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doi:10.2204/iodp.proc.301.105.2005

Igneous and metamorphic petrology

Core curation and shipboard sampling

To preserve important features and structures, core sections containing igneous rocks were examined before the core was split. Whole-round samples taken on the core receiving platform for microbiological studies were examined and photographed when possible. Contacts were examined for evidence of chilling, baking, and alteration. Each piece was numbered sequentially from the top of each core section and labeled on the outside surface. Broken core 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). Composite pieces sometimes occupied more than one section. Plastic spacers were placed between pieces with different numbers. 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 to the label pointing toward the top of the section.

Nondestructive physical property measurements, such as magnetic susceptibility and natural gamma ray (NGR) emission, were made before the core was split (see "Physical properties"). The pieces were split with a diamond-impregnated saw in such a way that important compositional and structural features were preserved in both the archive and working halves. After splitting, the archive half was described on VCD forms and photographed. Digital images of the core were taken using the Geotek DIS before they were described. To minimize contamination of the core with platinum group elements and gold, 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 analysis of physical properties, paleomagnetic studies, thin sections, XRD, inductively coupled plasma–atomic emission spectroscopy (ICP-AES), and shore-based studies.

The depth interval and length of each piece were entered in a piece log (see Table T2 for an example). Lithologic units and subunits were then identified on the basis of the presence of contacts, chilled margins, changes in primary mineralogy (occurrence and abundance), color, grain size, and structural or textural variations (see Table T3 for an example). Unit boundaries were generally chosen to indicate different volcanic cooling units, although we were forced by limited recovery in some cases to arbitrarily decide the exact location of a unit boundary within an interval where the lithology above and below the interval was different. In order to preserve important information about the volcanology without defining an unreasonable number of units within a single core, subunits were designated in cases where there were frequent changes in texture without accompanying changes in mineralogy (for example, several pieces containing glass within 50 cm of core, all of which are mineralogically similar).

Hard rock visual core descriptions

Hard rock VCD forms (HRVCDs) (Fig. F5) were used to describe each section of the igneous rock cores. A key to the symbols used on VCDs is given in Figure F6, and definitions of the terms used are provided in Table T4. From left to right on the HRVCD, the following are displayed:

1. Photograph of the archive half of the core

2. Scale from 0 to 150 cm

3. Piece number

4. Graphical representation of the rock with structural details

5. Piece orientation

6. Location of samples selected for shipboard studies

7. Boundaries of lithologic units

8. Structures

9. Presence of glass or altered glass

10. Phenocryst abundance and mineralogy

11. Groundmass grain size

12. Alteration intensity

In the graphical representation (column 4), chilled margins were indicated by using the symbol shown in Figure F6. A horizontal line across the entire width of column 4 denotes a plastic spacer, reflecting the curator's interpretation of the boundaries between different pieces of core. Vertically oriented pieces are indicated on the form by an upward-pointing arrow to the right of the appropriate piece (column 5). The locations of samples selected for shipboard studies are indicated in the column headed Shipboard Studies using the following notation:

• XRD = X-ray diffraction analysis

• ICP = ICP-AES analysis

• TS = petrographic thin section

• PP = physical property analysis

• TC = thermal conductivity

• PM = paleomagnetic analysis

• MB = microbiological analysis

The Lithologic Unit column displays the location of the boundaries between units and subunits and the unit designator (e.g., 1, 2A, 2B, etc.). The Structure column displays the graphical representations of structural types from the key in Figure F6.

The boundaries of the lithologic units and subunits were drawn on the HRVCD across columns 4–12 (solid lines = unit boundaries; dotted lines = subunit boundaries) and numbered consecutively within each hole. HRVCDs also contain a text description of each unit in each section of core that includes the following:

1. Expedition, site, hole, core number, core type, and section number

2. Depth of the top of the section in meters below seafloor

3. Unit number (consecutive downhole; subunits are designated by letters after the unit number [e.g., 1, 2A, 2B, etc.])

4. Rock name

5. Summary description of the unit as it appears in the section, including a brief rock name and the rock type (e.g., pillow basalt or sheet flow)

6. Piece numbers included in the unit

7. Type of contacts

8. Munsell color

9. Phenocryst minerals, abundance, and size

10. Groundmass grain size

11. Vesicle abundance

12. Nature of the alteration

13. Information about abundance and mineralogy of veins

14. Description of structures in the rock

15. Additional comments

Units and subunits were named on the basis of the groundmass texture and the abundance of primary minerals. Basalts were described based on the identification of phenocrysts in hand sample:

• Aphyric = <1% phenocrysts

• Sparsely phyric = 1%–5% phenocrysts

• Moderately phyric = 5%–10% phenocrysts

• Highly phyric = >10% phenocrysts

Rock names were further classified by the types of phenocrysts, where present (e.g., sparsely plagioclase-olivine phyric, in which the amount of olivine exceeds the amount of plagioclase). Rock color was determined on a wet, cut surface of the rock using the Munsell color chart. Groundmass character was determined by measuring average groundmass grain size (width of elongated grains) with a binocular microscope. Grain size was identified as follows:

• mg = medium grained (average groundmass grain size 1 mm)

• fg = fine grained (grains = 0.2–1 mm)

• µx = microcrystalline (groundmass crystals = 0.1–0.2 mm)

• cx = cryptocrystalline (crystals = <0.1 mm)

• G = glassy

An estimate of the percentage of vesicles and their average sizes was made and included in the comments on the HRVCDs. Mineral abundance was used in determining the rock name. The igneous unit and contact logs are included (see Table T3).

Pillow basalts were identified by curved chilled margins oblique to the vertical axis of the core or, when these margins were absent, by variolitic texture, curved fractures, and microcrystalline or cryptocrystalline grain size. For glassy or chilled pieces lacking definitive indications of pillow structures (e.g., curved glassy margins), we designated these units as "basalt flows," which could be interpreted as pillow basalts or sheet flows. Massive basalt flows were identified by sections of core bounded by planar subhorizontal chilled margins, with the same lithology and grain size that increased toward the center of the unit. Other rock types distinguished were breccias and hyaloclastites.

Igneous unit and contact logs

The first step in describing the core was the selection of unit boundaries, as described in "Hard rock visual core descriptions" in "Igneous and metamorphic petrology." Subunits are designated on the HRVCD, and their descriptions are included within the overall written description of the unit. The igneous unit and contacts log (Table T3) provides information about the unit boundaries and a brief description of each unit. The table lists the following for each unit:

• Core number

• Section number

• Piece number(s)

• Location (in meters below seafloor) of the upper contact, calculated from the curated depth of the top of the core and the length of the pieces in the core above the upper contact

• Type of the upper contact (listed in Table T3)

• Minimum thickness of the unit, calculated from the piece lengths

• Rock type of each unit

Thin sections

Thin sections of igneous rocks were studied to complete and refine the hand-specimen observations. This included textural features that were not identified in hand specimen; precise determination of grain size of phenocrysts and groundmass; the mineralogy, abundance, and type of 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), glomeroporphyritic (containing clusters of phenocrysts), hypocrystalline (100% crystals) to hypohyaline (100% glass), variolitic, intergranular (olivine and pyroxene grains between plagioclase laths), intersertal, subophitic, and ophitic were used to describe the textures of the mesostasis. The same terminology was used for thin section descriptions and the macroscopic descriptions. An example of the thin section description form is given in Table T5, with a key in Table T4. Thin section descriptions are included in "Core Descriptions" and are also available from the IODP database. Digital photomicrographs were taken during the cruise to document features described in the thin sections. A list of available images, any of which can be obtained from the IODP data librarian, is given in the Site U1301 chapter.

Alteration

All igneous rocks recovered during Expedition 301 have undergone alteration and veining. On the HRVCD forms, rocks were graded according to whether they are fresh (<2% by volume alteration products) or have slight (2%–10%), moderate (10%–50%), high (50%–90%), or complete (90%–100%) alteration. Alteration and vein description logs on a piece-by-piece scale were tabulated to provide a consistent characterization of the rocks and to quantify the different alteration types (see Tables T6, T7). Descriptions are based mostly on hand-specimen observations, and specific secondary minerals are not generally distinguished, except where crystal morphology allows unequivocal identification. Where additional mineralogical evidence is available from either thin section descriptions and/or X-ray diffractograms, these identifications were integrated into the alteration and vein logs and the HRVCDs. We recorded the following information in the logs:

• Alteration log (e.g., Table T6): This log records bulk rock alteration. Each entry records the igneous unit; identifiers for the core, section, piece, and subpiece; the locations of top and bottom of each piece; the length of each piece; and the depth below seafloor of the top of the core. The alteration type (as represented by rock color and calibrated by thin section observations), the abundance (in percent) and fill-mineralogy of vesicles, the abundance (in percent) of glass, and the percent alteration of glass are documented for each piece or group of pieces. A column for comments is included.

• Vein log (e.g., Table T7): This log records the presence, location, width, and mineral content of veins observed on the cut surface of the cores. Each entry records the igneous unit and the identifiers for the core, section, piece, and subpiece. For each vein, the location of the top and bottom of the feature is recorded, and the mineralogy, vein width (in millimeters), presence or absence of a related alteration halo, and the width (in millimeters) of the halo on one side of the vein are recorded. For breccia and vein nets, recorded data include the centimeter interval, the percentage of nonbasalt material (e.g., veins and cement), and the percentages of secondary minerals. A column for comments is included.

Structures

This section outlines the techniques used for macroscopic and microscopic description of structural features observed in hard rock basement cores. Conventions for structural studies established during previous ODP hard rock drilling legs (Shipboard Scientific Party, 1989, 1991, 1992a, 1992c, 1992d, 1993a, 1993b, 1995, 1999, 2003a) were generally followed during Expedition 301.

Graphical representation and terminology

All material from both working and archive halves was examined, although the sketches of the structures and orientation measurements were made on the archive half. The most representative structural features in the cores recovered during Expedition 301 are summarized on the HRVCD form (see "Core Descriptions"). For each section, more detailed structural information is described and sketched on a separate Structural Geology Description form (Fig. F7). Structural data were tabulated in the structure log (Table T8).

To maintain consistency of core descriptions we used a set of structural feature "identifiers." Brittle deformation identifiers include joint, vein, shear vein, fault, and breccia. Identification of these features is based on the presence of fractures, filling phases, and evidence of shear displacement. The terminology adopted generally follows that of Ramsay and Huber (1987), Twiss and Moores (1992), and Passchier and Trouw (1996) and is consistent with the terminology used during Leg 153 for brittle deformation (Shipboard Scientific Party, 1995). Some of the terms commonly used in the structural description are sketched in Figure F8:

J = joints (fractures where the two sides show no differential displacement [relative to the naked eye or 10x pocket lens] and have no filling material)

V = veins (extensional open fractures filled with epigenetic minerals)

SV = shear veins (obliquely opening veins with minor shear displacement, filled with slickenfibers or overlapping fibers)

F = faults (fractures with kinematic evidence for shear displacement across the discontinuity or with an associated cataclasite; we adopted the term microfault when the scale of the offset is millimetric)

This subdivision of the structures does not imply that all features fall into distinct and exclusive categories. We prefer to use the term veins for all the healed fractures, avoiding the usual subdivision based on fracture width (e.g., Ramsay and Huber [1987] defined veins as having >1 mm filling material), mainly to be consistent with the vein log (see "Alteration" in "Igneous and metamorphic petrology"). There are not rigid boundaries between the adopted structural categories; where necessary, details of specific structural features are illustrated with comments and sketches.

Geometric reference frame

Structures are measured on the archive half relative to the core reference frame used by IODP. The plane normal to the axis of the borehole is referred to as the horizontal plane. On this plane, a 360° net is used with a pseudo-south (180°) pointing into the archive half and a pseudo-north (0°) pointing out of the archive half and perpendicular to the cut surface of the core. The cut surface of the core, therefore, is a vertical plane striking 90°–270°. The strike of planar features across the cut face of the archive half was measured with 0° down the vertical axis of the core, and the dip was measured using the right-hand rule (Fig. F9). The orientations were then rotated into the IODP reference frame using the Stereonet (version 6.3.0X) Macintosh program by Rick Allmendinger.

Hard rock geochemical analyses

Representative samples from selected igneous units were analyzed for major and trace elements during Expedition 301 using ICP-AES. Approximately 10 cm³ samples were cut from the core with a diamond saw blade. All outer surfaces were ground on a diamond-impregnated disk to remove surface contamination and altered rinds resulting from drilling. Each cleaned sample was placed in a beaker containing trace metal-grade methanol and was ultrasonicated for 15 min. The methanol was decanted and the samples were ultrasonicated twice in deionized water for 10 min and then ultrasonicated 10 min in nanopure deionized water. The clean pieces were then dried for 10–12 h at 65°C.

The clean, dry whole-rock samples were fragmented to chips <1 cm by crushing them between two disks of Delrin plastic in a hydraulic press. They were then ground to a fine powder in a tungsten carbide (WC) mill by a SPEX 8510 shatterbox. A 1.0000 ± 0.0005 g aliquot of the sample powder was weighed on a Scientech balance and ignited to determine weight loss on ignition (LOI).

ODP Technical Note 29 (Murray et al., 2000) describes in detail the shipboard procedure for dissolution of rocks and ICP-AES analysis of samples. The following protocol is an abbreviated form of this with minor changes and additions. After determination of LOI, 100.0 ± 0.2 mg aliquots of the ignited whole-rock powders were weighed and mixed with 400.0 ± 0.5 mg of LiBO₂ flux that had been preweighed on shore. Standard rock powders and full procedural blanks (400 mg LiBO₂) were included with the unknowns in each ICP-AES run. During ODP Leg 206, a grinding blank of pure SiO₂ was analyzed as a check on grinding contamination contributed by the WC mills (Table T9), using a grinding blank that was processed using the shatterbox that appeared dirtiest and was therefore a "worst-case" scenario. The Leg 206 analysis gives an indication of the potential grinding contamination in Hole U1301B samples. All samples and standards were weighed to ±0.20 mg on the Scientech balance, and weighing errors are estimated to be ~0.05 mg.

We added 10 µL of 0.172 mM aqueous LiBr solution to the flux and rock powder mixture as an antiwetting agent to prevent the cooled bead from sticking to the crucible. Samples were then individually fused in Pt-Au (95%:5%) crucibles for ~3 min at a maximum temperature of 1050°C in a Bead Sampler NT-2100. After cooling, beads were transferred to 125 mL high-density (HD) polypropylene bottles and dissolved in 50 mL of 10% HNO₃, aided by shaking with a Burrell reciprocal bottle shaker for 1 h. From Run 2 onward, the samples were ultrasonicated for ~1 h after shaking to ensure complete dissolution of the glass bead. After digestion of the glass bead, all of the solution was passed through a 0.45 µm filter into a clean 60 mL wide-mouth HD polypropylene bottle. Next, 2.5 mL of this solution was transferred to a plastic vial and diluted with 17.5 mL of 10% HNO₃ to bring the total volume to 20 mL. The final solution-to-sample dilution factor for this procedure was ~4000×.

Major (Si, Ti, Al, Fe, Mn, Mg, Ca, Na, K, and P) and trace (Sc, V, Cr, Ni, Sr, S, Y, Zr, Nb, and Ba) element concentrations of standards and samples were determined with the JY2000 Ultrace ICP-AES, which routinely measures wavelengths between ~100 and 800 nm. Specific analytical conditions for each sample run during Expedition 301 are provided in Table T10.

The JY2000 plasma was ignited at least 30 min before each sample 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 BAS 148 standard (basalt standard created during ODP Leg 148, Hole 504B; Bach et al., 1996), or the BAS 206 (basalt interlaboratory standard created during ODP Leg 206; Shipboard Scientific Party, 2003a) prepared in 10% HNO₃. During the initial setup, an emission profile was selected for each peak, using standard 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 BHVO-2.

ICP-AES data presented in "Hard rock geochemistry" in "Igneous and metamorphic petrology" in the "Site U1301" chapter were acquired using either the Gaussian or maximum mode of the Windows 5 JY2000 software. Gaussian mode fits a curve to points across a peak and integrates the area under the curve to determine element intensity. Gaussian mode was used for Si, Ti, Al, Fe, Ca, Na, K, Sc, V, Sr, Zr, and Nb, and maximum mode was used for elements with asymmetric emission peaks (Mn, Mg, P, Y, Cr, Ni, and Ba). Intensity is integrated using the maximum intensity detected. Each unknown sample was run at least twice, nonsequentially, within a given sample run.

A typical hard rock ICP-AES run (Table T11) during Expedition 301 included the following:

• A set of five certified rock standards (JA-1, JR-1, BIR-1, AII, and BOB-1) analyzed twice each throughout the sample run

• Up to seven unknown samples run in duplicate

• A drift-correcting sample, BCR-2, analyzed every fourth sample position and at the beginning and end of each run

• Blank solutions run near the beginning and end of each run

• A check standard (i.e., standard run as an unknown), typically BAS-148 or BAS-206, although Run 1 consisted primarily of check standards and also included standards JA-3, JGb-1, and JB-2

• A 10% HNO₃ wash solution run for 90 s between each analysis.

Following each sample run, the raw intensities were transferred to a data file and all samples were corrected first for drift and then for the full procedural blank. The drift correction was applied to each element by linear interpolation between drift-monitoring solutions run approximately every fourth analysis. Following drift correction and blank subtraction, calibration curves were constructed based on five certified rock standards (JA-1, JR-1, BIR-1, AII, and BOB-1). Unknown concentrations were then calculated from the calibration line. Estimates of accuracy and precision for major and trace element analyses were based on replicate analyses of check standards (usually BAS-148 and BAS-206), the results of which are presented in Table T12. In general, run-to-run relative precision by ICP-AES was <3.5% for the major elements. Run-to-run relative precision for trace elements was generally <9%. Exceptions typically occurred when the element in question was near background values.

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