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Sediments and rocks recovered during Ocean Drilling Program Leg 126 at Sites 792 and 793 in the Izu-Bonin forearc basin are described with a primary focus on clay mineralogy. Evidence for diagenetic hydrothermal alteration processes is present in the upper Oligocene to lower Miocene sediments at these sites. The vitric and pumiceous sand/sandstone and pumiceous gravel contain high concentrations of smectites, zeolites, and gypsum. Microscopic observations show that the volcanic glass and feldspars have been altered to smectites and zeolites. The authigenic mineral assemblages indicate that these minerals resulted from precipitation from circulating fluids, as well as from the alteration of glass and feldspar under temperature conditions that may have reached 200°-300°C. Mineral assemblages in microfractures display thermal gradients that possibly reflect cooling effects. INTRODUCTION To determine the history of forearc sedimentation, arc volcanism, microstructural deformation, and plate motion, and the petrology of the underlying basement, nine holes were drilled at three sites (Sites 787, 792, and 793) during Leg 126 in the Izu-Bonin forearc (Taylor, Fujioka, et al., 1990). The cores collected at these sites provided a unique opportunity to study the complete forearc sequence through the sedimentary section into the volcanic basement. The objectives of this mineralogical study of Izu-Bonin forearc sediments were (1) to determine temporal bulk and clay mineralogy variations, (2) to relate any changes in clay mineralogy to chemical and physical characteristics of the sediments, and (3) to document diagenetic and hydrothermal alteration processes of volcanic glass and feldspar. The qualitative mineralogical descriptions of the sediments recovered during drilling have been presented in the individual site reports found in the Leg 126 Initial Reports volume (Taylor, Fujioka, et al., 1990). In this paper, the shipboard semiquantitative X-ray results are described in detail along with electron microscopic observations from shore-based research. SAMPLES AND METHODS A total of 44 samples from Holes 792A, 792B, 792D, and 792E and a total of 132 samples from Holes 793A and 793B were analyzed by X-ray powder diffraction (XRD) methods aboard the JOIDES Resolution. The samples were analyzed on a Phillips ADP 3520 X-ray diffractometer using CuKα radiation with Ni-filter, a count time from 1 to 2 seconds (s), and an accelerating voltage of 40 kV, 35 mA(see "Explanatory Notes" chapter in the Leg 126 Initial Reports [Taylor, Fujioka, et al., 1990] for further details). The samples to be X-rayed were ground in water in a porcelain mortar; the resulting finest fraction was then decanted, and the suspension was allowed to settle. After about 0.5 hr, the supernatant was pipetted and sedimented onto a glass slide to produce oriented samples. Examination of the powder diffractograms of these samples revealed peaks for clay minerals, zeolites, feldspars, and volcanic glass in the interval between 2°-65° 2θ. Ethylene glycol was then 1 Taylor, B., Fujioka, K., et al., 1992. Proc. ODP, Sci. Results, 126: College Station, TX (Ocean Drilling Program). Department of Geology, Shimane University, Matsue, Shimane Prefecture 690, Japan. 3 Department of Geology, University of Western Ontario, London, Ontario N6A 5B7, Canada. added to the prepared slides to aid in the identification of smectite in the interval between 2° and 30° 2θ. Enough ethylene glycol was added to air-dried material to make it visibly moist. Relatively small amounts (5%) of smectite can be identified with this technique. Semiquantitative estimates of major mineral and clay mineral abundances were determined using the strongest peak intensity (measured in counts per second; cps) of each mineral. Smectite and chlorite abundances were estimated after glycolation. Minerals characterized by a strong 7-Å peak intensity (kaolin and chlorite) were identified after the sample was treated with acid. To refine the XRD estimations of mineral abundances in the samples, scanning electron (SEM) and transmission electron (TEM) microscopy analyses were conducted using a JEOL-JSM T220A scanning electron microscope (accelerating voltage of 15 kV), and a JEOL-JEM 2000EX transmission electron microscope (accelerating voltage of 160 kV). RESULTS X-ray Powder Diffractions Qualitative descriptions of the XRD data for samples from Sites 792 and 793 that were analyzed aboard the JOIDES Resolution during Leg 126 are summarized in the individual site reports in the Initial Reports volume (see Taylor, Fujioka, et al., 1990). The semiquantitative mineral analyses (in cps) of the Site 792 and 793 samples are plotted against depth below the seafloor in Figures 1 and 2. Examples of the strongest peak intensities used to determine the counts per second value for the minerals are shown in Table 1. Volcanic glass abundance was estimated from the intensity of the broad peak at 3.0-3.3 Å and confirmed by optical observation in thin sections. The color of the volcanic glass changes from clear in the upper (younger) part of each section to brown and green in the deeper (older) intervals. At Site 792, the abundance of volcanic glass decreases and the abundance of glauconite and palagonite (10-Å minerals on Fig. 1) increase with depth. At Site 793, no definitive downhole trend can be seen in the concentration of volcanic glass. Zeolites are present in middle Miocene and older intervals at Site 792 (below 330 mbsf; Fig. 1), whereas at Site 793 zeolites are found only in Oligocene sediments (below 750 mbsf; Fig. 1). Analcime, phillipsite, heulandite, and clinoptilolite are common at both sites. Wairakite was also identified at Site 792, suggesting higher postdepositional temperatures. Gypsum, zeolite, and prehnite were observed in microfractures at both Sites 792 and 793. Calcite counts range from 10 to 600 cps at Site 792 and from 10 to 1500 cps at Site 793. Counts in the nannofossil layers range up to

Diagenetic and Hydrothermal Mineral Alteration Observed in Izu-Bonin Deep-Sea Sediments, Leg 126