Water Contents Of The Picrites And Basalts And Constraints On The Genesis Of The Emeishan Large Igneous Province

Recent water content studies of komatiites and picrites and related calculations of the mantle potential temperature have claimed that the formations of Archaean and Phanerozoic large igneous provinces are both related to hydrous mantle plume.The Late Permian Emeishan large igneous provinces(ELIP),located on the western margin of the Yangtze craton in SW China,was divided into western zone,middle zone and eastern zone based on the geological,geochemical and geophysical works.The Dali and Binchuan picrites outcrop in western zone of ELIP,representing the melts derived from the center zone of the Emaishan plume.The previous study has demonstrated that these picrites are hydrous(H2O>3.4 wt.%),amounting to more than 6000 ppm H2O in their melting source,which is significantly higher than that of mid-ocean ridge basalts and ocean island basalts(MORB source: 50?250 ppm;OIB source: 300?1000 ppm),highlighting an important role of water in the genesis of ELIP.However,the existing water content data are still limited to those picrites from the western zone.In order to comprehensively understand the mechanism of the ELIP formation,we need to answer two questions:(1)whether the mantle source of the basalts/picrites from other zones were also hydrous?(2)What is the influence of water content in the mantle source on the geochemical variations of the erupted basalts?In this study,we analyzed the high-Ti samples from the lower part of Ertan section in the middle zone,Yongsheng and Lijiang picrites in western zone,and further calculated the water content of primary melt based on the method of water content in the clinopyroxene phenocryst.The result shows that the H2O content of clinopyroxene phenocrysts in Ertan basalts is 76?424 ppm,and the water content of primary magma is estimated to be 2.71±0.95 wt.%.The H2O content of clinopyroxene phenocrysts in Yongsheng picrites is 235?410 ppm,and the water content of primary magma is estimated to be 1.52±0.53 wt.%.The H2O content of clinopyroxene phenocrysts in Lijiang picrites is 115?338 ppm,and the water content of primary magma is estimated to be 1.08±0.38 wt.%.The estimated H2O content of Ertan basalts and Yongsheng and Lijiang picrites in mantle source are 1357 ppm,2584 ppm and 1836 ppm,respectively,all of which are significantly higher than that of mid-ocean ridge basalts and ocean island basalts.The high H2O content of basalts and picrites from different zone in ELIP thus indicate that water plays an important role in generating ELIP,and the Emeishan mantle plume was extensively hydrated.Combined with the water content of Dali and Binchuan picrites previously reported,there are negative correlations among the ratios of Ti/Y,Sm/Yb,TiO2/Al2O3 with H2O content in mantle source of ELIP,and these trends can go further when more data were collected from global other LIPs.The simulations show that these correlations can hardly be interpreted by varying extents of partial melting of one lithology(peridotite or pyroxenite),but mixed lithologies in melting source.We propose a scenario: during the decompressional melting of a hydrous mantle plume that contains recycled component(eclogite/pyroxenite),the difference in water content can affect the melting behavior of the mixed source,resulting in different contributions of each component-derived melts to the erupted basalts/picrites that thus show obvious geochemical variations.