Each basement rock core was divided into 1.5-m sections. Each section was numbered, and each piece of rock in a section was numbered sequentially beginning with the number 1 at the top of each section. Pieces that could be fitted together were assigned the same number followed by a consecutive letter (e.g., 1A, 1B, 1C, etc.). Plastic spacers were placed between pieces having different numbers but not between pieces having the same number but different letters. The presence of a spacer represents an unknown, and possibly substantial, interval of no recovery. Any piece that was longer than the diameter of the core liner (i.e., the piece could not have rotated about a horizontal axis in the liner) was marked with a red wax arrow on the label pointing to the top of the section. Each piece was then split into archive and working halves using a rock saw with a diamond blade. Core descriptions, photographs, and nondestructive measurements were performed on the archive halves. Samples for shipboard analyses (physical properties, paleomagnetism, XRD, and thin-section studies) and shore-based analyses were taken from the working half of the core.
Basement hard-rock cores were described on the VCD form (Fig. F4) specific to igneous rocks (see the "Core Descriptions" contents list). The VCD form for fine- and medium-grained igneous rocks includes the following information:
The left column of the VCD is a graphical representation of the archive half. A horizontal line across the entire width of this column denotes a plastic spacer glued inside the liner. The number of each piece is also indicated. Oriented pieces are indicated by an upward-pointing arrow in the orientation of the appropriate piece. Samples for shipboard studies are indicated in the Shipboard Studies column, using the following notation:
Core descriptions followed a checklist of macroscopic features to ensure consistent and complete descriptions. For each lithostratigraphic unit defined, the following checklist was used:
Igneous rocks were named mainly on the basis of mineralogy and texture. Basalts (fine grained) and diabase (medium grained) were termed aphyric if they contained <1% phenocrysts. If porphyritic, the rock may be sparsely phyric (phenocrysts content of 1%-2%), moderately phyric (>2%-10%), or highly phyric (>10%). Estimates of phenocryst proportions were based on those visible with a 10× hand lens. Basalts were further classified by phenocryst type: a moderately plagioclase-olivine phyric basalt contains >2%-10% total phenocrysts, most of which are plagioclase with lesser amounts of olivine.
Finally, any other miscellaneous comments were added, including continuity of the unit within the core and interrelationships among units. When the VCD form was complete, each record was checked by the database program for consistency and converted into a format that could be directly pasted onto the final VCD record for subsequent curatorial handling.
Petrographic descriptions, including estimates of the various mineral phases (both primary and secondary), were made on the igneous thin-section description form to complement the hand-specimen observations. Specifically, thin sections were used to describe (1) the texture and mineralogy of groundmass; (2) detail of phenocryst mineralogy including mineral type, morphology, size, and abundance; (3) the accessory minerals, such as chromium spinel, magnetite, zircon, and apatite, as well as the inclusions; and (4) the secondary mineral type, morphology, abundance, and the presence of vein vesicle and fracture filling. Identifications of very fine grained secondary phases, such as clays, zeolites, and infillings, were performed by XRD analysis. Modal data were collected using visual estimation by reference to standard charts. Crystal sizes were measured using a micrometer scale. A photomicrograph for each thin section was taken using a digital camera, and each picture was saved as a TIFF image file.
A Philips PW1710 X-ray
diffractometer was used for the XRD analysis of unknown, generally secondary,
mineral phases. Instrument conditions were as follows: CuK
radiation with monochrometer, 40 kV, 35 mA, and goniometer step scan from
2°-70°2
at steps of 0.02°2/s.
Samples were ground with an agate mortar and pestle and mounted in aluminum
sample holders. XRD data are compiled in a separate table in "Sedimentology"
in the "Site
1179" chapter.
During Leg 191, problems with the ICP-AES on board prevented shipboard geochemical analysis of basaltic rocks. The XRF analysis of selected shipboard igneous rock samples was performed after the cruise at Hiroshima University. Results from these analysis are included in Table T10 in the "Site 1179" chapter. We used a fully automated wavelength-dispersive Rigaku zsx-101e XRF system, equipped with a 3-kW generator and an Rh/W dual-anode X-ray tube, to determine the major and trace element abundances in the samples. Analytical conditions used are given in Table T1.
Following cutting by either a water-cooled diamond circular saw or a 1-in diameter diamond drill, the samples were polished using a diamond disk to remove saw marks or any unwanted material. The average sample taken weighed ~22 g. After polishing, the samples were cleaned in an ultrasonic bath in methanol and deionized water for 10 min each, followed by drying at 110°C. Larger pieces (~20 cm3) were reduced to <1 cm diameter by crushing between two disks of Delron plastic in a hydraulic press. The samples were then ground for ~1-5 min in a Spex 8510 shatterbox with a tungsten carbide barrel. We measured loss on ignition from weighed powders, which were heated for 2 hr at 1025°C. Fused glass discs were prepared for both major and trace element analysis using an alkali flux. The flux used was Johnson Matthey Spectroflux 100B, which is a mixture of lithium tetraborate (Li2B4O7) and lithium metaborate (LiBO2) with a mixing ratio of 2:8. The discs were prepared from 2.000 g of rock powder mixed with 4.000 g of dry flux. This mixture, with 0.6 g of LiNO3 as an oxidizer and 100 µL of 5% LiI solution added to prevent adhesion to the Pt-Au crucible, was then melted in air at 1200°C for ~7 min with constant agitation to ensure thorough mixing and then cooled.
Concentrations of all measured elements were computed from measured X-ray intensities using calibration curves derived from the measurement of 17 well-analyzed igneous rock reference standard samples provided by the Geological Survey of Japan (GSJ). The values recommended by Imai et al. (1995) are used for all elements. Data correction was made using the matrix effects compensation method, in which the matrix effects were corrected adopting the matrix compensation coefficients of major elements computed from the minute fluctuation method for theoretical X-ray intensity using the fundamental parameter method (Yamada et al., 1998). The analytical errors (relative standard deviation) based on the average of replicate analyses of the GSJ reference standard JB-1a are given in Table T1.
