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doi:10.2204/iodp.proc.322.102.2010

Lithology

Visual core descriptions

We followed conventional Ocean Drilling Program (ODP) and IODP procedures for recording sedimentologic information on VCD forms on a section-by-section basis (Mazzullo and Graham, 1988). Core descriptions were transferred to section-scale templates using J-CORES software and then converted to core-scale depictions using Strater software. Texture (defined by the relative proportions of sand, silt, and clay) follows the classification of Shepard (1954). The classification scheme for siliciclastic lithologies follows Mazzullo et al. (1988).

The Graphic Lithology column on each VCD plots to scale all beds that are at least 2 cm thick. Graphic patterns do not show persistent interlayers <2 cm thick, but such intervals are identified as laminae in the Sedimentary Structures column. It is difficult to discriminate between the dominant lithologies of silty claystone and clayey siltstone without quantitative grain size analysis. However, we tried to make a qualitative visual distinction between silty claystone and clayey siltstone on the basis that clay contains <25% silt (Shepard, 1954). We did not use separate patterns for more heavily indurated examples of the same lithologies (e.g., silty clay versus silty claystone) because the dividing line is arbitrary. In practice, we used terms for lithified rock because we only obtained RCB cores. Figure F3 displays graphic patterns for all sedimentary lithologies encountered during Expedition 322. Also shown are symbols for internal sedimentary structures, soft-sediment deformation structures, and severity of core disturbance in sedimentary rock.

Where calcareous sediments occur, we used the term "calcareous claystone" for clay-rich deposits that contain 10%–50% calcite (following the classification Mizutani et al., 1987) and "lime mudstone" where there is a mud-supported fabric with >90% carbonate component (Dunham, 1962; expanded by Embry and Klovan, 1972).

To emphasize the differences in the composition of volcanic sandstones, we modified the classification scheme of Fisher and Schmincke (1984). In general, coarser grained sedimentary rocks (63 µm to 2 mm average grain size) are named "sandstone" where volcaniclastic components are <25% of the total clasts. Volaniclastic grains can be (1) reworked and commonly altered heterogeneous fragments of preexisting volcanic rock, tuff, or tephra or (2) fresh, or less altered, compositionally homogeneous pyroclasts. Pyroclasts are produced by many types of processes associated with volcanic eruptions, without reference to the causes of the eruption or origin of the particles. Pyroclasts can includes crystals, glass shards, and rock fragments.

If the sedimentary rock contains >25% but <75% volcaniclasts, it is designated a "volcaniclastic sandstone." As a subset if volcaniclastic sandstone, if >25% but <75% of the volcaniclasts are vitric pyroclasts, then we used the term "tuffaceous sandstone." If the total clast composition is >75% pyroclasts, then the sediment is classified as "ash" or, if lithified, as "tuff." Depending on the grain size and degree of compaction, the nomenclature is adjusted accordingly (e.g., tuff versus ash), as shown in Table T2. Because of problems with accuracy, compositions close to the dividing lines of the classification scheme are problematic. In addition, with the exception of fresh glass shards in the population of pyroclasts, it is difficult to use smear slides to discriminate unequivocally between primary eruptive products and crystals or rock fragments created by erosion of fresh volcanic materials.

Bioturbation intensity in deposits was estimated using the semiquantitative ichnofabric index as described by Droser and Bottjer (1986, 1991). The indexes refer to the degree of biogenic disruption of primary fabric such as lamination and range from 1 for nonbioturbated sediment to 6 for total homogenization:

• 1 = No bioturbation recorded; all original sedimentary structures preserved.

• 2 = Discrete, isolated trace fossils; <10% of original bedding disturbed.

• 3 = Approximately 10%–40% of original bedding disturbed. Burrows are generally isolated but locally overlap.

• 4 = Last vestiges of bedding discernable, ~40%–60% disturbed. Burrows overlap and are not always well defined.

• 5 = Bedding is completely disturbed, but burrows are still discrete in places and the fabric is not mixed.

• 6 = Bedding is nearly or totally homogenized.

The ichnofabric index in cores was identified with the help of visual comparative charts (Heard and Pickering, 2008). Distinct burrows that could be identified as particular ichnotaxa were also recorded.

Smear slides and thin sections

Smear slides and thin sections are useful for identifying and reporting basic sediment attributes (texture and composition), but the results are only semiquantitative. We estimated the texture of the sediments with the help of a visual comparison chart (Rothwell, 1989). Errors can be large, however, especially for textural estimates of the fine silt and clay-size fractions. Smear slide analysis also tends to underestimate the amount of sand-size grains because they are difficult to incorporate evenly onto the slide. As during Expedition 315, we suspect that the counting errors can be considerable (e.g., "Lithology" in Expedition 315 Scientists, 2009) and easily up to ~20%. Nevertheless, in order to define lithologies, smear slides are one way to compare textures and composition by point counting.

Point counting on smear slides was made over at least four defined areas using 100× to 200× magnification. This method works well for sandstones and sandy siltstones in thin sections, and the theoretical 2σ error for a total of 200 counted particles in a thin section is between 3% and 7%, depending on the portion of the total inventory (van der Plas and Tobi, 1965). As mentioned above, errors are larger using smear slides because of uneven grain distributions, which makes reliable classification of some volcanic-rich sandstones difficult. This is particularly true if compositions are close to a boundary of the classification scheme. The various components were binned into several categories (e.g., feldspar, pyroxene, metamorphic lithics, sedimentary lithics, etc.) to facilitate the optimum reproducibility among different scientists (Table T3).

For fine-grained sediments (silt/siltstone and silty clay/claystone), rough estimations were made regarding the matrix, using the visual comparison chart after Rothwell (1989). Point counting of the coarser fraction was then added, and together they were normalized to 100% of the total component inventory. Because most of the point counting is based on estimations, 2σ error for the finer grained sediments is much higher and must be considered when interpreting the results. To reduce errors, we normalized the data for the finer grained sediments against three principal classes: for example, quartz or feldspar, as well as volcanic clastic or sedimentary lithic content were normalized to total mineral and total lithic contents, respectively. Additionally, the inventory of ash layers, ash pods, and dispersed ash layers was documented separately to account for the presence of glass shard textures, juvenile components, and other minerals. This method was not applied to lithified tuffs because of the crushing of many glass shards during slide preparation. Detailed results are given in the Tephra logs (see TEPHRA in "Supplementary material") and summarized in the smear slides (see "Core descriptions"). The relative major component abundance was also validated by bulk powder XRD (see "X-ray diffraction"), and the absolute weight percent of carbonate was verified by chemical analysis (see "Organic geochemistry").

The sample location for each smear slide was entered into J-CORES with a sample code of "SS," using the Samples application. The position of each specimen is shown on the VCD slide editor column of the VCD application.

X-ray diffraction

We completed routine XRD analyses of bulk powders using a PANalytical CubiX PRO (PW3800) diffractometer. Our principal goal was to estimate relative weight percentages of total clay minerals, quartz, plagioclase, and calcite using diagnostic peak areas. Most of the samples were selected from intervals adjacent to whole-round samples, and most are part of sampling clusters that included samples for physical properties, carbonate, and XRF. A few additional samples were collected periodically from unusual lithologies such as carbonate-cemented claystone and volcanic ash. Samples were freeze-dried, crushed with a ball mill (along with powder for XRF and carbonate), and mounted as random bulk powders. The instrument settings were as follows:

• Generator = 45 kV and 40 mA.

• Tube anode = Cu.

• Wavelength = 1.54060 Å (K_α1) and 1.54443 Å (K_α2).

• Step spacing = 0.005°2θ.

• Scan step time = 0.648 s.

• Divergent slit = automatic.

• Irradiated length = 10 mm.

• Scanning range = 2°–60°2θ.

• Spinning = yes.

In order for our results to match those of ODP Leg 190 and NanTroSEIZE Stage 1 as closely as possible, the choice was made to use MacDiff 4.2.5 software (www.ccp14.ac.uk/ccp/ccp14/ftp-mirror/krumm/Software/macintosh/macdiff/MacDiff.html) for data processing. Each peak's upper and lower limits were adjusted manually following the guidelines shown in Table T4. Calculations of relative mineral abundance utilized a matrix of normalization factors derived from integrated peak areas and singular value decomposition (SVD) (Table T5). As described by Fisher and Underwood (1995), calibration of SVD factors depends on the analysis of known weight-percent mixtures of mineral standards that are appropriate matches for natural sediments. SVD normalization factors were recalculated during Expedition 315 after the diffractometer's high-voltage power supply was replaced. Bulk powder mixtures for the Nankai Trough are the same as those reported by Underwood et al. (2003): quartz (Saint Peter sandstone), feldspar (Ca-rich albite), calcite (Cyprus chalk), smectite (Ca-montmorillonite), illite (Clay Mineral Society IMt-2, 2M1 polytype), and chlorite (Clay Mineral Society CCa-2). Examples of diffractograms for standard mixtures are shown in Figure F4.

In order to calculate average errors (SVD-derived estimates versus true weight percent), immediately prior to Expedition 322, 14 standard mineral mixtures were run three times each. The average precision (reproducibility) is total clay minerals = 1.7%, quartz = 1.1%, plagioclase = 1.3%, and calcite = 1.0%. The average accuracy (true% – calculated%) is total clay minerals = 4.0%, quartz = 2.0%, plagioclase = 1.8%, and calcite = 1.9%. In spite of its precision with standard mixtures, the SVD method is only semiquantitative, and results for natural specimens should be interpreted with some caution. One of the fundamental problems with any bulk powder XRD method is the difference in peak response between poorly crystalline minerals at low diffraction angles (e.g., clay minerals) and highly crystalline minerals at higher diffraction angles (e.g., quartz and plagioclase). Contents of clay minerals are best characterized by measuring the peak area, whereas peak intensity may be easier and more accurate to quantify quartz, feldspar, and calcite. Analyzing oriented aggregates rather than random orientations enhances basal reflections of the clay minerals, but this step is time consuming and requires isolation of the clay-size fraction to be effective. Errors also propagate as more minerals and peaks are added to the procedure. Counts for "plagioclase" may also include K-feldspar, so we refer to that value in natural specimens as "feldspar." For clay mineral assemblages, the two options are to individually measure one peak for each mineral and add the estimates together (thereby propagating the error) or to measure a single composite peak at 19.4°–20.4°2θ. Chlorite does not contribute to this composite peak. Another source of error is contamination of mineral standards by impurities such as quartz (e.g., the illite standard contains ~20% quartz). For trace quantities of a mineral and peaks with low intensity, use of negative SVD normalization factors may result in negative values of absolute weight percent. In such cases, we inserted the numerical value of 0.1% as a proxy for "trace."

In the final assessment, calculated values of a mineral's weight percent should only be regarded as relative percentages within a four-component system where clay minerals + quartz + feldspar + calcite = 100%. How close those estimates are to their absolute percentages within the mass of total solids will depend on the abundance of amorphous solids (e.g., biogenic opal and volcanic glass), as well as the total of all other minerals that occur in minor or trace quantities. For most natural samples, the absolute errors are probably between 5% and 10%. Thus, the primary value of bulk powder XRD data should be to identify spatial and temporal trends in sediment composition and to assist with core-log integration.

X-ray fluorescence

Whole-rock quantitative XRF spectrometry analysis was undertaken on core samples for the major elements. Samples of 10 cm³ were taken from every interstitial water cluster sample. Additional samples were collected from the working half of the core next to samples for XRD and carbonate analysis. Samples were freeze-dried, crushed with a tungsten-ball mill, and fused with 4.5 g of SmeltA12 flux for 7 min at 1150°C to create glass beads. Loss on ignition was measured using weight changes on heating at 1000°C for 3 h. Analyses were performed on the wavelength dispersive XRF spectrometer Supermini (Rigaku) equipped with a 200 W Pd anode X-ray tube at 50 kV and 4 mA.

Major elements were measured using the fused glass bead method and are presented as weight percent oxide proportions (Na₂O, MgO, Al₂O₃, SiO₂, P₂O₅, K₂O, CaO, TiO₂, MnO, and Fe₂O₃) (Table T6). Rock standards of the National Institute of Advanced Industrial Science and Technology (Geological Survey of Japan [GSJ]) were used as the reference materials for quantitative analysis. Table T7 lists the results and standard deviations for standard samples. Processed data were uploaded into J-CORES as a comma-delimited data file. Data are reported as total counts on the peak and also as semiquantitative oxide weight percents.

X-ray fluorescence scanning

We undertook whole-rock semiquantitative XRF spectrometry analysis for major elements and trace elements on core material (Na, Mg, Al, Si, P, S, K, Ca, Ti, V, Cr, Mn, Fe, Cu, Ni, Zn, and Zr). Semiquantitative element scanning of core sections was undertaken using the onboard JEOL TATSCAN-F2 energy dispersive spectrometry (EDS)-based split-core scanner, equipped with a cryogenic Si semiconductor and a 76 mm berylium window (Sakamoto et al., 2006). The target for the X-ray generation is rhodium (Rh) that can generate X-rays with five times higher intensity compared to the standard EDS-based scanner. The diameter of the collimeter that detects the incident X-ray beam is 0.8 mm, and the space for irradiation on the sample is ~1.1 mm. Although the distance between the sample surface and detector critically affects the intensity of X-rays, the TATSCAN-F2 has a function that moves the X-ray unit automatically to maintain a 1 mm distance between the core surface and measurement window, thereby reducing the problem. Data are reported as total counts on the peak. Semiquantitative oxide weight percents for some elements were calculated from background-corrected integrated peak intensities using software provided by the vendor (JEOL) for the TATSCAN. Further theoretical details and operational procedures are outlined in Richter et al. (2006).

Prior to scanning, the core surface was carefully cleaned with a brush between each scanned section. The core sections were scanned with 3.0 mm spacing at 30 kV (1000 µA), with a 150 s sampling time. After measurement, the XRF spectral data were saved and converted into a CSV file using the ElementStation2 software. The results were uploaded to J-CORES and displayed at the corresponding depth on the Composite Log Viewer program. All raw data were also saved to the onboard share server.

Basement description

The methods used for volcanic basement description were based on VCD and thin section analyses. Lava morphology was identified by visual description of the core, initially from whole-round core sections. Visual description (using a binocular microscope) also helped distinguish between the different types of volcanic/igneous rock. Thin sections were produced for each volcanic/igneous rock unit. Rock texture, grain size, and phenocryst mineralogy together with abundance were determined using thin section analysis. Massive lava units were divided into individual cooling units based on the presence of chilled margins.

All basement core descriptions and associated shipboard analyses were archived electronically in J-CORES. Figure F5 displays graphic patterns for all basement lithologies encountered during Expedition 322. When describing and assigning a name to a rock interval, cores were divided based on changes in mineralogy, texture, grain size, composition, and the occurrence of chilled margins. For igneous petrology, all rocks were assigned to porphyritic, aphanitic, or phaneritic groups. Aphanitic rocks were divided into glassy, aphyric, and phyric groups based on their phenocryst abundance, whereas phaneritic rocks were defined as fine, medium, and coarse grained (Table T8). At the microscopic scale, rock texture was defined according to the degree of crystallinity (Table T8), with all other textural terms used being based on the definitions in McKenzie et al. (1982). Textural features within glass-rich zones were described using definitions in ODP Leg 147 Initial Reports volume (Shipboard Scientific Party, 1993). All classification and textural data were recorded in J-CORES.

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