Abstract In this paper we summarize the present knowledge on bioalteration of basaltic glass from pillow lava rims of former oceanic crust, based on the study of four ophiolite complexes. These complexes range in age from Late Cretaceous to Mid-Proterozoic, and their metamorphic grades vary from non-metamorphosed to low greenschist- to low amphibolite-facies metamorphism. In the non-metamorphosed volcanic part of ophiolite complexes, in which undevitrified glass is still present in pillow lava rims and/or hyaloclastites, biogenerated textures are common. These textures (termed granular and tubular) 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 metamorphic and completely recrystallized examples, the textural evidence of bioalteration is generally obliterated, although it may still be visible in little-deformed volcanic domains of low-grade greenschist-facies metamorphism. Where biogenerated textures are present, element mapping invariably reveals the presence of organic carbon, and sometimes nitrogen, sulphur and phosphorus. Carbon-isotope signatures (δ 13 C) in the pillow lava rims from all four investigated ophiolites show lower values than those of the adjacent crystalline parts. This phenomenon 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.

Abstract The alteration of basaltic glass in the volcanic part of the oceanic crust is, to a substantial extent, biologically mediated. Evidence of microbial interaction with basaltic glass can be provided by a number of independent observations, such as: (1) Textures at the alteration front, generated by dissolution of the glass and subsequent precipitation. These bio-generated textures can be defined as a granular type (dominant) and a tubular type, and show size and form which are compatible with microbial etching. (2) Filament-like structures, representing organic remains, appear in connection with bio-generated textures. (3) Within areas of the bio-generated textures, particularly at the alteration front, DNA and ribosomal RNA have been demonstrated to be present in relatively young samples. (4) X-ray mapping shows that carbon and nitrogen invariably appear within the bio-generated textures, in young samples most strongly enriched at the alteration front. (5) Carbon isotopes (δ 13 C) in carbonates extracted from the glassy margin of pillows show highly variable values which can be explained in terms of bio-fractionation of the 12 C and 13 C isotopes. Estimates of the proportion of bio-genetic alteration products of basaltic glass, on the basis of textural relationships, suggest that bio-alteration is dominant compared to abiotic alteration in the upper 300 m of the oceanic crust, and declines to become insignificant at a depth of about 500 m.