Stable Isotope Geochemical Characteristics Of Nitrate In Turpan-hami, Xinjiang, China And Its Geological Significance

Massive nitrate deposits have recently been declared in Turpan-Hami basin,Xinjiang, China, rivaling with those world famous deposits in Atacama Desert, Chile.On the basis of previous research, a new strategy coupling extensive sampling andlaboratory mineral identification with isotope analysis has been utilized to constrainthe geological mineralogical and isotopic compositions of the nitrate deposits inTurpan-Hami basin. This can help understand the geological significance of differenttypes of nitrate deposits. The main conclusions are as follows:(1)The distribution of nitrate deposits in Turpan-Hami basin is mainly driven bytectonics, exhibiting a zonal pattern. There are two major types of nitrate deposits: saltflat-typed deposits and salt lake-typed deposits. The salt flat-typed deposits mianlyoccur in Ouaternary Holocene stratigraphy consisting of loose sand and gravel,featured by abundant salt minerals including nitratine, halite, thenardite and so on.Salt lake-typed nitrate deposits mainly distribute in the low-lying areas of theTurpan-Hami basin, containing three main categories of salt minerals, i.e. nitrate(dominated by potassium nitrate), chloride and sulfate minerals, followed by aralatively small amount of sodium saltpeter, darapskite and humberstonite.(2)As for the geochemical composition, salt flat-typed nitrate deposits arecharacterized by low sulfafe but high chlorite contents, with the main nitrate mineralsincluding nitratine and darapskite; instead, compared to typical halite deposits, thechlorine content in the salt lake-typed nitrate deposits is significantly lower, but withhigher sulfate and insouble solid contents.(3)In Turpan-Hami basin,the δD values of brine waters range from-167.7‰to-7.5‰, while the δ18O values are in the range of-23.46‰～2.29‰.These δD andδ18O generally fall along the evaporation line (with the slope of8), sugestting thebrine waters in Turpan-Hami basin are mainly originated from precipitation. However,as an inland basin with strong evaporation, the brine water in Turpan-Hami basin, theresidual liquid from evaporation, typically has larger δD and δ18O values than thelocal precipitation because of the strong evaporation in Turpan-Hami basin.(4)The Δ17O and δ15N values of nitrate vary greatly different regions inTurpan-Hami basin. Δ17O values range from9.5‰to17.1‰, indicating thatatmospheric deposition is an important source of the nitrate deposits; δ15N values are3.4～12.5‰with the majority in the range of3.4to7.4‰that is consistent with theisotopic characteristics of atmospheric nitrate. There are significant differences inΔ17O and δ15N values between two different types of nitrate deposits: the saltflat-typed nitrate deposits have higher Δ17O values but lower δ15N values than the saltlake-typed nitrate deposits because microbial activities can be limited in salt flats withan absence of water compared to in salt lakes. Therefore, nitrogen and oxygenisotopic differences in nitrate deposits in different regions can be attributed topost-depositional processes.