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doi:10.2204/iodp.proc.314315316.201.2011

Application of the method

The goal of refining the XRD methods for chlorite and kaolinite was to improve the detail of large data sets generated for NanTroSEIZE, without adding excessive amounts of time and complexity to sample preparation and data reduction. To test the utility of the doubled half-peak area approach, we took advantage of previously unpublished XRD data from ODP Sites 1177 and 1178 for which the reflections at ~3.55 Å had been recorded (Table T2). Kaolinite and chlorite contents for those specimens (in central and western Nankai Trough) were initially reported as one undifferentiated value, ranging from 0% to 32% of the clay-size fraction (Steurer and Underwood, 2003; Underwood and Steurer, 2003). Those authors, as well as Underwood et al. (1993), using samples from ODP Site 808, treated a small number of representative samples with acid to resolve how much kaolinite contributes to the composite 7 Å peak area. Results from Sites 808 and 1173 show that kaolinite makes up <20%, and averages ~14%, of the chlorite-kaolinite composite (based on acid-treatment calculations). Three values from Site 1177 range from 10% to 60% kaolinite in the chlorite-kaolinite composite. Only one sample (190-1177A-46R-1, 120 cm) was analyzed by both methods and yielded a value of 60% for the ratio of treated to untreated peak area, but it is worth noting that the total chlorite + kaolinite in this clay-size fraction is only 4% (Steurer and Underwood, 2003). For the same sample, we obtained a 50:50 ratio of kaolinite to chlorite using the doubled half-peak area method (Table T2).

One justification for expanding the data set for the previously analyzed ODP samples from Nankai Trough is to ascertain whether or not the contents of kaolinite and chlorite changed systematically with time or stratigraphic position within the Shikoku Basin and Nankai trench wedge. Any such basin-wide changes in the central and western Nankai Trough might have some bearing on interpretations of clay provenance and sediment dispersal for coeval strata within the NanTroSEIZE transect. To begin this comparison, we picked paleomagnetic polarity and calcareous nannofossil data from Sites 1177 and 1178 (Shipboard Scientific Party, 2001a, 2001b) and constructed composite age-depth models. Figure F6 shows how relative abundances of kaolinite and chlorite in the clay-size assemblage change as a function of time, as constrained by the age-depth models.

Most values of kaolinite are <10% of the total clay-size fraction, and chlorite content is typically <20%. Strata at Site 1177 cover an age range of 1.6 to 18.6 Ma, so those samples provide a better view of temporal trends. At Site 1177, the kaolinite and chlorite contents are significantly lower for strata that are older than ~11 Ma, averaging only ~2% of the clay-size fraction. This depletion occurs is because the supply of smectite was much higher during the early to middle Miocene (Steurer and Underwood, 2003). The ratio of chlorite to kaolinite is also higher in these older strata. At Site 1178, chlorite values don’t show any systematic stratigraphic trend, although this scatter of results over time is complicated by occurrences of several thrust faults and an unconformity. Over comparable ranges of time at the two sites, the relative abundance of kaolinite gradually decreases by roughly 8 wt% with age.

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