REASSESSING AND RECLASSIFYING THE ROLE OF THE APEX CHERT IN ASTROBIOLOGY

Microstructures from the 3.5 Ga Apex Chert have been proposed as the oldest evidence of life on Earth. Astrobiologists have also used these microstructures as analogs for ancient fossil life on Mars. However, while these structures have been studied for over 30 years with a multitude of geochemical techniques, their biogenicity and environment of formation is still controversial. Continued exploration of these questions is crucial to understanding the history of life on Earth, and perhaps on other planets.

To this end, new samples were collected from the original Apex Chert locality in order to deduce the nature of the microstructures and study the diagenesis and subsequent alteration of the rocks. While the microstructures have been previously interpreted as carbonaceous microfossils based on observations and Raman spectroscopy of very thick (300 µm) paleontological thin sections, these new 30 µm thin sections revealed previously obscured petrographic details about these microstructures and highlighted some of the difficulties in employing surface techniques like Raman spectroscopy. Thin-section based paragenetic description illustrates at least 5 generations of post-depositional fracturing and mineralization, while mineral identification by Raman spectroscopy delineates at least two alteration events: a sulfate-rich barite-forming fluid and a later oxidizing, iron-rich fluid that dissolved previous phases and precipitated hematite. In contrast to earlier studies, together, the data collected suggest that these microstructures are neither biologic nor carbonaceous.

Nonetheless, the Apex Chert is still a useful Mars analog. For instance, thin sections contain hematite textures similar to that of hematite spherules found by the Opportunity Rover on the Meridiani Planum. Additionally, as future Mars rover missions plan to use camera-based imaging techniques and point-spectra Raman spectroscopy to identify potential fossils and mineral assemblages, understanding the abilities and limitations of these techniques is crucial to accurately interpret remotely-sensed data. As this study of the Apex Chert uses techniques similar to those proposed for Mars rover missions, it provides a cautionary note about the limitations of these techniques when identifying ancient life and mineral assemblages.