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doi:10.2204/iodp.proc.348.203.2017

Methods

Sample preparation

All of the samples analyzed in this study were selected from cuttings measuring 1–4 mm in effective diameter (concentrated shipboard by wet sieving). Each extracted interval of cuttings included a companion specimen for shipboard bulk-powder XRD; those scans provided estimates of the relative abundance of total clay minerals, quartz, feldspar, and calcite (Strasser et al., 2014b; see the “Site C0002” chapter [Tobin et al., 2015b]). Isolation of the clay-size fraction for XRD analyses started with air drying and gentle hand-crushing of the mudstone with mortar and pestle, after which specimens were immersed in 3% H₂O₂ for at least 24 h to digest organic matter. After adding ~250 mL of Na-hexametaphosphate solution (concentration of 4 g/1000 mL distilled H₂O), the beakers were inserted into an ultrasonic bath for several minutes to promote disaggregation and deflocculation. After visual confirmation of disaggregation, samples were washed by two passes through a centrifuge (8200 revolutions per minute [rpm] for 25 min; ~6000 g) with resuspension in distilled-deionized water after each pass. The suspended sediment was then transferred to a 60 mL plastic bottle and resuspended by vigorous shaking and a 2 min application of an ultrasonic cell probe. The clay-size splits (<2 µm equivalent settling diameter) were separated by centrifugation (1000 rpm for 2.4 min; ~320 g). Preparation of oriented clay aggregates followed the filter-peel method (Moore and Reynolds, 1989a) using 0.45 µm filter membranes. To saturate expandable clay minerals with ethylene glycol, slides were transferred to a closed vapor chamber and heated to 60°C for at least 24 h prior to XRD analysis.

X-ray diffraction

Expedition 348 cuttings samples were analyzed at the New Mexico Bureau of Geology and Mineral Resources using a Panalytical X’Pert Pro diffractometer with Cu anode. Scans of oriented clay aggregates were run at generator settings of 45 kV and 40 mA. The continuous scans cover an angular range of 3° to 26.5°2θ, with scan step time of 1.6 s, step size of 0.01°2θ, and the sample holder spinning. Slits are 1.0 mm (divergence) and 0.1 mm (receiving). MacDiff software (version 4.2.5) was used to establish a baseline of intensity, smooth counts, correct peak positions offset by misalignment of the detector (using the quartz (100) peak at 20.95°2θ; d-value = 4.24 Å), and calculate integrated peak areas (total counts). This program also calculates peak width at half height. A glitch in this procedure was created by a move of the X-ray diffractometer to a new building at New Mexico Tech; technicians from Panalytical mistakenly installed a 15 mm mask rather than the 20 mm mask that had been used previously. That switch reduced the area of the X-ray beam, which resulted in lower peak intensities and smaller dimensions of peak width at half height. The switch occurred after scanning cuttings samples 65 through 134 and before scanning samples 138 through 323.

Calculations of mineral abundance

The most accurate analytical methods for XRD analyses require calibration with internal standards, use of single-line reference intensity ratios, and some fairly elaborate sample preparation steps to create ideal random particle orientations (e.g., Srodon et al., 2001; Omotoso et al., 2006). Given the unusually large number of samples to analyze throughout the NanTroSEIZE project, our strategy has been to obtain reliable semiquantitative accuracy with optimal efficiency. To accomplish that for the clay-size fraction, we recorded the integrated areas of a broad smectite (001) peak centered at ~5.3°2θ (d-value = 16.5 Å), the illite (001) peak at ~8.9°2θ (d-value = 9.9 Å), the composite peak of chlorite (002) + kaolinite (001) at 12.5°2θ (d-value = 7.06 Å), and the quartz (100) peak at 20.85°2θ (d-value = 4.26 Å).

Underwood et al. (2003) describe how mineral standards are analyzed to calculate a matrix of singular value decomposition (SVD) normalization factors (Table T1). The mixtures for those standards consist of smectite + illite + chlorite + quartz. The average errors (measured weight percent versus computed weight percent) are 3.9% for smectite, 1.0% for illite, 1.9% for chlorite, and 1.6% for quartz. The chlorite (002) and kaolinite (001) peaks overlap almost completely. A refined version of the Biscaye (1964) method (see Guo and Underwood, 2011) allows separation between the kaolinite (002) and chlorite (004) peaks. The average error of accuracy for the chlorite/kaolinite ratio is 2.6%, and that ratio was used to compute individual mineral percentages from the undifferentiated weight percent of chlorite (002) + kaolinite (001). To calculate the abundance of individual clay minerals as weight percent in the bulk mudstone, we multiply each relative percentage value among the clay minerals (where smectite + illite + chlorite + kaolinite = 100%) by the percentage of total clay minerals within the bulk powder (where total clay minerals + quartz + feldspar + calcite = 100%), as determined by shipboard XRD analyses of collocated specimens (Strasser et al., 2014b; see the “Site C0002” chapter [Tobin et al., 2015b]). To facilitate direct comparisons with other published data sets from the region, this report also includes the weighted peak area percentages for smectite, illite, and undifferentiated chlorite + kaolinite as computed by Biscaye (1965) weighting factors [1× smectite (001), 4× illite (001), and 2× chlorite (002) + kaolinite (001)]. Errors of accuracy using that method are usually substantially greater (±10% or more) than errors using SVD normalization factors (Underwood et al., 2003).

For documentation of clay diagenesis, the saddle/peak method of Rettke (1981) was used to calculate percent expandability of smectite and illite/smectite (I/S) mixed-layer clay. This method is sensitive to the proportions of discrete illite (I) versus I/S mixed-layer clay; the curve for 1:1 mixtures of I and I/S provides the best match for the range of Nankai specimens. A complementary measure of the proportion of illite in the I/S mixed-layer phase is based on the 2θ angle of the (002/003) peak (following Moore and Reynolds, 1989b). We use the quartz (100) peak to correct peak position for misalignment of the detector and/or sample holder. The I/S (002/003) peak tends to be broad and low in intensity, so the center of the peak needs to be picked manually. Values of illite crystallinity (Kübler) index are reported here as peak width at half height (Δ°2θ) for the (001) reflection.

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