中国东部陆下岩石圈地幔中的再循环地壳流体组分

Recycled Crustal Volatiles in the Subcontinental Lithospheric Mantle beneath Eastern China

地幔捕虏体中的流体组分记录了地幔演化的信息,可用来认识地幔中再循环地壳组分的性质和来源。采用分步加热质谱法测定中国东部二辉橄榄岩捕虏体组成矿物中流体挥发分的碳、氢和氧同位素组成,结合化学组成(Zhangetal.,2004)综合分析表明,早期流体包裹体、矿物晶格缺陷和空隙中的流体挥发分主要在高温段(800～1200℃)释出,CO2和CO显示较轻的δ13C值,与世界其它地区地幔捕虏体明显不同;晚期流体包裹体中的流体挥发分主要在低温段(300～600℃)释出。根据中国东部地幔演化事件和地幔矿物性质认为陆下岩石圈地幔中保存的三种的流体挥发分中均存在来源不同的再循环地壳组分:①地幔初始流体:主要为耐熔矿物橄榄石在800～1200℃释出的流体组分。华北克拉通地幔初始流体的化学组成(主要为CO)与华南克拉通(主要为CO2)明显不同,反映二者拼合前各自演化时期捕获的流体组分。地幔特征的δD、δ13CCH4和δ18OCO2反映初始流体组分可能为元古代克拉通大陆型岩石圈地幔成分,CO2和CO较轻的δ13C值揭示初始流体中存在部分再循环地壳流体,可能在1.9Ga的大陆碰撞过程中混入。②地幔交代流体:指斜方辉石和单斜辉石800～1200℃释气峰的流体组分,以H2为主。华北克拉通交代流体主要组分H2的含量(80.73mm3/g)明显低于华南克拉通(138.91mm3/g),地幔特征的δDH2和δ13CCH4表明交代流体主体为地幔来源,较轻的δ13CCO2,CO和较重的δ18OCO2揭示其中存在再循环陆壳流体,其可能来源于华北与华南中生代拼合过程中的壳幔相互作用。③岩石圈减薄流体:指二辉橄榄岩捕虏体组成矿物在400～600℃释放出的流体,华北与华南克拉通在流体组成方面相似,壳源特征的CO2、CO和CH4的δ13C值,以及较轻的δ18OCO2和δDH2O值指示该流体可能为岩石圈地幔减薄过程中引起的再循环洋壳流体,可能与中国东部转换带中水平俯冲的太平洋岩石圈(或其前身)脱气有关。

The volatiles in mantle xenoliths can provide the clues for mantle evolution and are used to reveal the nature and origin of recycled crustal volatiles（RCVs） in Earth＇s mantle.The carbon,oxygen and hydrogen isotopic compositions of volatiles in the olivine（Olv）,orthopyroxene（Opx） and clinopyroxene（Cpx） of the lherzolite xenoliths from eastern China have been measured by stepwise heating mass spectrometers.The results integrated with chemical composition of volatiles（Zhang et al.,2004） show that the volatiles resided in the early-stage fluid inclusions and structural defects and vacancies were mainly released at a high temperature（800～1200℃）,and display lighter δ13C values of CO2 and CO,which are distinct from mantle xenoliths in other places of the world.The volatiles trapped in the late-stage fluid inclusions were released at a low temperature（300～600℃）.The mantle evolution events beneath eastern China and mineral properties show that recycled crustal volatiles with different origins occur in three kinds of fluid volatiles in the subcontinental lithospheric mantle.① Initial volatiles of lithospheric mantle are released at 800～1200℃ from refractory minerals like olivine,and show significant differences in chemical compositions between in the North China craton（NCC,dominated by CO） and in South China craton（SCC,dominated by CO2）,which represent the volatiles trapped in distinct development stage of NCC and SCC prior to their combination.They are characterized by δDH2,H2O,δ13CCH4 and δ18OCO2 of mantle origin with lighter δ13C values of CO2 and CO,suggesting that a little of RCVs mixed into initial volatiles of old craton continental mantle during Proterozoic continental collision（1.9Ga）.②The mantle metasomatism volatiles are released at 800～1200℃ from Opx and Cpx and dominated by H2.H2 contents in NCC（80.73 mm3/g） are significantly lower than those（138.91 mm3/g） in SCC.δDH2 and δ13CCH4 with mantle characteristics indicate that the main part of metasomatic volatiles are derived mainly from mantle,while lighter δ13C values of CO2 and CO,heavier δ18O of CO2 suggest that a recycled crustal fluid with terrigenous features have been involved possibly by a crust-mantle interaction during the Mesozoic combination process of NCC and SCC.③The volatiles of subcontinental lithosphere thinning are released at 400～600℃ from Olv,Opx and Cpx of mantle xenoliths,and composed mainly of CO2 with no significant differences between in NCC and in SCC,suggesting a common development condition between NCC and SCC.They are characterized by crustal δ13C of CO2 and CO,biogenic δ13C of CH4,lighter and variable δ18O of CO2,and lighter δD of H2,implying a recycled oceanic crustal volatile which could be derived from dehydration of the subducted paleo-Pacific lithosphere（or predecessor） that lies horizontally in the transition zone beneath eastern China.