Aging of the Glass to Altered-Glass Boundary in Seamount Basalts

In oceanic basalts, fresh volcanic glass reacts with seawater, resulting in the loss of cations from the silica network and the formation of altered glass that retains insoluble cations and potassium. Microorganisms are associated with the glass to altered-glass (GAG) boundary, although their role is not known. It has been hypothesized that microorganisms promote the alteration of volcanic glass to take advantage of the reducing power of iron and manganese in the silicate glass, or alternatively, that microorganisms colonize the GAG boundary to use iron and manganese that are released during alteration. We examined the chemical composition of the GAG boundary of seamount basalts from the Gulf of Alaska that span the age range of <0.1 Ma to 49 Ma. All samples in the study were collected by manned or unmanned submersibles from outcrops on the sea floor, and were presumably exposed to bottom seawater from the time of eruption to the time of collection. The underlying assumptions of this study are that (1) the glass would become less reactive as it aged due to the closing off of fluid pathways as fractures became filled and as the surface of the basalt became coated with manganese crust; (2) the diminished fluid movement would result in the extinction of the original community of microorganisms, or in a change in the microbial community at the GAG boundary; and (3) the changing fluid flow and biological conditions in the rocks would result in changes in the composition of the altered glass at the GAG boundary. The younger basalts (<7 Ma) have the granular texture at the GAG boundary that has been associated with microorganisms. The older samples in this study (30 Ma and 49 Ma) have no, or only minor, granular texture. The altered glass of the older samples have higher Ti and lower Al, Mg, and Mn than the altered glass of the younger samples. Fe is enriched in the altered glass relative to the glass in samples of all ages. Phosphorus, an element that could be elevated by the presence of microorganisms, is only enriched in one area of altered glass in the 4 Ma sample. The aging of the GAG boundary does result in changes in composition of the altered glass, however, the influence, if any, of microorganisms on this change is not known.