Localized TiO2 accumulation in Archean stromatolites: implications for titanite microtube formation and possible UV shielding

Titanite-filled microtubular structures associated with carbonaceous material occur within stromatolites of the 2.72 Ga Tumbiana Formation, Pilbara Craton. Petrographic, crystallographic, nanoscale structural and compositional characterization, and geochemical analyses suggest a genetic relationship between microtubular structures, organic matter, Ti enrichment, and titanite mineralization. These microtubular structures are interpreted as mineralized ambient inclusion trails formed through localized dissolution associated with carbonaceous material and pyrite during burial rather than microbial tunnels or dendritic titanite growth. Elemental mapping using TEM and Atom Probe shows that Ti enrichment is spatially associated with carbon-rich stromatolitic laminae, and nanoscale TiO 2 (anatase) particles occur within titanite-bearing domains, suggesting localized Ti concentration within organic-rich microenvironments, potentially mediated by microbial biomass and extracellular polymeric substances. Titanite likely formed during burial and low-grade metamorphism through reactions potentially involving TiO 2 , quartz, calcite, and methane-bearing fluids, preserving microtubular textures and associated carbonaceous material. These results indicate that Ti-bearing mineral phases associated with stromatolitic environments may represent previously underrecognized archives of microbe–mineral interactions in Archean rocks. The occurrence of TiO 2 nanoparticles within organic-rich stromatolitic domains indicates localized Ti concentration prior to titanite formation and raises the possibility that the UV-absorbing properties of TiO 2 may have been relevant in Archean surface environments subjected to intense ultraviolet radiation.