6. Site 1191¹

Shipboard Scientific Party²

INTRODUCTION

Hole 1191A is located among chimneys in the Satanic Mills high-temperature hydrothermal site (see Fig. F1 in the "Leg 193 Summary" chapter) and was cored during time made available while preparation for deployment of a casing string was under way. Drilling attained a depth of only 20.1 meters below seafloor (mbsf) because of borehole instability.

We drilled within the hard crust of vesicular dacite/rhyodacite found at the previous sites without passing through it. Drilling intersected fresh to mildly altered dacite-rhyodacite (70 wt% SiO₂ on an anhydrous basis) for the entire hole depth to 20.1 mbsf. This is moderately vesicular and aphyric in hand specimen. Large vesicles, as long as 4 cm, are severely stretched.

Alteration increases gradually downward, from slight to moderate. It consists of both replacement of the vitreous matrix and vesicle and fracture fillings. The igneous groundmass is replaced in patches by cristobalite and clay. Vesicles become lined by a silica + sulfide + clay coating, in which, for most vesicles, euhedral zeolites grow (possibly clinoptilolite and, much less abundant, phillipsite). Below ~11 mbsf, fractures and nearby vesicles are lined by subhedral marcasite, which includes framboids of pyrite. One marcasite veinlet has a selvage of more intense cristobalite-clay alteration. This alteration style differs from that found below fresh dacites at the top of previous holes drilled through hydrothermally active sites (Sites 1188 and 1189) in two main aspects: the absence of anhydrite (abundant at Sites 1188 and 1189) and the presence of abundant zeolites. This alteration style is believed to reflect passage of hydrothermal fluids almost to the seafloor.

High-temperature hydrothermal chimneys surround Site 1191; thus, the alteration found may be considered weak. The absence of anhydrite may be explained by a variety of causes, including (1) alteration at temperatures below 150°C, (2) noninvolvement of unmodified seawater in the alteration, and (3) later dissolution by circulation of cooler fluid. The zeolites may have formed separately, perhaps during a cooling, oxidative episode after eruption of the lava.

One of the main results of Site 1191, coupled with the other sites, was to establish the general presence of the hard crust of fresh to weakly altered dacite/rhyodacite throughout Pual Ridge. Site 1191 samples provided a reliable estimation of the magnetic inclination (-16°), which is significantly different from the present-day value of ~7.7° for the region. Whereas this may result from a slight deviation during drilling with the rotary coring system, the more or less consistent magnetic inclination among sites and the young age of Pual Ridge suggests the presence of a tilted block.

Active bacterial cells were found by direct counting on two samples from depths of ~10 and 15 mbsf, respectively. Anaerobic bacterial cultivation tests on both samples yielded positive results at temperatures as high as 90°C.

The main objective of drilling at this high-temperature venting site was to test for similarity or otherwise of its subsurface characteristics without more extensive characterization of the Roman Ruins hydrothermal site (Site 1189). Limited penetration resulted in only recovering the uppermost part of the lithologic section at Site 1191. However, as little time was effectively consumed drilling here (profiting from a necessity for other drill-rig operations), we consider the exercise as rather valuable, as it provided both additional fresh volcanic samples and a small-scale example of near-seabed hydrothermal activity.

Hole 1191A

After operations at Hole 1188B, we chose to use the time while the drilling team was preparing for our casing deployment to attempt a single bit hole at one of our alternate high-temperature sites. During a 1.25-hr camera survey of the seafloor, we located a suitable position for Site 1191 (Table T1). We initiated Hole 1191A at 2045 hr on 27 November 2000, with the bit tagging the seafloor at 1705 meters below rig floor (mbrf) in an area of small standing and fallen chimneys. The vibration-isolated television sleeve was recovered, and continuous rotary core barrel coring continued through Core 193-1191A-3R to a depth of 1725.1 mbrf (20.1 mbsf). At that point, the driller lost rotation of the drill string. The pipe stuck with the bit only 17 mbsf. After working for 1.25 hr, the driller freed the pipe with 170 klb of overpull. However, upon deploying the next core barrel, the driller noted that the pump pressure was low, indicating that something might be keeping the barrel from landing and seating properly. Following two deployments of a bit deplugger, an inspection of the latch and landing shoulder assembly indicated that the barrel was indeed landing and latching properly. Pump pressures were normal with the bit off bottom, but lower than expected with the bit in contact with the bottom of the hole. After 10 hr of attempting to ream through 3 m of hard fill, we finally regained our original depth of 20.1 mbsf.

However, when we could not advance while cutting Core 193-1191A-4R even after 3 hr of rotation, we decided to terminate coring and move on to our casing attempt at Site 1188. On our ensuing wireline run, we were not able to engage the core barrel, so we tripped the pipe. The end of the drill string arrived on deck without the bit, the bit disconnect assembly, or the core barrel; apparently, the mechanical bit release had failed because of excessive wear during coring. Unfortunately, we had also deployed and lost the Lamont-Doherty Earth Observatory drill string accelerometer tool housing on our final coring attempt, and because it was unique, we did not have the opportunity for more temperature measurements using this tool. A total of 3.36 m of core was recovered for an overall average of 16.7% of the section cored (Table T1).

Targeted at the western side of the Satanic Mills hydrothermal site, the bit descended over rough volcanic outcrops. After we dropped a beacon, the bit was moved 20 m east across volcanic talus then descended 3 m to an area of chimney debris. Continuing 15 m farther (descending another 2 m), we passed a large (10 m) fallen chimney and several clusters of small standing chimneys venting shimmering fluid, before moving southwest for 10 m and spudding on fine chimney debris.