Preservation of biosignatures in metaglassy volcanic rocks from the Jormua ophiolite complex, Finland

Evidence of microbially-mediated alteration of basaltic glass is preserved in originally glassy basalts (rims of pillow lavas, hyaloclastite breccias, and chilled margins of dykes) from the well-preserved 1.95 Ga Jormua ophiolite complex (JOC) of Northeastern Finland. Although textural evidence of microbial alteration is commonly observed in relic glass from recent oceanic crust and some ophiolites, these textures have been destroyed during greenschist to lower amphibolite facies regional metamorphism and deformation of the JOC. However, another robust biosignature is found in the generally depleted δ13C values of disseminated carbonate extracted from originally glassy basalts, relative to crystalline samples. The same distribution of δ13C values is well documented in samples from recent oceanic crust as well as ophiolites of Phanerozoic age. This characteristic contrast in the δ13C values of disseminated carbonate is interpreted to result from microbe-induced fractionation during oxidation of organic matter. X-ray mapping of initial alteration zones has identified residual carbon associated with highly-concentrated S that is unrelated to carbonate. We attribute these biosignatures to microbially-mediated alteration of originally glassy material prior to ophiolite emplacement.