X-RAY DIFFRACTION METHODS

Analyses of sediment samples by X-ray diffraction (XRD) have been a routine part of shipboard and shore-based measurements by the ODP and the Deep Sea Drilling Project (DSDP). 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 with meaningful accuracy (Moore, 1968; Cook et al., 1975; Heath and Pisias, 1979; Johnson et al., 1985; Fisher and Underwood, 1995; Underwood et al., 2003). The most common approach for analyzing marine clays has been to apply the Biscaye (1965) peak area weighting factors (smectite = 1x, illite = 4x, chlorite = 2x). Several previous workers used those factors to characterize clays from Cascadia Basin and vicinity (Duncan et al., 1970; Knebel et al., 1968; Karlin, 1980). Errors in such data can be substantial, however, and they change significantly with the absolute abundance by weight of each mineral (Underwood et al., 2003). Results are also affected by differences in sample disaggregation technique, chemical treatments, particle size separation, and the degree of preferred orientation of clay mounts (Moore and Reynolds, 1989; McManus, 1991). Even though data reproducibility might be very good, accuracy is usually no better than ±10% unless the analytical methods include calibration with internal standards. Errors are also smaller if, by good fortune, the absolute proportions of minerals within clay mixtures are close to equal.

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

The XRD laboratory at the University of Missouri utilizes a Scintag Pad V X-ray diffractometer with CuK radiation (1.54 Å) and Ni filter. Scans of oriented clay aggregates were run at 40 kV and 30 mA over a scanning range of 2° to 23°2, a rate of 1°2/min, and a step size of 0.01°2. Slits were 0.5 mm (divergence) and 0.2 mm (receiving). We processed the digital data using MacDiff software (version 4.2.5) to establish a baseline of intensity, smooth counts, correct peak positions caused by misalignment of the detector (using the quartz [100] peak at 20.95°2; d-value = 4.24 Å), and calculate integrated peak areas (total counts).

Figure F4 shows a representative diffractogram for a clay-sized aggregate. The weighting factors of Biscaye (1965) apply to the integrated areas of a broad smectite (001) peak centered at ~5.3°2 (d-value = 16.5 Å), the illite (001) peak at ~8.9°2 (d-value = 9.9 Å), and the chlorite (002) peak at 12.5°2 (d-value = 7.06 Å). Because of interference between small amounts of kaolinite (001 reflection) and chlorite (002 reflection), we report that relative abundance as chlorite (+ kaolinite). As an indicator of clay diagenesis, the saddle/peak method (Rettke, 1981) was used to calculate the percent expandability of smectite within illite/smectite (I/S) mixed-layer clay.