ALTERATION AND WEATHERING

All igneous rocks recovered during Leg 183 have undergone secondary alteration or weathering in subaerial, shallow subaqueous, or deep marine environments. Many features of low-temperature hydrothermal alteration (both submarine and subaerial) and subaerial weathering appear similar and are difficult to distinguish in the drilled core. In these descriptions, alteration is defined as the chemical transformation of mineral assemblages caused by interaction with hydrothermal fluids. Weathered materials differ from altered materials in that they form at the Earth's surface at ambient temperatures and pressures and are affected by the percolation of rainwater and meteoric fluids that have not been heated. The products of subaerial weathering may be further modified on the seafloor. Postcruise mineralogic and chemical analysis may help discriminate between subaerial weathering and hydrothermal processes. As a consequence, we did not make this distinction while logging core.

On the HRVCD forms, rocks were graded according to whether they were fresh (<2% by volume alteration/weathering products) or have slight (2%-10%), moderate (10%-40%), high (40%-80%), very high (80%-95%), or complete (95%-100%) alteration/weathering. We determined the types, forms, and distributions of secondary alteration/weathering effects, as well as abundances of veins, vesicles, and their mineral fillings, using a selection of precision tools. Any changes in alteration/weathering styles throughout a section or an igneous unit were also recorded on the HRVCD. Features related to subaerial weathering are also noted on the HRVCD. Unconsolidated parts of regolith intervals are also recorded on the sedimentary VCDs.

Alteration and vein-core description logs were tabulated to provide a consistent characterization of the rocks, as well as provide the information required to make quantitative estimates of the extent of alteration. Cores were described on a piece-by-piece scale. Alteration and vein logs for each hole are presented (see the "Core Descriptions" contents list). Descriptions are based mostly on hand-specimen observations, and specific clay, zeolite, and carbonate minerals are not generally distinguished, except where crystal morphology allows unequivocal identification. Where additional mineralogic 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.

Table T9 provides a list of abbreviations used in the alteration and vein logs.

We recorded the following information in the databases:

  1. The alteration log (e.g., Table T10) was used to record the bulk-rock alteration. Each entry records the igneous unit; identifiers for the core, section, piece, subpiece; the length of each piece; and the depth below seafloor of the top of each piece. Visual estimates of the percentage of altered groundmass, color, the abundance (in percent), diameter (in millimeters), mineral fillings and halo widths (in millimeters) of vesicles, and the proportion of altered phenocrysts with the precursor and secondary minerals are documented for each piece. Data recorded for breccias include percentages of matrix and clasts, and the total percentage of secondary minerals and sediments. A column for comments is included.
  2. The vein/structure log (e.g., Table T11) was used to record the presence, location, and mineral content of veins observed on the cut surface of the Leg 183 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 mineral fillings, vein width (in millimeters), apparent orientation on the cut face (0-90; horizontal to vertical), presence or absence of a related alteration halo, and the half width (in millimeters) of the halo are described. A column for comments is included.

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