Assessing the biogenicity and syngenicity of candidate bioalteration textures in pillow lavas of the ∼2.52 Ga Wutai greenstone terrane of China

Abstract

Microorganisms that inhabit sub-seafloor lavas are capable of etching volcanic glass and creating micron-sized tunnels and pits. Mineralization of these bioalteration traces ensures that these textures survive deformation and transformation of the host glass to metamorphic minerals. The fossil record of such bioalteration textures extends far beyond volcanic glass from in-situ oceanic crust to include meta-volcanic glass from ophiolites and Precambrian greenstone belts. Investigation of petrographic thin section reported here from ∼2.52 Ga tholeiitic pillow lavas from the Wutai Group of N. China found filamentous micro-textures. Laser Raman spectroscopy confirmed that these textures are mineralized by titanite. Moreover, the Wutai micro-textures are comparable in size, morphology and distribution to bioalteration textures from Archean greenschist facies pillow lavas. In-situ U-Pb dating of the titanite by laser ablation multi-collector inductively coupled plasma mass spectrometry (LA-MC-ICP-MS) gives an age of 1.81 ± 0.12 Ga (2σ, n=22, 206Pb/238U weighted average). This provides a minimum age for the mineralization of these candidate bioalteration textures and corresponds to a regional metamorphic event. This also represents a minimum age estimate for the timing of bioalteration and is compatible with the existence of a Late Archean-Proterozoic sub-seafloor biosphere.