大陆下地壳和岩石圈地幔含水性的差异——山东莒南玄武岩中深源包体的初步观察

对产于莒南晚中生代玄武岩中的镁铁质麻粒岩和橄榄岩包体矿物进行了傅里叶变换红外光谱(FTIR)分析.结果显示,麻粒岩矿物和全岩中水含量分别为:单斜辉石300×10-6～1 180×10-6,斜方辉石80×10-6～169×10-6,斜长石717×10-6～1 239×10-6,全岩525×10-6～855×10-6;橄榄岩矿物和全岩中水含量分别为:单斜辉石466×10- 6～746×10-6,斜方辉石187×10-6～304×10-6,橄榄石6×10-6～15×10-6,全岩81×10-6～245×10-6.从单矿物看,麻粒岩和橄榄岩之间水含量的差距不是很明显,但麻粒岩的全岩水含量明显高于橄榄岩,表明大陆深部岩石圈的水含量在垂向上具有不均一性.

Water content difference between continental lower crust and lithospheric mantle: granulite and peridotite xenoliths hosted in basalts from Junan, Shandong Province

The deep continental lithosphere,which includes the lower crust and the lithospheric mantle,is a very important layer in the earth’s interior.The minerals comprise lower continental crust and lithospheric mantle are mainly nominally anhydrous minerals(NAMs) such as clinopyroxene,orthopyroxene,feldspar,olivine and garnet,and could contain very small and variable amounts of water in the structure as hydrogen-related minor defects;nevertheless,such trace amounts of water can disproportionally affect many physical and chemical properties and play a vital role in geodynamical processes.Knowledge of water distribution in the deep earth may help us understand the interior processes and the evolution of the earth.In this study,using Fourier transform infrared spectrometer(FTIR) analysis,the authors obtained the water content of such minerals as clinopyroxene,orthopyroxene and plagioclase in continental lower crustal granulite xenoliths and mantle peridotite xenoliths hosted by alkali basalts in Junan,Shandong Province.H2O content ranges from 300×10-6 to 1 180×10-6 for clinopyroxene,80×10-6 to 169×10-6 for orthopyroxene,717×10-6 to 1 239×10-6 for plagioclase,and 525 ×10-6 to 855×10-6 for bulk rock in continental lower crustal granulite xenoliths;and from 466×10-6 to 746×10-6 for clinopyroxene,187×10-6 to 304×10-6 for orthopyroxene,6×10-6 to 15×10-6 for olivine,and 81×10-6 to 245×10-6 for bulk rock in mantle peridotite xenoliths.The measured water concentrations of the minerals and bulk rock can roughly reflect the distribution of water in its sources,as evidenced by the following observations: ① EMP and FTIR measurements show no inter-and intragrain heterogeneities for individual minerals in each sample;② the cores of the clean,crack-and inclusion-free grains of relatively larger sizes were selected for FTIR measurements;③ the studied samples were usually fresh and subjected to no alteration.In agreement with previous investigations of similar samples from Hannuoba and Nushan in North China Craton,water content of the continental lower crust is obviously higher than that of the underlying lithospheric mantle.