奥陶系海相碳酸盐锶同位素组成及受成岩流体的影响

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

综合了已有的全球奥陶系海相碳酸盐的锶同位素分析结果及演化趋势,测试了塔里木盆地北部奥陶系4种类型共计51个非沉积碳酸盐矿物的锶同位素组成,通过对照研究,得出如下认识:(1)奥陶纪全球海水87Sr/86Sr比值具有随时间单调下降的总体趋势,其原因与晚寒武世—奥陶纪的全球淹没事件,以及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;(2)塔里木盆地塔中地区海相碳酸盐的锶同位素演化趋势总体上与全球一致,说明全球海平面变化仍然是塔里木盆地海相碳酸盐锶同位素组成与演化的主要控制因素;(3)塔里木盆地塔中地区上奥陶统部分海相碳酸盐87Sr/86Sr比值的降低可能与深部流体的影响有关;(4)塔里木盆地塔中地区上奥陶统部分海相碳酸盐样品87Sr/86Sr比值的升高与晚奥陶世盆地抬升过程造成的表生成岩环境中近地表大气淡水作用带来的富放射性成因锶的影响有关;(5)分布于塔里木盆地北部奥陶系风化壳中非沉积碳酸盐具有很高的87Sr/86Sr比值,最低值也显著高于0.7079左右的晚奥陶世末海水的锶同位素组成,说明大气淡水及壳源组分在奥陶系水岩反应中具有极为重要的意义;(6)奥陶系风化壳中的4种非沉积成因方解石中,与不整合面附近古喀斯特作用有关的方解石的87Sr/86Sr比值最高,显著高于各种成因的裂隙(构造或非构?

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.