Almost all the rocks recovered during Leg 193 have undergone hydrothermal alteration. On the hard-rock VCD forms and the alteration and mineralization logs, rocks were graded according to whether they are fresh (<2% by volume alteration products) or have slight (2%-<10%), moderate (10%-<40%), high (40%-<80%), very high (80%-<95%), or complete (95%-100%) 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 vein abundance and alteration intensity.
Alteration/mineralization and vein/structure descriptions for each hole are found in the "Core Descriptions" contents list. These were based mostly on visual and binocular microscope observations of cut, wet surfaces, and specific alteration minerals were only distinguished where it was considered that unequivocal identification could be made. Where additional mineralogical evidence was available from RI and/or XRD measurements (see below), these identifications were integrated into the alteration/mineralization spreadsheets and the VCD forms. However, we did not enter these data into the vein/structure spreadsheet. Table T2 provides a list of abbreviations used in alteration/mineralization and vein/structure descriptions.
We recorded the following information in the alteration/mineralization and vein/structure description sheets:
Shipboard geochemical analyses on altered rocks collected during Leg 193 were conducted with the ICP-AES using the same suite of elements as that analyzed for fresh igneous rocks (see "Geochemistry"). However, except where noted with the data, the analytical methodology differs during sample preparation in one respect. Prior to heating to 1050°C in the furnace, the crushed powder was heated to 600°C and roasted at this temperature for 6 hr to allow oxidation of any sulfide material prior to fusion. All other analytical procedures are identical to those applied to fresh igneous rocks.
Bulk-rock XRD analysis was undertaken on a large number of samples to determine the mineralogical composition of altered rocks. Standard XRD operating procedure and conditions were adhered to, and an interactive software package (R. Petschick, Macdiff 3.1, 1995) was used to help identify the main minerals. Minerals are termed major, minor, or trace based on peak height in the XRD spectra. However, peak heights may be strongly influenced by factors other than abundance, so no quantitative measurement is implied. In particular, the abundances of clay and phyllosilicate minerals are not quantitatively estimated and likely to be underestimated.
To provide a semiquantitative estimate of anhydrite abundance, XRD spectra were taken for a number of quartz-anhydrite mixtures of known proportions. The relative peak heights were used to provide a calibration for the estimation of the anhydrite content of quartz-bearing altered rocks. The calibration does not account for the effect of other minerals, and therefore, values should be considered indicative at best.
The RI measurements provided a simple, quick method to distinguish between alteration phases with otherwise similar properties (most notably barite and anhydrite and quartz and cristobalite). The methodology followed was identical to that described in "Igneous Petrology". Preliminary petrological observations using crushed particles mounted in oil were used in several instances where quick information was required for description purposes or where thin-section manufacture was impractical or not intended.
During Leg 193, short-wave infrared (SWIR) spectrometry was conducted to aid with the identification of fine-grained alteration minerals and in the delineation of the alteration mineral assemblage. The SWIR spectral range was useful in this alteration study as phyllosilicates, hydroxylated silicates, sulfates, and carbonates all have distinctive spectral patterns in this range. Many members of these mineral groups were encountered in the core.
The analysis was conducted using an Integrated Spectronics PIMA (portable infrared mineral analyzer) II. The PIMA II emits a beam of infrared light onto the sample and measures the intensity of the light reflected in each wavelength interval. Certain minerals preferentially absorb particular wavelengths of light. The spectral signatures generated by the differing positions and intensities of absorption features combine to create an overall absorption pattern for the sample, which may be used diagnostically to qualitatively identify the dominant mineralogy of the sample with respect to the mineral groups mentioned above.
The PIMA II works in the spectral range from 1300 to 2500 nm. Measurements are taken incrementally every 2 nm. A 1-cm2 area of the core representing each alteration type was analyzed with additional measurements taken of features of interest (e.g., vein selvages and patchy alteration).
All spectral results were recorded in digital format and can be found in the "Supplementary Materials" contents list. Each of these files has been named by the hole number followed by a three digit number, which identifies the order in which the sample was acquired for that hole (e.g., 1188A001.TXT). The first column of the data gives the wavelength (stepped at 2-nm intervals from 1300 to 2500 nm), and the second column records the reflectance intensity (from 0 to 1). All data in these files are tab delimited. An example of this data format is given below.
Additional data for all the samples are present in the "PIMALOG.TXT" file. This file gives the file name, site, hole, core, section, piece number, and comments for each sample acquired. Specific features are also noted, when analyzed.
The very fine aphyric nature of the majority of the rocks drilled during Leg 193 made mineral identification and estimation of percentages in hand specimen extremely difficult. In many cases, subsequent examination of thin sections and XRD analyses indicated that the relative proportions of minerals, as estimated in hand specimen, were erroneous. Additionally, as it was necessary to distinguish alteration types based on hand-specimen properties, the three principal types of alteration (bleaching, silicification, and green silica-clay [GSC] alteration) were separated mainly by using color and hardness. An unfortunate side effect of this necessarily simplistic approach was the misidentification of moderately altered volcanic rocks in the lower parts of Holes 1188F and 1189B as completely silicified units, based on hardness.
Because of the time constraints, it was not always possible to correct the alteration logs (see the "Core Descriptions" contents list) to reflect additional information obtained by more sophisticated methods. In particular, mineral abundances listed on the logs should be considered to be indicative, at best. Thin-section descriptions provide the more reliable estimate of mineral abundances.