How Far Back in the Geological Record Is Bioalteration of Volcanic Glass Evident?

Ubiquitous alteration by microbes of volcanic glass from pillow lavas and tuffs has been documented in the upper 500m of in-situ modern oceanic crust. Bioalteration may be extensive enough to control the element balance between the upper oceanic crust and the hydrosphere. Such microbial processes deep within the modern crust raise questions relevant to the possible origin of life at mid-ocean ridges as well as sampling strategies for other planets. However, the evolutionary history of glass bioalteration or its preservability in the geological record is not known. We searched for evidence of ancient bioalteration of glass in L. Cretaceous to M. Proterozoic ophiolites ranging in metamorphic grade from non-metamorphosed to low greenschist to low amphibolite facies. Bio-generated textures are common in Cretaceous ophiolites where undevitrified glass is still present. These textures are similar to those found in volcanic glass of recent to old (170 Ma) in-situ ocean floor, and mimic microbes in terms of size and shape. In the metamorphosed and completely recrystallized examples, the textural evidence of bioalteration is generally obliterated, though they may still be visible in pillow rims from little-deformed volcanic domains of low-grade greenschist facies metamorphism. Where bio-generated textures are present, element mapping invariably reveals in them organic carbon, and sometimes N, S and P. We suggest these clots may be metamorphosed remains of Ordovician microbes. Carbon-isotope signatures in the pillow lava rims from all the investigated ophiolites, show lower values than those of the adjacent crystalline parts. This phenomenon, also observed in modern pillows, may be attributed to bio-induced fractionation of carbon isotopes during preferential bioalteration of the pillow lava glass, and may further represent a feature that seems to survive metamorphism and deformation. Perhaps the Proterozoic pillows also suffered some bioalteration.