Microbialite cathodoluminescence: A tool for investigating palaeoredox conditions, alteration histories and primary textures across time

Abstract

Microbialites have long been utilised by geologists as palaeoenvironmental indicators, despite outstanding questions regarding their formation and preservation in the rock record. Here, we leverage cathodoluminescence (CL) microscopy, a technique commonly used to investigate carbonate formation and diagenetic alteration, to better understand the textural characteristics, formation mechanisms and diagenetic histories of microbialites. We compare CL features to gain insight into palaeoredox conditions and alteration histories for a suite of six microbialite samples spanning from the Proterozoic to modern, finding a strong degree of similarity amongst samples regardless of age or depositional environment. CL reveals that microbialites typically have complex microfabrics that include other accessory minerals and grains, all of which provide insight into their unique formation and palaeoredox histories. We find that the modern microbialite sample showed the greatest difference in CL characteristics compared to the other microbialite samples, most probably because of its aragonitic composition and incomplete lithification. In contrast, the ancient microbialite samples preserve a distinct and most probably primary, mottled luminescence texture despite spanning more than 500 Myr; this mottled texture may typify ancient microbialite fabrics that formed in shallow water settings. We also distinguish a variety of CL characteristics that support previously proposed formation and/or diagenetic histories in these samples. Lastly, we use energy-dispersive X-ray spectroscopy to compositionally identify rare grains observed with CL, highlighting the utility of CL as a possible screening tool for both geological and non-geological components within samples. Our analyses demonstrate the power of using classic CL techniques to answer modern questions in microbialite research.