The Chemical And Isotopic Compositions And Their Origin Of Upper Mantle Volatiles In Eastern China

The volatiles in Earth's mantle are important agency in material and energy transfer on earth interior, and play the important roles in the fractionation and evolution processes of the Earth's mantle and are derived from various reservoirs. Mantle xenoliths hosted in Cenozoic alkali basalts in eastern China are relicts of lithospheric mantle after mantle partial melting, metasomatism etc. deep processes, in which initial volatiles of upper mantle, metasomatic volatiles and recycled crustal volatiles etc are preserved without surface air contamination and can provide successful probe of mantle fluids. The chemical, stable isotopic and noble gas isotopic compositions of these volatiles have made important contributions to understanding the origins and nature of volatiles in Earth's mantle. The unaltered lherzolite xenoliths were collected from Wudalianchi (WL), Jiaohe (JH), Huinan (HN), Damaping.(MP), Dafangshan (DF), Liuhefangshan (LF), Xilong (XL), Mingqing (MQ) and Mingxi (MX) in eastern China, and the stepwise-heating volatile extraction mass-spectrometry method has been extensively used for concentration and isotope analysis of volatiles in mantle xenoliths. An online vacuum stepwise-heating MAT-271 mass spectrometer system was employed to determine the chemical compositions of the volatiles in mineral separates in lherzolite and pyroxenite xenoliths, carbon isotopes of CO₂, CO and CH₄ were analyzed by a GC-C-Delta plus XP mass spectrometer system employing a stepwise heating extraction procedure, hydrogen isotopes of H₂O and H₂ and carbon and oxygen isotopes of CO₂ were analyzed by the vacuum stepwise heating zinc metal reducing method using MAT252 and MAT251 mass spectrometers, and the online vacuum stepwise heating mass spectrometer (MM5400) are employed to determine noble gas elemental abundances and isotopic compositions of mineral separates. The types, nature and origin of lithospheric mantle volatiles beneath eastern China were revealed based on the chemical, stable isotopic and noble gas isotopic compositions of these volatiles released from constitute minerals at different heating temperature, and following conclusions were obtained.1. Volatiles are hosted in silicate mineral structures and different type of fluid inclusions, Fluid inclusions in constitute minerals of lherzolite and pyroxenite xenoliths from eastern China can be divided into two types based on shape, size and distribution. Type-1 fluid inclusions are characterized by oval or negative crystal etc regular shapes, small size (0.005-0.02mm across) and...