EVIDENCE FOR BIOALTERATION IN PILLOW LAVAS OF PRECAMBRIAN OPHIOLITES

Bioalteration of basaltic glass in pillow lavas and volcaniclastics from in-situ oceanic crust and well-preserved Phanerozoic ophiolites has been documented in several independent ways. 1) Distinct alteration textures are generated during etching of the glass when microbes colonize along fractures forming tubular or granular textures; 2) microbial activity leaves behind chemical fingerprints in the form of elevated levels of the typical bioelements C, N, S, P and K; 3) fossil remains of microbes such as filaments and biofilms are preserved and identified by their typical morphology; and, 4) highly depleted carbon isotope ratios of disseminated carbonate in the glass suggest metabolic oxidation of organic matter.

We have searched for biosignatures in pillow lavas from two Precambrian ophiolites: the Middle Proterozoic (1.95 Ga) Jormua ophiolite (Finland), and the Early Archean (3.5 Ga) Jamestown ophiolite (South Africa). The Jormua ophiolite contains all the principal components of a standard ophiolite. The volcanic rocks contain well-preserved pillow structures with easily identifiable selvages despite being metamorphosed to lower amphibolite facies and the development of a well-defined foliation. The Jamestown ophiolite is dominated by mafic to ultramafic pillow lavas, sheet flows, and intrusions, but lacks a typical sheeted dike complex. The volcanic rocks are virtually undeformed and the metamorphic grade varies from zeolite through greenschist facies.

The d¹³C of disseminated carbonate in pillow lava rims from both the Jormua and Jamestown ophiolites are shifted to lower values (as low as –17 per mil) when compared to the adjacent crystalline pillow cores. Element mapping in the pillow rims reveals the presence of organic C, and sometimes N and S. In the undeformed pillow lavas of the Jamestown ophiolite there are features that strongly resemble fossilised filaments or biofilm. We suggest microbial alteration proceeded immediately after pillow formation, in a similar fashion to what invariably can be observed in pillow lavas of modern ocean crust. We regard these features as evidence of biological activity in the Precambrian deep seas and suggest that volcanic glass–microbe interaction was a process already established by Early Archean time.