Previous   |    Next

doi:10.2204/iodp.proc.348.204.2017

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 (see the “Site C0002” chapter [Tobin et al., 2015b]). Isolation of clay-size fractions 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), beakers were inserted into an ultrasonic bath for several minutes to promote disaggregation and deflocculation. After visual confirmation of complete disaggregation, suspensions 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, 1989b) using 0.45 µm filter membranes. A closed vapor chamber was heated to 60°C for at least 24 h to saturate the aggregates with ethylene glycol.

X-ray diffraction

The cuttings samples from Expedition 348 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°–26.5°2θ with a scan step time of 1.6 s, step size of 0.01°2θ, and the sample holder spinning. Slits were 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.

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 optimal 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 goal for the clay-size fraction, we recorded the integrated areas of the following peaks (Fig. F3):

• Smectite (001) centered at ~5.3°2θ (d-value = 16.5 Å);
• Illite (001) at ~8.9°2θ (d-value = 9.9 Å);
• Composite chlorite (002) + kaolinite (001) at 12.5°2θ (d-value = 7.06 Å); and
• Quartz (100) at 20.85°2θ (d-value = 4.26 Å).

Underwood et al. (2003) described the procedure for analyzing standards and computing 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 using this method on standard mineral mixtures 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, so a refined version of the Biscaye (1964) method was used to discriminate kaolinite (002) from chlorite (004), as documented by Guo and Underwood (2011). 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 SVD weight percent of undifferentiated chlorite (002) + kaolinite (001). To calculate the abundance of individual clay minerals in the bulk mudstone, we multiplied 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 co-located specimens (see the “Site C0002” chapter [Tobin et al., 2015b]). To facilitate direct comparisons with other published data sets from the region, this report also lists the weighted peak-area percentages for smectite, illite, and undifferentiated chlorite + kaolinite using Biscaye (1965) weighting factors (1× smectite [001], 4× illite [001], and 2× chlorite [002] + kaolinite [001]). Errors of accuracy using that method can be 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 (Fig. F3). 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 provided 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 I/S (002/003) peak (following Moore and Reynolds, 1989a), using the quartz (100) peak to correct peak positions for misalignment of the detector and/or sample holder. The I/S (002/003) peak tends to be broad and low in intensity (Fig. F3), so the center of the peak needs to be picked manually. As diagenesis advances, that peak also shifts progressively closer to the flank of the illite (002) peak, making the center peak position more difficult to judge. Values of illite crystallinity (Kübler) index are reported here as peak width at half height (Δ°2θ) for the (001) reflection.

Top of page   |    Previous   |    Next