祁连造山带玉石沟地幔橄榄岩中挥发分的流体化学和稳定同位素组成及古大洋岩石圈演化意义  

Chemical and stable isotopic compositions of volatiles in mantle peridotites of the Yushigou ophiolite in Qilian orogenic belt,western China

作　　者：胡沛青 张铭杰 汤中立 LI Chusi 王记周 

机构地区：[1]Lanzhou Univ,Key Lab Environm Syst Western China,MOE,Lanzhou 730000,Peoples R China [2]Indiana Univ,Dept Geol Sci,Bloomington,IN 47405 USA

出　　处：《岩石学报》2007年第1期169-176,共8页Acta Petrologica Sinica

摘　　要：Fluid inclusions in olivine and orthopyroxene of mantle peridotites from the Yushigou ophiolite can be divided into three types based on decrepitation temperature,shape and distribution.Type-1 fluid inclusions are characterized by oval or negative crystal shapes and small size(<5μm across).They occur in the cores and mantles of the host crystals,and decrepitated at>840℃.Type- 2 fluid inclusions have irregular or tabular shapes with relatively large size(10～100μm in length).They occur in irregular or circular healed micro-fractures in the host crystals,and decrepitated at 612～710℃.Type-3 fluid inclusions have size and shape,similar to type-2 fluid inclusions but occur in micro-fractures restricted to the margins of the host crystals,and decrepitated at much lower temperature from 190℃to 340℃.The three different types of fluid inclusions are interpreted to represent primary,metasomatic (pseudo-secondary)and secondary inclusions,respectively.Stepwise heating reveals three concentration peaks of volatiles at 200～400℃,400～800℃and 800～1200℃released from olivine and orthopyroxene in harzburgite and dunite from the Yushigou ophiolite, which are considered to correspond to the decrepitation of secondary,metasomatic and primary fluid inclusions at similar temperature ranges.CO2 is a major constituent in the volatiles released at three different temperature intervals.Trace amounts of H_2 and N_2 are present in the volatiles released at<800℃and trace amounts of H_2O and SO_2 are mainly present in the volatiles at 400～800℃.TheδD(-95.2‰,-306.3‰)of H_2O and theδ^(13)C(-15.5～-12.5‰)andδ^(18)O values(1.4～1.9‰)of CO_2 released at<800℃are lower than normal mantle values and suggest the mixing origin of crustal fluids( sedimentary organic)with ocean water,implying that Yushigou AOLM had undergone an intensive metasomatism by a fluid composed of CO_2.H_2O and SO_2,and followed by degassing. In contrast,the volatiles released at 800～1200℃are characterized by trace amounts of H_2 andFluid inclusions in olivine and orthopyroxene of mantle peridotites from the Yushigou ophiolite can be divided into three types based on decrepitation temperature, shape and distribution. Type-1 fluid inclusions are characterized by oval or negative crystal shapes and small size （ 〈 5/μm across）. They occur in the cores and mantles of the host crystals, and decrepitated at 〉 840℃. Type-2 fluid inclusions have irregular or tabular shapes with relatively large size （ 10 - 100/μm in length ）. They occur in irregular or circular healed micro-fractures in the host crystals, and decrepitated at 612 -710℃. Type-3 fluid inclusions have size and shape similar to type-2 fluid inclusions but occur in micro-fractures restricted to the margins of the host crystals, and decrepitated at much lower temperature from 190℃ to 340℃. The three different types of fluid inclusions are interpreted to represent primary, metasomatic （pseudo-secondary） and secondary inclusions, respectively. Stepwise heating reveals three concentration peaks of volatiles at 200～ 400℃ , 400 ～ 800℃ and 800～1200℃ released from olivine and ortopyroxene in harzburgite and dunite from the Yushigou ophiolite, which are considered to correspond to the decrepitation of secondary, metasomatic and primary fluid inclusions at similar temperature ranges. CO2 is a major constituent in the volatiles released at three different temperature intervals. Trace amounts of H2 and N2 are present in the volatiles released at 〈 800℃ and trace amounts of H20 and SO2 are mainly present in the volatiles at 400 -800℃. The 6D （ - 95.2%o, - 306.3%0） of H20 and the 613 C （ - 15.5 - - 12.5%o） and 6TM O values （ 1.4 - 1.9%o） of CO2 released at 〈 800℃ are lower than normal mantle values and suggest the mixing origin of crustal fluids （sedimentary organic） with ocean water, implying that Yushigou AOLM had undergone an intensive metasomatism by a fluid composed of CO2. H2 and SO2, and followed by degassing. In contrast, the volat

关 键 词：祁连造山带 玉石沟地幔 橄榄岩 挥发分 流体化学 稳定同位素组成 古大洋 岩石圈 演化意义 

分 类 号：P588.125[天文地球—岩石学]