EVIDENCE FOR BIOALTERATION IN PILLOW LAVAS OF PRECAMBRIAN OPHIOLITES
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 glassmicrobe interaction was a process already established by Early Archean time.