Proc. IODP, 333, Data report: clay mineral assemblages in the upper Shikoku Basin, results from IODP Expedition 333, Sites C0011 and C0012

doi:10.2204/iodp.proc.333.205.2017

Methods

Sample preparation

All of the samples analyzed in this study came from “clusters” that included companion specimens for shipboard bulk powder XRD, X-ray fluorescence, and carbon-carbonate; those shipboard XRD scans provided estimates of the relative abundance of total clay minerals, quartz, feldspar, and calcite (see the “Site C0011” and “Site C0012” chapters [Expedition 333 Scientists, 2012b, 2012c]).

We isolated the clay-size (<2 µm) fractions by 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 used an ultrasonic bath for several minutes to promote disaggregation and deflocculation. Washing consisted of two passes through a centrifuge (8200 rpm for 25 min; ~6000 g) with resuspension in distilled deionized water after each pass. Each sample was resuspended by vigorous shaking and 2 min exposure to an ultrasonic cell probe immediately before centrifugation (1000 rpm for 2.4 min; ~320 g). The clay-size splits (<2 µm equivalent settling diameter) were mounted on glass discs following the filter-peel method (Moore and Reynolds, 1989) using 0.45 µm filter membranes. Clay aggregates were saturated with ethylene glycol in a closed vapor chamber heated to 60°C for at least 24 h prior to XRD analysis.

X-ray diffraction

Clay-size specimens from Expedition 333 were analyzed using two X-ray diffractometers. When the project began, the XRD laboratory at the University of Missouri was equipped with a Scintag Pad V X-ray diffractometer with CuKα 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 3° to 26.5°2θ, a rate of 1°2θ/min, and a step size of 0.01°2θ. Slits were 0.5 mm (divergence) and 0.2 mm (receiving). The Department of Geological Sciences shut down that facility before our study was finished. The remaining samples were analyzed at the New Mexico Bureau of Geology and Mineral Resources, using a Panalytical X’Pert Pro diffractometer with Cu anode. Those continuous scans were completed with generator settings of 45 kV and 40 mA over an angular range of 3° to 26.5°2θ, scan step time of 1.6 s, and step size of 0.01°2θ. Slits were fixed at 1.0 mm (divergence) and 0.1 mm (receiving), and the sample holder was set to spinning. The digital data were processed using MacDiff software (version 4.2.5) to draw 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), calculate integrated peak area (total counts), and measure peak width at half height (Δ°2θ).

Calculations of mineral abundance

Given the unusually large number of samples throughout the NanTroSEIZE project, our priority has been to obtain semiquantitative accuracy with optimal efficiency. To accomplish that for the clay-size fraction, we first analyzed standard mineral mixtures and computed a matrix of normalization factors using singular value decomposition (SVD). Underwood et al. (2003) provided a full description of the standards and a thorough analysis of the error. Average errors using this method are 3.9% for smectite, 1.0% for illite, 1.9% for chlorite, and 1.6% for quartz. Because of differences in X-ray tubes and instruments, we needed to solve for three sets of normalization factors (Table T1) and then use them during computations. Figure F3 shows representative examples of 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). For simplicity, the term “smectite” is used here as a label for mixtures of several possible detrital and authigenic minerals within the smectite group, plus small amounts of smectite-rich illite/smectite (I/S) mixed-layer clay. Figure F3 also provides an example of the computation array for SVD normalization factors.

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). Chlorite is the dominant mineral, so the relevant power-function regression is

%chlorite = 100 × (PA/TA)^1.433,

where PA = 2× half-peak area for chlorite (004) and TA = total peak area for composite chlorite (004) + kaolinite (002). Analyses of standard mineral mixtures showed that the average error of accuracy for the chlorite/kaolinite ratio is 2.6% (Guo and Underwood, 2011). We computed individual mineral percentages using that ratio and the SVD weight percent of undifferentiated chlorite (002) + kaolinite (001).

To calculate the abundance of individual clay minerals in the bulk sediment (e.g., smectite), we multiplied each relative weight percent value among the clay minerals (where smectite + illite + chlorite + kaolinite = 100%) by the weight percent 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 C0011” and “Site C0012” chapters, [Expedition 333 Scientists, 2012b, 2012c]). To facilitate direct comparisons with other published data sets from the region, we also report the weighted peak area percentages for smectite, illite, and undifferentiated 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 the errors using SVD factors (Underwood et al., 2003).

The saddle/peak method of Rettke (1981) was used to calculate percent expandability for assemblages of smectite and smectite-rich 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. The values of expandability should not be overinterpreted, however, because each represents a collective response to natural mixtures of detrital smectite, authigenic smectite, and smectite-rich detrital mixed-layer I/S. We also report values of illite crystallinity (Kübler) index as peak width at half height (Δ°2θ) for the (001) reflection; the illite peak typically narrows as levels of thermal maturity increase.

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