MESOSCALE X-RAY FLUORESCENCE (XRF) MAPPING OF STROMATOLITES REVEALS GROWTH PROCESSES FOR AN NSUZE GROUP STROMATOLITE

Stromatolites are formed by physical, biological, and chemical processes, and combinations of specific processes produced the great variety of textures and morphologies found in the geologic record. We here introduce a new technique for mesoscale chemical mapping for stromatolite interpretation. In particular, we use x-ray fluorescence microscopy (µXRF) to identify spatial associations between various elements (“fluorescence facies”) to infer the physical, biological, and chemical processes that produced the observed compositional distributions.

As a test case, elemental distributions from µXRF scans of stromatolites from the Nsuze Group (2.98 Ga) were analyzed to develop a fluorescence facies growth model. There are six major fluorescence facies: dolomitic laminae, chert laminae, clotted dolostone and chert, stromatolite clast breccia, Mn-rich dolomitic cavity fill, and calcitic cavity fill. Dolomitic laminae are composed of microlaminated ferroan dolomite with isolated quartz, rutile, zircon grains, and unidentified Fe-rich grains. Individual dolomitic laminae contain syndepositional slump or fault structures, or contain current structures, reactivation surfaces, or stranded ripples. These laminae were likely formed by deposition from bed load, and by trapping and binding by microbial mats. Chert laminae are composed of chert with minor fine-grained phyllosilicates, heavy minerals, and rare sand-sized feldspar grains. Pinnacle structures, roll-ups, and oversteepened laminations suggest that this fluorescence facies was produced by the silicification of microbial mats. The clotted dolostone and chert fluorescence facies is composed of intergrown chert and dolomite, which may appear as cryptically laminated clots. It forms high-relief cauliform mounds and had high early porosity that was frequently filled with Mn-rich dolomitic cavity fill. Stromatolite clast breccias are matrix-supported breccias containing a mixture of stromatolitic clasts, and heavy mineral grains. This facies was likely formed as storm lag deposits filling erosional scours and interstromatolite spaces. Cavities in the clotted dolostone and chert and stromatolite clast breccia fluorescence facies were filled by an initial generation of Mn-rich cavity fill and a later generation of calcitic cavity fill.