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doi:10.2204/iodp.proc.348.203.2017

Results

Table T2 shows the peak-area values (total counts) for common minerals in the clay-size fraction, separated by lithologic unit. Table T2 also includes the weight percent values of mineral abundance calculated via SVD normalization factors and the weighted peak-area percentages using Biscaye (1965) factors. Relative abundances of clay-size smectite within lithologic Unit IV range from 53 to 28 wt% (Fig. F4), with a mean value (µ) of 41.7 wt% and a standard deviation (σ) of 7.1. Values for illite in the clay-size fraction range from 39 to 27 wt% (µ = 32.3; σ = 3.2). Percentages of undifferentiated kaolinite + chlorite in Unit IV range from 24 to 15 wt% (µ = 19.6; σ = 2.6); in all cases, chlorite is the dominant mineral over kaolinite. Percentages of clay-size quartz average 3.9 wt% (σ = 4.2). Within Unit V, the values for clay-size smectite range from 42 to 27 wt% (µ = 33.5; σ = 3.7). Abundances of illite range from 45 to 27 wt% (µ = 33.8; σ = 4.0), whereas the content of kaolinite + chlorite ranges from 27 to 17 wt% (µ = 23.2; σ = 2.9). The average percentage of clay-size quartz is 6.1 wt% (σ = 3.1). Comparable values from Hole C0002F (Underwood and Song, 2016) show no systematic offsets or changes in scatter relative to the results from C0002N (Fig. F4).

Figure F5 illustrates how relative mineral abundances change within the bulk sediment as a function of depth. Within Unit IV, bulk-sediment smectite ranges from 35 to 11 wt% (µ = 25.5, σ = 5.7). Most of those percentages are significantly lower than what Underwood and Guo (2013) documented at Sites C0011 and C0012 in the Shikoku Basin (i.e., the Nankai subduction inputs) (Fig. F1), where coeval (5 to 6 Ma) Miocene strata contain averages of 30 to 33 wt% smectite. Illite in the bulk sediment of Unit IV ranges from 25 to 15 wt% (µ = 19.5, σ = 2.3), and the amount of kaolinite + chlorite ranges from 16 to 8 wt% (µ = 11.9, σ = 1.9). Within lithologic Unit V, the abundance of bulk-sediment smectite is reduced more, ranging from 27 to 17 wt% (µ = 21.4, σ = 2.1). Illite in the bulk sediment ranges from 27 to 15 wt% (µ = 21.6; σ = 2.1), and kaolinite + chlorite ranges from 20 to 10 wt% (µ = 14.9; σ = 2.5). As with the clay-size fraction, parallel plots of the bulk-sediment values from Hole C0002F (Underwood and Song, 2016) show no systematic offsets or changes in scatter relative to results from similar depths in C0002N (Fig. F5). Thus, it appears as though compositional data for Site C0002 have not been affected by the expanded and contracted vertical intervals over which cuttings were mixed within the respective boreholes.

Indicators of clay diagenesis are tabulated in Table T3 and plotted as a function of depth in Figure F6. Values of the illite crystallinity (Kübler) index range from 0.62 to 0.25Δ°2θ; the average value is 0.36Δ°2θ. As a frame of reference, the boundary between advanced diagenesis and anchizone metamorphism is set at 0.52Δ°2θ, and the achizone-epizone boundary (incipient greenschist facies) is 0.32Δ°2θ (Warr and Mählmann, 2015). A pronounced shift toward an apparent enhancement of clay crystallinity (narrower peak width) in Hole C0002N is found at about 1495 mbsf, and a mismatch occurs below that depth between values from C0002F and C0002N (Fig. F6). That shift, however, is probably an artifact of having different masks installed on the diffractometer, as described in “Methods”. The expandability of illite/smectite mixed-layer clays ranges from 89 to 59%. The average value is 64% (σ = 2.1), and expandability decreases with increasing depth (Fig. F6). The proportion of illite in illite/smectite mixed-layer clays ranges from 16 to 53%, with an average value of 35% (σ = 7.9). Those values are scattered, but the proportion of illite in I/S increases as depth increases (Fig. F6).

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