SITE 1188Site Objectives
The principal objectives of drilling at Site 1188 were to establish subsurface alteration and mineralization patterns, and their variation with depth, beneath this area of low-temperature diffuse venting and acid sulfate alteration at the seafloor. Other objectives included defining fluid pathway structures, testing the possible existence of "subhalative" massive sulfide layers, establishing the volcanic architecture, if allowed by the alteration, and delineating the extent and characteristics of subsurface microbial life. These objectives required relatively deep penetration, our aim being 500 m with the possibility of even deeper drilling if conditions were suitable.
Holes 1188A and 1188F are only ~30 m apart (Fig. 8), so together they provide a vertical section to a depth of 386.7 mbsf of the lithologic architecture beneath the low-temperature diffuse Snowcap Knoll hydrothermal field located within the PACMANUS hydrothermal area of Pual Ridge (Fig. 9, Fig. 10). Volcanic rocks from the upper part of Hole 1188A are unaltered rhyodacite (Fig. 11). Plagioclase (optically identified as labradorite) is the most common phenocryst phase with subordinant titanomagnetite (Fig. 12). Unaltered plagioclase crystals in the deeper altered rocks also have the optical properties of labradorite. Most of the rocks are either vesicular or amygdaloidal, with instances of open or incompletely filled vesicles persisting to some of the deepest units (Fig. 13). The groundmass of all the igneous rocks is uniformly fine grained, and porphyritic rocks contain relatively few and relatively small plagioclase phenocrysts (generally only 1%2% and 12 mm long). Thus, all of the rocks are interpreted as volcanic, there being no coarser-grained rocks or textures such as microgranitic ones that might indicate a hypabyssal origin.
Occurrences of perlitic texture (Fig. 14), spherulitic texture, flow banding (Fig. 15), and volcanic brecciation and autobrecciation all attest to the success of the coring program in sampling various coherent and brecciated portions of the volcanic rocks that built up the upper 387 m of Pual Ridge. Occurrences of hydrothermal breccia and pseudoclastic textures illustrate details about the subsequent lithologic modifications that overprint the volcanic rocks when they are subjected to subseafloor hydrothermal activity.
Alteration varies with depth and is complicated by overprinting relationships (Fig. 16). A 34 m-thick cap of fresh rhyodacite to dacite, possibly intercalated with altered units that have not been recovered, is underlain by a sequence of deeply altered volcanic rocks from that depth in Hole 1188A to the bottom of Hole 1188F (387 mbsf). The original volcanic nature of the rock can often be recognized (Fig. 17). Pseudoclastic textures are frequently developed (Fig. 18). The earlier, more pervasive alteration (greenish) comprises silica forms and polymorphs varying successively from opaline silica (above) through cristobalite to quartz (below) accompanied by clays. Clays are mostly illite (Fig. 19) with chlorite, the latter especially at depth. Mixed-layer chlorite-smectite and pyrophyllite are also common. A second type of alteration (generally white), seldom truly pervasive and generally as veins or vein controlled, is composed of silica (as above), clays, pyrophillite, and variable amounts of pyrite and anhydrite (Fig. 18). Deeper in the site (Hole 1188F), quartz and chlorite become more important, as a later silicification event (+ clays) overprints the two previous ones. Late anhydrite-pyrite-quartz veins are ubiquitous throughout (Fig. 20).
Sulfide and Oxide Petrography
Pyrite is the dominant sulfide mineral at Site 1188. The first appearance of pyrite correlates with the onset of hydrothermal alteration of the uppermost dacitic volcanics in Hole 1188A. Within the hydrothermally altered rocks, pyrite is predominantly present in trace amounts (0%5%) within four distinct settings: associated with anhydrite-silica-filled fractures (most common), as vesicle linings associated with anhydrite and silica, in silica-anhydrite-magnetite veins, and disseminated throughout the variably altered igneous protoliths.
Chalcopyrite is a minor phase throughout the core from Site 1188. Chalcopyrite first appears in trace amounts within a chloritized interval as small inclusions in quartz. Deeper within the section, chalcopyrite is present as isolated anhedral grains and, in a few places, as partial replacements of pyrite (Fig. 21). Chalcopyrite is also present in a few vesicles.
Pyrrhotite is a trace mineral in cores from Hole 1188F (but not in samples from Hole 1188A). Pyrrhotite is present exclusively as small (0.006 to 0.01 mm), pink, anhedral to equant subhedral inclusions within pyrite grains (Fig. 22).
Sphalerite is rare, observed only macroscopically in vesicles, in some cases perched on pyrite
crystals that it clearly postdates. Iron-poor honey-yellow crystals are most common, but a black
variety was also described, in one case in the same vesicle as the honey-yellow type.
Titanium magnetite is an accessory and, in places, a liquidus phase of the relatively unaltered dacitic rocks that cap the sequence of highly altered rocks. Hydrothermal magnetite, some with octahedral habit and typically 0.2 mm in diameter, is first found in the lower part of Hole 1188A within altered volcanic rocks. The magnetite is invariably observed within veinlets with silica and anhydrite and in vugs together with green clay, anhydrite, and pyrite. The downhole distribution of magnetite in thin section shows maxima at 150165 m in Hole 1188A and 355375 m in Hole 1188F. These maxima coincide with peaks in pyrite abundance, probably because of the prevalence of both veins and, in the case of magnetite, vein halos.
The structures identified in these cores are primary volcanic layering, orientations of veins, and vein relationships. Original layering was identified from the orientation of elongate flattened and stretched vesicles in some of the massive lavas and, in other parts of the core, from millimeter scale color banding attributed to flow (Fig. 13). Several features of the veins are worthy of note. Anhydrite and pyrite are the most common mineral assemblages in the veins and are present from 48 mbsf in Hole 1188A and to 378.5 mbsf in Hole 1188F (Fig. 23). Cristobalite-bearing veins were only encountered in the upper part of the system (i.e., above ~126 mbsf). Magnetite-bearing veins are at two depth intervals: between 146 and 184 mbsf in Hole 1188A and between 322 and 378.50 mbsf in Hole 1188F. Crack-seal veins and alteration halos showing multiple zonation were only observed below 218 mbsf. Furthermore, the alteration halos around the veins consisting of quartz and clay minerals tend to be more intense in Hole 1188F compared to Hole 1188A. Crosscutting relationships between veins are more abundant in Hole 1188F than in Hole 1188A. Brecciation and network veining are only present in Hole 1188A above 110 mbsf, and such structures were not observed in Hole 1188F. There are no systematic trends with respect to dips of veins with depth, and the vein paragenesis is very complex as a result of several episodes of fluid infiltration (Fig. 24).
Onboard chemical analyses were performed on 42 altered rocks from Site 1188, mostly of homogeneous rocks but including some with anhydrite-dominated veins or breccia matrices. Some analyses of the latter display elevated contents of CaO. More generally, CaO is similar to or lower than its content in fresh dacites depending on whether relict igneous plagioclase has been preserved, and Na2O shows a similar behavior. Enrichment in MgO relative to likely parents is a common phenomenon, particularly in chloritic rocks. Depending on the relative abundance of illite, K2O varies from severely depleted to modestly enriched. On a relative basis, MnO is always extremely depleted. Sulfur is distinctly enriched reflecting the presence of pyrite, or of pyrite and anhydrite. Both enrichment and depletion of iron (total as Fe2O3) occur, but some pyritic samples have similar Fe contents to those of unaltered dacites and rhyodacites, which indicates that pyrite has commonly formed by a sulfidation process. Immobile behavior is apparent for Al2O3, TiO2, Zr, and Y and less certainly for P2O5.
Both direct bacterial counting and adenosine-triphosphate (ATP) analysis established the presence of microbial biomass within core samples from ~34 and 49 mbsf, respectively, but not on a sample from 60 mbsf or others deeper in Holes 1188A and 1188F. Samples were successfully cultivated at various temperatures under both aerobic and anaerobic conditions to 25° and 90°C from samples taken at 10 and 34 mbsf and at 25°C under anaerobic conditions from the 49 mbsf sample. Aerobic microbes were also cultivated at 25°C from very deep rock samples at 222 and 225 mbsf, but these are believed to arise from seawater contamination.
Magnetic susceptibility varies greatly over the length of the recovered core, but generally increases downsection. The average compressional wave velocity for all samples from this site is 4.1 km/s; however, the more massive volcanic rocks commonly have higher compressional velocities than the brecciated and flow-banded rocks. Thermal conductivity is higher in brecciated rocks (average = >2 W/(m·K) than in massive altered dacite (usually < 2 W/(m·K)). Average measured solid density differs somewhat between Holes 1188A and 1188F (2.65 and 2.82 g/cm3 , respectively). This may reflect a higher abundance of veins (anhydrite + pyrite) in samples from the lower part of the cored interval. Porosity is highly variable, from < 1% to nearly 45%, but with an overall trend toward decreasing porosity with depth.
The top 35 m of the cored material is characterized by high magnetic susceptibility and remanent intensity. This depth corresponds to the relatively fresh dacite-rhyodacite section at the top of the core. The magnetic measurement values show a considerable drop from 35 to ~135 mbsf, representing the high alteration in Hole 1188A. A sudden increase in both the susceptibility and remanent intensity is present below 135 mbsf. The cause of this change is unclear. However, the presence of a large amount of paramagnetic minerals, such as pyrite, can increase the magnetic susceptibility. Thermal demagnetization experiments indicate the dominant magnetic carrier in the upper part of the section is titanomagnetite with variable degrees of alteration. Deeper in the core, magnetite and possibly maghemite may be the important sources of magnetization. One of the most notable features in the magnetic character of this section is a sharp rise in the susceptibility values below 275 mbsf. The high magnetization intensity at the bottom of the hole is consistent with the presence of magnetite, which was identified by X-ray diffraction and optical microscopy.
Maximum borehole temperature during the drilling of Hole 1188A was 4°C as recorded with the developmental Lamont-Doherty Earth Observatory core barrel temerature tool (CBTT). Borehole temperatures were also measured with the wireline logging temperature tool in Hole 1188F, registering 100°C a few hours after coring ended. Five days later the UHT-MSM tool measured a maximum temperature of 304°C in the bottom of Hole 1188F. A final temperature run in Hole 1188F with the UHT-MSM tool on the last day of operations recorded a maximum of 312°C. Water samples were collected from Hole 1188B and 1188F. The sample from Hole 1188B was taken only a few meters below seafloor because of a blockage in the hole, and a concomitant temperature measurement was 6°C. Water samples also followed each UHT-MSM run in Hole 1188F, but difficulties in rapidly estimating the temperature gradient to avoid high temperatures that would have damaged the WTSP resulted in water samples taken at 12°C (107 mbsf) and 22°C (206 mbsf). Hole 1188B was drilled with the LWD/RAB BHA, and the data provide a 360° image of the borehole resistivity characteristics that might be used for lithologic correlation. Wireline logging in Hole 1188F indicates that the borehole has a much larger diameter (in excess of 17 in) than we expected from drilling with the 7.5-in-diameter diamond bit. High U in spectral gamma-ray measurements was recorded at 197209 mbsf, and a smaller peak was recorded at 239245 mbsf.
Site 1189 | Table of Contents