Strontium Isotope Composition and Its Characteristics Analysis of Cambrian-Ordovician Carbonate in Tazhong District, Tarim Basin

In this paper, ICP-MS analytic technology is employed to test the strontium and manganese contents of 109 samples collected from four wells in Tazhong area, Tarim basin. In addition, strontium isotope composition tests have also been conducted, with VG354 solid isotope mass spectrograph, on 25 samples from Zhong 1 and Zhong 4 wells. The analysis of strontium and manganese contents, the research into the characteristics of strontium isotope composition from Zhong 1 and Zhong 4 wells, the comparison between the results from strontium analysis of the global Ordovician marine carbonate and its evolution trend, and the combination of sedimentary facies characteristics of isolated wells in this area conclude the following five points: (1) The marine carbonate strontium isotope curve in the Tazhong area of Tarim basin is consistent with the global evolution trend generally descending with time, the direct reason of which is the evolution of paleogeographic environment. The Ordovician paleogeographic environment ranged from the restricted platform to the open platform and then to the shallow marine shelf in Tazhong area, Tarim basin. Dolomitization is another secondary reason, and the inversion of fluid with high manganese can lead to heavy strontium; (2) The ⁸⁷Sr/⁸⁶Sr ratios of the Upper Ordovician in Tazhong area characterized by an apparent mono-decline trend that goes with the time, similar to the global strontium isotope ratio, implies the starting point of the variation of the Upper Ordovician sea level, the overall trend of which is rising; (3) Compared with the global seawater strontium ratios, the ⁸⁷Sr/⁸⁶Sr ratio of the Lower Ordovician is much higher, and the ⁸⁷Sr/⁸⁶Sr ratio of the Lower Ordovician is often increasing, with its maximum value determined at 0.709 727. The dolomitization, the evaporation of the supratide, and the reflux of high manganese haline, closely associated with such an increasing ratio, can lead to the increase in the content of the strontium and the rise in the ⁸⁷Sr/⁸⁶Sr ratio; (4) The manganese content varies little, and is affected little by dolomitization; (5) Large variations occur in the strontium values of Ordovician carbonate in the Tazhong district. Apart from paleogeographic environment and petrography, other factors that can affect the values of strontium are the content of the terrestrial clastics and the diagenisis modification which may result in the further change in the strontium values.