LOW TEMPERATURE, OXIDATIVE ALTERATION OF BASALT: SPECIAL IMPLICATIONS FOR BIOCORROSION

Microbial life in Earth’s oceanic crust resides in mafic materials: under sediments, pillow basalts are underlain by dyke complexes and extensive massive gabbros. In these rocks, over the course of Wilson cycles, alteration by seawater and biological agents occurs, generating increasingly complex combinations of alteration textures. Trace mineral production serves as an indicator (ephemeral or long lasting) of water-rock system status. In this work, we align experimental and modeling results of the alteration of basalt by (i) seawater and (ii) hydrogen peroxide (H2O2) solution to parse distinct mineral products linked to common terrestrial water action on this rock type. Experimentally, Ward’s Scientific basalt (Twin Falls, Idaho) and gabbro (Duluth, Minnesota) were sectioned into coupons, polished, and subjected to aqueous alteration for 1 to 28 weeks; coupons were set in flow-through seawater tanks or reacted in 12% H2O2 in Petri dishes. At selected time steps, coupons were removed, rinsed, dried, sub-sampled for x-ray diffraction (XRD) to target chemically attacked corners/edges, and examined using scanning electron microscopy with energy dispersive x-ray spectroscopy (SEM-EDS). Textures and trace mineral detections were logged. To contextualize the observed corrosion, Geochemist’s Workbench (GWB) was employed in REACT mode to model similar aqueous alteration. GWB model results suggest that in the earliest reaction stages, dolomite, talc, goethite, and saponite form but may be lost; brucite, magnetite, antigorite, pyrite and strontianite may accumulate later. Trace minerals that may be rare in terms of detection, but important indicators of ongoing reactions, included Na-saponite, Na-nontronite, chalcocite, mordenite, pyrolusite, chalcopyrite, sphalerite, and possibly other Cu-Fe-oxide solids. A review of the common low temperature alteration minerals in mafic rocks is presented to aid the interpretation of GWB findings, and together inform trace mineral analyses of the experimental products. Implications for deep crustal biosphere monitoring and seabed industrial infrastructure concerns are discussed.