DISCOVERING THE ORIGINS OF CARBONACEOUS MATERIALS IN SILICEOUS DEPOSITS: MICRO RAMAN SPECTROSCOPIC CHARACTERIZATION OF QUARTZ FABRICS IN CHERTS

A wide variety of microbes flourish in modern hydrothermal environments on Earth, and the biosignatures of their ancient ancestors are preserved in siliceous deposits such as cherts. Unfortunately, ancient terrestrial deposits also contain conflicting signatures, since microfossils are sometimes misidentified, and graphitized carbon and many biominerals are also formed abiotically. In some cases, multiple episodes of hydrothermal alteration can obscure otherwise syngenetic biosignatures and likewise create false signatures in the form of secondary carbon emplacement or diagenetic phase changes. Carbonaceous microfossils are most often entombed within large quartz grains. In some cases the interstices between quartz grains are also carbon-rich, but the origin of this carbon is often ambiguous. With the goal of unraveling these inconsistencies, we combined high-resolution micro Raman spectroscopic imaging with spectral deconvolution methods to characterize the quartz fabrics and macromolecular carbon in a suite of chert samples that include the ~400 Ma Rhynie chert, 1.88 Ga Gunflint Formation, and ~3.49 Dresser Formation. Using this diverse set of samples, we are able to do the following: 1) detect and map subtle differences in Raman spectra for separate generations of quartz in each sample set; 2) differentiate between different pools of carbon, one that is syngenetic and the other that was likely post-depositionally emplaced; 3) discern the affects of carbon on quartz grain orientation. Hydrothermally derived silicate deposits are of particular relevance to Mars analog studies and possible landing sites, and the unambiguous detection of biosignatures in these rocks is important for the success of upcoming missions.