Structural characterization by Raman hyperspectral mapping of organic carbon in the 3.46 billion-year-old Apex chert, Western Australia

The 3.46 billion years old Apex Chert, Pilbara Craton, Western Australia, is well-known for hosting the oldest, highly disputed microfossils on Earth. This rock has a complex history of thermal alteration that includes circulation of hydrothermal fluids, lower greenschist-facies regional metamorphism, and post-metamorphic weathering by meteoric fluids. Carbonaceous material occurs in the sedimentary stratiform part of the chert as well as the underlying intruding hydrothermal black chert veins. In order to identify a least-altered remnant of early life it is necessary to develop a method that enables rapid evaluation of CM structural order on a small spatial scale. Here we present the detailed characterization of CM in the Apex chert by Raman hyperspectral mapping. It is shown that this approach gives better estimates of average Raman band ratios than individual point analyses, and it is demonstrated that significant differences in structure exist between CM in the stratiform part of the Apex chert and CM in an underlying black chert vein. The large Raman map-based datasets also reveal that significant mixing took place between these two end members CM's, indicating that the Apex chert has been thoroughly altered by hydrothermal fluid circulation. At the brecciated intersection between the stratiform chert and the intrusive hydrothermal chert vein very poorly ordered CM was found that is not in line with lower greenschist-facies regional metamorphism. It is speculated here that this CM represents an organic fraction that was introduced or thoroughly altered by late stage meteoric fluids. Alternatively, the ubiquitous presence of hematite in this sample caused a perturbation in the Raman spectra of the CM, leading to an artifact in the calculated Raman-based band ratios. Overall it can be concluded that the best preserved CM occurs in the stratiform parts of the Apex chert, while earlier discussions on organic microfossils in this rock often focused on parts of the chert that either represented the hydrothermal veins or the brecciated intersection between the veins and the stratiform part.