双辽新生代玄武岩及地幔捕虏体内流体的组成、碳同位素特征及其来源

Chemical and carbon isotopic compositions of volatiles in Shuangliao Cenozoic basalts and related mantle xenoliths:Implications for origins of volatiles

幔源玄武岩及地幔捕虏体中所含的地幔流体挥发分对于认识岩浆源区特征及岩石圈地幔演化具有重要意义。本文对东北地区双辽新生代玄武岩及其携带的地幔捕虏体中流体挥发分进行研究,探究地幔流体特征与来源。双辽玄武岩橄榄石斑晶和地幔捕虏体内发育早期、晚期两类流体包裹体,早期流体包裹体中的挥发分代表矿物形成时捕获的岩浆原始流体挥发分,主要在高温段(600~1200℃)释出,晚期流体包裹体中的挥发分代表矿物形成后捕获的交代或蚀变流体挥发分,主要在低温段(200~600℃)释出。地幔捕虏体原始流体挥发分主要为H_2O、CO和H_2,其次为CO_2,并含有少量CH_4等烃类。地幔捕虏体交代流体挥发分与双辽玄武岩橄榄石斑晶原始流体挥发分相似,主要为CO_2和H_2O,其次为CO和H_2,含有少量烃类。地幔捕虏体原始流体挥发分、交代流体挥发分及玄武岩橄榄石斑晶原始流体挥发分中,CO_2和烃类碳同位素均具有有机成因特征,表明岩石圈地幔及玄武岩岩浆源区内均存在有机质热解产物的混入,玄武岩岩浆源区及岩石圈地幔交代流体中的有机成因挥发分主要来自于俯冲太平洋板片之上的沉积有机质。双辽玄武岩斜长石斑晶和基质流体挥发分含量较高,主要为H_2O和CO_2,相对于橄榄石原始流体挥发分具有较轻的CO_2碳同位素组成,这种现象表明岩浆上升过程中发生了明显的脱气作用。

The mantle fluids hosted in basalts and mantle xenoliths are important for us to understand characteristics of magmatic source and evolution of lithospheric mantle. We have investigated the features of volatiles extracted from basaltic rocks and related xenoliths collected from Shuangliao volcanic field in order to reveal the characteristics and geneses of mantle fluids. Two types of fluid inclusions can be recognized in olivine phenocrysts of Shuangliao basalts and constituent minerals of xenoliths：the early stage and late stage fluid inclusions. The volatiles in early stage of inclusions were trapped during crystallization of minerals and represent the primary volatiles of magmas. The volatiles in late stage of inclusions were trapped by minerals during alteration and metasomatism processes and represent the secondary volatiles. The volatiles in early and late stage of inclusions were released at high temperature interval （600~1200℃） and low temperature interval （200~600℃）, respectively. Primary volatiles of xenoliths are composed mainly of H2O, CO and H2 with minor CO2 and CH4. Metasomatic volatiles of xenoliths and primary volatiles in olivine phenocrysts are dominated by H2O and CO2 with minor CO, H2 and CH4. The primary and metasomatic volatiles in mantle xenoliths and primary volatiles in olivine phenocrysts all contain biogenic CO2 and hydrocarbons. It indicates that biogenic volatiles from organic matters were introduced into lithospheric mantle and magma source of Shuangliao basalts. The biogenic volatiles in magma source of Shuangliao basalts and metasomatic fluids in lithospheric mantle were most likely derived from the sedimentary organic matters in the subducted Pacific slab. The plagioclase phenocrysts and matrix of Shuangliao basalts contain much more volatiles than minerals mentioned above. The volatiles are composed mainly of CO2 and H2O. The δ13C values of CO2 are much lower than that extracted from olivine phenocrysts, which can be interpreted as a result of degassing during ascent of magma.