Studies Of The Mineralgy And Stable Isotope Of Bloedite

Bloedite is precipitated from brine as a secondary sulfate mineral with hydration water.It is not only a dominated mineral of evaporate sulfate deposits,but also can be formed into a bloedite deposit.The mineralogical and isotopic characteristics of bloedite are of great significance for origins,weathering and climate changes in Qaidam Basin.As one of non-traditional isotopes,δ²⁶Mg of salt minerals bearing hydration water are few reported.In this study,five bloedite samples from a core SG-1 in Qaidam Basin were examined for its mineralogical and isotopic features(δ²⁶Mg,δD andδ¹⁸O of hydration water,andδ¹⁸O-_SO4 andδ³²O-_SO4)by X-ray diffraction,ion chromatograph)and Mass spectrometer,and differential thermal analysis（DTA）.Main conclusions are following as.（1）Its cell parameters are:a=5.5376?,b=8.247?,c=11.117?andβ=100.761?.The ion contents of Mg²⁺,Na⁺and SO₄^2-are 5.2～9.3 mg/g,12～20.2mg/g and 567.45～1223.38 mg/g,respectively.The dehydration temperature of bloedite ranged from 135 to180℃in the first stage,and from 200 to 298℃in the second stage and lattice variation at 620℃.（2）Isotopic compositions of hydration water areδD=-12.10～3.72‰with an average of 2.27‰andδ¹⁸O=13.07～15.34‰with an average of 15.82‰,respectively.The linear relationship betweenδ¹⁸O andδD is different with those of the whole and local meteoric water line,and theδ¹⁸O of hydration water is similar with those of deep brines and surface brines,which indicated that meteoric water was the original source of the bloedite hydration water with strong evaporation.（3）δ²⁶Mg of bloedite at 0.36 Ma is-0.072‰,and are-1.163‰和-1.098‰at0.39 Ma.The brine at 0.36 Ma is richer in heavy Mg isotopes than those at 0.39Ma.Its possible reasons are that the minerals-bearing Mg deposited early in 0.36Ma combined with light Mg isotopes,heavy Mg isotopes were remained in brines when bloedite deposited;and/or most of the heavy Mg isotopes of bloedite were from the basic and ultro-basic rocks,as a result of weak weathering in 0.36 Ma.（4）Theδ³⁴S_-SO4 values of bloedite are 11.4～15.5‰,δ¹⁸O_-SO4 are 14.5～15.3‰.Evaporation and the resource of sulfur possible are the main reason that controlled the changes ofδ³⁴S.