HR: 1340h
AN: B13A-0217    [Abstracts]
TI: Tracing Alteration Textures in Young to Ancient Pillow Lavas: A Petrographic Signature of Early Life on Earth
AU: Furnes, H
EM: harald.furnes@geo.uib.no
AF: University of Bergen, Department of Earth Science, Bergen, 5007 Norway
AU: * Staudigel, H
EM: hstaudigel@ucsd.edu
AF: University of California, Scripps Institution of Oceanography, La Jolla, CA 92093-0225 United States
AU: Banerjee, N R
EM: banerjee@ualberta.ca
AF: University of Bergen, Department of Earth Science, Bergen, 5007 Norway
AU: Banerjee, N R
EM: banerjee@ualberta.ca
AF: University of Alberta, Department of Earth and Atmospheric Sciences, Edmonton, AB T6G2E3 Canada
AU: Muehlenbachs, K
EM: karlis.muehlenbachs@ualberta.ca
AF: University of Alberta, Department of Earth and Atmospheric Sciences, Edmonton, AB T6G2E3 Canada
AU: de Wit, M
EM: maarten@cigces.uct.ac.za
AF: University of Cape Town, Department of Geological Sciences, Rondebosch, 7701 South Africa
AB: Alteration of basaltic glass in pillow lava rims and hyaloclastites in the upper oceanic crust may involve a combination of abiotic and biotic processes depending on thermal conditions. Abiotic alteration yields smooth fronts of banded dark yellow to brown material of approximately equal thickness on both sides of fractures. Textures we ascribe to a biogenic origin occur as granular and tubular types, and differ markedly from the characteristic abiotic features. The granular type comprises individual coalesced spherical bodies 0.2-0.6 microns in diameter, and appears as solid bands, semicircles or irregular patches protruding into the fresh glass. The tubular type consists of thin, sometimes branching tubes, with widths of 1-4 microns, and minimum lengths commonly between 10-100 microns, that define a highly irregular alteration front. The granular and tubular structures have the right size, shape and distribution to have resulted from microbial corrosion of the glass. Further alteration causes them to be filled by authigenic minerals. The microbial alteration textures always occur in places where seawater could penetrate into the glass, such as along fractures, around vesicle walls and near varioles. Their biogenicity is substantiated by carbon isotope data, element distribution of C, N, P and S, as well as the presence of DNA in young samples. The granular texture is the most common of the two types and occurs at any depth down to 500m into the lava pile; both types are, however, most abundant in the 100-300m depth range. Even though these microstructures are easily destroyed during deformation and metamorphic mineral growth, an ultimate goal of our research is to trace biotextures in pillow lava as far back in time as possible. For that purpose we have investigated undeformed and low grade metamorphosed pillow lavas and interpillow hyaloclastite from the $\sim$3.5 Ga Barberton Greenstone Belt in South Africa. Petrographically the original glass rim of the pillow lavas and the shards consist of predominantly fine-grained chlorite hosting fracture-rooted titanite-filled tubular structures. These structures are of similar width and length as those found in pillow lava rims of modern oceanic crust, produced by microbial corrosion. The tubular structures are commonly partly overgrown by chlorite and are thus clearly pre-metamorphic. We suggest these tubular structures represent traces of microbial activity formed during etching of the original glassy material as microbes colonised the surface along fractures soon after eruption on the sea floor $\sim$3.5 billion years ago.
DE: 9619 Precambrian
DE: 3030 Micropaleontology
DE: 3035 Midocean ridge processes
DE: 0400 Biogeosciences
SC: Biogeosciences [B]
MN: 2004 AGU Fall Meeting