Alteration of submarine basaltic glass from the Ontong Java Plateau: A STXM and TEM study

Frequent observations of tubular to vermicular microchannels in altered basalt glass have led to increasing appreciation of a possible significant role of microbes in the low-temperature alteration of seafloor basalt. We have examined such microchannel alteration features at the nanoscale in basalt glass shards from the Ontong Java Plateau using a combination of focused ion beam milling, transmission electron microscopy and scanning transmission X-ray microscopy. Three types of materials were found in ultrathin cross-sections cut through the microchannels by FIB milling: fresh basalt glass, amorphous Si-rich rims surrounding the microchannels, and palagonite within the microchannels. X-ray absorption spectroscopy at the C K-edge and Fe L2,3-edges showed the presence of organic carbon in association with carbonates within the microchannels and partial oxidation of iron in palagonite compared with basalt glass. Although these observations alone cannot discriminate between a biotic or abiotic origin for the microchannels, they provide new information on their mineralogical and chemical composition and thus better constrain the physical and chemical conditions prevailing during the alteration process.