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doi:10.2204/iodp.proc.324.203.2013

Methods and materials

Preliminary results of IODP Expedition 324 (Expedition 324 Scientists, 2010a) reported that primary minerals of Shatsky Rise basalts at Sites U1346 and U1349 (clinopyroxene + plagioclase ± olivine + Fe-Ti oxides) were altered and mainly replaced by clays associated with abundant calcite and minor zeolites. Mineral identification was based on optical microscopy on thin sections and XRD analyses of bulk rock powder. However, precise characterization of clay minerals onboard was complex, and their names were essentially based on their color (e.g., brown clays, green clays, etc.).

The objectives of our study were thus to identify precisely the nature of clay minerals and their characteristics for a selection of samples collected at Sites U1346 and U1349. A total of 20 samples, 8 samples from Site U1346 and 12 samples from Site U1349, chosen at regular intervals within each hole and as being representative in terms of alteration, were analyzed by XRD (Table T1) (Guillaume et al., 2004; Köster, 1995). After reducing basalt samples to a fine-grained powder with a Fritsch Pulverisette equipped with an agate mill at Laboratoire Géosciences Environnement Toulouse (France), a two-step procedure was applied on all selected samples to identify clay minerals. First, XRD patterns of nonoriented bulk rock powders were obtained with a CPS 120 diffractometer (CoKα radiation at 0.178897 nm, operated at 40 kV and 25 mA; Laboratoire Géosciences Environnement Toulouse). Each XRD pattern was acquired on 1024 channels over 110°2θ and with a counting time of 15 min. Based on these patterns, the presence of clay minerals was highlighted by a broad peak at 14–15 Å (i.e., 6.6°–8.2°2θ; Fig. F1). However, because of high concentrations of carbonate in the samples (Delacour et al., 2012), peaks of clay minerals appear small on XRD patterns and are consequently difficult to identify precisely. Therefore, in a second step, decarbonation of the samples was applied at pH 5 (acetic acid–sodium acetate in solution to prevent alteration of clay particles), and the finest fraction (<2 µm) of each sample was separated and prepared as oriented sections. These sections were then analyzed with an INEL G3000 diffractometer (CuKα radiation at 0.15418 nm, operated at 30 kV and 40 mA with a step interval of 0.032°2θ and counting times of 3 s; Laboratoire Géosciences Environnement Toulouse) in the region of short angles to identify the clay minerals under air-dried condition (AD), after saturation with ethylene glycol (EG), and after heating at 550°C (H). The nature of the clay minerals was determined by the position of the (001) peak (Fig. F2). Chlorite is characterized by a peak at 14 Å on the AD, EG, and H pattern. Smectite is characterized by a peak at 15 Å on the AD pattern, which shifts to 17 Å on the EG pattern and to 10 Å on the H pattern. Kaolinite is characterized by a peak at 7 Å on the AD and EG patterns, whereas no peak is observed on the H pattern because the mineral is destroyed by heating. A peak attributed to illite at 10 Å is present on the AD, EG, and H pattern. The nature of smectite is determined using the relative position of the (060) peak on XRD patterns of the nonoriented bulk rock powders (Figs. F1, F3).

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