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doi:10.2204/iodp.proc.338.206.2016

Methods

Sample preparation

All of the samples analyzed in this study were selected from cuttings measuring 1–4 mm in effective diameter (concentrated by wet sieving). Each extracted interval of cuttings included a companion specimen for shipboard bulk-powder XRD, which provided estimates of the relative abundance of total clay minerals, quartz, feldspar, and calcite (see the “Site C0002” chapter [Strasser et al., 2014b]). Isolation of the clay-size fraction for XRD analyses began 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. We added ~250 mL of Na-hexametaphosphate solution (concentration of 4 g/1000 mL distilled H₂O) and inserted the 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; ~6000 g) with resuspension in distilled-deionized 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-size splits (<2 µm equivalent settling diameter) were then separated by centrifugation (1000 rpm for 2.4 min; ~320 g). Oriented clay aggregates were prepared using the filter-peel method (Moore and Reynolds, 1989) and 0.45 µm membranes. We saturated the clay aggregates with ethylene glycol vapor using a closed vapor chamber heated to 60°C for at least 24 h prior to XRD analysis.

X-ray diffraction

Our analyses of the cuttings samples from Expedition 338 were completed 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 a scan step time of 1.6 s and step size of 0.01°2θ. Slits were 1.0 (divergence) and 0.1 mm (receiving), and the sample holder was spinning. We processed 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). This program also calculated 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., Środoń et al., 2001; Omotoso et al., 2006). Our primary goal throughout the NanTroSEIZE project has been to obtain accurate values for the clay-size fraction from a large suite of samples. To accomplish that goal efficiently, 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 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). We then applied a matrix of singular value decomposition (SVD) normalization factors (Table T1), calculated after analyzing standard mineral mixtures (Underwood et al., 2003). Those standards consisted of smectite + illite + chlorite + quartz. The average errors using this method were 3.9% for smectite, 1.0% for illite, 1.9% for chlorite, and 1.6% for quartz. The kaolinite (001) and chlorite (002) reflections overlap almost completely, so we followed a refined version of the Biscaye (1964) method, as documented by Guo and Underwood (2011). The average error of accuracy for the chlorite/kaolinite ratio is 2.6%. To calculate the abundance of individual clay minerals 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 (see the “Site C0002” chapter [Strasser et al., 2014b]). To facilitate direct comparisons with other published data sets from the region, we also report the weighted peak area percentages for smectite, illite, and chlorite + kaolinite using Biscaye (1965) weighting factors (1 × smectite, 4 × illite, and 2 × chlorite + kaolinite). Errors of accuracy using that method can be substantially greater (±10% or more) as compared to SVD normalization factors (Underwood et al., 2003).

For documentation of clay diagenesis, we utilized the saddle/peak method of Rettke (1981) to calculate percent expandability of smectite and illite/smectite mixed-layer clay. This method is sensitive to the proportions of discrete illite versus illite/smectite mixed-layer clay; we chose the curve for 1:1 mixtures of discrete illite and illite/smectite. A complementary measure of the proportion of illite in the illite/smectite mixed-layer phase considers the center peak position (2θ angle) of the (002/003) peak (following Moore and Reynolds, 1989), using the quartz (100) peak to correct for misalignment of the detector and/or sample holder. We also report illite crystallinity (Kübler) index as values of peak width at half height (Δ°2θ) for the (001) reflection.

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