Seafloor alteration of basaltic glass: Textures, geochemistry, and endolithic microorganisms

Abstract

The incipient low‐temperature alteration of glassy margins of recent seafloor lavas from the Mohns Ridge is characterized by scanning electron microscopy and bulk chemical analyses of major and trace elements. These investigations are designed to evaluate the endolithic microbial biomass, to identify relations between alteration textures and microorganisms, and to identify the chemical processes that take place during alteration. The basaltic glass along intersecting fractures and around vesicles is typically altered to concentrically zoned, yellow‐brown amorphous gel‐palagonite. In most fractures microorganisms are observed at the outer surfaces of palagonite rims, within porous zones of the rims, and frequently also at the glass‐palagonite interfaces. The cells act as nucleation sites for precipitation and become encrusted and embedded in palagonite with time. Zones of porous palagonite containing numerous hollow, fossilized cells alternate with zones of compact palagonite lacking distinct cell structures, which together indicate that the microbial growth is discontinuous. The palagonite has an average organic carbon content of 0.9 wt% (δ13Corg: −22‰), which derive from both living and fossilized biomass. The microbial growth and biomineralization are major controls on the porosity and texture of the palagonite and thus likely on the chemical exchange between glass and seawater. Pit marks in the glass in fractures both with and without microbes indicate that microbial as well as abiotic processes mediate pitting. Elemental and isotope data show that the transformation of glass to gel‐palagonite at seafloor conditions with high water/rock ratios results in near complete loss of Si and alkali elements to seawater; formation of Fe‐ and Al‐oxyhydroxides; complete exchange of alkaline earth elements like Sr with seawater; retention of V, Cu, Y, Pb, Th, U, and trivalent REEs, accompanied by a strong gain of these and other elements from seawater. Higher Fe/Ti ratios in the gel‐palagonite compared to the parental glass suggest that the palagonite represents a mixture of Fe‐Ti oxyhydroxides derived from both the glass and seawater.