PRESERVATION OF MICROBIAL ICHNOFOSSILS IN BASALTIC GLASS BY TITANITE MINERALIZATION

Abstract

Glassy volcanic rocks and their metamorphosed equivalents are a recently realized setting in the search for traces of early life on Earth, as revealed by ichnofossils. Numerous authors have demonstrated that the record of microbial colonization of submarine basaltic glass extends to at least 3.5 Ga. Microbes rapidly colonize the glassy surfaces along fractures and cracks exposed to water, producing characteristic granular and tubular bioalteration textures. We present results of a mineralogical study of the materials mineralizing bioalteration structures in oceanic crust samples from the Ontong Java Plateau (OJP), and subaqueously emplaced, formerly glassy, basaltic rocks from the Abitibi greenstone belt (AGB). Previously reported Ca-and Ti-rich material in the OJP tubules was determined to be very fine-grained crystalline titanite (CaTiSiO 5 ) by micro X-ray diffraction (μXRD). Formerly glassy pillow lavas and interpillow hyaloclastites in AGB samples contain tubular and granular structures that also have been preserved by titanite mineralization, as confirmed by μXRD. Titanite mineralization associated with bioalteration in the OJP samples strongly suggests that mineralization of these trace fossils occurred penecontemporaneously with bioalteration. Early precipitation of titanite within the alteration structures enables their preservation in the geological record, potentially for billions of years in the absence of penetrative deformation. Mineralogical identification of titanite by in situ non-destructive μXRD has provided essential phase information, which is complementary to other microscopic and microanalytical data.