High-resolution P wave tomography shows that the subducting Pacific slab is stagnant in the mantle transition zone and forms a big mantle wedge beneath eastern China. The Mg isotopic investigation of large numbers of mantle-derived volcanic rocks from eastern China has revealed that carbonates carried by the subducted slab have been recycled into the upper mantle and formed carbonated peridotite overlying the mantle transition zone, which becomes the sources of various basalts. These basalts display light Mg isotopic compositions(δ26 Mg = –0.60‰ to –0.30‰) and relatively low87 Sr/86 Sr ratios(0.70314–0.70564) with ages ranging from 106 Ma to Quaternary, suggesting that their mantle source had been hybridized by recycled magnesite with minor dolomite and their initial melting occurred at 300-360 km in depth. Therefore, the carbonate metasomatism of their mantle source should have occurred at the depth larger than 360 km, which means that the subducted slab should be stagnant in the mantle transition zone forming the big mantle wedge before 106 Ma. This timing supports the rollback model of subducting slab to form the big mantle wedge. Based on high P-T experiment results, when carbonated silicate melts produced by partial melting of carbonated peridotite was raising and reached the bottom(180–120 km in depth) of cratonic lithosphere in North China, the carbonated silicate melts should have 25–18 wt% CO2 contents, with lower Si O2 and Al2 O3 contents, and higher Ca O/Al2 O3 values, similar to those of nephelinites and basanites, and have higher εNdvalues(2 to 6). The carbonatited silicate melts migrated upward and metasomatized the overlying lithospheric mantle, resulting in carbonated peridotite in the bottom of continental lithosphere beneath eastern China. As the craton lithospheric geotherm intersects the solidus of carbonated peridotite at 130 km in depth, the carbonated peridotite in the bottom of cratonic lithosphere should be partially melted, thus its physical characters are similar to the
Despite the significant improvement on spatial resolution,Nano SIMS still preserves relatively high mass resolution,sensitivity,and analytical precision.It has become an important analytical platform to determine chemical compositions of solid materials,and has been widely used in space,earth,life,and materials sciences,etc.By using a Cs+ion beam with a size as small as 50nm scanning over sample surfaces,we are able to obtain high spatial resolution images of up to 7 species simultaneously.When utilizing Faraday cup,high analytical precision of 0.3‰–0.5‰(1SD)for C,O and S isotopic analysis can be achieved.Although this precision level is still lower than that of conventional SIMS,it already meets the major requirements of Earth Sciences.In 2011,the first Nano SIMS of China(Cameca Nano SIMS 50L)was installed at Institute of Geology and Geophysics,Chinese Academy of Sciences.Based on the working mechanism and analytical modes of the instrument,this paper will systematically introduce the analytical methods established with the Nano SIMS and their potential applications in earth sciences.These methods include trace element distribution images in mineral zoning,high spatial resolution(2–5?m)Pb-Pb and U-Pb dating,water content and H isotopic analysis for silicate glass and apatite,C isotopic analysis for diamond and graphite,O isotopic analysis for carbonate,S isotopic analysis for sulfides.In addition,the specific requirements for sample preparation will also be introduced in order to facilitate domestic earth scientists’use.
YANG WeiHU SenZHANG JianChaoHAO JiaLongLIN YangTing
