IGNEOUS PETROLOGY

Core Curation and Shipboard Sampling

To preserve important features and structures, core sections containing igneous rocks were examined prior to splitting. Each piece was numbered sequentially from the top of the 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, and 1C). Plastic spacers were placed between pieces with different numbers. The presence of a spacer may represent a more substantial interval of no recovery than that represented by the spacer. If it was evident that an individual piece had not rotated about a horizontal axis during drilling, an arrow was added pointing to the top of the section. The pieces were split with a diamond-impregnated saw so that important compositional and structural features are preserved in the archive and working halves.

Nondestructive physical properties measurements, such as magnetic susceptibility and natural gamma-ray emission, were made on the core before it was split (see "Physical Properties"). After the core was split and described, the working half was sampled for shipboard physical properties, magnetic studies (see "Paleomagnetism"), thin sections, XRD, and XRF studies. The archive half was described on the HRVCD form and was photographed before storage. To minimize contamination of the core with Pt-group elements and Au, describers removed jewelry from hands and wrists before handling.

Visual Core Descriptions

We used HRVCD forms to document each section of the igneous rock cores (see the "Core Descriptions" contents list). The left column on the form, adjacent to the core photograph, represents the archive half graphically. 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. Location of samples selected for shipboard studies are indicated in the column headed "Shipboard Studies" with the following notation: XRD = X-ray diffraction analysis; XRF = X-ray fluorescence analysis; TS = petrographic thin section; PP = physical properties analysis; and PM = paleomagnetic analysis.

We subdivided the core into consecutively numbered lithologic units (mostly representing single lava flows) on the basis of changes in color, structure, grain size, and mineral occurrence and abundance. Some units were divided into subunits (A, B, C, and so forth) because of uncertainty as to the number of individual flows involved or to bring attention to major internal divisions within a flow. Breccia or mylonite zones within units led to subdivision into, for example, A, B (brecciated zone), and C subunits. Larger intercalated sediment horizons were designated as independent lithologic units.

Written descriptions accompany the schematic representation of the core sections. This information includes

  1. The leg, site, hole, core, type, and section number (e.g., 183-1135A-15R-3).
  2. The unit number (consecutively downhole), the rock name (see below), and the number of pieces. Additional detailed descriptions of igneous lithology are reported in the hard-rock core description log.
  3. Contact relations with neighboring lithologic units.
  4. Phenocrysts: the types of minerals visible (with a hand lens or binocular microscope), their distribution within the unit, and, for each phase, its abundance (in volume percent), size range (in millimeters), shape, degree of alteration, and further comments if appropriate.
  5. Groundmass texture and grain size: glassy, aphanitic, fine grained (<1 mm), medium grained (1-5 mm), or coarse grained (>5 mm). Grain size changes within units were also noted.
  6. Vesicles: abundance (nonvesicular = <1%, sparse = 1%-5%, moderate = 5%-20%, high >20% by volume), distribution, size, shape, and mineral linings and fillings. Additional detailed descriptions of vesicle distribution through units are reported in "Physical Volcanology".
  7. Color name and code (for the dry rock surface) according to the Munsell color charts.
  8. The rock structure: whether the unit is massive, flow-banded, brecciated, scoriaceous, pillowed, hyaloclastic, or tuffaceous. Note that we generally did not distinguish between syn- and postemplacement brecciation of the lava flows.
  9. Alteration: the alteration was graded as fresh (<2% by volume alteration products), 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. Additional detailed descriptions of alteration and weathering of igneous units are reported in "Alteration and Weathering".
  10. The presence of veins and fractures, including their abundance, width, mineral linings and fillings, and, where possible, their orientation. Additional detailed descriptions of vein materials are reported in "Alteration and Weathering".
  11. Additional comments, including notes on the variability of the unit.

We assigned provisional rock names on the basis of hand-specimen observation (hand lens and binocular microscope) and later checked these with thin-section studies and XRF major element analyses, where necessary. Porphyritic rocks were named by phenocryst type, using mineral names in order of decreasing abundance. The term "phenocryst" was used for a crystal that was significantly larger (typically at least five times) than the average size of the groundmass crystals. Many porphyritic basalts recovered during Leg 183 exhibited a range of crystal sizes (seriate texture), making estimation of phenocryst populations approximate. Descriptors were defined as follows:

These descriptors were further modified by including the names of phenocryst phases, in order of decreasing abundance. Thus, a "highly olivine-plagioclase phyric basalt" contains more than 10% (by volume) phenocrysts, the dominant phenocryst being olivine, with lesser amounts of plagioclase. The prefix includes all of the phenocryst phases that are in the rock, as long as the total content exceeds 1%.

Thin-Section Descriptions

We examined thin sections 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 phases, either estimated visually or determined by point counting, and textural descriptions are reported in "Thin Sections" (see the "Core Descriptions" contents list). The textural terms used are defined in MacKenzie et al. (1982). For some porphyritic basalts, the thin section and visual core descriptions differ slightly, typically because small plagioclase laths in a rock with seriate texture are visible only in thin section. Thus, a rock visually described as olivine-plagioclase-phyric may be plagioclase-olivine-phyric according to the thin-section description. Because not all units were examined in thin section, this discrepancy has been accepted and retained to maintain consistency of the visual records. Where possible, plagioclase compositions were estimated optically using the Michel-Levy Extinction Angle Method from at least 10 separate crystals.

X-Ray Fluorescence Analysis

We selected representative samples of major lithologic units and samples with specific characteristics for shipboard XRF analysis. Large pieces (~20 cm3) were reduced to smaller fragments (<1 cm in diameter) by crushing between two disks of Delrin plastic in a hydraulic press. The sample was then ground for ~5 min in a Spex 8510 shatterbox with a tungsten carbide barrel. Contamination of the samples with Nb during grinding was investigated before the start of Leg 152, and none was detected at that time (Larsen, Saunders, Clift et al., 1994).

We used a fully automated wavelength-dispersive ARL8420 XRF system equipped with a 3-kW generator and a Rh-anode X-ray tube to determine the major and trace element abundances in the samples. Analytical conditions used are given in Table T6. The spectrometer was calibrated using a suite of 30 well-analyzed reference standards. The values recommended by Govindaraju (1989) were used for all elements except for Zr and Nb. A subset of the standards, with concentrations recommended by Jochum et al. (1990), was used for these two elements. Precision estimates, based on replicate shipboard analyses of the USGS reference standard BHVO-1, are given in Table T7 (major elements) and Table T8 (trace elements). Several mechanical failures of the instrument produced a hiatus in analyses after Site 1138 samples, requiring recalibration, so we report statistics for analyses before and after this interruption. Precision for Ba and Ce decreased significantly during the Site 1138 sample analyses, so abundances for these elements are not reported in that chapter. Variability in the measured standard Nb abundance also increased, so for this period we corrected the measured unknown Nb by a normalization factor (measured Nb/accepted Nb for BHVO-1).

We analyzed major elements using fused lithium tetraborate glass disks doped with lanthanum oxide as a heavy absorber (Norrish and Hutton, 1969). The discs were prepared from 600 mg of rock powder that had been ignited for 2 hr at ~1025C and mixed with 7.2 g of dry flux consisting of 80% lithium tetraborate and 20% lanthanum oxide. This mixture, with 20 mL of LiBr (8.6M) added to prevent adhesion to the Pt-Au crucible, was then melted in air at 1150C for ~4 min with constant agitation to ensure thorough mixing and then cooled. The 12:1 flux:sample ratio and the use of the heavy absorber makes matrix effects insignificant over the normal range of igneous rock compositions. Hence, the relationship between X-ray intensity and element concentration is linear. Iron contents are reported as Fe2O3 as a consequence of oxidation during the fusion process. We measured loss on ignition from weighed powders heated for 4 hr at 1025C, then reweighed. CO2 and H2O contents were determined with a Carlo Erba NA 1500 analyzer.

We determined trace elements using pressed-powder pellets. These were made by mixing 5 g of rock powder with 30 drops of a solution of Chemplex polymer in methylene chloride (100 mg/cm3) and then pressing the mixture into an aluminum cap under a load of 8 T. A pellet made with 5 g of basalt powder should appear infinitely thick to the shortest wavelengths used in the analysis. X-ray intensities were corrected for line overlap and interelement absorption effects. The latter corrections were based on the relationship between mass absorption coefficient and the intensity of the Rh-K Compton scatter line (Reynolds, 1963, 1967; Walker, 1973).

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