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.