古代碳酸盐岩同位素保持性及其地质应用

Stable Isotope Preservation in Ancient Carbonate Rocks and Its Geological Applications

摘要:

北京大学郑淑惠指出:“古代碳酸盐岩的地质应用,只有解决了矿物同位素数据的保存性问题,才能更好地阐明地质过程^①”。很多地球化学家研究现代、亚近代和古代沉积时,不但研究全岩,而且研究沉积物组份、胶结物和各种生物化石的碳、氧稳定同位素组成,并且进行彼此间的对比,进而探讨矿物岩石同位素变化规律和地质应用。

Abstract:

This paper discusses the preservation of stable isotopes from carbonate rocks based on stable isotope analysis at Permian-Triassic boundary sections in South China, and from the top and bottom of the‘mung bean rock’ units between the Lower and Middle Triassic. The results are examined in combination with the regional geological setting and stratigraphy, palaeontological features, and sedimentary facies and environments. Though initial isotope values from open marine, sabkha and salt lake deposits show some variation during early submarine diagenesis, epidiagenesis, neometamorphism and anadiagenesis, the original isotope signature generally remains clearly preserved. For example, δ¹³C values remain unaltered, δ¹⁸O values, however, show great changes. The unbalanced variations in δ¹⁸O values may be attributed to dedolomitisation and desulphatisation processes. It is important not to ignore this factor when applying stable isotope analysis methods in ancient carbonate rocks to stratigraphic devesion, and the reconstruction of sedimentary environments and sea-level fluctuations.