QUANTIFYING BIOEROSION IN HAWAI'IAN HYALOCLASTITES: DEPTH, MII, AND PARAGRNESIS IN THE HSDP CORE

Semi-quantitative measurements of abundance describe distribution and features of euendolithic microborings (elmb) in basalt hyaloclastites in the core from the Hawaii Scientific Drilling Program #2 phase 1 (HSDP #2₁) boring. Coupled with paragenesis of glass alteration, the new observations help to constrain the history of microbial activity and the conditions under which it occurred.

The HSDP #2₁ core reaches 3062.1 meters below sea level (mbsl). Samples studied here were from Mauna Kea-sourced submarine deposits, below 1080 mbsl. Hyaloclastite makes up ~50% of the submarine rock column, is dominant down to 1990 mbsl, and is interbedded with pillow lavas deeper in the core. A microendolithic ichnofabric index (MII), adapted for the size of the microborings and their situation in glass, quantified abundance of elmb during visual observation. Clays, zeolites, Ca-silicate, palagonite, and elmb, form in three stages of alteration increasing with depth: incipient (I, 1080 to 1462 mbsl), smectitic (1478 to 1573 mbsl), and palagonitic (P, below 1573 mbsl).

Abundance of elmb is below 1.2 (average of 50 views in each sample) on the MII scale at depth of <1320 m (MII of 1 = no disruption of glass) in the incipient (I) zone, and is 3 (= 10 to 40% disruption) or less in all views from that interval. In the interval from 1320 to 2125 mbsl, individual views in samples have MII as high as 4 (40 to 60% disruption) and 5 (>60% disruption). Average abundance reaches maxima of 2.26 and 2.44 at depths of 1397 (I) and 2117 mbsl (P), but ranges as low as 1 in some samples. At depth >2125 mbsl, no samples had MII greater than 1.2. In fact, 15 of 34 samples display no borings, only 4 have individual MII views of 3 and one had one view of 4.

All samples with average MII greater than 1.2 have well developed smectite coating on surfaces, suggesting smectite formation (and the earlier fracturing) preceded most boring activity. Later smectite formation and pore-filling zeolites reduced porosity and permeability to very low values, probably terminating boring. No such terminating event occurred for samples from 1320 to 1478 mbsl. It is unclear why samples below 2125 mbsl, with visibly identical alteration to those above 2125, are not bored, although they lie below the 1^st pillow-lava interval.