SITE 1189Site Objectives
In addition, Site 1189 was designed as a test of the nature and extent of microbial life, particularly hyperthermophilic bacteria, at such a high-temperature hydrothermal site, together with delineation of the conditions conducive to such deep biomass.
Holes 1189A and 1189B (Fig. 25) provide a first-order understanding of the volcanic origin of the upper 200 m of Pual Ridge at the Roman Ruins high-temperature black smoker vent field. The only fresh rocks recovered are aphyric dacite in the uppermost core from Hole 1189A. Although all other cores from this site show high to complete alteration, abundant evidence for extrusive volcanic features shows that the whole sequence is either volcanic or volcaniclastic in origin.
Sparse relict plagioclase phenocrysts or pseudomorphs thereof are preserved in several of the Holes 1189A and 1189B lithologic units (Fig. 26). Clinopyroxene (almost always replaced) and titanomagnetite are much more rare. Fresh plagioclase, especially in Hole 1189B, is characteristically rounded (Fig. 27), suggesting a period of phenocryst instability and dissolution prior to or during eruption. Many coherent volcanic units from Site 1189 are vesicular, with the degree of vesicularity ranging up to~20 vol% (Fig. 28). Hydrothermal alteration has commonly resulted in vesicle filling by secondary minerals, resulting in an amygdaloidal texture. The lithologic succession at Hole 1189A is comprised of alternating coherent volcanic rock units and brecciated units. Several units show fragmental textures that have been logged as hydrothermal breccia because conclusive evidence for a volcaniclastic origin (e.g., grading, layering, rounding, or polymict composition) is lacking.
In the cored part of Hole 1189B, below 31 mbsf, the rocks recovered alternate between coherent volcanic units and brecciated units. Above ~70 mbsf, many of the breccias have stockwork-like veining. Below 70 mbsf, and particularly below 137 mbsf, the volcanic rocks are, on average, less altered and richer in relict plagioclase. One moderately vesicular rock unit from Hole 1189B (from ~138 mbsf) is noteworthy for its highly stretched vesicles, all of which have steep stretching orientations ranging from ~70° to 90°, indicative of subvertical flow. An upper limit on the thickness of eruptive units can be estimated at ~50 m by the presence of several intervals of polymict breccias, indicating paleoseafloor positions. Based on this evidence, it appears Pual Ridge was built up in at least five eruptive episodes.
The two holes cored (Holes 1189A and 1189B), although very close together (~30 m) show rather distinct alteration features with respect to each other, under a thin crust of fresh, hard dacite (cored in Hole 1189A). The main alteration features are summarized in Figure 29.
Alteration in Hole 1189A is similar to that found at Site 1188, but the presence of cristobalite is restricted here to the upper 25 mbsf, suggesting a higher geothermal gradient. Below the cristobalite-bearing interval, there is abundant quartz in all three main associations found at Site 1189 (early pervasive greenish quartz + clay, subsequent white quartz + clay ± anhydrite, and late silicification + clay). Small amounts of pyrite (1%5%) are found in all samples and assemblages. As at Site 1188, late quartz-pyrite-anhydrite veins are ubiquitous.
Hole 1189B intersected the only massive sulfide mineralization found during Leg 193. Very poor recovery precludes detailed interpretations, but massive and semimassive sulfides are hosted in anhydrite (±gypsum) at least partially replacing volcanic rock (Fig. 30). Underneath, there is a zone of stockwork-related alteration (to 119 mbsf) in which the rock clasts show complete alteration into greenish quartz + clay and are embedded in vein networks variably rich in anhydrite, quartz, pyrite, and hematite. Anhydrite predominates down to ~60 mbsf. Hematite is locally present disseminated in quartz as jasperoidal cement (Fig. 31). The amount of silicification increases downward through the upper sequence (especially below 70 mbsf).
Hole 1189B intersected a lower sequence below 127 mbsf comprising moderately to highly altered coherent volcanic rocks, highly to completely altered monomict breccias, and polymict volcaniclastic breccias and sandstones. In Hole 1189B, cristobalite is restricted to the least altered (generally coherent) volcanic rocks, rich in relict plagioclase, and is interpreted to be the result of simple devitrification. However, some cristobalite-bearing breccias contain complex alteration assemblages that include different clays (illite, chlorite, smectite, and mixed-layer varieties). Intense alteration produced some striking textures, including contrasting color banding attributed to the enhancement of flow banding and pseudoclastic textures (Fig. 32).
Sulfide and Oxide Petrography
In the cores from Holes 1189A and 1189B, pyrite is the dominant sulfide. Pyrite is disseminated within the groundmass of the volcanic rocks, within quartz-anhydrite veins, and finally as linings and cores to vesicle fill. Some accessory sulfides, namely sphalerite and chalcopyrite, are more abundant in Hole 1189B than in Hole 1189A. Marcasite, galena, tennantite, and covellite are rare, and the latter have only been identified in cores from Hole 1189B. In addition, the oxides magnetite, hematite, and possibly ilmenite (one example) are present. Semimassive sulfide mineralization comprising ~50% sulfides was recovered in Section 193 1189A-12R-1, 120128 cm (Fig. 33). The relationships between the accessory sulfides and pyrite provide evidence for at least two generations of pyrite-quartz precipitation (Fig. 34). Chalcopyrite is also observed in some late crosscutting veins. Pyrite is both intergrown with and includes chalcopyrite. Sphalerite in Hole 1189B provides convincing evidence for two generations of sphalerite deposition, before and after a period of pyrite formation. Nevertheless, the overall sulfide content of this site is low and should be considered as only a very weakly mineralized stockwork sequence.
The structures identified in Holes 1189A and 1189B were primary volcanic layering, brecciation of volcanic rocks, orientation of veins, and age relationship between veins. There are several interesting similarities and differences between the two holes with respect to vein structures.
The brecciated rocks in the two holes are very similar, consisting of variably altered volcanic fragments, crosscut by vein networks of quartz with pyrite and minor anhydrite. However, the volcanic rocks recovered from Hole 1189B are more brecciated than the rocks from Hole 1189A. Additionally, the vein intensity is higher and intervals of vein network (stockwork) are thicker in Hole 1189B (Fig. 35, Fig. 36). In contrast to Hole 1189A, magnetite and hematite in Hole 1189B are in the networks as minor components and sphalerite and chalcopyrite are present as trace minerals. In both holes, late coarse-grained anhydrite veins crosscut vein networks and brecciated rocks. More than 95% of the veins recovered from both holes were < 1 cm thick, indicating that we did not intersect any major fluid pathways feeding the sulfide deposits.
One fresh volcanic rock from Hole 1189A was analyzed and is dacitic in composition, similar to fresh samples from other sites but with distinctly lower SiO2 (64.25 wt%) and Zr/TiO2 (~130 vs. ~250). All other analyses were performed on altered rocks, and these analyses generally reflect the transformations described in the alteration mineralogy sections of this report. In Hole 1189A, green, silica-clay altered rocks have high water and sulfur contents and increases in Fe2O3 and MgO correspond to increases in reported chlorite, clay minerals, and pyrite. In Hole 1189B, SiO2, CaO, and Na2O increase with depth, in agreement with the observed increase in plagioclase content.
Bacteria were not detected by direct count below 50 mbsf in cores from Hole 1189A, and no bacteria were detected by this method in any core below the first core (31 mbsf) from Hole 1189B. Enrichment cultivations of samples from Hole 1189A showed growth of bacteria in both aerobic and anaerobic conditions to 25°C and as high as 90°C in anaerobic conditions. Samples from Hole 1189B exhibit growth of bacteria in anaerobic cultures from as deep as 130 mbsf and at temperatures to 90°C.
Magnetic susceptibility decreases from top to bottom in the cored section from Hole 1189A. A similar pattern is expressed in data from the lower part of Hole 1189B, albeit the overall susceptibility is distinctly higher. Compressional wave velocity averages 4.4 km/s at ambient pressure. Thermal conductivity is relatively constant at ~2 W/(m·K), with the exception of one pyrite-rich sample that has a value of >5 W/(m·K). Solid rock density is constant in all samples measured from both holes, averaging 2.7 g/cm3. As in samples from Site 1188, porosity is highly variable, from 15% to nearly 70%.
In terms of rock magnetic behavior, Site 1189 shares many common features with Site 1188. The uppermost part of the section at both sites is characterized by high susceptibility and high remanent intensity caused by abundant magnetite and titanomagnetite. At both sites, the upper few tens of meters are underlain by an interval of low remanent intensity. Below this is a zone of high susceptibility and high remanent intensity. Inasmuch as the maximum remanent intensity is present in this lowermost interval, it is conceivable that this lower zone is equally important, if not more so, as the source of the magnetic anomalies measured from the sea surface. Some significant differences also exist between Sites 1188 and 1189 in terms of magnetic behavior. The highest measured magnetization intensity is deeper at Site 1189 than at Site 1188. However, susceptibility and remanent intensity are both generally lower at Site 1189. This may reflect more intense high temperature alteration at Site 1189.
Site 1189 will likely turn out to be the highlight of downhole measurements for Leg 193. Continuous wireline logs were collected in Hole 1189B with excellent tool response as a result of good borehole condition. Formation MicroScanner (FMS) data are particularly striking, imaging different patterns of fracturing and local dissemination of sulfide minerals. Our second LWD/RAB experiment, Hole 1189C, was drilled to 166 mbsf, past a distinct lithology change described in cores and potentially imaged in Hole 1189B wireline logging data. Following completion of RAB drilling, we dropped a FFF and conducted wireline logging of the same hole. This marks the first time in ODP history that we have the opportunity to correlate directly between wireline logging and RAB data from the same drilled interval in hard rock.
Site 1190 | Table of Contents