This section describes the technologies used for defining the lithologic units and subunits in the igneous cores. Major units correspond to major lithologic changes and their respective number (e.g., Unit U4) and will refer to the classification established during Leg 170, reference Holes 1039B and 1039C. Sequences that were not described during Leg 170, such as those within basement, are identified and labeled following the same classification. The lithologic units are defined by VCDs, thin section petrography, and geochemical variations based on shipboard ICP-AES analyses.
Core sections brought on board were examined to identify important features and structures before the core was split. Each piece was numbered sequentially from the top of each core section and labeled on the outside surface. Core, section, piece, and size information are recorded in the piece log (Fig. F5). For the microbiology study, subsections of cores were first taken to a cool area and specific whole-round samples were taken. At the sampling location, spacers were installed in the core section. Digital photographs were taken before, during, and after microbiology sampling to assist in reconstructing the piece after microbiology sampling. Where microbiological samples were taken, no core samples remain. The core adjacent to the microbiology samples was returned to the core liner. A specific column was added to the piece log to note whether a sample was taken for microbiology purposes. Contacts were examined for evidence of chilling, baking, and alteration. Broken 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). Plastic spacers were placed between pieces with different numbers. Note that 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 pointing toward the top of the section. Lithologic units and subunits (flow units, breccia, or interflow sediment) were identified on the basis of the presence of contacts, chill margins, changes in primary mineralogy (presence and abundance), color, grain size, and structural or textural variations. The pieces were later split with a diamond-impregnated saw in such a manner that important compositional and structural features were preserved in both the archive and working halves.
Nondestructive physical property measurements, such as magnetic susceptibility and natural gamma ray emission, were made on the core before it was split (see "Physical Properties"). After splitting, the archive half was described on VCD forms and photographed. Digital images of the core were taken using the digital imaging system. To minimize contamination of the core with Pt-group elements and Au, 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 property, magnetic, thin section, structural, XRD, ICP-AES, and shore-based studies. Microbiology technologies are described in "Microbiology".
VCD forms (Fig. F6) were used to document each section of the igneous rock cores. From left to right on the VCD form 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 numbers, (4) a graphical representation of the pieces and their relationships to each other, (5) the orientation of pieces, (6) the location of shipboard analyses, (7) the lithologic units and any boundaries between, (8) the observed deformation structures (see "Structural Geology"), (9) the percentage of phenocrysts, (10) the grain size, (11) the proportion of vesicles, (12) the degree of alteration, and (13) the presence of mineralization. To the right are more detailed descriptions of selected hand samples.
In the graphic representation, a horizontal line across the entire width of this column denotes a plastic spacer. Vertically oriented pieces are indicated on the form by an upward-pointing arrow to the right of the appropriate piece.
The location of samples selected for shipboard studies is indicated in the column headed "Shipboard studies," using the following notation:
The VCD forms also display the number of the lithologic unit and the location of the unit boundaries. Units were defined based on major changes in lithology, texture, structure, and mineralogy. After we made lithologic descriptions, we attempted to integrate the observations to define unit boundaries. The boundaries often reflect major physical changes in the core (e.g., pillowed vs. massive or cumulate vs. massive) that were also observed in the physical property and downhole measurements.
The VCD entry for structure refers to whether the unit is cumulate, massive, pillowed, hyaloclastic, banded, brecciated, scoriaceous, or tuffaceous but also refers to all structural features such as normal fault, reverse fault, veins, fractures, and voids. Symbols used to describe deformation features are defined in "Structural Geology". For veins and fractures, we described their abundance, width, orientation, and mineral linings and fillings. Where possible, the minerals filling the veins were identified in the "voids" portion of the VCD.
In the phenocryst column, we present the types of minerals visible with a hand lens or binocular microscope, their distribution within the unit, and, for each phase, their abundance (volume percent) and size range (millimeters) with further comments as appropriate. The classification is described as follows:
This is intended to more accurately define the name of the rock by the types of phenocrysts present (e.g., sparsely plagioclase-olivine phyric, in which the amount of plagioclase exceeds the amount of olivine or, in the case of gabbro, pyroxene-plagioclase phyric, where the percentage of pyroxene is higher than that of plagioclase).
Groundmass grain size was identified as medium grained (MG) if the average grain size was 1 mm or greater, fine grained (FG) if the grains could be identified and were <1 mm, microcrystalline (M) if the groundmass crystals could be seen but were too fine to identify, cryptocrystalline (C) if crystals could not be distinguished, hypocrystalline (HY) if glass was present with crystals and crystal abundance exceeded glass abundance, and hypohyaline (HH) if glass abundance exceeded crystals. The same terminology was used to describe the groundmass of gabbro. Mineral morphology was indicated as anhedral (an), subhedral (su), or euhedral (eu).
Vesicularity was described as vesicle abundance (visual estimates of the volume fraction of vesicles were supplemented by observations using a binocular microscope), size, shape (sphericity and angularity), and also whether the vesicles are empty or filled and the nature of the filling. We use the following classification:
We graded the degree of alteration as
Changes of alteration through a section or a unit were also noted. Where possible, we identified the secondary minerals such as carbonate, clay, zeolite, or iron oxide.
Finally, we also documented the presence of mineralization and described its abundance, size, orientations, shape, and mineral linings.
Each section of core was examined separately by two describers for igneous and metamorphic characteristics. The boundaries of the lithologic units were drawn on the VCD form, and for Hole 1253A the units were numbered continuously from the end of Leg 170, starting with Unit 4 (gabbro). Units were divided into subunits (A, B, etc.) on structural and mineralogical grounds; subunits were further subdivided on the basis of detailed observations discussed in "Petrology" in the "Site 1253" chapter. The VCDs also contain a text description of each section of core that includes the (1) leg, site, hole, core number, core type, section number; (2) depth of the top of the section in meters below seafloor; (3) unit number (consecutive downhole), number of pieces in the unit in the section, and rock name; and (4) groundmass, grain size, Munsell color, vesicle abundance and size, structure, nature of the alteration, information about abundance and filling of fractures, and additional comments. The legend for the VCDs is shown in Figure F7.
Thin sections of igneous rocks were studied in transmitted light to complete and refine the hand-specimen observations and are summarized in the format shown in Figure F8. Observations 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 aggregates (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), aggregate (containing clusters of crystals), hypocrystalline (100% crystals) to hypohyaline (100% glass), and intergranular (olivine and/or pyroxene grains between plagioclase laths) were used to describe the textures of the groundmass. The same terminology was used for thin section descriptions and the megascopic descriptions.
In addition to macroscopic description of alteration, alteration as observed in thin sections is presented in the thin section log (Figure F8), where we present the amount of secondary minerals, their size if measurable, and any further information on their occurrence (e.g., Fe oxides replacing glass). We also provided a description of voids in term of abundance, location, size, morphology, and filling.
All information gathered on the primary minerals and their alteration products are summarized within the comment section to highlight all specific features. Digital photomicrographs are used to illustrate representative characteristics. We provide a photomicrograph of the entire thin section and then close-up pictures of specific features. Each digital photomicrograph is numbered according to the thin section number and the close-up number (e.g., picture of thin section 1 with three close-up photographs are numbered 1-A, 1-B, and 1-C). The location of the close-up pictures is shown in the photomicrograph of the entire thin section.
Chemical analyses of basalt and gabbro were determined during Leg 205 using ICP-AES. We selected representative samples of gabbro from Hole 1253A for shipboard ICP-AES analysis. Large whole-rock pieces were first cut with a diamond-impregnated saw blade and ground on a diamond wheel to remove surface contamination. Samples were washed in an ultrasonic bath containing methanol for ~10 min, followed by three consecutive ~10-min washes in an ultrasonic bath containing nanopure deionized water, and then dried for ~12 hr in an oven at 110°C. The cleaned whole-rock samples (~20 cm3) were reduced to fragments <1 cm in diameter by crushing between two disks of Delrin plastic in a hydraulic press and ground for ~5 min in a Spex 8510 shatterbox with a tungsten carbide barrel. The sample powders were weighed on a Scientech balance and ignited to determine weight loss on ignition.
We weighed 100 ± 2-mg aliquots of the ignited whole-rock powders and mixed them with 400 ± 0.4 mg of lithium metaborate (LiBO2) flux that had been preweighed on shore. Standard rock powders and full procedural blanks were included with the unknowns for each sample run. All samples and standards were weighed on the Cahn Electro balance. Weighing errors are conservatively estimated to be ±0.01 mg.
Mixtures of flux and rock powders were fused in Pt-Au crucibles at 1050°C for 10-12 min in a Bead Sampler NT-2100. A total of 10 µL of 0.172-mM aqueous lithium bromide (LiBr) solution was added to the mixture, before fusion, as an antiwetting agent to prevent the cooled bead from sticking to the crucible. Cooled beads were transferred to 125-mL polypropylene bottles and dissolved in 50 mL of 2.3-M HNO3 by shaking with a Burrell wrist-action bottle shaker for an hour. After digestion of the glass bead, all of the solution was filtered to 0.45 µm into a clean 60-mL widemouthed polypropylene bottle. Next, 2.5 mL of this solution was transferred to a plastic vial and diluted with 17.5 mL of 2.3-M HNO3 to bring the total volume to 20 mL. The solution-to-sample dilution factor for this procedure is ~4000. Dilutions were conducted using a Brinkmann Instruments dispensette (0-25 mL).
Major and minor (Si, Ti, Al, Fe, Mn, Mg, Ca, Na, K, and P) and trace (Zr, Y, Sr, Ba, Ni, Cr, Sc, and V) element concentrations of powder samples were determined with the JY2000 Ultrace ICP-AES. The JY2000 sequentially measures characteristic emission intensities (with wavelengths between ~100 and 800 nm). Murray et al. (2000) developed protocols for dissolution and ICP-AES analysis of rock powders (see also Shipboard Scientific Party, 2001). The JY2000 plasma was ignited 30 min before each run to allow the instrument to warm up and stabilize. After the warm-up period, a zero-order search was performed to check the mechanical zero of the diffraction grating. After the zero-order search, the mechanical step positions of emission lines were tuned by automatically searching with a 0.002-nm window across each emission peak using a U.S. Geological Survey (USGS) Basalt Hawaiian Volcano Observatory standard (BHVO-2) or Basalt Columbia River (BCR-2) prepared in 2.3-M HNO3, analogous to sample preparation for unknowns. The only exception is P, which was automatically searched by using a single-element standard. During the initial setup, an emission profile was collected for each peak, using BHVO-2, to determine peak-to-background intensities and to set the locations of background points for each element. The JY2000 software uses these background locations to calculate the net intensity for each emission line. The photomultiplier voltage was optimized by automatically adjusting the gain for each element using the standard (BHVO-2 or BCR-2) (see Gladney et al., 1990; Gladney and Roetlands, 1988) with the highest concentration for that element. Before each run, a profile of BHVO-2 was collected to assess the performance of the instrument from day to day.
All ICP-AES data presented in the site reports were acquired using the Gaussian analytical mode of the Windows 5 JY2000 software. This mode fits a Gaussian curve to a variable number of points across a peak and then integrates to determine the area under the curve. Each sample and standard was run as an unknown at least twice, nonsequentially, in all sample runs, and labeled as A and B.
A typical ICP-AES run included (1) a set of five certified rock standards of which four are basaltic (BHVO-2, BIR-1 [USGS Icelandic (Reykjannes) basalt], BCR-2 [USGS], and JB-2 [Geological Survey of Japan (GSJ)]) and one is gabbroic (Jgb-1 [GSJ]), run at the beginning, middle, and end of the sample run; (2) up to eleven unknown samples; (3) a drift-correcting sample (BHVO-2 or BCR-2 standard) analyzed every fifth sample position and at the beginning and end of each run; and (4) a blank solution run near the beginning, middle, and end of each run. A 2.3-M HNO3 wash solution was run for a minimum of 90 s between each of the samples and standards.
Following each sample run, the raw intensities were transferred to an Excel data file, and data reduction was completed using Kaleidagraph software. The enhanced plotting functions in Kaleidagraph allowed quick assessment of drift, the repeatability of standards and the blank, and identification of occasional discordant measurements. This ensured proper control over standardization and drift correction. Once transferred, a drift correction was then applied to each element and to the blank by linear interpolation between drift-monitoring solutions run before and after a particular batch of samples, assuming that the time between measurements during autosampling was constant. The drift correction was applied to the blank after noticing for some elements a substantial systematic drift for the blank itself, consistent with the trend of the BHVO-2 or BCR-2 drift monitor. The average of the three drift-corrected procedural blank measurements was then deducted from the drift-corrected intensities of all samples. Following drift correction and blank subtraction, concentrations for each sample were calculated from linear regressions using the average intensity per unit concentration for the five standards measured, each one being measured twice during the run. The blank was also included in the regression with both its intensity and concentration set at zero. This calibration is based on several standards rather than normalization to a single standard, such as BHVO-2 only. The regression technique gave excellent correlation coefficients (>0.99) for most oxides and trace elements (except Ni and P at ~0.8, and Si and Al at 0.95). It also revealed either important discrepancies between standards or problems with sample preparation in the cases of Cu and Zn, which accordingly are not reported in the site chapters. In addition, concentration variations of Si, Al, and Ca are also observed from time to time between duplicate analyses of some samples and standards. The cause of these variations remains unclear.
Estimates of accuracy and precision for major and trace element analyses were based on fits to the regressions for BHVO-2, BIR-1, JB-2, BCR-2, and JGb-1, the results of which are presented in Table T3. To assess the reproducibility, all standards were run as an unknown and the measured values were compared to the certified values. In general, run-to-run relative precision by ICP-AES was better than 3% for the major elements. Run-to-run relative precision for trace elements was generally <5%. Exceptions typically occurred when the element in question was near the detection limit of the instrument.
Identifications of secondary minerals, such as vein and void fillings, were determined by XRD using a Philips PW1729 X-ray diffractometer. Samples were taken from altered bulk rock and small spots in veins or cavity fillings to identify materials with distinctive visual characteristics. The samples were freeze-dried overnight and crushed to fine grain size using a mortar and pestle, and the powders were mixed in water slurries on glass slides. Detailed setups for XRD are described in "X-Ray Diffraction" in "Lithostratigraphy."