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doi:10.2204/iodp.proc.327.102.2011 Petrology, hard rock geochemistry, and structural geologyCore curation and shipboard samplingCore handling protocols are described in “Core handling and analysis” in the “Introduction.” To preserve important features and structures, core sections containing igneous rocks were examined before the core was split, allowing orientation of individual pieces. To minimize contamination of the core with platinum group elements and gold, the describers removed jewelry from their hands and wrists before handling the core. The archive half of each core was described using the DESClogik core description program, and a summary VCD was produced for each section. Lithologic units and subunits were identified on the basis of the presence of contacts, chilled margins, changes in primary mineralogy (occurrence and abundance), color, grain size, and structural or textural variations. Unit boundaries were generally chosen to indicate different volcanic cooling units, although limited recovery in some cases resulted in arbitrary placement of a unit boundary between samples having different lithologies. In order to preserve important information about volcanology without defining an unreasonable number of units within a single core, subunits were designated when there were frequent changes in texture without accompanying changes in mineralogy (for example, several pieces containing glass within 50 cm of core, all of which are mineralogically similar). Hard rock visual core descriptionsHard rock VCD (HRVCD) forms were generated for each section of basement core recovered. A key to the symbols used on the HRVCDs is provided in Figure F2. Each HRVCD form contains the following information (from left to right):
Upward arrows on the HRVCD forms indicate vertically orientated pieces of core. The location and purpose of samples taken for shipboard analyses are marked with the appropriate notation (Fig. F2). The lithologic unit column records the position of unit and subunit boundaries by solid (unit) and dashed (subunit) horizontal lines. Lithologic units are numbered and increase downhole; unit numbers followed by capital letters denote subunits. The structural column has symbols to represent the location and type of observed structural features. Structural measurements (indicated by boxes) can be obtained from the database. In addition to the graphical logs, a short unit summary is provided on the right-hand side of the HRVCD. This summary contains the following information:
Igneous petrologyEach unit and subunit was described in terms of groundmass, primary mineralogy, color, vesicles, and igneous contacts. Each of these observations was recorded using the DESClogik program and uploaded to the LIMS database. Units and subunits were defined on the basis of groundmass texture and abundance of primary minerals. Basalts were defined in hand specimens by the identification and abundance of phenocrysts:
Igneous units were further classified by the type of phenocryst present. For naming purposes, phenocrysts are listed in order of lowest to highest abundance (e.g., “plagioclase-olivine phyric” indicates that olivine abundance is greater than plagioclase abundance). Rock color was determined on a wet cut surface of the core using the Munsell color chart. Groundmass character was assessed using a binocular microscope and defined by average grain size (the width of elongated grains) and classified as follows:
The abundance of vesicles was estimated and ranked using the following scale:
The size and shape (sphericity and roundness) of vesicles were also recorded. Description of vesicle-filling phases can be found in “Alteration.” For graphical simplicity, documentation of vesicles in the graphical logs of the HRVCDs was limited to occurrences of high vesicularity. Breccia samples were described by clast and matrix. Clast abundance, size, shape, sorting, composition, and alteration were noted. Matrix was described in terms of volume, composition, structure, and alteration, and cement was described in terms of volume and composition. Each breccia was classified as hydrothermal, magmatic, or tectonic in origin:
Classification of igneous units as pillow basalt, basalt flows, and sheet flows was based on the nature of the recovered contacts and margins. Pillow basalts were characterized by curved margins (oblique to the vertical axis of the core) or by the presence of variolitic texture, curved fractures, or micro- to cryptocrystalline groundmass grain size. Basalt flows were assigned to units with chilled margins that lacked definitive pillow structure (curved margins). Sheet flows were identified by planar subhorizontal chilled margins and an increase in grain size in the center of the unit. Other rock types identified were breccias (magmatic and tectonic). AlterationVirtually all igneous rocks recovered during Expedition 327 have undergone alteration. This alteration manifests in four general forms: (1) replacement of groundmass, (2) replacement of phenocrysts, (3) hydrothermal veins and alteration halos, and (4) lining and filling vesicles. Each of these alteration types was described and recorded separately and combined into a brief summary on the HRVCD forms. In addition, an alteration log recording the background alteration color and halo color for each individual piece is available in ALTLOG in “Supplementary material.” Background alteration type was defined by rock color and calibrated by thin section observations. Alteration intensity was assigned using the following scale:
The abundance of volcanic glass, both total and fresh percents, was recorded. The abundance and composition of infilling by secondary minerals was described for vesicles. Where possible, the successive infilling of vesicles was used to deduce the order of precipitation of secondary phases. Halos surrounding infilled vesicles were described by size, color, and most abundant secondary mineral, where distinguishable. A detailed vein log recording the presence, location, width, crosscutting relationships, shape, composition percent, color, and width of associated halos of each vein was made (see Site U1362 vein log in “Core descriptions” and VEINLOG in “Supplementary material”). Structural measurements undertaken on the veins were recorded and are described in “Structural geology.” Thin sectionsThin sections of basement rocks were studied to complete and refine igneous and alteration hand specimen observations. At least one representative thin section was produced for each unit. All thin section observations were recorded in the DESClogik program and entered into the LIMS database. These observations included textural features that were not identified in hand specimens; precise determination of the grain size of phenocrysts and groundmass; mineralogy, abundance, and type of glomerocrysts; presence of inclusions within phenocrysts; and presence of minor phases such as spinel, oxides, and sulfides. Crystal sizes (minimum, maximum, and average) of all primary phases were measured, and mineral morphologies and textural features were recorded. When replacement of primary phases by secondary phases was observed, abundance, composition, and textural occurrence were recorded. The terms “heterogranular” (different crystal sizes), “seriate” (continuous range in grain size), “porphyritic” (indicating the presence of phenocrysts), or “glomeroporphyritic” (containing clusters of phenocrysts) were applied to groundmass texture. Groundmass was also classified as hypocrystalline (100% crystals) to hypohyaline (100% glass), variolitic, intergranular (olivine and pyroxene grains between plagioclase laths), intersertal, subophitic, or ophitic. The same terminology was used for thin section and macroscopic descriptions. An example of the thin section description form is given in Figure F3. Thin section descriptions are included in this volume (see “Core descriptions”) and available from the IODP database. Digital photomicrographs were taken during the cruise to document features described in the thin sections. Selected images are available in the “Site U1362” chapter and in PHOTOMIC in “Supplementary material.” Modal estimation of phenocryst phases in thin section was made using scanned images of the whole thin section and Adobe Photoshop software. This method is described in detail in the Expedition 309/312 Proceedings volume (Expedition 309/312 Scientists, 2006) and is briefly summarized here. This method is less labor intensive and more accurate for estimating phenocryst abundances. As with any method based on a single thin section, the applicability of the method to larger volumes depends on an assumption of homogeneity that may or may not be valid. For each thin section, a full-page scanned image was made, and phenocryst phases were marked on an acetate overlay. The acetate overlays were scanned and saved as native Adobe Photoshop files. The threshold function was used to separate the phenocrysts, marked by black, from the background. The histogram function of Photoshop was then used to determine the total number of pixels and the number of pixels represented by the phenocryst phase. Using this ratio the modal percentage of the phenocryst phase was calculated. This method can also be used for multiple phenocryst phases by erasing and subtracting the pixels associated with each phase. Structural geologyThis section outlines the techniques used for the description of structural features observed in hard rock basement cores. Conventions for structural studies adopted during previous ODP hard rock drilling legs (Shipboard Scientific Party, 1989, 1991, 1992a, 1992c, 1992d, 1993a, 1993b, 1995, 1999, 2003) were generally followed during Expedition 327. Graphical representation and terminologyAll material from both working and archive halves was examined, although all structure and orientation measurements were made on the archive halves. The most representative structural features in the cores are summarized on the HRVCD forms (see “Core descriptions”). All structural data were entered into the DESClogik program. To maintain consistency in core descriptions we used a set of structural feature identifiers. Brittle deformation identifiers include “joint,” “vein,” “shear vein,” “fault,” and “breccia.” Identification of these features was based on the presence of fractures, filling phases, and evidence of shear displacement. The terminology adopted generally follows that of Ramsay and Huber (1987), Twiss and Moores (1992), and Passchier and Trouw (1996) and is consistent with the terminology used during previous ODP/IODP legs and expeditions (e.g., ODP Leg 153 terminology was used for brittle deformation [Shipboard Scientific Party, 1995]). Some of the terms commonly used in the structural descriptions are illustrated in Figure F4 (Expedition 301 Scientists, 2005a):
This division of structures does not imply that all features fall into distinct and exclusive categories. We prefer to use the term “veins” for all healed fractures, avoiding the usual division based on fracture width (e.g., Ramsay and Huber [1987] defined veins as having >1 mm filling material). Rigid boundaries between the adopted structural categories do not exist. Where necessary, details specific to structural features are illustrated with comments and sketches. Geometric reference frameStructures were measured on archive halves relative to the core reference frame used by IODP. The plane normal to the axis of the borehole was referred to as the horizontal plane. On this plane, a 360° net was used with a pseudosouth (180°) pointing into the archive half and a pseudonorth (0°) pointing out of the archive half and perpendicular to the cut surface of the core. The cut surface of the core, also called “core face” or “cut face,” therefore is a vertical plane striking 90°–270°. The strike of planar features across the cut face of the archive half was measured with 0° down the vertical axis of the core, and the dip was measured using the right-hand rule (Fig. F5). The measured orientations were rotated into the IODP reference frame using stereonet software. Hard rock geochemistryShipboard analysesSample preparationRepresentative samples of igneous rocks were analyzed for major and trace elements during Expedition 327 using ICP-AES. Samples of ~10 cm3 were cut from the core with a diamond saw blade. All outer surfaces were ground on a diamond-impregnated disk to remove altered rinds and contamination resulting from cutting and drilling processes. Cleaned samples were then placed individually into beakers containing trace metal-grade methanol and were washed ultrasonically for 15 min. The methanol was decanted, and the samples were washed twice in an 18 MΩ deionized water ultrasonic bath for 10 min. The clean pieces were dried for 10–12 h at 110°C. The clean, dry whole-rock samples were crushed to <1 cm chips between two disks of Delrin plastic in a hydraulic press. The chips were ground to a fine powder in a tungsten carbide SPEX 8000M mixer/mill or, for larger samples, a SPEX 8515 shatterbox. A 1.0 ± 0.0005 g aliquot of the sample powder was weighed on a Mettler Toledo dual balance system and ignited to determine weight loss on ignition (LOI). ODP Technical Note 29 (Murray et al., 2000) describes in detail the shipboard procedure for digestion of rocks and ICP-AES analysis of samples. The following protocol is an abbreviated form of this procedure, with minor modifications. After determination of LOI, 100.0 ± 0.2 mg aliquots of the ignited whole-rock powders were weighed and mixed with 400.0 ± 0.5 mg of LiBO2 flux that had been preweighed on shore. Standard rock powders and full procedural blanks were included with unknowns in each ICP-AES run. All samples and standards were weighed ±0.2 mg on the Cahn C-31 microbalance (designed for onboard measurements), with weighing errors estimated to be ±0.05 mg under relatively smooth sea-surface conditions. Next, 10 mL of 0.172 mM aqueous LiBr solution was added to the mixture of flux and rock powder as an antiwetting agent to prevent the cooled bead from sticking to the crucible. Samples were then fused individually in Pt-Au (95:5) crucibles for 12 min at a maximum temperature of 1050°C in a Bead Sampler NT-2100. After cooling, beads were transferred to 125 mL high-density polyethylene (HDPE) bottles and dissolved in 50 mL of 10% HNO3. Following digestion of the bead, all of the solution was passed through a 0.45 µm filter into a clean 60 mL wide-mouth HDPE bottle. Next, 2.5 mL of this solution was transferred to a plastic vial and diluted with 17.5 mL of 10% HNO3 to bring the total volume to 20 mL. The final solution-to-sample dilution factor was ~4000. AnalysesMajor (Si, Ti, Al, Fe, Mn, Mg, Ca, Na, K, and P) and trace (Ba, Sr, Zr, Y, V, Sc, Cu, Zn, Co, Cr, and Ni) element concentrations of standards and samples were determined with a Teledyne Leeman Labs Prodigy ICP-AES instrument. Specific analytical conditions for each sample run are provided in Table T2. For several elements, measurements were made at two wavelengths (e.g., Si was measured at 251.611 and 288.158 nm). In such cases, the reported values were obtained by averaging the data. The plasma was ignited at least 30 min before each sample run to allow the instrument to warm up and stabilize. A peak profile alignment was then performed using basalt laboratory standards BHVO-2 (US Geological Survey) or BIR-1 (Govindaraju, 1989) in 10% HNO3. During the initial setup, an emission profile was selected for each peak to determine peak-to-background intensities and to set the locations of background levels for each element. The Prodigy software uses these background locations to calculate the net intensity for each emission line. The photomultiplier voltage was optimized by automatically adjusting the gain for each element using BHVO-2. ICP-AES data presented in “Geochemistry” in the “Site U1362” chapter were attained by integrating the area under the curves for wavelength versus response plots. Each sample was analyzed three times from the same dilute solution within a given sample run. For elements measured at two wavelengths we either used the wavelength giving the better calibration line in a given run or, if the calibration lines for both wavelengths were of similar quality, used the data for both and reported the average concentration. A typical ICP-AES run (Table T2) included
Data reductionFollowing each sample run the measured raw intensity values were transferred to a data file, corrected for instrument drift, and then corrected for the full procedural blank. Drift correction was applied to each element by linear interpolation between the drift-monitoring solutions run every fifth analysis. Following drift correction and blank subtraction, a calibration line for each element was calculated using the results for the certified rock standards. Element concentrations in the samples were then calculated from the relevant calibration lines. Estimates of accuracy and precision for major and trace element analyses were based on replicate analyses of international standards (BCR-2), the results of which are presented in Table T3. In general, run-to-run relative precision by ICP-AES was <3% for the major elements. Run-to-run relative precision for trace elements was <10%. Exceptions typically occurred when the element in question was near background values. Shore-based analysesEpidote samples recovered from the drill bit in Hole U1362A were analyzed using a tabletop Hitachi TM1000 scanning electron microscope (SEM) at the National Oceanography Centre, University of Southampton, UK, following methods described by Hunt et al. (2010). This SEM energy dispersive spectrometry methodology enables nonpretreated samples to be analyzed, akin to the EAGLE III µXRF. |
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