Strontium isotope composition of marine carbonate and the influence of diagenetic fluid on it in Ordovician

This paper synthesizes the analytic data for strontium isotope of the global Ordovician marine carbonate and its evolution trend. The authors tested ()~(87)Sr/()~(86)Sr ratio for 4 types of nondepositional (carbonate) minerals (totally 51 samples) Ordovician in the north of Tarim Basin. By the comparative method of studying, they got the following cognitions: (1)The ()~(87)Sr/()~(86)Sr ratio of the global Ordovician (seawater) has an overall descending trend with time. The reason is relevant not only to the continental flooding of Late Cambrian and Ordovician, but also to the process that the sources of radiogenic Sr were covered by sediments deposited from vast epicontinental seas. (2)The marine carbonate strontium isotope curve in the Tazhong area of Tarim Basin is consistent to the global evolution trend, suggesting that the variation of global sea-level is the main factor to control the marine carbonate strontium isotope composition and its evolution trend in Tarim Basin. (3)The lowering of ()~(87)Sr/()~(86)Sr ratio of the Upper Ordovician marine carbonate may have a relation to the influence of the mantle source fluid. (4)The increase of ()~(87)Sr/()~(86)Sr of some uppermost Ordovician samples in the Tazhong area of Tarim Basin has a relation to the radiogenic Sr leached out by meteoric water from the Late Ordovician carbonate in the epigenesis environment. This (environment) was produced by the rising of Tarim Basin resulted from the tectonic movement. (5)The (nondepositional) carbonate which is distributed in the weathering zone of Ordovician in the north of Tarim Basin has a very high ()~(87)Sr/()~(86)Sr record. The lowest record is still higher than 0.7079 or so, which is the (()~(87)Sr/()~(86)Sr) ratio of Late Ordovician seawater. The fact shows that the meteoric water has very important (significance) in interaction of fluid and rock of Ordovician. (6)In 4 types of nondepositional carbonate (distributed) in the weathering zone of Ordovician, the ()~(87)Sr/()~(86)Sr ratios of the calcite related to the palaeo-(karstification) near the unconformity have the highest records which are much higher than that of any other kinds of calcite, suggesting that the meteoric water near the unconformity plays the most important role in the non-marine fluid influence.