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: