All igneous and metamorphic descriptions and measurements during Leg 210 were made on the archive halves of the cores except where otherwise noted.
The methods used here were designed following the "Explanatory Notes" chapters of the Initial Reports volumes for earlier ODP legs, mainly 149 (Shipboard Scientific Party, 1994), 173 (Shipboard Scientific Party, 1998), and 209 (Shipboard Scientific Party, in press). Modifications were made as appropriate in order to meet the specific needs and problems encountered during Leg 210.
Prior to splitting igneous and metamorphic rock cores, the whole cores were examined for structural features. Each contiguous "piece" was then numbered sequentially from the top of each core section and labeled. Broken core fragments that could be fit together were reassembled into composite pieces; each fragment was lettered consecutively from the top down (e.g., 1A, 1B, 1C, etc.). Composite pieces sometimes occupied more than one core section. Plastic spacers were placed between pieces with different numbers. If it was evident that an individual piece or fragment had not rotated about a horizontal axis during drilling, an arrow was added to the label pointing to the top of the section. The pieces were split with a diamond saw into archive and working halves. Cores were split so as to allow important features and structures to be represented in both the working and archive halves.
A VCD form was designed to record petrology and structural observations of both igneous and metamorphic rocks (Fig. F6). The left column is a graphic representation of the archive half of the core. A horizontal line across the entire width of the Graphic Representation column denotes a plastic spacer. Pieces that could be oriented vertically are indicated on the form by an arrow pointing upward to the right of the piece. Symbols were designed to graphically illustrate the mineralogy and structures observed in the cores (Fig. F7). During core description, representative samples of the main lithologies were taken from the working half for XRD analysis to confirm mineral identities. For each lithologic unit, representative samples were examined in the working half of the core (where recovery permitted) for shipboard physical property analysis, magnetic studies, ICP-AES, and polished thin section studies. Samples were oriented with respect to the core face. Where feasible, cubes or minicores for physical property measurements were taken immediately adjacent to samples used for chemical analyses and polished thin sections.
Shipboard samples and studies are indicated in the column headed Shipboard Studies, using the following notation:
Digital core images were recorded for all archive halves, and images were used to aid petrological description. For VCDs of igneous and metamorphic rocks, see the "Visual Core Descriptions" for each site.
The classification of igneous and metamorphic rocks is based on texture, grain size, mineral occurrence and abundance, rock composition, and rock clast type. When modal analyses could be reliably obtained, igneous rocks were classified according to essential primary minerals and/or chemical analyses following standard International Union of Geodesy and Geophysics nomenclature (Streckeisen, 1976; Le Bas et al., 1986). Rocks for which the protolith is completely obscured by metamorphism or alteration processes were given separate lithologic names. If the original rock type was discernible, the prefix "meta" or the term "altered" was used as a modifier with the name of the protolith. Any postemplacement changes in mineralogy or structure associated with elevated temperatures or pressures were classified as "metamorphic." We reserved the term "altered" for rocks that have undergone low-temperature hydrothermal alteration or seafloor weathering. Igneous minerals were termed "primary." Minerals affected by hydrothermal alteration or metamorphic processes were termed "secondary." Rock classification and description of mafic and ultramafic rocks are referenced to the "Igneous Petrology" and "Metamorphic Petrology" sections in the "Explanatory Notes" chapter of the Leg 209 Initial Reports volume (Shipboard Scientific Party, in press). Modifications were made as appropriate in order to meet the specific needs and problems encountered during Leg 210.
The primary and secondary rock-forming minerals were recorded in a short description added to the VCD. The primary rock-forming minerals found at Sites 1276 and 1277 include olivine, orthopyroxene, clinopyroxene, spinel, Fe-Ti oxide, plagioclase, and amphibole. Mica, apatite, oxide and sulfide minerals, and other minerals of magmatic origin such as zircon were recorded as accessory minerals. Secondary minerals include clay minerals (smectite, kaolinite, chlorite, and serpentine), analcime, pyrite, calcite, and quartz. For each rock type, the following data were recorded in a log accompanying the VCD:
On the VCD forms the modal percentage of the mineral includes both the fresh and altered parts of the mineral recorded in the piece. Grain size is recorded for the primary minerals and refers to the average long dimension of the minerals and is given in millimeters, as are the minimum and maximum crystal sizes. The crystal shape of the primary minerals describes the aspect ratio of the grains and is used when the deformation has modified the original crystal morphology. The aspect ratio is the ratio of the long to the short dimension of the crystal. The terms euhedral, subhedral, anhedral, and interstitial are used to describe the shapes of crystals preserving an igneous morphology. The shapes are divided into four classes: (1) equant (<2:1 aspect ratio), (2) subequant (2:1–3:1 aspect ratio), (3) tabular (3:1–5:1 aspect ratio), and (4) elongate (>5:1 aspect ratio).
When describing the texture and structure of the igneous cores, a separate description is required for diabase and gabbroic rocks and for basaltic rocks. In diabase and gabbroic rocks, the texture is defined by the grain size and the extent of igneous foliation that is developed. Diabases have grain sizes that range from too small to be discerned by the unaided eye to fine grained; microgabbros are fine grained; gabbros are medium grained or coarser. The textural distinction between diabases and microgabbros is based on the modal homogeneity of the sample. Diabases are defined as having a felty texture with randomly distributed plagioclase laths throughout and are modally homogeneous. Microgabbros can have felty textured plagioclase with distinctive modal segregations of plagioclase laths into a framework in the sample, or they can be more massive.
When describing basaltic rocks, the proportions and characters of phenocrysts and vesicles define the following textures:
Where segregations, bands, and veins were observed in the cores, the orientation, frequency, and mineralization were recorded on the VCD. The word "vein" was used to describe features formed by precipitation from nonmagmatic fluids, and the abundance, connectivity, and dimension of the veins was noted. Segregation bands are formed by different chemistries of the magma as it flowed through the igneous intrusive unit.
The terms metamorphism, metasomatism, and hydrothermal alteration are used loosely and synonymously without making implications about open- vs. closed-system behavior. The VCDs (alteration log) provide information on the degree of hydrothermal mineral alteration seen in the pieces and in thin section. This takes into account the extent of replacement of igneous minerals by secondary minerals and the nature and approximate modes of secondary mineral assemblages. These data were merged with the estimated primary modes to calculate the total extent of rock alteration. Further, the extent to which metamorphic minerals contribute to any subsolidus fabric was recorded in the comments on the VCD. Alteration intensity was classified as follows: fresh (<2%), slight (2%–10%), moderate (10%–40%), high (40%–80%), very high (80%–95%), and complete (>95%).
Thin sections were examined to complement and refine the hand specimen observations. The same terminology used for megascopic descriptions was used for thin section descriptions. The percentages and textural descriptions of individual phases are reported. Thin section descriptions are included in the "Core Descriptions" contents list.