IGNEOUS PETROLOGY AND VOLCANOLOGY³

To describe important mineralogic and structural features we examined both the archive and working halves of core sections containing igneous rocks. Each piece was numbered sequentially from the top of each core section and labeled on the outside surface. Pieces that could be fitted together were assigned the same number and were lettered consecutively (e.g., 1A, 1B, 1C, etc.). Plastic spacers were placed between pieces with different numbers. The presence of a spacer may represent a substantial interval of no 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.

Nondestructive physical properties measurements, such as color imaging and natural gamma ray emission, were made on the core before it was split (see "Core Physical Properties"). After the core was split, lithologic descriptions were made of the archive half. The working half was sampled for shipboard physical properties measurements (see "Core Physical Properties"), magnetic studies (see "Paleomagnetism and Rock Magnetism"), thin section analysis, and X-ray diffraction and inductively coupled plasma-atomic emission spectrometry (ICP-AES) analyses. The archive halves were described on visual core description (VCD) forms and photographed.

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. A horizontal line across the entire width of the column denotes a plastic spacer. Oriented pieces are indicated on the form by an upward-pointing arrow to the right of the piece. A description of the symbols, colors, and other notations used on the VCDs is provided in Figure F12.

Locations of samples selected for shipboard studies are indicated in the "Shipboard studies" column with the following notation: ICP = ICP-AES analysis, TSB = petrographic thin section, PP = physical properties measurements, and PMAG = paleomagnetic measurements.

Structural features are noted in the "Volcanology/Structure" column and include vesicularity, where NV = nonvesicular (vesicle content <1%), SV = sparsely vesicular (vesicle content = 1%-5%), MV = moderately vesicular (vesicle content = 5%-20%), HV = highly vesicular (vesicle content >20%), G = unaltered glass, and (G) = altered glass.

The "Alteration" column is used to denote the presence of veins (V).

We determined that the hard rock cores contained only one lithologic unit, on the basis of similar color, structure, brecciation, grain size, vesicle abundance, mineral occurrence, and abundance. Written descriptions accompany the schematic representation of the core sections, and include the following:

1. The leg, hole, core, core type, and section number (e.g., 196-1173A-15R-3), as well as the top of the core section measured in meters below seafloor.
2. The unit number (consecutive downhole), rock name, and piece numbers. We assigned provisional rock names on the basis of hand specimen observation (using a hand lens and binocular microscope) and later checked these assignments by examining thin sections. Porphyritic rocks were named by phenocryst type, where the term "phenocryst" was used for a crystal that was significantly (typically five times) larger than the average size of the groundmass crystals and/or generally euhedral in shape. Phenocryst abundance descriptors were further modified by including the names of phenocryst phases, in order of decreasing abundance. Thus, a "moderately plagioclase-olivine-phyric basalt" contains >10% (by volume) phenocrysts, the dominant phenocryst being olivine, with lesser amounts of plagioclase. As long as the total content >1%, the minerals named include all of the phenocryst phases that occur in the rock.
3. Contact relations and unit boundaries. 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) that were also observed in the physical properties and downhole measurements. Intervals of sediment and/or hyaloclastite, changes in vesicularity, alteration, volume fraction, and type of matrix also define lithologic contacts. Where possible, whole-rock ICP-AES analyses (see "ICP-AES Analysis") were used to investigate chemical differences between units. Note that, whereas every effort was made to have unit boundaries reflect individual lava packages, the term "unit" should not necessarily be considered synonymous with "lava flow" in this volume.
4. Phenocrysts. This entry describes the types of minerals visible with a hand lens or binocular microscope, their distribution within the unit, and, for each phase, its abundance (volume percent), size range (millimeters), shape, and degree of alteration, with further comments if appropriate.
5. Groundmass texture and grain size, where the grain-size categories used are glassy, aphanitic, fine grained (<1 mm), medium grained (1-5 mm), or coarse grained (>5 mm). Changes in grain size and proportions of crystals and glass within units were also noted.
6. Vesicles. This entry records vesicle abundance (visual estimates of the volume fraction of vesicles were supplemented by observations using a binocular microscope), size, shape (sphericity and angularity), and whether the vesicles are empty or filled and the nature of the filling.
7. Color name and code (for the dry rock surface) according to the Munsell rock color charts (Rock-Color Chart Committee, 1991).
8. Structure. This entry refers to whether the unit is massive, pillowed, hyaloclastic, banded, brecciated, scoriaceous, or tuffaceous. We sought to produce an integrated picture of the style of volcanism and environmental setting of the drill site by identifying features that are diagnostic of specific physical processes. Pillowed sequences were inferred using the presence of glassy margins and/or groundmass grain-size variations. A section was described as massive if there was no evidence for pillows, even though it may be part of a pillowed sequence. The interpretation of the lavas involved three steps: (1) the observed features were tied to physical processes, (2) the emplacement style of individual flows was inferred, and (3) the environmental setting (e.g., subaerial or submarine) for the whole sequence was discussed.
9. Alteration. We graded the degree of alteration as unaltered (<2% of alteration products by volume), slight (2%-10%), moderate (10%-40%), high (40%-80%), very high (80%-95%), or complete (95%-100%). Changes of alteration through a section or a unit were also noted. Where possible, we identified the secondary minerals as carbonate, clay, zeolite, or iron oxide.
10. Veins and fractures. We described their abundance, width, orientation, and mineral linings and fillings. Locations of veins were indicated by V on the VCDs. The minerals filling the veins were identified as in the "Alteration" portion of the VCDs.
11. Any additional comments.

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 phenocryst, groundmass, and alteration phases were estimated visually, and textural descriptions were reported (see "Site 1173 Thin Sections"). The textural terms used are defined by MacKenzie et al. (1982). For some porphyritic basalts, the thin section and visual core descriptions may differ slightly, typically because small plagioclase laths in a rock with seriate texture are visible only in thin section. Thus, a rock described visually as olivine-plagioclase-phyric may be described as plagioclase-olivine-phyric in the thin section description.

We selected representative samples of major lithologic units 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 0.100 ± 0.002 g aliquots of the ignited whole-rock powders and mixed them with 0.4000 ± 0.0004 g of Li-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 ±0.00001 g.

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. Ten microliters of 0.172-mM aqueous lithium bromide (LiBr) solution was added to the mixture before fusion, as an anti-wetting 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 microns into a clean 60-mL wide-mouth 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 Brinkman dispensette (0-25 mL).

Major (Si, Ti, Al, Fe, Mn, Mg, Ca, Na, K, and P) and trace (Zr, Y, Nb, Sr, Ba, Ni, Cr, Sc, V, Co, Cu, and Zn) 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). ICP-AES protocols for dissolution and analysis of rock powders were developed by Murray et al. (2000) (see also Shipboard Scientific Party, 2001). The elements analyzed, emission lines used, and the specific analytical conditions for each sample are provided in Table T5.

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 the University of Massachusetts Kilauea basalt laboratory standard K-1919, prepared in 2.3-M HNO₃. The only exception is P, which was automatically searched for using a single element standard. During the initial setup, an emission profile was collected for each peak, using K-1919, 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, JB-1a, BIR-1, or K1919) with the highest concentration for that element. Before each run, a profile of K-1919 was collected to assess the performance of the machine from day to day. A typical sample run lasted ~12-14 hr, depending on the number of samples and replicate analyses.

All ICP-AES data presented in the site report were acquired using Mode 2 of the JY2000 software, except for Fe, Mg, Mn, Ba, Cr, Sc, V, and Y data, which were acquired in Mode 5. In Mode 5, the intensity at the peak of an emission line is measured and averaged over a given counting interval repeated three times sequentially. Mode 2 fits a Gaussian curve to a variable number of points across a peak and then integrates to determine the area under the curve. The parameters for each run are given in Table T5. Each unknown sample was run at least twice, nonsequentially, in all sample runs.

A typical ICP-AES run includes (1) a set of three certified rock standards (BHVO-2, BIR-1, and JB-1a; Table T6) run at the beginning, middle, and end of the sample run; (2) up to 11 unknown samples; (3) a drift-correcting sample (the K-1919 standard) analyzed every fourth sample position; 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 a data file and data reduction was completed using a spreadsheet to ensure proper control over standardization and drift correction. Once transferred, intensities for all samples were corrected for the full procedural blank. A drift correction was then applied to each element by linear interpolation between drift-monitoring solutions run before and after a particular batch of samples. The interpolation was calculated using the lever rule. Following blank subtraction and drift correction, concentrations for each sample were calculated from the average intensity per unit concentration for the standard BHVO-2, which was analyzed twice during the run.

Estimates of accuracy and precision for major and trace element analyses were based on replicate analyses of BHVO-2, BIR-1, and JB-1a, the results of which are presented in Table T6. In general, run-to-run relative precision by ICP-AES was better than 2% 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 (see Table T5 for machine detection limits).

³This section was written during Leg 197. Leg 197 contributor addresses can be found under "Leg 197 Contributors" in the preliminary pages of the volume.