To preserve important compositional and structural features in both the archive and working halves, we generally examined the whole core sections containing igneous rocks prior to cutting them with a diamond-impregnated saw. Nondestructive physical properties measurements, such as magnetic susceptibility and natural gamma-ray emission, were made on the core before it was split (see "Physical Properties"). After the core was split and described, the working half was sampled for shipboard physical properties measurements, magnetic studies (see "Rock Magnetism"), thin sections, XRD, and ICP-AES analyses. The archive half was described on the visual core description (VCD) form and photographed. To minimize contamination of the core with platinum-group elements and gold, scientists and technicians removed jewelry from hands and wrists prior to handling.
To provide a preliminary estimate of the composition of igneous rock units, a refractive index (RI) measurement was made where fresh glass was encountered in core. Small chips were gently crushed and an appropriate size fraction (20-50 µm) obtained by rolling across paper. Aliquots were immersed successively in standard Cargille RI oils (0.002 spacings) and examined under transmitted light with a petrological microscope until the RI was matched or bracketed using the color-dispersed Becke line method in unfiltered white light. Because we expected to be dealing with a consanguineous lava sequence where most major elements are correlated with silica, RI and SiO2 content were linearly related. A calibration curve (Fig. F3) based on analyzed rocks from the Eastern Manus Basin dredge hauls (R.A. Binns, unpubl. data) was then used to estimate composition. From prior experience, as much as 20% of microlites can be present in the vitreous groundmass without seriously affecting this relationship insofar as it is used to classify the rock type. The following weight percent SiO2/RI boundaries (volatile-free basis) were adopted, following the International Union of Geological Sciences (IUGS) scheme (Le Maitre, 1976):
The term rhyodacite is preferred over rhyolite because of the Na/K values and the general lack of quartz phenocrysts.
Where appropriate, mineral phenocryst compositions were similarly estimated using RI calibration charts of Deer et al. (1992, fig. 159). Preliminary petrological observations using crushed particles mounted in oil were also used in several instances where quick information was required for description purposes or where thin-section manufacture was impractical or not intended.
All of the igneous rocks cored during Leg 193 were felsic lavas. Felsic lavas are composed of both coherent and volcaniclastic facies, corresponding to the insulated central part of the erupted melt and the outer brecciated part of the flow, respectively. The relative proportions of coherent and clastic facies of individual flows are variable depending on factors such as temperature, volatile content, viscosity, shear stress, and eruption rate. In subaerial settings, brecciation of the outer part of the lava is a result of autoclastic fragmentation caused by the movement of the ductile center of the flow. In contrast, quench fragmentation is generally considered to be the dominant process in the submarine environment generating hyaloclastite (Figs. F4, F5).
However, surficial lava structures on the crest of Pual Ridge indicate a fluid style of eruption with little evidence of quench fragmentation (Waters et al., 1996).
Lavas may show a porphyritic texture with phenocrysts set in a fine-grained or glassy groundmass, or otherwise they may be aphyric, as are most dacites previously sampled from the crest of Pual Ridge. Because phenocrysts are formed in the magma chamber prior to eruption, they are a good indicator of geochemical composition and may be used for correlation and discrimination of individual flows.
The groundmass of felsic lavas can be glassy, vesicular to pumiceous, spherulitic, microcrystalline, or a mixture of several texturally distinctive domains (Fig. F6). In particular, the coherent facies, representing the slowly cooled central part of a flow, may show considerable textural variation. At slow cooling rates, high-temperature devitrification can take place, resulting in the formation of spherulites (round, nodular, or lenticular aggregates of fine quartz and feldspar needles). At even slower cooling rates, an interlocking mosaic of quartz and feldspar crystals would form in the groundmass, but no such samples were encountered in Leg 193 cores. After solidification, glassy parts of the lava are readily altered by interaction with circulating solutions. Perlitic cracks are the result of hydration and volume expansion of the glass representing an early stage of alteration. Typical alteration minerals in diagenetically altered felsic volcanic glass are zeolites, clays minerals, and/or secondary feldspar, depending on temperature and burial depth.
We used VCD forms to document each section of the igneous rock cores. The left column on the form, adjacent to the core photograph, represents the archive half graphically. A horizontal line across the entire width of the column denotes a plastic spacer (see "Core Handling"). Oriented pieces are indicated on the form by an upward-pointing arrow to the right of the piece. Locations of samples selected for shipboard studies are indicated in the column headed "Shipboard studies" with the following notation: XRD = X-ray diffraction analysis; ICP-AES = inductively coupled plasma-atomic emission spectrometry analysis; TSB = polished petrographic thin section billet; PP = physical properties measurements; and PMAG = paleomagnetic measurements. Lithologic, alteration, and mineralization features are indicated with graphics on the form in three labeled columns (Fig. F7).
We subdivided the core into consecutively numbered lithologic units on the basis of changes in color, structure, brecciation, grain size, vesicle abundance, mineral occurrence, and abundance. Where possible, unit boundaries were chosen to reflect primary igneous or volcanologic characteristics. However, where processes of alteration obliterated the primary features, units were delineated according to alteration characteristics.
Written descriptions accompany the schematic representation of the core sections. They include the following:
a. Vesicle abundance (i.e., nonvesicular [no visible vesicles], sparsely vesicular [<5%], and moderately vesicular [5%-20%]). Visual estimates of the volume fraction of vesicles were supplemented by observations using a binocular microscope. Filled vesicles, or amygdules, were noted as well, using the analogous terms sparsely amygdaloidal and moderately amygdaloidal.
b. Size and size distribution, including multiple modes if present.
c. Shape (sphericity and angularity).
We examined thin sections from the core intervals noted on the VCD forms to complement and refine the hand-specimen observations. In general, the same terminology was used for thin-section descriptions as for the VCDs. The percentages of individual phases were estimated visually, and reported in the thin-section descriptions (see the "Core Descriptions" contents list). The textural terms used are defined by MacKenzie et al. (1982). Thin sections of volcaniclastic rocks that were sufficiently fresh to allow the discrimination of vesicles, phenocrysts, and groundmass were point counted using an automatic point counter and a regular rectangular grid.
For some porphyritic rocks, the thin-section analyses and VCDs differ slightly, typically because small plagioclase laths in a rock with seriate texture are visible only in thin section. Thus, for example, a rock described visually as olivine-plagioclase-phyric may be plagioclase-olivine-phyric in the thin-section description. Similarly, vitric and lithic components may be estimated incorrectly in volcaniclastic hand samples, and therefore a different rock name assigned to a thin section as a result. Thin sections were described for as many units as was practical, although it was not possible to produce sections for every unit defined.