On the role of microbes in the alteration of submarine basaltic glass: a TEM study

The alteration of submarine basaltic glass that has been attributed to microbial activity was investigated via transmission electron microscopy in order to document glass alteration and to understand the chemical reactions that bacteria could utilize. The features that were examined occur within fresh glass at the alteration front and consist of 0.1–2.3 μm patches or tubes of phyllosilicates rimmed by 0.1–0.3 μm wide zones of altered glass. The altered glass has lost significant Mg, Fe, Ca and Na, slight amounts of Al and Mn, and exhibits an order of magnitude increase of K. This incongruent dissolution occurred at neutral to acid pH and temperatures <90°C. Phyllosilicates consist of montmorillonite–saponite with berthierine interlayers, plus a mica component. The phyllosilicates directly replace altered glass along the perimeters of the phyllosilicate areas, but the centers of many of these areas may be filled pore space formerly occupied by microbes that facilitated glass alteration. If so, the sizes of the bacteria ranged to less than 0.1 μm and into the range of nano-organisms. Iron in secondary products is essentially all ferrous, and is most likely mobilized from glass as the ferrous ion, possibly with the reduction of minor primary ferric iron in the glass. Small amounts of sulfide in secondary pyrite associated with altered glass may be derived from microbial reactions or from the glass itself. Net bulk chemical changes for altered glass plus phyllosilicates include losses of Si, Al, Fe, Mn, Ca and Na, plus possible slight losses of Ti. This material is redistributed into fracture-filling smectite along with significant K and Mg from seawater. The uptake of K requires the circulation of large amounts of seawater through fractures in the glass (>100 fracture volumes), which may be important for the supply of nutrients for microbial activity.