Hole 1191A is situated in the Satanic Mills hydrothermal site between the Snowcap (Site 1188) and Roman Ruins (Site 1189) hydrothermal sites. Drilling intersected fresh to moderately altered, moderately vesicular, aphyric rhyodacite down to a depth of ~15 mbsf. Fresh, black rhyodacite (Fig. F1) occupies the uppermost part of Section 193-1191A-1R-1, and the refractive index (RI) measurements indicate an SiO2 content of ~71 wt%. Alteration intensity increases from slight to moderate downhole and is marked by patchy to pervasive replacement of the black glassy to microcrystalline groundmass by silica and clay leading to a grayish, spotty appearance (Fig. F2). Zeolites (mainly clinoptilolite) are common within the vesicles and form radiating clusters on the inner vesicle walls. Below a depth of ~11 mbsf, several sulfide-rich hairline veins cut across the altered rhyodacite.
All the recovered volcanic rocks from Site 1191 have been logged as one unit because there is no change in primary lithologic features and alteration intensity increases gradually (Table T2). Therefore, we inferred that the rhyodacite was homogeneous in composition prior to alteration.
Primary volcanic features recognized in the Site 1191 specimens include vesicles, minor phenocrysts, and quenched or variably altered groundmass.
Round to elongate or tabular vesicles constitute 8-15 vol% of the recovered rocks. They show a wide range of dimensions from a fraction of a millimeter to several centimeters. Commonly, large vesicles as long as ~4 cm define a stretching direction parallel to the flow direction of the lava (Fig. F2). In some cases, the orientation of the stretching direction is normal to the long axis of core, indicating that flow was parallel to the current seafloor surface. In several other cases of oriented pieces, however, the flow directions indicated by stretched vesicles dip steeply (from ~45° to 90°).
The rocks have been classified as aphyric, based on hand specimen observation. However, thin-section examination shows that they contain scant plagioclase, clinopyroxene, and magnetite microphenocrysts, accounting for a trace to 0.4 vol% (Table T3). Plagioclase laths reach a maximum length of 1.2 mm, and stubby prismatic clinopyroxene crystals measure as wide as 0.4 mm. Magnetite grains, possibly microphenocrysts, are typically 10-40 µm across.
In thin section, the groundmass of unaltered rhyodacite in Hole 1191A is brown and consists of fine feldspar microcryst needles and volcanic glass in roughly equal proportions (Fig. F3). The microcrysts are typically aligned in a preferred orientation defining a flow lamination.
This volcanic groundmass is replaced during alteration by a very fine grained mixture of silica and clay. Samples of slightly to moderately altered rhyodacite show a progressive change in the composition of the groundmass. Slightly altered samples contain irregular, semiconnected domains of unaltered groundmass surrounded by altered groundmass, whereas more intensely altered samples only contain remnant, irregular isolated "islands" of unaltered groundmass (Fig. F4). Vesicles in these rocks have a halo of altered groundmass indicating that pervasive alteration proceeded outward from these primary cavities.
Locally, where pervasive alteration is advanced, the remnants of the original groundmass are preserved only in rare isolated domains. These domains may have angular outlines with sharp margins to the surrounding altered groundmass and may be misinterpreted as xenoliths on superficial examination (Fig. F5).