【作者】 马锦龙；

【导师】 陶明信；

【作者基本信息】 中国科学院研究生院（兰州地质研究所） ， 地球化学， 2002， 博士

【摘要】 地幔流体在研究地幔物质组成、物质来源和演化等方面有重大意义，其中的稀有气体则是研究相关问题的重要方法和有效途径之一，二者均为国际地学的热点领域。玄武岩、特别是其中的幔源包体和巨晶是目前获取地幔流体的最理想样品，国内已有较多此类样品中流体研究的成果，但稀有气体的研究尚处于初步阶段。笔者通过野外考察和采样，对沂沭断裂带及其邻近地区的该类样品做了流体和稀有气体同位素研究，取得如下成果和认识： 1、通过分步及一步加热法在MAT-271质谱仪上对沂沭地区幔源包体做了流体组成测试与研究。结果表明，研究区幔源包体的流体组成以CO₂、CO和H₂为主，次为CH₄、N₂等气体；气体主要组分的释出特征是：CH₄总在500℃出现释气峰，H₂的释出量随温度升高而增加，CO₂的释出的总趋势是随温度升高，释出量降低，多数样品中CO在500～650℃出现释气低谷。分析表明，不同地区或不同类型包体间流体组成特征的差异可能与其所处环境的氧逸度(f₀₂)有关；分步加热中流体组成特征的变化则反映了地幔流体演化的阶段性。 2、利用熔融法在MM5400质谱仪上对沂沭地区幔源岩类的稀有气体做了较系统的研究。结果表明，沂沭地区地幔包体（全岩、单矿物）的³He／⁴He值主要介于0.115～2.7Ra之间，巨晶为3.4～11.3Ra，寄主玄武岩³He／⁴He值极低(均为10^-8量级)；各类岩矿⁴⁰Ar／³⁶Ar值介于296.5～864间，且多在400以下，以玄武岩略高；所有岩矿的²⁰Ne／²²Ne值（10.19～13.10）都高于大气值（9.8）；各类岩矿的Kr、Xe同位素所测出比值均与大气值相似。研究表明，不同岩矿中稀有气体同位素丰度和比值的这种分布特征反映了其各自不同的成因与来源：包体单矿物在He、Ne、Ar同位素的多种图解上均呈现出明显的大气-地幔源的混合特征；包体全岩的稀有气体同位素还反映有放射成因⁴He、⁴⁰Ar的加入；玄武岩则既反映出流体的扩散分馏效果、又反映了放射成因组分的加入。 3、在国内、外数据资料基础上，总结归纳了地幔、沉积壳层和大气三个源区的CO₂、CH₄、N₂等组分与稀有气体的比值指标体系。沂沭地区幔源包体单矿物的CO₂／³He=28～1050×10⁹、CH₄／³He值主要为11～1642×10⁷、N₂／Ar=927～56596、N₂／³He=2.47～26.73×10⁹，在多种综合判识指标图解上均呈现地幔-洋壳-大气三者的混合特征，反映了板块俯冲作用中洋壳和大气对该区地幔流体的影响。 4、对世界范围幔源流体稀有气体数据的分析表明，不同构造环境幔源岩中的轻稀有气体同位素比值明显不同。总体而言，我国东部幔源包体的³He／⁴He值总体上偏低（1～10Ra）、⁴⁰Ar／³⁶Ar值很低（多在1000以下）、²⁰Ne／²²Ne值（9.8～12.0）具明显的幔源特征，与受大洋板块俯冲影响的构造环境类似，表明其地幔流体中洋壳及大气的影响。 5、综合上述研究结果及前人研究成果表明，沂沭乃至中国东部幔源包体的流体代表曾受古洋壳及大气混染的“地幔源区”流体。据此，笔者提出了关于中国东部幔源流体的新成因模式：中、新生代古太平洋板块的俯冲，将大量洋壳物质和大气带入中国东部陆下地幔并形成了混合源区。在混合源区演化过程中，不同演化阶段的“地幔流体”在构造作用过程中或以矿物包裹体形式，或通过断裂排气而上升，从而在地表或近地表形成成因上相关的包体流体和气藏。更多还原

【Abstract】 An amount of mantle-derived xenoliths and megacryst, which are excellent samples for research of the mantle, were widely found in Yishu areas of Shandong Province. And more and more attentions of Chinese geologists were drew to the fluids degassed from these rocks because of its importance on the research of mantle structure, compositions and evolving, etc, and the noble gas research of these fluids is a new and useful method for these researches.We analyzed the fluid compositions of mineral separates of xenoliths by spectrograph of MAT271. The primary compositions are CC2 CO and HI in the mineral separates of mantle-derived xenoliths in YiShu areas, and CH4, N2, C2H4, C2H6, etc, take its second place. There are usually more fluids in pyroxene than in olivine for the same xenolith sample, and the fo2 is always different for olivine and pyroxene in the same xenoliths that were sampled from the fault zone, but aren’t at outer of faults. There are only one top for fluid degassing of olivine and pyroxene. The degassing characters of primary compositions are: 500C being always top for CH4; degassing of H2 increasing with increasing temperature, but just the reverse for CO2, Three parts which maybe correlate to the evolving stages of mantle fluids, were distinguished according to the fluid compositions at different temperatures.The noble gas isotopes of whole rocks, mineral separates, megacrysts and basalts were also analyzed by spectrograph of MM5400. The results suggest that there are abundance differences in noble gas isotopes among different rocks and separates, and also among different areas. As a whole, the abundance of 4He, 20Ne, 40Ar 84Kr,132Xe are always highest in the basalts, and often lower in separates, but usually mediate in whole rocks. And 40Ar and 84Kr are most striking in the abundance difference for different areas.As for the ratios of isotopes, 3He/4He are often between 0.115 and 2.7Ra in whole rocks and mineral separates, and always lower in whole rocks than in its mineral separates; 3.4-11.3Ra of 3He/4He were got in megacryst, and 10-8 of 3He/4He, which are as same as mediate ratio of crust, were got in basalts. Ratios of 40Ar/36Ar are usually lower in all rocks and separates, and between 296.5-864, but often inadequate to 400. There are little difference for ratios of Ar/ Ar in different rocks and mineral separates, and the higher ratios were also got in basalts. All ratios of 20Ne/22Ne are higher in the rocks and mineral separates than in the atmosphere, and between 10.19 and 13.10, which are higher than other areas of eastern China, and as same as MORB and OIB. The isotopic ratios of Kr, Xe in rocks and separates are not distinguished from the atmosphere, and no difference in different rocks.The noble gas isotopes suggest that there are some radiogenic 4He and 40Ar in whole rocks; and both radiogenic and fractionated parts were found in basalts. And the fluids of mineral separates, which represent the typical parts of the mantle-derived fluids, show a mixed character of atmosphere-mantle at the isotopic map of He-Ne, He-Ar and Ne-Ar.The studies on the alliance of noble gas and CO2, CH4, N2 are very valid for discussionon the source. Based on the ratios of 3He/4He, CO2/3He, CH4/3He and N2/Ar, we can distinguish three sources of mantle, atmosphere and crust. And these ratios of the mineral separates of Yishu area are successively: CO2/3He=28-1050x109, CH4/3He=11-1642x107, N2/Ar=927-56596, N2/3He=2.47-26.73 x 109, and imply a mixed source at differently allied map.The ratios and abundance of noble gas isotopes of the mantle fluids are constrained by its tectonic-settings. And the characters of the noble gas, especially the elements of He and Ar, are distinct in different tectonic-settings. According to the isotopic ratios of He, Ne and Ar, the tectonic-settings of Middle Ocean Ridge(MORB), Ocean Island(OIB), Subduction Zone, Continental Mantle, Continental Hotspot and Margin of Continental Plate, which separately stand for different sources, may be recognized. And three regions更多还原