Rare earth elemental and Sr isotopic evidence for seawater intrusion event of the Songliao Basin 91 million years ago

Petrogenesis of lacustrine dolostone is closely related with paleo-lake water conditions.Here we report the high spatial-resolution petrographic and geochemical results of a lacustrine dolomite nodule from the Qingshankou Formation,the Songliao Basin.Sedimentary and elemental signatures confirm the protogenetic origin of this nodule and its effectiveness in recording geochemical characteristics of paleo-lake water during dolomitization.The low Y/Ho ratios,middle rare earth element(MREE)enrichment and subtle positive Eu anomalies within the nodule indicate a fresh water source.However,the Sr isotope values in the core of the nodule(0.7076-0.7080)are close to contemporaneous seawater(0.7074),yet different from the modern river(0.7120)and the host black shale(0.7100).On the premise of excluding the influence of hydrothermal fluids,the significantly low strontium isotope values of the lacustrine dolomite might be caused by seawater interference during dolomitization.Our findings demonstrate that lacustrine dolomite within black shales is not only a faithful tracer of diagenetic water environment,but also a novel and easily identified mineralogical evidence for episodic seawater intrusion event(91 Ma)in the Songliao Basin,which supplements other paleontological and geochemical evidence.

Effects of U-ore on the chemical and isotopic composition of products of hydrous pyrolysis of organic matter

In order to investigate the impact of U-ore on organic matter maturation and isotopic fractionation,we designed hydrous pyrolysis experiments on Type-II kerogen samples,supposing that the water and water–mineral interaction play a role.U-ore was set as the variable for comparison.Meanwhile,anhydrous pyrolysis under the same conditions was carried out as the control experiments.The determination of liquid products indicates that the presence of water and minerals obviously enhanced the yields of C（15＋） and the amounts of hydrocarbon and nonhydrocarbon gases.Such results may be attributed to waterorganic matter reaction in the high-temperature system,which can provide additional hydrogen and oxygen for the generation of gas and liquid products from organic matter.It is found that δD values of hydrocarbon gases generated in both hydrous pyrolysis experiments are much lower than those in anhydrous pyrolysis.What is more,δD values are lower in the hydrous pyrolysis with uranium ore.Therefore,we can infer that water-derived hydrogen played a significant role during the kerogen thermal evolution and the hydrocarbon generation in our experiments.Isotopic exchange was facilitated by the reversible equilibration between reaction intermediaries with hydrogen under hydrothermal conditions with uranium ore.Carbon isotopic fractionations of hydrocarbon gases were somehow affected by the presence of water and the uranium ore.The increased level of i-C4/n-C4ratios for gas products in hydrous pyrolysis implied the carbocation mechanism for water-kerogen reactions.

The primary dolostone in the Meso-Neoproterozoic:Cases study on platforms in China

Meso-Neoproterozoic dolostone is abnormally well-developed in the platform areas in China,detailly,in the Mesoproterozoic Calymmian Wumishan Formation of North China Basin,Neoproterozoic Ediacaran Qigbulak Formation of Tarim Basin,and the Neoproterozoic Ediacaran Dengying Formation of Sichuan Basin.The outcrops and drilling cores from these basins show that these dolostones are mostly thick-bedded to massive,with mud-sized to silt-sized crystalline dolostone reaching 86%-97%of the total dolostone thickness.These dolostones basically suffered no secondary metasomatism,regardless of containing microorganisms(such as algae and bacteria)or not.The analyses of sedimentary palaeogeographic features indicate that:(1)the Meso-Neoproterozoic dolostone nearly covered the entire area basins,and was widely deposited in supratidal to intertidal zones,as well as in open platform environments;(2)the dolostone was subdivided according to the crystalline size,with the distribution controlled by geomorphology,i.e.,the pure mud-sized crystalline dolostone developing in depression areas,whereas the grainy mud-sized crystalline dolostone or the mound stromatolitic mud-sized crystalline dolostone developing in uplift areas;(3)deep-water basins developed between the carbonate platforms,and the seismic profiles show that these basins were formed by syngenetic deep faults in the Meso-Neoproterozoic strata.These syngenetic faults increased Mg^(2+)concentration in the seawater by injecting rich-Mg2+-bearing hydrothermal fluids into the carbonate platforms.The analyses of geochemical and paleoclimatic indexes reveal that there is no obvious difference in carbon and oxygen isotope ratios between dolostones having different textures,with the ratios similar to those of the global seawater of the Meso-Neoproterozoic,indicating that the silt-sized crystalline dolostone has the same sedimentary origin as the mud-sized crystalline dolostone,i.e.,the former is from authigenic recrystallization of the latter rather than secondary metasomatism.Therefore,whatever the crystalline sizes are,the dolostones are all originated from sedimentation.All the petrological,sedimentary and geochemical data suggest that the dolostones covering almost the whole Meso-Neoproterozoic typical platform areas of China are of relatively primary sedimentary origin.