ALTERATION AND WEATHERING

All igneous rocks recovered during Leg 197 have undergone secondary alteration or weathering in subaerial, shallow subaqueous, or deep marine environments. Many features of low-temperature submarine alteration and subaerial weathering appear similar and are indistinguishable in the cores. In these descriptions, alteration is representative of chemical transformations of mineral assemblages induced by interactions with hydrothermal fluids. Weathered materials are formed at the Earth's surface (ambient temperature and pressure) by interactions with meteoric fluids. Note that the products of subaerial weathering may be further modified through alteration on the seafloor.

On the igneous rock visual core description forms, rocks were graded (based on volume of alteration products) as follows:

f = unaltered (<2%).

s = slightly altered (2%-10%).

m = moderate alteration (10%-40%).

h = highly altered (40%-80%).

vh = very highly altered (80%-95%).

c = complete alteration (95%-100%).

We determined the types, forms, and distribution of secondary alteration/weathering effects as well as abundances of veins and vesicles along with associated secondary minerals. Features related to any changes in the alteration/weathering styles through a section or an igneous unit are reported on the VCD. Alteration and vein core description logs 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, and specific clay, zeolite, and carbonate minerals are not generally distinguished, except where crystal morphology allows unequivocal identification. As the main objectives of the leg are centered on paleomagnetism, we paid special attention to the occurrence of secondary iron oxide minerals and alteration of primary igneous opaque phases (e.g., titanomagnetite altered to maghemite). Where additional mineralogical 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 VCDs. XRD analyses were conducted on board the ship, but instrumental problems were encountered. The data presented in this volume should therefore be used with caution as a general guide for mineral recognition rather than for precise determinations. Table T8 provides a list of abbreviations used in the alteration and vein logs. Colors were taken from Munsell Rock Color Charts or Munsell Soil Color Charts (Munsell Color Co., 1991, 1975, respectively).

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

1. The alteration log (Fig. F19) was used to record bulk rock alteration and vesicle filling. Each entry records the igneous unit and identifiers for the core, section, and the depth below seafloor of the top of each piece. Also recorded were visual estimates of the degree of alteration based on a scale ranging from 0 for unaltered samples to 5 for complete alteration, presence of secondary minerals such as Fe oxides and clay, visual estimates of vesicularity based on a scale ranging from 0 for nonvesicular samples to 3 for highly vesicular samples, and mineral fillings of vesicles.
2. The vein log (Fig. F20) was used to record the presence, location, apparent orientation, width, and mineral content of veins observed on the cut surface of the cores. Each entry records the igneous unit and identifiers for the core, section, piece, and the depth below seafloor of the top of each piece. For each vein, the depth, location of the top and bottom, vein width (in millimeters), apparent orientation, mineral fillings, and proportions of the feature are recorded. If a related alteration halo is present, its color, half-width (in millimeters), alteration mineralogy, and proportions are described. A column for comments is included.