Elongate, fine tubes, ~1 µm wide and up to 200 µm long, extend from fractured surfaces, vesicle walls, and internal fractures into fragments of basalt glass in samples from the Hawaii Scientific Drilling Project #2 phase 1 (HSDP #21) core and the Hilina slope, Hawaii. Several features indicate that these tubes are microbial endolithic microborings: the tubes resemble many described microborings from oceanic basalt glass, their formation is postdepositional but restricted to certain but different ranges of time in the two sets of samples, and they are not uniformly distributed throughout glass fragments. Microtubules record several characteristic behaviors including boring into glass, mining, seeking olivine, and avoiding plagioclase. They also are highly associated with a particular form of glass-replacing smectite. Evidence of behavior should join morphological and geochemical criteria in indicating microbial alteration of basalt glass.
In some samples, steeply conical tubes, ~10–20 µm in diameter tapering to 1 µm and commonly filled with smectite, appear to be modifications or elaborations of the microtubules. These also curve toward olivine and are associated with replacement smectite. In HSDP #21 samples, microtubules initiated at margins of shards before palagonite replaced those margins and are preserved during palagonitization. In fact, microtubules appear to have provided routes that enhanced the efficiency of water's reaching of unaltered glass. In Hilina Slope samples, the microtubules appear to postdate palagonitization because they initiate at the boundary between palagonite and unaltered sideromelane.
Preservation of microtubules during palagonitization in samples together with recognition of other associated characteristics representing behavior suggests that such features may be recognizable in more heavily altered ancient rocks.