IODP Proceedings    Volume contents     Search


Igneous petrology

Core curation and sampling for shipboard measurements

To preserve important features and structures, all cores were passed through X-ray CT scan, and examined before being cut with a diamond-impregnated saw. Each piece was numbered sequentially from the top of the core section and labeled on the outside surface. Broken pieces that could be fit together were assigned the same number and were lettered consecutively (e.g., 1A, 1B, 1C, etc.). Rarely, composite pieces may have occupied more than one section. 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 pointing to the top of the section was added.

Nondestructive physical property measurements by multisensor core logger (MSCL) were made on the core before it was split (see “Physical properties”). 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, digital images of the core were taken before description. The archive half was described on the VCD form for igneous characteristics, alteration, and structure. The working half was sampled for shipboard physical property measurements (see “Physical properties”), thin sections, and XRD. No samples were taken onboard for shore-based studies.

Visual core descriptions and barrel sheets for igneous rocks

We used general description VCD forms (Fig. F17) to document each section of the igneous rock cores. Symbols used on the VCDs are given in Figure F18. On the VCDs, the following are displayed from left to right:

  • A scale from 0 to 150 cm,

  • Piece number,

  • Photograph of the archive half of the core,

  • Piece orientation,

  • Locations of samples selected for shipboard studies,

  • Boundaries and number of lithologic units,

  • Structures,

  • Phenocrysts, and

  • Alteration intensity.

Copies of VCDs are available from IODP upon request.

Vertically oriented pieces are indicated on the form by an upward-pointing arrow to the right of the appropriate piece. Locations of samples selected for shipboard studies are indicated in the Shipboard Studies column, using the following notation: XRD = X-ray diffraction analysis, XRF = X-ray fluorescence, TSB = petrographic thin section, PP = physical property analysis, or PMAG = paleomagnetic analysis. The Lithologic unit column displays the locations of boundaries between units and subunits and the unit designator (consecutive downhole subunits are designated by letters after the unit number; e.g., I, IIA, IIB, etc.). The Structure column displays graphical representations of structural types from the key in Figure F18.

During Expedition 333, we defined igneous rocks with glassy to fine-grained (average groundmass grain size = <1 mm) characteristics as volcanic rocks and the root term “basalt” was used. Basalt designates all igneous rocks of basaltic composition in the size range glassy to fine grained.

VCDs of volcanic rocks contain a text description of each unit in each section of core that includes:

  • Summary description of unit as it appears in the section (e.g., pillow basalt or sheet flow);

  • Rock name; and

  • Additional comments, including phenocryst mineral abundance and size; groundmass grain size and texture, vesicle abundance, nature of alteration, description of structures in the rock, and any other additional comments.

In cryptocrystalline to microcrystalline rocks, there is a clear distinction between phenocrysts and groundmass crystals. These were described based on the identification of phenocrysts in hand sample following the criteria listed below.

  • Aphyric (<1% phenocrysts),

  • Sparsely phyric (1%–5% phenocrysts),

  • Moderately phyric (5%–10% phenocrysts), and

  • Highly phyric (>10% phenocrysts).

Rock names were further classified by types of phenocrysts present (e.g., sparsely plagioclase-olivine phyric, in which the amount of olivine exceeds the amount of plagioclase). An estimate of the percentage of vesicles and their average size was included in the VCDs. Vesicularity is described according to the abundance, size, and shape (sphericity and angularity) of the vesicles. The subdivision was made according to the following.

  • Nonvesicular (<1% vesicles),

  • Sparsely vesicular (1%–5% vesicles),

  • Moderately vesicular (5%–20% vesicles), and

  • Highly vesicular (>20% vesicles).

Pillow lavas were identified by curved chilled margins oblique to the vertical axis of the core or, when these margins were absent, curved fractures, and microcrystalline or cryptocrystalline grain size. Sheet flows were defined as thick intervals with the same rock type and grain size that increased toward the center of the flows. The most common types of contacts are those with chilled margins.

Thin section description of igneous rocks

During Expedition 333, five thin sections from the core intervals noted on the VCD forms were examined 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 are reported in table format. The textural terms used are defined by MacKenzie et al. (1982).

Igneous units

The first level of core description is the definition of unit boundaries on the basis of the presence of contacts, chilled margins, changes in primary mineralogy, color, grain size, and structural or textural variations. Ideally, unit boundaries within volcanic rocks are meant to represent successive cooling units, but, when recovery is limited, unit boundaries may only represent changes in lithology.