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doi:10.2204/iodp.proc.314315316.202.2012

Methods

Calculations of mineral abundance

Sediment samples can be analyzed by XRD in many ways. The presence of a specific detrital and/or authigenic mineral can be detected easily through visual recognition of characteristic peak positions. It is more problematic, however, to estimate the relative abundance of a mineral in bulk sediment or the clay-size fraction with meaningful accuracy (e.g., Moore, 1968; Heath and Pisias, 1979; Johnson et al., 1985). The most common semiquantitative approach for analyzing marine clays has been to apply the Biscaye (1965) weighting factors to the peak areas of basal reflections (McManus, 1991). Errors in such data can be substantial, however, and accuracy changes significantly in response to the absolute abundance by weight of each mineral (Underwood et al., 2003). XRD results are also affected by sample disaggregation technique, chemical pretreatments, particle size separation, crystallinity and chemical composition of minerals, peak-fitting algorithms, and the degree of preferred orientation of crystallites (e.g., Moore and Reynolds, 1989; Ottner et al., 2000). Even though data reproducibility might be very good, accuracy is usually no better than ±10% unless the analytical methods include calibration with internal standards, use of single-line reference intensity ratios, and some fairly elaborate sample preparation steps (Środoń et al., 2001; Omotoso et al., 2006).

Figure F3 shows representative X-ray diffractograms for three clay-size aggregates from the Nankai Trough. To obtain semiquantitative estimates of mineral abundance in the clay-size fraction, we measured the peak areas and applied a matrix of singular value decomposition (SVD) normalization factors, as documented in full detail by Underwood et al. (2003). We apply a matrix of weighting factors (Table T1) to 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 chlorite (002) + kaolinite (001) peak at 12.5°2θ (d-value = 7.06 Å), and the quartz (100) peak at 20.85°2θ (d-value = 4.26 Å). The average errors for standard mineral mixtures using this method are approximately 3% for smectite, 1% for illite, 2% for chlorite, and 1.4% for quartz (Underwood et al., 2003). Because of interference between the kaolinite (001) and chlorite (002) reflections, we first calculate that relative abundance as undifferentiated chlorite + kaolinite and then solve for the proportion of each mineral using the overlapping double peak at ~25°2θ (Fig. F3). The kaolinite (002) and chlorite (004) reflections are centered at ~24.8°2θ and ~25.1°2θ, respectively, and we follow a refined version of the Biscaye (1964) method, as documented fully by Guo and Underwood (2011). Judging from the analysis of standard mineral mixtures, the average error for the chlorite/kaolinite ratio is 2.6%. To provide an estimate of the abundance of individual clay minerals in the bulk mud(stone), we also multiply each relative percentage among the clay minerals (i.e., excluding quartz) by the abundance of total clay minerals as determined by shipboard bulk-powder XRD analyses of colocated “cluster” specimens (see, for example, the “Expedition 315 Site C0001” chapter [Expedition 315 Scientists, 2009a]). To facilitate comparisons with the many other published data sets from the region, we report the weighted peak area percentages for smectite, illite, chlorite, and kaolinite using both SVD normalization factors and Biscaye (1965) weighting factors. It is important to stress here that these values are relative percentages, and they should regarded as semiquantitative.

As an indicator of clay diagenesis, we use the saddle/peak method (Rettke, 1981) to calculate the 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. Our calculations follow a curve established for 1:1 mixtures of I and I/S.

Sample preparation

Isolation of clay-size fractions starts with air drying and gentle hand-crushing of the mud/mudstone with mortar and pestle, after which specimens are immersed in 3% H₂O₂ for at least 24 h to digest organic matter. We then add ~250 mL of Na hexametaphosphate solution (concentration of 4 g/1000 mL distilled H₂O) and insert the beakers into an ultrasonic bath for several minutes to promote disaggregation and deflocculation. This step (and additional soaking) is repeated until visual inspection indicates complete disaggregation. Washing consists of two passes through a centrifuge (8200 revolutions per minute [rpm] for 25 min; ~6000 g) with resuspension in distilled-deionized water after each pass. After transferring the suspended sediment to a 60 mL plastic bottle, each sample is resuspended by vigorous shaking and a 2 min application of a sonic cell probe. The clay-size splits (<2 µm equivalent settling diameter) are then separated by centrifugation (1000 rpm for 2.4 min; ~320 g). We prepare oriented clay aggregates using the filter-peel method (Moore and Reynolds, 1989) and 0.45 µm membranes. The clay aggregates are saturated with ethylene glycol vapor for at least 24 h prior to XRD analysis, using a closed vapor chamber heated to 60°C in an oven.

X-ray diffraction parameters

The XRD laboratory at the University of Missouri (USA) utilizes a Scintag Pad V X-ray diffractometer with CuKα radiation (1.54 Å) and Ni filter. Scans of oriented clay aggregates are run at 40 kV and 30 mA over a scanning range of 3° to 26.5°2θ, a rate of 1°2θ/min, and a step size of 0.01°2θ. Slits are 0.5 mm (divergence) and 0.2 mm (receiving). We process the digital data using MacDiff software (version 4.2.5) 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).

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