| Mantle xenoliths of Jiaohe,including spinel pyroxenites,spinel peridotites and garnet pyroxenites xenoliths,were analyzed by EMPA(Electron probe x-ray micro-analyzer),LA-ICP-MS(Laser ablation-inductively coupled plasma-mass spectrometry)and Micro-FTIR(micro-Fourier transform infrared spectroscopy),to evaluate the water distribution of lithospheric mantle of Jiaohe area.Compared to pyroxenites from Hannuoba,Hebei province,the spinel pyroxenites of Jiaohe have clinopyroxene(cpx)Mg#range from 88.4 to 89.3,higher than those of high-pressure cumulative pyroxenites of Hannuoba,but lower than those of spinel peridotites.Spinel pyroxenites also behave differently from high-pressure cumulative pyroxenites and spinel peridotites in the covariance diagram between Ti O2and Mg#in cpx,but with the characteristics similar to Cr-pyroxenites in Hannuoba.Such a phenomenon indicates that the spinel pyroxenites in Jiaohe were fromed by metasomatism.But the variation of water content and H2O/Ce ratio of Jiaohe spinel pyroxenites show a narrower range than that of Hannuoba Cr-pyroxenites.Such a discrepancy between Jiaohe spinel pyroxenites and Hannuoba Cr-pyroxenites may be related to the degree of metasomtasim they experienced.The Cr2O3?Al2O3 contents of cpx in spinel peridotites,which range 1.7 to 14.2 wt.%,well correlated with their Mg#.These correlations are attributed to partial melting.The water contents and H2O/Ce ratio(which range from 8.0 to 66.0 ppm,22.0 to 325.4 distinctively)of these cpx shows no rigorous relation with thier Mg#.The(Ba/Th)N(“N”indicate normalized with primitive mantle vauses)in cpx correlates with the H2O/Ce of whole rock,which indicates that fluid-related metamsotism can increase the H2O/Ce ratio of mantle rocks.Based on the positive Eu anomaly and disequilibrium O isotopic fractionations resembling olivine-bearing cumulative gabbros,the garnet pyroxenites are considered as remnants of oceanic lower crust.The water content of the garnet pyroxenites derived from remnants of oceanic lower crust is 55.6-100.9 ppm,and the H2O/Ce ratio is 37.0-742.39.Compared with the pyroxenites of Hannuoba and Hawaii,the garnet pyroxenites in Jiaohe have low water content and extremely wide H2O/Ce ratio.Such features may be formed in the subduction,which can drive dowm its water content by dehydration,or need low H2O/Ce ratio and low water content end member addtion.Comparing with water content and H2O/Ce ratio of melts(those melts equilibriumed with xenoliths and formed by their patial melting)related with spinel pyroxenites,spinel peridotites and garnet pyroxenites in Jiaohe,the high H2O/Ce ratio end member of basalt in Shuangliao may be related with garnet pyroxenites in Jiaohe. |
PDF Full Text Request