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doi:10.2204/iodp.proc.309312.102.2006

Alteration

Alteration characteristics of the drill core were determined using visual inspection of the core, microscopic thin section descriptions, and XRD analyses.

Core description

All igneous rocks recovered during Expedition 309/312 have undergone alteration. On the hard rock VCD forms, rocks were graded according to alteration intensity as follows:

  • Fresh (<2% by volume alteration products)
  • Slight (2%–10%)
  • Moderate (10%–50%)
  • High (50%–90%)
  • Complete (90%–100%)

The approximate frequency (rare = 0–5/m, several = 5–15/m, common = 15–30/m, and very common = >30/m) and the thickness of veins are recorded, and their minerals are given in order of abundance. The frequency (rare, several, common, and very common) and types of alteration halos are recorded on the VCD forms, and their volume percents are given in the alteration log.

Alteration and vein core 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 T2, T3). Alteration of plutonic rocks was described using the plutonic rock alteration log (Table T4). Where both basalts and gabbros are found in the same section, the appropriate alteration log sheet was used for intervals of each rock type. A comment was added, noting which intervals are different, including any additional information required. Veins in plutonic rocks were recorded using the standard vein log (see 309VEIN.XLS and 312VEIN.XLS in “Supplementary material”). Descriptions are based mostly on hand specimen observations; 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 are integrated into the alteration and vein logs and the VCDs.

We recorded the following information in logs:

  • Alteration log (e.g., Table T2): This log was used to record bulk rock alteration, either piece by piece or for a given group of pieces. Each entry records identifiers for core, section, piece(s), and interval (centimeters); length of each piece or group of pieces; depth below seafloor (in meters) of the top; and igneous unit. Information on alteration type (as represented by rock color and calibrated by thin section observations) for background rock, alteration patches, and alteration halos is provided. Four additional entries are included: (1) percentages of amygdules with mineral composition, (2) percentages of vesicles with mineral composition, (3) percentages of fresh and altered glass, and (4) comments.
  • Vein log (e.g., Table T3): This log was used to record the location, apparent area, and mineralogy of veins, vein nets, and breccias and the alteration halos observed on the cut surfaces of the cores. Each entry records identifiers for core, section, piece, and subpiece. For each vein, the location of the top and bottom of the feature is recorded, along with vein width (in millimeters), mineral fillings, presence of a related alteration halo, and width (in millimeters) of the halo on one side of the vein. Veins that do not intersect both sides of the cores are indicated with a “V” in the Vertical? column. For breccia and vein nets, recorded data include centimeter interval, percentages of nonbasaltic material (veins and cement), and percentages of secondary minerals within the latter. One column indicates veins and breccias studied by XRD (with mineral composition). Two columns provide the width and type of alteration halos adjacent to the vein. A column for comments is included.
  • Plutonic rock alteration log (e.g., Table T4): This log was used to record bulk rock alteration of gabbros and other coarse-grained holocrystalline rocks, either piece by piece or for a specified group of pieces. Each entry records identifiers for core, section, piece(s), and interval; length of each piece or group of pieces; and igneous unit. Total modal percentage of secondary minerals were estimated in hand specimen, as were proportions of major primary igneous minerals (olivine, clinopyroxene, plagioclase, orthopyroxene, and oxides) and the secondary minerals by which they are replaced. A column for comments is included.

Thin section description

Thin sections of basement rocks recovered during Expedition 309/312 were examined in order to

  • Confirm macroscopic identifications of secondary minerals;
  • Determine their mode of occurrence in terms of vesicle and void fillings, vein composition, and primary mineral replacement;
  • Identify chronological relationships between different secondary minerals or parageneses;
  • Establish distribution, occurrences, and abundance of secondary minerals downhole; and
  • Quantify the amount of alteration.

Modal compositions were estimated for each thin section after calibration by point counting on a selection of sections. Digital photomicrographs were taken during the expeditions to document features described in thin sections.

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