塔中地区寒武系-奥陶系碳酸盐岩Sr元素和Sr同位素特征

使用VG354固体同位素质谱仪对中1、中4井的25个碳酸盐岩样品做了Sr同位素测试, 并利用电感耦合等离子质谱仪(ICP-MS) 对塔中地区4口井共109个碳酸盐岩样品测试了Sr、Mn元素的含量.通过对Sr、Mn元素含量分析, 及中1、中4井碳酸盐岩的Sr同位素组成分析, 对比全球奥陶系海相碳酸盐的Sr同位素分析结果及演化趋势, 得出了如下认识: (1) 塔中地区奥陶纪87Sr/86Sr比值与全球海水Sr同位素演化趋势基本一致, 具有随时间下降的总体趋势, 这与广阔陆表海和有关的沉积物对放射性成因锶的封存作用有关, 说明海平面变化和白云岩化作用仍然是该区海相碳酸盐岩锶同位素组成与演化的主要控制因素; (2) 塔中地区早奥陶世的87Sr/86Sr比值与全球海水Sr同位素比值相当, 说明该区早奥陶世碳酸盐岩成岩环境为正常海水, 且早奥陶世87Sr/86Sr比值有单调降低的规律, 说明与海平面变化有关; (3) 塔中地区晚奥陶世87Sr/86Sr比值比全球海水高, 其原因是白云岩化作用和晚奥陶世盆地抬升近地表水带来高87Sr/86Sr比值, 且晚奥陶世87Sr/86Sr比值为整体升高的趋势; (4) 塔中地区奥陶纪碳酸盐岩中Mn元素含量变化不大, 反映了塔中地区奥陶纪成岩环境主要为浅水相, 但也有深水相, 白云岩化对其影响不大; (5) 塔中地区奥陶纪碳酸盐岩中Sr元素含量变化较大, 反映该时期该区碳酸盐岩成岩流体主要为海水, 但也有混合水, 白云岩主要为Ⅲ类白云岩和Ⅰ类白云岩. 

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.