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Hydrothermal Evolution, and Mineralogical and Biological Formation of Palagonite |
1 Geological institute, University of Bergen, Allegt.41, 5007 Bergen, Norway Harald.Furnes{at}geol.uib.no
2 Department of Microbiology, University of Bergen, Jahnebk. 5 5007 Bergen, Norway
3 Department of Geology, University of Alberta, Edmonton, Alta T6G 2E3, Canada
4 Scripps Institution of Oceanography, University of California, La Jolla, CA 92093-0225, USA
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 (
13C) in carbonates extracted from the glassy margin of pillows show highly variable values which can be explained in terms of bio-fractionation of the 12C and 13C 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.
