To preserve important features and structures, core sections containing igneous rocks were examined before the core was split. Contacts were examined for evidence of chilling, baking, and alteration. Each piece was numbered sequentially from the top of each core section and labeled on the outside surface. Broken core pieces that could be fitted together were assigned the same number and were lettered consecutively from the top down (e.g., 1A, 1B, and 1C). Composite pieces sometimes occupied more than one section. Plastic spacers were placed between pieces with different numbers. The presence of a spacer may represent a substantial interval without recovery. If it was evident that an individual piece had not rotated about a horizontal axis during drilling, an arrow was added to the label pointing to the top of the section.
Nondestructive physical property measurements, such as magnetic susceptibility, natural gamma ray (NGR) emission, and digital imaging of the exterior of the whole-core pieces were made before the core was split (see "Physical Properties"). The pieces were split with a diamond-impregnated saw in such a way that important compositional and structural features were preserved in both the archive and working halves. After splitting, the archive half was described on VCD forms and photographed. Digital images of the core were taken using the Geotek digital imaging scanner before describing. To minimize contamination of the core with platinum group elements and gold, the describers removed jewelry from their hands and wrists before handling. After the core was split and described, the working half was sampled for shipboard physical properties, paleomagnetic studies, thin sections, XRD, inductively coupled plasma-atomic emission spectroscopy (ICP-AES), and shore-based studies.
Each section of core was examined consecutively by three teams of describers, focusing first on igneous characteristics, then alteration, and finally structure. Each team described all sections of the hard rock cores. The igneous team recorded the depth interval and length of each piece in a piece log (see Table T2 for an example). Lithologic units and subunits were then identified on the basis of the presence of contacts, chilled margins, changes in primary mineralogy (occurrence and abundance), color, grain size, and structural or textural variations (see Table T3 for an example). Unit boundaries were generally chosen to reflect different volcanic cooling units, although we were forced by limited recovery in some cases to arbitrarily decide the exact location of a unit boundary within an interval where the lithology above and below the interval was different. In order to preserve important information about the volcanology without defining an unreasonable number of units within a single core, subunits were designated in cases where there were frequent changes in texture without accompanying changes in mineralogy (for example, several pieces containing glass within 50 cm of core, all of which are mineralogically similar).
VCD forms (Fig. F6) were used to describe each section of the igneous rock cores. A key to symbols used on VCDs is given in Figure F7, and the definitions of the terms used are provided in Table T4. From left to right on the VCD the following are displayed: (1) a photograph of the archive half of the core, (2) a scale from 0 to 150 cm, (3) the piece number, (4) a graphical representation of the rock with structural details, (5) the piece orientation, (6) the location of samples selected for shipboard studies, (7) the boundaries of lithologic units, (8) structures, (9) presence of glass or altered glass, (10) phenocryst abundance and mineralogy, (11) groundmass grain size, and (12) alteration intensity. In the graphical representation (column 4), chilled margins were indicated by using the symbol shown in Figure F7. A horizontal line across the entire width of column 4 denotes a plastic spacer, reflecting the curator's interpretation of the boundaries between different pieces of core. Vertically oriented pieces are indicated on the form by an upward-pointing arrow to the right of the appropriate piece (column 5). The locations of samples selected for shipboard studies are indicated in the column headed Shipboard Studies, using the following notation:
The Lithologic Unit column displays the location of the boundaries between units and subunits and the unit designator (e.g., 1, 2a, 2b, etc.). The Structure column displays the graphical representations of structural types from the key in Figure F7.
The boundaries of the lithologic units and subunits were drawn on the VCD across columns 4-12 (solid lines denote unit boundaries; dotted lines denote subunit boundaries) and numbered consecutively within each hole. VCDs also contain a text description of each unit in each section of core that includes (1) the leg, site, hole, core number, core type, and section number; (2) the depth of the top of the section in meters below seafloor; (3) the unit number (consecutive downhole; subunits are designated by letters after the unit number, e.g., 1, 2a, 2b, etc.) (sedimentary units within the basement were also numbered consecutively downhole but were numbered separately from igneous units and were distinguished from them with the letter "S;" e.g., S1, S2); (4) the rock name; (5) a summary description of the unit as it appears in the section, including a brief rock name and the rock type (e.g., pillow basalt or sheet flow); (6) the piece numbers included in the unit; (7) the type of contacts; (8) the Munsell color; (9) the phenocryst minerals, abundance, and size; (10) the groundmass grain size; (11) vesicle abundance; (12) the nature of the alteration; (13) information about abundance and filling of veins; (14) a description of structures in the rock; and (15) any additional comments.
Units and subunits were named on the basis of the groundmass texture and the abundance of primary minerals. Basalts were described based on the identification of phenocrysts in hand sample:
Rock names were further classified by the types of phenocrysts, where present (e.g., sparsely plagioclase-olivine phyric, in which the amount of olivine exceeds the amount of plagioclase). In cases where the groundmass grain size was fine grained or larger, we did not use these modifiers because it was difficult to distinguish phenocrysts from groundmass crystals in hand sample. (e.g., we used "fine-grained basalt" rather than "aphyric fine-grained basalt"). Rock color was determined on a wet, cut surface of the rock using the Munsell color chart. Groundmass character was determined by measuring average groundmass grain size (width of elongated grains) with a binocular microscope. Grain size was identified as
An estimate of the percentage of vesicles and their average sizes was made and included in the comments on the VCDs. Mineral abundance was used in determining the rock name. The igneous unit and contact logs are included (see Table T3).
Pillow basalts were identified by curved chilled margins oblique to the vertical axis of the core or, when these margins were absent, by variolitic texture, curved fractures, and microcrystalline or cryptocrystalline grain size. For glassy or chilled pieces lacking definitive indications of pillows (for example, with subhorizontal chilled margins), we designated the units as "flow margins," which could be interpreted either as pillow basalts or as the tops or bottoms of sheet flows or massive flows. Sheet flows were identified by sections of core <3 m thick with the same lithology and grain size that increased toward the center of the unit. Massive units were identified by continuous intervals >3 m thick of similar lithology that increased in grain size toward their center. Other rock types distinguished were dikes, pillow breccias, breccias, and hyaloclastites.
The first step in describing the core was the selection of unit boundaries, as described in "Visual Core Descriptions". Subunits are designated in the VCD, and their descriptions are included within the overall written description of the unit. The igneous unit and contacts log (Table T3) provides information about the unit boundaries and a brief description of each unit. The table lists for each unit the core number; section number; piece number(s); location (in meters below seafloor) of the upper contact, calculated from the curated depth of the top of the core and the length of the pieces in the core above the upper contact; the type of the upper contact (listed in Table T3); the minimum thickness of the unit, calculated from the piece lengths; and the rock type of each unit.
Thin sections of igneous rocks were studied to complete and refine the hand-specimen observations. This included textural features that were not identified in hand specimen; precise determination of grain size of phenocrysts and groundmass; the mineralogy, abundance, and kind of glomerocrysts; the presence of inclusions within phenocrysts; and the presence of spinel, oxides, and sulfides. Crystal sizes of all primary phases were measured. In addition, mineral morphologies, grain sizes, and textural features were described. The terms heterogranular (different crystal sizes), seriate (continuous range in grain size), porphyritic (indicating presence of phenocrysts), glomerophyric (containing clusters of phenocrysts), hypocrystalline (100% crystals) to hypohyaline (100% glass), variolitic, intergranular (olivine and pyroxene grains between plagioclase laths), intersertal, subophitic, and ophitic were used to describe the textures of the mesostasis. The same terminology was used for thin section descriptions and the megascopic descriptions. An example of the thin section description form is given in Figure F8, with key in Table T4. Thin section descriptions are included in this volume (see the "Core Descriptions" contents list) and are also available from the ODP Janus database. Digital photomicrographs were taken during the cruise to document features described in the thin sections. A list of available images, any of which can be obtained from the ODP Data Librarian, is given in Table T30 in the "Site 1256" chapter.
All igneous rocks recovered during Leg 206 have undergone alteration. On the hard rock VCD forms, rocks were graded according to whether they are fresh (<2% by volume alteration products) or have slight (2%-10%), moderate (10%-50%), high (50%-90%), or complete (90%-100%) alteration. Alteration and vein core description logs on a piece-by-piece scale were tabulated to provide a consistent characterization of the rocks and to quantify the different alteration types (see Tables T5, T6). Descriptions are based mostly on hand-specimen observations, and specific secondary minerals are not generally distinguished, except where crystal morphology allows unequivocal identification. Where additional mineralogical evidence is available from either thin section descriptions and/or X-ray diffractograms, these identifications were integrated into the alteration and vein logs and the VCDs.
We recorded the following information in the logs: