安徽女山下地壳麻粒岩包体中的水:红外光谱分析

Structural water in lower crustal granulite xenoliths from Nushan volcano:Micro-FTIR investigations.Acta Petrologica

运用显微傅立叶变换红外光谱技术,对安徽女山新生代玄武岩中的10个麻粒岩包体的主要组成矿物(单斜辉石、斜方辉石、斜长石)进行了观察。结果显示,这些名义上的无水矿物中都普遍含有以 OH^-形式赋存的结构水,其含量(H_2Owt.)分别可以高达～2340ppm(单斜辉石)、～1590ppm(斜方辉石)和～910ppm(斜长石)。不同样品间矿物的结构水含量具有明显的不均一性的分布特点,这很可能是对其原岩性质的一种反映。根据单矿物的水含量和各自的体积分数计算出的全岩水含量为125～963ppm。根据二辉石温度计计算的女山麻粒岩包体的平衡温度为～800～900℃,对应于女山最下部地壳(～25～35km),因此本文数据为大陆下地壳含水与否提供了初步的最直接的证据,并且其水含量可能具有不均一性的分布特点。

Nominally anhydrous minerals （clinopyroxene, orthopyroxene, plagioclase） of granulite xenoliths in Cenozoic basahs from Nushan, eastern China have been investigated by Micro-FTIR. The results indicate that water occurs in all these nominally anhydrous minerals as structural OH with the concentrations up to, expressed by H2O wt. , -2340 ppm for clinopyroxene, - 1590 ppm for orthopyroxene and -910 ppm for plagioclase, respectively. The vast variation of water contents within these minerals among different samples suggests that water distribution of the protoliths may be very heterogeneous. On the basis of water concentrations in the minerals and their proportions, the bulk water contents are calculated to range from 125 to 960 ppm （ H2O wt. ）. Equilibrium temperatures calculated by two-pyroxene thermometers are mostly concentrated in the range of 800 - 900℃, corresponding to the lowermost crust at Nushan （about 25 -35km）. Based on these results, it is suggested that quite amount of hydrogen can be stored within the nominally anhydrous minerals in granulites, which may constitute the dominant reservior of hydrogen in the lower continental crust, and that the estimated amount is around 5 × 10^18 kg H2O. Therefore, this study provides the direct evidence that the lower continental crust is not “dry”.