PETROGRAPHY OF PERIDOPHYLLIC ENDOLITHIC MICROBORINGS FROM HYALOCLASTITES OF KILAUEA'S HILINA SLOPE: COMPARISON WITH MICROBORINGS IN HSDP HYALOCLASTITES

Hyaloclastite samples collected during JAMSTEC dives on the Hilina slope of Hawaii contain a variety of endolithic microborings resembling those from the submarine portion of the Hawaii Scientific Drilling Project #2 Phase 1 (HSDP 2₁) core. Microborings from HSDP 2₁ samples accept stains with DAPI. Other workers have reported extracting DNA from archea from one HSDP 2₁ sample. Similar microborings, reported to be of organic origin, are common in basaltic glass samples from marine environments. Alteration sequence of hyaloclastites in Hilina slope samples resembles that in HSDP 2₁: early isopachous and replacement smectites are followed by palagonitization and formation of zeolite cements. As a tribute to R.L. Folk in his 80th year, this contribution emphasizes information learned from petrographic study of the microborings.

Microborings extend into glass from any surface. Such borings are initially about 1 μm in diameter and 10s to over 100 μm long, and initially taper only where they break into branches. Many types have finials or other ornaments at their end. Subsequent alteration reshapes the microborings into steep cones, frequently with smectite linings or fillings. Microborings facilitate alteration of basalt glass to smectite. Some microborings in both occurrences are distinctly peridophyllic: they curve toward and focus on olivine (Ol) crystals in the hyaloclastite shards. Where no Ol is close to the margin of the shard, microborings follow curving or irregular paths into the glass or may branch into a tree-like pattern.

Borings in Hilina samples differ from those in HSDP 2₁ samples in one key respect. In Hilina samples, most borings initiated after palagonite formed and extend from the interface between unaltered glass and palagonite that has replaced the margin of the shard. In HSDP 2₁ samples, borings originated early in the alteration history at the primary margin of the shard and are preserved during palagonitization. As formation of palagonite is a stage in the progressive alteration of basaltic glass, the timing of infection with microboring organisms suggests that the Hilina samples were buried too quickly for effective infection or significant growth, like samples in the lower part of the HSDP 2₁ core. Borings formed in Hilina samples only after exposure on the slope by mass failure.