×10~(-6),根据矿物百分比含量估算的全岩水含量为46×10~(-6)~137×10~(-6);元素地球化学特征表明,本次研究的橄榄岩包体是岩石圈地幔经历较低程度部分熔融的残余;低(La/Yb)N值(0.22~0.57)以及高Ti/Eu比值(4 076~6 772)暗示橄榄岩可能经历了以硅酸盐熔体为交代介质的微弱地幔交代作用;单斜辉石的微量元素组成比较单一,整体表现出高场强元素、大离子亲石元素以及轻稀土元素的同步亏损;结合中国东部地区已经发表的橄榄岩包体含水量数据来看,滇东南马关木厂岩石圈地幔具有明显富水的特征,可能与该区自中生代以来遭受的新特提斯洋壳大规模俯冲流体交代作用有关;与华北克拉通含水量的明显差异有可能反映的是两地岩石圈地幔正处于不同的演化阶段。

Water content of peridotite xenoliths in Cenozoic basalt from Muchang of Maguan, southeastern Yunnan Province

Nominally anhydrous minerals, i.e., clinopyroxene (Cpx), orthopyroxene (Opx) and olivine (Ol), from peridotite xenoliths hosted by Cenozoic basalts in Muchang, Maguan, southeastern Yunnan Province, were investigated by micro-Fourier transform infrared spectrometry (FTIR). Most of Ol and all the Cpx and Opx grains contain structural water in the form of OH. The range of the average core water content (H₂O) of multi grains (>10) in the same sample from the analyzed Muchang peridotites is 160×10^-6~557×10^-6, 85×10^-6~207×10^-6 and 5×10^-6~12×10^-6 for Cpx, Opx and Ol respectively. The whole-rock water content recalculated according to mineral volume proportions varies from 46×10^-6 to 137×10^-6, much richer than that in the North China Craton. Elemental fractionation suggests that the peridotite xenoliths are relicts of primary mantle with partial melting. Low (La/Yb)_N (0.22~0.57) and high Ti/Eu (4 076~6 772) of clinopyroxenes suggest that Muchang lithospheric mantle has undergone low mantle metasomatism by silicate melt. Apparently, the large-scale Neo-Tethys oceanic crust subduction and fluid metasomatism contributed to the rich water in Muchang, which implies that the mantle in Muchang was the new proliferative mantle and the geological evolution differed from that of the North China Craton in Cenozoic.