BEHAVIOR RECORDED IN ENDOLITHIC MICROBORINGS IN BASALT GLASS FROM HAWAII

Endolithic microborings in basalt glass from oceanic settings are trace fossils that indicate behavior of microorganisms. Samples from the Hilina Slope (offshore, SE of Kilauea) and from the core from the Hawaii Scientific Drilling Project #2 well at Hilo, display such endolithic microborings. They are present in basalt glass in hyaloclastites and pillows. Tubes are initially ~1 μm wide, and 10s to over 100 μm long. They extend into basalt glass from former free surfaces, e.g. margins of hyaloclastite grains or pillows, fractures, or vesicles, or from alteration fronts.

Several lines of evidence suggest that the tubes are indeed microborings, i.e. they are the results of microbial activity (review in Banerjee et al., 2006). Various studies have reported the presence of DNA and other vital compounds; have described clustered distribution, both in individual samples and in large volumes of rock; and have established that timing of formation of microborings precludes magmatic or quench-related processes.

Key behaviors are presumably related to the search for nutrients. Activities include 1) boring into glass, 2) peridophilia, or attraction to olivine, and 3) avoidance of plagioclase. In the absence of olivine, microborings appear to preserve a random mining behavior. Some microborings are more ornate than simple tubes; they seem to expand or become more complex with time, becoming steep cones that may be 10's of μm wide and show small branches. Some effects of boring activity are also characteristic. For example a specific alteration product of the glass, replacive smectite with micronodules of Ca-Ti silicate (not titanite), is highly associated with endolithic microborings.

Basalt glass is unstable and alters to palagonite or various mineral phases. During the process of palagonitization, the microborings indicating characteristic behaviors are pseudomorphed and smectitic replacement is preserved. Such preservation suggests that further alteration may not obliterate trace-fossil evidence of microbial activity. If so, one criterion for detecting organic activity in Archean or extraterrestrial rocks could be specific behavior represented by trace fossils (c.f. Furnes et al., 2004).

References: Banerjee et al. (2006) Earth & Planet. Sci. Lett. 241:707. Furnes et al. (2004) Science 304:578.