海相碳酸盐矿物的阴极发光性与其成岩蚀变的关系

引言众所周知,原始的海相碳酸盐矿物为文石、高镁方解石和低镁方解石,但它们对成岩蚀变极度敏感,在大气淡水成岩环境中,文石和高镁方解石都会转变为成岩低镁方解石,同时,原始的低镁方解石也会有不同程度的蚀变。在进行古代碳酸盐岩的沉积环境和古海洋学研究时,往往需要测定岩石中的微量元素及氧、碳、锶等同位素组成,用以判断古海水

CATHODOLUMINESCENCE AND DIAGENETIC ALTERATION OF MARINE CARBONATE MINERALS

Aragonite, high-Mg calcite (HMC)and Iow-Mg calcite (LMC) in present-day sea water and diagenetic Iow-Mg calcite (dLMC) in ancient limestones have been investigated using cathodoluminescence. All the present-day marine carbonate minerals (aragonite, high-and Iow-Mg calcite) are non-luminescent under cathodoluminescence due to low concentration of Mn and the differences of partition cofficients of Mn in sea water and meteoric water, whereas the diagenetic Iow-Mg calcite in ancient limestones is luminescent to a great extent, from very dark red to very bright red cathodoluminescence resulting from diagenetic alteration in meteoric water with high Mn and Fe concentration. Most of the skeletons (e.g. brachiopods) whose primary compositions are low-Mg calcite are more resistant to diagenetic alteration than other carbonate minerals and therefore non-luminescent. Their isotopic and chemical compositions may be interpreted to represent the initial chemical compositions in equilibrium with those of sea water. The presence or absence of cathodoluminescence may be employed as the criteria for recognition of alteration of ancient carbonate minerals.