ALTERATION

All igneous rocks recovered during Leg 192 have undergone low-temperature alteration, either halmyrolysis or anoxic alteration. On the hard-rock VCD forms, rocks are graded according to whether they are unaltered (<2 vol% alteration products) or have slight (2-10 vol%), moderate (10-40 vol%), high (40-80 vol%), very high (80-95 vol%), or complete (95-100 vol%) alteration. Alteration and vein/structure 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. Descriptions are based mostly on hand-specimen observations of cut wet surfaces; specific clay, zeolite, and carbonate minerals are not generally distinguished, except where crystal morphology allows unequivocal identification. For example, we commonly noted white veins containing only calcium carbonate in the vein/structure logs, although we observed zeolites in some thin sections. When additional mineralogical evidence was available from either thin section descriptions and/or X-ray diffractograms, we integrated these identifications into the alteration logs and the VCDs. However, because of the large number of veins described, we did not enter thin section and XRD data into the vein/structure log. Table T4 provides a list of abbreviations used in the alteration and vein/structure logs. Bulk rock and halo colors are from the Munsell Soil Color Charts (1975).

We recorded the following information in the alteration and vein/structure logs:

1. The alteration log (Table T5) records bulk-rock alteration. Each entry identifies the igneous unit; the core, section, piece, and interval; the length of each piece; and the depth below the seafloor of the top of each piece. Visual estimates of the alteration type (as represented by rock color and calibrated by thin section observations), the abundance (in percent), size (in millimeters), and mineral fillings of vesicles and miarolitic cavities, and the proportion of altered groundmass and phenocrysts (with the precursor and secondary minerals) are documented for each piece. A column for comments is included.
2. The vein/structure log (Table T6) records the presence, apparent orientation, location, width, and mineral content of veins observed on the cut surface of the core. Each entry identifies the igneous unit and the core, section, piece, and depth below the seafloor of the top of each piece. For each vein, the locations of the top and bottom, depth below the seafloor, mineral fillings and proportions, vein width (in millimeters), and apparent orientation are recorded. We also recorded the presence or absence of a related alteration halo and the color, half width (in millimeters), and alteration mineralogy of halos. A column for comments is included.