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doi:10.2204/iodp.proc.345.102.2014 Alteration and metamorphismThe characteristics of hydrothermal alteration of rock recovered during Expedition 345 were described macroscopically and microscopically. X-ray diffraction (XRD) analyses were used to confirm mineral identification of vein-filling materials and rock coatings. Shipboard observations of altered and metamorphosed rock were recorded using the DESClogik worksheet interface and uploaded to the LIMS database. Alteration and metamorphic features of igneous rock were described in terms of pervasive background alteration, localized alteration patches (zones of more intense alteration), hydrothermal veins and alteration halos, and cataclastic zones. Alteration was generally described on a depth-interval basis. Wherever possible, the depth of any feature was defined by the intersection with the center line of the split face of the archive half. For features that did not intersect the center line, the highest and lowest observed points were recorded. Sections of Leg 147 plutonic cores were redescribed using the techniques and methods outlined below to ensure consistency in descriptions between Expedition 345 and previous ODP legs. As was done during Legs 176, 206, and 209 and Expeditions 304/305 and 309/312 (Shipboard Scientific Party, 1999a, 2003, 2004; Expedition 304/305 Scientists, 2006; Expedition 309/312 Scientists, 2006), core descriptions were made by the entire metamorphic petrology team working together. Each member was responsible for one or more aspects of the description (macroscopic description of background alteration and texture, veins and halos, and thin section identification of microscale features) and data entry on the entire core. This approach ensured consistency of recorded observations for all cores. Usually the entire team worked together, particularly when defining metamorphic intervals. Core characterization was based on individual pieces from a recovered section of core when continuity of core pieces could not be confirmed. In cases where core was continuous, core characterization was tied to igneous units based on magmatic features unless distinct alteration intervals were observed. Macroscopic descriptions were compiled in the DESClogik workbook tabs. A summary comment of the overall nature of the alteration including background and vein halo alteration, as well as the structural characteristics of vein morphology, is captured in the Alteration comment field of the Unit summary tab in DESClogik. This information was then used for the production of VCDs, which summarize the description of each section of core as well as provide a downhole plot of overall alteration intensity expressed as a rank, as described below. Macroscopic core descriptionAll metamorphic descriptions created during Expedition 345 were made on the archive halves of the cores. Alteration and vein core description logs on a piece-by-piece or interval scale were tabulated to provide consistent characterization of the rock and to quantify the different alteration types. Metamorphic domains are defined as regions with significantly different alteration characteristics. Examples of types of domains include background alteration, vein halos, and cataclastic zones but can also be the same as lithologic intervals, as alteration style and intensity commonly vary with primary igneous mode. Alteration intervals are defined on the basis of major changes in alteration mineralogy and degree of recrystallization or replacement. Plutonic rockAlteration of plutonic rock was described systematically in the Plutonic alteration tab in DESClogik. First, we estimated the proportion of different features defined below:
Each of these features (if present) is described separately line by line (e.g., halo is on one line, background is on another) within DESClogik. For each alteration textural type, the volume percentage of each primary mineral replaced and the volume percentage of each secondary mineral replacing each primary mineral were documented. Rather than assign a percentage to these proportions, an overall rank was assigned based on a range of volume percentages in order to encompass the uncertainty associated with macroscopic mineral identification. The rank scale used for documenting the proportion of each primary mineral (by percentage of volume alteration products) replaced and plotted on the VCD is as follows:
The overall volume percentage of alteration in the entire alteration interval was estimated, integrated over all features, and reported as alteration intensity using the rank scale outlined above for estimating the volume percentage of each secondary mineral replacing each primary mineral. Alteration intensities are independent of changes in lithologic intervals defined by igneous features. Sulfides and oxides were noted as alteration products where the relationships between a primary phase and sulfide or oxide were obvious. If the relationship was not clear or the oxide or sulfide phase occurred within veins, the abundances were noted in separate columns. Recrystallized patches are described in terms of their shape and area and the modal abundance of the replacing minerals when we cannot identify the original mineralogy. We conclude with a general comment about the alteration in the metamorphic domain. Where observed, the following distinctive alteration textures and relationships were also noted in the DESClogik template as
An alteration summary description was entered in DESClogik so that it could be added to the VCDs. The alteration intensity plotted on the VCD corresponds to the background alteration intensity. The VCDs also contain a summary statement of the alteration characteristics for each lithologic interval in a core. Volcanic and hypabyssal rockAlteration in volcanic and hypabyssal rock was described systematically in the Volcanic alteration tab in DESClogik following methodology similar to that described above for plutonic and ultramafic rock. For volcanic and hypabyssal rock, we distinguish between groundmass alteration and phenocryst replacement, as well as alteration of glass. If vesicles were present, we made an estimate of the abundance and nature of vesicle filling alteration. Alteration intensity plotted in the VCDs is based on the intensity of phenocrysts. Veins and alteration halosVeins were defined as linear cracks partially or completely filled by secondary minerals. Fractures were defined as linear cracks with no mineral fill. The term halo was used to describe alteration spatially related to cracks. Note that both veins and fractures may have halos. Veins and halos were described with the same methods for all rock types. Data on abundance, width (in millimeters), orientation, texture, connectivity, color, and vein-filling minerals were recorded for each piece containing one or more veins. These observations in cores were recorded in the Veins and halos tab of DESClogik. In pieces with more than one vein, veins were numbered sequentially from the top of the piece; vein numbers are correlated with structural observations of veins (see “Alteration veins”). Data recorded related to vein-related alteration halos included halo type (based on mineralogy), halo width, and abundance of halos as a percentage of the piece containing them. Linear vein density is described in “Structural geology” of the hole chapters; see also “Structural geology”). Veins were measured on the archive half unless otherwise noted. The classification used for vein geometry, texture, and connectivity is shown in Figure F12. The orientations of veins in oriented core pieces were systematically measured by the structural geology team (see “Structural geology”) in close liaison with the metamorphic petrology team to ensure consistency of vein classification. Alteration halos representing zones of increased alteration adjacent to veins were described by width and color in the halo Comments column of the Vein and halo log, and the halo alteration mineralogy was described as a domain in the Background alteration log. Vein networks and breccia/cataclastite were recorded on the Vein and halo log in the Alteration feature column, noting the depth interval, total volume percent of secondary minerals, and percentages of individual secondary phases. A numerical estimate was also made of the percentage of the metamorphic interval that was vein material. Vein types were categorized according to mineralogy, allowing vein types to be illustrated downhole. Plots of vein type with depth are included in “Structural geology” in the hole chapters. Vein-related symbols used in the VCDs are shown in Figure F7. Thin section descriptionThin sections of rock recovered during Expedition 345 were examined to confirm macroscopic identifications of secondary minerals and establish the occurrence, abundance, and distribution of secondary minerals with depth in the core. Each thin section was photographed in both plane-polarized light and under crossed polars (see thin section images in WEBIMAGE in “Supplementary material”). Descriptions of alteration in thin sections for all rock types sampled are described in the Thin section alteration tab within DESClogik. Where a thin section contained areas with different alteration-related lithology, mineralogy, and/or texture, these features were defined as domains (e.g., Domain 1, Domain 2, etc.). For thin sections with multiple alteration domains, a map of the domains is shown on the full thin section photomicrograph (Fig. F13). The different domains were described separately, and the relative abundance of the domains was noted. Thin section descriptions closely follow the procedure for macroscopic core description, except that mineral abundances and alteration are expressed as percentages rather than as ranks. For each thin section, we first determined the relative abundance and number of different alteration domains we could distinguish and described each domain line by line in the Thin section alteration tab within DESClogik. Observations about the alteration domain, including mineralogy, textural features, veins, and deformation were recorded for both plutonic and volcanic rocks within the Thin section alteration tab within DESClogik. We summed the alteration phases replacing each primary mineral to establish the total alteration of that domain. A weighted average of each domain yielded the total alteration of the rock in thin section. Chronological relationships between different secondary minerals or parageneses were recorded as comments when observed. Comments on any mineral-specific alteration observations (i.e., coronas, mesh texture, and halos) and general comments on the thin sections were included in the thin section reports (see “Core descriptions”). Alteration intensity for thin section descriptions of each primary mineral is defined as
Sediment descriptionDrill cuttings were inspected in hand samples by the same techniques as used for macroscopic core. In addition, grain mounts were made of each of the drill cuttings intervals, and each slide was point-counted using a mechanical stage. X-ray diffractionPhase identification of vein material and, as time permitted, powders for whole rocks and patches were aided by XRD analyses using a Brucker D-4 Endeavor diffractometer with a Vantec-1 detector using nickel-filtered CuKα radiation. XRD was performed on small amounts of powder (usually ~20 mg) that were freeze-dried, crushed, and mounted as smear slides or pressed onto depressions in sample holders. Mineral identification was achieved with the interactive Diffrac.Suite EVA version 1.4 software package (2010) using the powder diffraction file database associated with the program. Identifications were based on multiple peak matches. Instrument conditions were as follows:
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