Site 1190 is an area without any indication of hydrothermal activity on the surface and was expected to provide a section through relatively unaltered volcanic rocks as a background reference. Only shallow penetrations were achieved. Holes 1190A and 1190B yielded one core each, with 6 and 45 cm of curated rock recovery, respectively. Samples from Hole 1190C come from three cores, for a total of 59 cm of curated core representing 13.4 m of drilled depth below seafloor.
Almost all of the rocks recovered are fresh, black, glassy, moderately vesicular dacite (Fig. F1). However, Pieces 2 and 4 from Section 193-1190B-2R-1 exhibit slight bleaching. Originally logged as aphyric (<1% phenocrysts), thin-section analysis indicates that the rocks are commonly sparsely to moderately porphyritic with as much as 3% plagioclase, clinopyroxene, and rare magnetite phenocrysts (commonly <1 mm in maximum dimension). The siliceous composition of the samples has been confirmed by measurements of the refractive index of the glass on Samples 193-1190A-1R-1 (Piece 1) and 193-1190B-2R-1 (Piece 1), which indicate a SiO2 content of about 71-72 wt% SiO2. In the absence of any substantial lithologic heterogeneity, all samples are assigned to Unit 1 of their respective holes (Table T2). There are no significant differences between the rocks from the three holes, suggesting that all three holes sampled the same lava flow at the surface of Site 1190.
Primary volcanic features recognized in the Site 1190 specimens include vesicles, phenocrysts, quenched glassy groundmass, and flow banding.
The vesicularity of samples was estimated in hand specimen to be ~5% for Unit 1 of Hole 1190A, between 5% and 15% for Unit 1 of Hole 1190B, and between 5% and 20% for Unit 1 of Hole 1190C. These figures are supported by vesicularity estimates of representative thin sections, determined by point counting, of between 7% and 15% (Table T3).
Most vesicles are generally millimeter scale in their cross-sectional dimension; however, elongate tubes and flattened shapes are very common. Minimum vesicle sizes are just a fraction of a millimeter, whereas maximum sizes reach several centimeters along the longest axis, and there appears to be a broad unimodal vesicle size distribution.
Three phenocryst types are present at Site 1190. Plagioclase is the most common phenocryst (1-2 vol%), forming euhedral to subhedral, blocky and elongate lath-shaped crystals. They vary from 0.1 to 2.0 mm in long dimension, averaging ~0.8-1.0 mm long. The plagioclase, which is not significantly zoned, commonly contains melt inclusions and partly or wholly encloses magnetite grains.
Elongate prisms of euhedral to subhedral clinopyroxene are less abundant as phenocrysts than plagioclase (<1 vol%). Reaching a maximum dimension of 1.4 mm in Hole 1190A, they are more commonly 0.1 to 0.8 mm long, with an average of 0.5 mm. Some clinopyroxene crystals enclose magnetite.
Minor magnetite phenocrysts (<0.2 vol%) are euhedral to anhedral, ranging in size from ~0.04 to 0.3 mm, and averaging 0.1 mm across. Although very small, these grains nonetheless exceed the typical groundmass microlite opaque grains, which are typically 0.001-0.010 mm across.
Clusters of several phenocrysts, including two or all three phenocryst phases, are common. Notably, the glassy groundmass that occupies interstitial and concave spaces in and around these clusters is usually free or nearly free of microlites, in contrast to the microlite-rich glassy mesostasis elsewhere in the rocks (Fig. F2). Apparently, microlites are difficult to nucleate in these areas, perhaps because the melt is locally enriched in the chemical components rejected by the growing phenocrysts. High-viscosity and low-diffusion coefficients, characteristic of cool felsic magmas, could explain why these areas might be chemically distinct from the bulk groundmass.
The groundmass visible in thin sections is typically comprised of tan glass packed with felted, clear, acicular microcrysts (probably plagioclase, possibly also pyroxene) measuring ~0.025 mm long and 0.001-0.002 mm wide and disseminated 0.001-0.010 mm granular opaque minerals (magnetite). Locally, the groundmass grades to microlite-free tan glass in and around clustered phenocrysts, as described previously.
Faint flow banding defined by subtle changes in groundmass color can be observed locally in hand specimen. In thin section, bands of dark brown groundmass domains have a lensoidal to wavy geometry with diffuse margins against a more common light brown groundmass (Fig. F3A). In detail, the dark bands are enriched in plagioclase microlite needles, whereas light groundmass contains about equal proportions of volcanic glass and microlites (Fig. F3A). Microvesicles (<0.1 mm in diameter) are abundant in dark bands suggesting that phase separation and the formation of gas bubbles in the melt was favored in the microlite-rich domains.