SITE 1191Site Objectives
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 (6970 wt% SiO2 on an anhydrous basis) for the entire hole depth to 20.1 mbsf. This is moderately vesicular and aphyric in hand specimen (Fig. 39). Large vesicles, up to 4 cm long, are severely stretched (Fig. 40).
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 (Fig. 41). Vesicles become lined by a silica + sulfide + clay coating, in which, in most vesicles, euhedral zeolites grow (possibly clinoptilolite and, much less abundant, phillipsite). Below ~11 mbsf, fractures and vesicles are lined by subhedral marcasite, which incorporate framboids of pyrite or greigite (Fig. 42). The alteration differs from that found near the top of previous holes drilled through hydrothermally active sites in two main aspects: the absence of anhydrite (abundant at Sites 1188 and 1189) and the presence of abundant zeolites.
The structures identified in the core from Hole 1191A were primary volcanic layering, elongation of vesicles, and marcasite-pyrite veins. 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. The stretching and flattening of vesicles most probably were formed during flow of the viscous lava and thereby define the direction of flow. Only 16 silica-pyrite veins were intersected, all of which were 1 mm or less in thickness.
The samples from Hole 1191A have similar compositions to the unaltered dacite-rhyodacites from Sites 1188, 1189, and 1190. A CIPW normative mineralogy calculated for these samples gave a quartz-normative composition.
Active bacterial cells were found by direct counting on two samples from depths of ~10 and 15 m, respectively. Anaerobic bacterial cultivation tests on both samples yielded positive results at temperatures reaching 90°C.
Magnetic susceptibility is high in all samples, although only small amounts of titanomagnetite were observed in thin sections. The natural gamma measurement values from this site are similar to those of Hole 1188A on average and are significantly lower than those of Hole 1189A. Thermal conductivity measurements are consistent with measurements for unaltered dacite from Holes 1188A and 1189A (average = 1.45 W/(m·K)). There is little variation in the solid density data, ranging from 2.5 to 2.6 g/cm3, again consistent with measurements for unaltered dacites from Sites 1189 and 1190. The porosity values range from 7.1% to 11.5%.
Site 1191 samples provided a reliable estimation of the magnetic inclination (16°) significantly different from the present-day value of ~7.7° for the region. Although this may result from 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.
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