Discovery of Mass Independent Oxygen Isotopic Compositions in Superscale Nitrate Mineral Deposits from Turpan-Hami Basin, Xinjiang, China and Its Significance

The Turpan-Hami Basin in eastern Xinjiang is one of the driest regions on Earth and a premier environment to form and preserve nitrate. A large nitrate ore field in this basin was found recently. It is estimated there are about 2.5 billion tons of resources of nitrate, and the amount is as much as the Atacama Desert super-scale nitrate deposit in Chile. Nitrate is one of a few minerals with mass-independent fractionation (MIF), and the oxygen isotope MIF is an effective method to determine the source of nitrate. The delta 17O, delta 18O of nitrate were measured by fluorination and thermal decomposition method. The date indicated that this is the first time that oxygen isotope MIF has been located in inland nitrate minerals. The results obtained by two methods are similar, delta 17O=delta 17O-0.52x delta 18O=12 parts per thousand-17 parts per thousand. The experiment and observation data proved that oxygen isotope MIF of nitrate are the result of photochemical reactions in the troposphere and stratosphere. Thus, evidence from MIF oxygen isotopic compositions indicate that long term atmospheric deposition of nitrate aerosol particles produced by photochemical reactions is the source of the deposits.