Geochemical characterization of tubular alteration features in subseafloor basalt glass

Abstract

There are numerous indications that subseafloor basalts may currently host a huge quantity of active microbial cells and contain biosignatures of ancient life in the form of physical and chemical basalt glass alteration. Unfortunately, technological challenges prevent us from observing the formation and mineralization of these alteration features in situ, or reproducing tubular basalt alteration processes in the laboratory. Therefore, comprehensive analysis of the physical and chemical traces retained in mineralized tubules is currently the best approach for deciphering a record of glass alteration. We have used a number of high-resolution spectroscopic and microscopic methods to probe the geochemical and mineralogical characteristics of tubular alteration features in basalt glasses obtained from a suite of subseafloor drill cores that covers a range of different collection locations and ages. By combining three different synchrotron-based X-ray measurements – X-ray fluorescence microprobe mapping, XANES spectroscopy, and μ-XRD – with focused ion beam milling and transmission electron microscopy, we have spatially resolved the major and trace element distributions, as well as the oxidation state of Fe, determined the coordination chemistry of Fe, Mn and Ti at the micron-scale, and constrained the secondary minerals within these features. The tubular alteration features are characterized by strong losses of Fe2+, Mn2+, and Ca2+ compared to fresh glass, oxidation of the residual Fe, and the accumulation of Ti and Cu. The predominant phases infilling the alteration regions are Fe3+-bearing silicates dominated by 2:1 clays, with secondary Fe- and Ti-oxides, and a partially oxidized Mn-silicate phase. These geochemical patterns observed within the tubular alteration features are comparable across a diverse suite of samples formed over the past 5–100Ma, which shows that the microscale mineralization processes are common and consistent throughout the ocean basins and throughout time. The distributions of Ti and Cu are distinct between tubular mineralization and the crack-filling minerals and thus delineate sequential stages of fluid–rock interaction. The preserved chemistry of clay and oxide mineralization in the tubular alteration then represents a common precursor state (e.g. Ti accumulation), that has not yet undergone recrystallization (e.g. titanite formation) as observed in many older, metamorphosed examples of tubular alteration.