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doi:10.2204/iodp.proc.324.102.2010 Alteration and metamorphic petrologyAlteration characteristics of cores were described on the archive halves of the core, using petrographic description of thin sections and XRD analyses (bulk rock, veins, and vesicle infillings). Alteration was then presented as a component of the VCDs. DESClogik: the new core description software for descriptive data captureA new software program, DESClogik, first implemented and assessed during Expedition 320T for core logging, thin section description, and database compilation, was used during Expedition 324. Prior to drilling operations, two spreadsheet templates were constructed and customized in order to record alteration characteristics. These spreadsheet templates were based on the methods and guided by the recorded observations of ODP Leg 192 and IODP Expedition 304/305 (Mahoney, Fitton, Wallace, et al., 2001; Blackman et al., 2006). The first of these spreadsheet templates was used to record alteration by visual core description (Fig. F12), and the second template was used to record descriptions of veins (Fig. F13). Core loggingDescription of alteration of the archive half of the core provides information on alteration of primary igneous features including phenocrysts and groundmass secondary mineralogy, veins, and infilling material in vesicles. Information on the extent of replacement of igneous minerals and groundmass by secondary minerals and, where possible, the nature and approximate modes of secondary mineral assemblages are recorded using DESClogik. The presence of veins or vesicle infill and their description and mineralogy are also recorded using DESClogik. Visual estimates of alteration degree, type, color, textures (e.g., halos and patches), abundance (%) of mineral filling vesicles, and the proportion of altered groundmass and phenocrysts were recorded in the alteration and vein logs of DESClogik. Most of the secondary mineral assemblages were composed of clays that were difficult to identify macroscopically; therefore, a general descriptive term such as "brown clay" was used. Alteration degree is defined and reported graphically on the VCDs according to various ranges of intensity (Fig. F8):
Alteration color was defined based on Munsell Soil Color Charts (Munsell Color Company, 1994) and converted to a more intuitive color name. Precise color identification using Munsell Soil Color Charts was recorded in the DESClogik template of the Igneous Petrology group (see "Igneous petrology"). Quantification of individual mineral modes was estimated by investigating the archive-half core under a binocular microscope. A distinction was made between overprinting alteration assemblages and assemblages localized by preexisting lithology changes. For example, a core of altered basalt containing a subinterval with vein halos would be described using two rows, one long interval of altered basalt within the alteration tab of DESClogik (Fig. F12) and a shorter subinterval containing the overprinting vein halo assemblage within the vein tab of DESClogik (Fig. F13). Veins are recorded according to their location, apparent orientation, width, length, and mineralogy (Fig. F13). Textures were recorded in DESClogik using a series of codes (Fig. F14) in terms of vein shape (straight, sigmoidal, irregular, pull-apart, and fault vein), connectivity (isolated, single, branched, and network), texture (massive, cross-fiber, slip-fiber, vuggy, and polycrystalline), structure (simple, composite, banded, haloed, and intravenous), and geometry (en echelon, ribbon, and cross fractures). Thin section descriptionThin section descriptions were recorded using DESClogik and subsequently uploaded to the database. Secondary mineral assemblages and replacement relations to primary phases were described, as well as secondary modes (Fig. F11). Mineralogy of veins and vesicles, as well as cement when present in basaltic breccia, was also reported in the database. The modal estimates of the secondary minerals allowed characterization of alteration intensity. X-ray diffractionPhase identification in selected samples of whole-rock shipboard powders and vein and vesicle material was aided by XRD analyses using a Bruker D-4 Endeavor diffractometer with a Vantec-1 detector using nickel-filtered CuKα radiation. Instrument conditions were | |||
