【作者】 胡沛青；

【导师】 汤中立； 张铭杰；

【作者基本信息】 兰州大学 ， 地质工程， 2006， 博士

【摘要】 地幔流体是地球内部物质和能量交换最活跃的组分，在地幔分异演化等过程中发挥着重要作用。中国东部新生代碱性玄武岩中的地幔捕虏体为岩石圈地幔的残片，其中气体不易散失而且很少受地表大气的污染，保留了地幔源区的最直接的信息，是进行地幔流体研究最直接的样品。地幔捕虏体中流体的化学组成、稳定同位素及稀有气体同位素组成是认识地幔不同类型流体性质、来源的重要手段。本文采集了中国东部五大连池、蛟河、辉南、张家口、大方山、六合方山、明溪等地的二辉橄榄岩捕虏体和辉石岩捕虏体，以二辉橄榄岩捕虏体和辉石岩捕虏体中橄榄石、斜方辉石和单斜辉石为对象，采用分步加热MAT-271质谱计测定了流体的化学组成，MAT-252和GC-C-Delta-plus XP质谱计测定CO₂的碳、氧同位素，以及CO和烷烃的碳同位素，用Zn还原封管法测定了H₂O和H₂的氢同位素、采用MM5400稀有气体质谱计测定了He、Ne和Ar的丰度和同位素组成。根据地幔捕虏体不同组成矿物在不同温度释出流体化学组成特征、稳定同位素特征及稀有气体同位素特征探讨了中国东部陆下岩石圈地幔流体的类型、性质及来源，得出以下科学认识： 1、地幔捕虏体中流体组分的赋存形式主要有不同类型的气液包裹体和矿物晶格中混溶的流体。根据流体包裹体的形态、大小及分布特征可区分出两种类型的流体包裹体：第一类为早期流体包裹体：形状规则，呈球形、卵圆形和负晶形等规则形状，个体较小（0.005-0.02mm），成群产出于矿物的中心或核部，均一温度为861-1074℃。第二类为晚期流体包裹体：形状不规则，呈树枝状和管状，个体较大（0.01-0.1mm），边部存在一些显微裂缝，分布于矿物裂隙或边缘。分步加热实验表明中国东部地幔捕虏体组成矿物中的流体在300-600℃、600-1000℃和1000-1200℃形成三个释气峰，300-600℃低温释气峰释放的流体主要来自边部存在一些显微裂缝的晚期流体包裹；600-1000℃释气峰释放的流体主要来自和均一温度范围相近的早期流体包裹体；1000-1200℃高温释气峰的流体主要来自矿物晶格中的流体。 2、中国东部地幔捕虏体的流体化学组成以H₂、CO₂和CO为主，SO₂、CH₄、H₂S和N₂为次要成分。流体组成随岩石类型、矿物种类和释出温度不同而显示不同特征。二辉橄榄岩捕虏体中橄榄石在800-1200℃释放出的流体以CO、CO₂和H₂为主要成分。斜方辉石和单斜辉石在800-1200℃释放出的流体以H₂和CO为主，平均分别为8.22mm³／g和7.35mm³／g。辉石岩捕虏体组成矿物800-1200℃释出较更多还原

【Abstract】 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更多还原