Wet and dry resistivities along three orthogonal directions of ODP Holes 185-801C and 185-1149B

The results of shore-based three-axis resistivity and X-ray computed tomography (CT) measurements on cube-shaped samples recovered during Leg 185 are presented along with moisture and density, P-wave velocity, resistivity, and X-ray CT measurements on whole-round samples of representative lithologies from Site 1149. These measurements augment the standard suite of physical properties obtained during Leg 185 from the cube samples and samples obtained adjacent to the cut cubes. Both shipboard and shore-based measurements of physical properties provide information that assists in characterizing lithologic units, correlating cored material with downhole logging data, understanding the nature of consolidation, and interpreting seismic reflection profiles.

Chemistry of low-temperature hydrothermal altered basement of ODP Site 129-801

Low-temperature hydrothermal alteration of basement from Site 801 was studied through analyses of the mineralogy, chemistry, and oxygen isotopic compositions of the rocks. The more than 100-m section of 170-Ma basement consists of 60 m of tholeiitic basalt separated from the overlying 60 m of alkalic basalts by a >3-m-thick Fe-Si hydrothermal deposit. Four alteration types were distinguished in the basalts: (1) saponite-type (Mg-smectite) rocks are generally slightly altered, exhibiting small increases in H2O, d18O, and oxidation; (2) celadonite-type rocks are also slightly altered, but exhibit uptake of alkalis in addition to hydration and oxidation, reflecting somewhat greater seawater/rock ratios than the saponite type; (3) Al-saponite-type alteration resulted in oxidation, hydration, and alkali and 18O uptake and losses of Ca and Na due to the breakdown of plagioclase and clinopyroxene; and (4) blue-green rocks exhibit the greatest chemical changes, including oxidation, hydration, alkali uptake, and loss of Ca, Na, and Mg due to the complete breakdown of plagioclase and olivine to K-feldspar and phyllosilicates. Saponite- and celadonite-type alteration of the tholeiite section occurred at a normal mid-ocean ridge basalt spreading center at temperatures <20°C. Near- or off-axis intrusion of an alkali basalt magma at depth reinitiated hydrothermal circulation, and the Fe-Si hydrothermal deposit formed from cool (<60°C) distal hydrothermal fluids. Focusing of fluid flow in the rocks immediately underlying the deposit resulted in the extensive alteration of the blue-green rocks at similar temperatures. Al-saponite alteration of the subsequent alkali basalts overlying the deposit occurred at relatively high water/rock ratios as part of the same low-temperature circulation system that formed the hydrothermal deposit. Abundant calcite formed in the rocks during progressive "aging" of the crust during its long history away from the spreading center.

Geochemistry of basaltic rocks from ODP Site 129-801

Middle Jurassic basaltic lavas obtained from Site 801 in the western Pacific Pigafetta Basin represent ocean crust from the oldest segment of the present-day Pacific Ocean. A composite 131 m section shows the basement to be composed of an upper alkalic basalt sequence (about 157 Ma) with ocean island basalt chemical features and a lower tholeiitic basalt sequence (about 167 Ma) with typical normal-type mid-ocean ridge basalt features. The basalt sequences are separated by a quartz-cemented, yellow goethite hydrothermal deposit. Most basalts are altered to some degree and exhibit variable, low-grade smectite-celadonite-pyrite-carbonate-zeolite assemblages developed under a mainly hydrated anoxic environment. Oxidation is very minor, later in development than the hydration assemblages, and largely associated with the hydrothermal deposit. The tholeiitic normal-type mid-ocean ridge basalt has characteristically depleted incompatible element patterns and all compositions are encompassed by recent mid-ocean ridge basalt from the East Pacific Rise. Chemically, the normal-type mid-ocean ridge basalt is divided into a primitive plagioclase-olivine +/- spinel phyric group (Mg* = 72-60) and an evolved (largely) aphyric group of olivine tholeiites (Mg* = 62-40). Both groups form a single comagmatic suite related via open-system fractionation of initial olivine-spinel followed by olivine-plagioclase-clinopyroxene. The alkalic ocean island basalt are largely aphyric and display enriched incompatible element abundances within both relatively primitive olivine-rich basalts and evolved olivine-poor hawaiites related via mafic fractionation. In gross terms, the basement lithostratigraphy is a typical mid-ocean ridge basalt crust, generated at a spreading center, overlain by an off-axis seamount with ocean island basalt chemical characters.

Physical properties and AMS characteristics of ODP Site 185-1149 sediments

During Ocean Drilling Program Leg 185, we studied progressive changes of microfabrics of unconsolidated pelagic and hemipelagic sediments in Holes 1149A and 1149B in the northwest Pacific at 5818 m water depth. We paid particular attention to the early consolidation and diagenetic processes without tectonic deformation before the Pacific plate subduction at the Izu-Bonin Trench. Shape, size, and arrangement of pores were analyzed by scanning electron microscope (SEM) and were compared to anisotropy of magnetic susceptibility (AMS) data. The microfabric in Unit I is nondirectional fabric and is characterized by large peds of ~10-100 µm diameter, which are made up of clay platelets (mainly illite) and siliceous biogenic fragments. They are ovoid in shape and are mechanically packed by benthic animals. Porosity decreases from 0 to 60 meters below seafloor (mbsf) in Unit I (from 60% to 50%) in association with macropore size decreases. The microfabric of coarser grain particles other than clay in Unit II is characterized by horizontal preferred orientation because of depositional processes in Subunit IIA and burial compaction in Subunit IIB. On the other hand, small peds, which are probably made of fragments of fecal pellets and are composed of smectite and illite (3-30 µm diameter), are characterized by random orientation of clay platelets. The clay platelets in the small peds in Subunit IIA are in low-angle edge-to-face (EF) or face-to-face (FF) contact. These peds are electrostatically connected by long-chained clay platelets, which are interconnected by high-angle EF contact. Breaking of these long chains by overburden pressure diminishes the macropores, and the clay platelets in the peds become FF in contact, resulting in decreases in the volume of the micropores between clay platelets. Thus, porosity in Subunits IIA and IIB decreases remarkably downward. The AMS indicates random fabric and horizontal preferred orientation fabric in Units I and II, respectively. This result corresponds to that of SEM microfabric observations.In Subunit IIB, pressure solutions around radiolarian tests and clinoptilolite veins with normal displacement sense are seen distinctively below ~170 mbsf, probably in correspondence to the transition zone from opal-A to opal-CT.

Argon composition and new combined chronologies of ODP Sites 185-1149 and 191-1179

Despite the different scientific objectives of Legs 185 and 191, the sedimentary sections recovered from Sites 1149 and 1179 are the two most complete sections recovered from the northwestern Pacific Basin by either the Deep Sea Drilling Project (DSDP) (i.e., Legs 6, 20, 32, and 86) or ODP (i.e., Legs 185 and 191). During Leg 185, a complete sedimentary section (410 m) and an additional 133 m of highly altered volcanic basement were recovered. The Miocene to Pleistocene section (i.e., upper ~150 m) recovered from Site 1149 includes lithostratigraphic Unit I (0-118.2 meters below sea floor [mbsf]) and Subunit IIA (118.2-149.5 mbsf) of Plank, Ludden, Escutia, et al. (2000, doi:10.2973/odp.proc.ir.185.2000) and consists of ash- and biogenic silica- bearing clay, radiolarian-bearing clay, silt-bearing clay, ash-bearing siliceous ooze, and diatomaceous clay, with numerous discrete volcanic ash layers (Plank, Ludden, Escutia, et al., 2000, doi:10.2973/odp.proc.ir.185.2000). During Leg 191, a near-continuous 375-m-thick sedimentary section was recovered in addition to 100 m of basaltic basement. The upper 221.5 m of the sedimentary section at Site 1179 (i.e., within lithostratigraphic Unit I of Kanazawa, Sager, Escutia et al. [2001, doi:10.2973/odp.proc.ir.191.2001]) consists of upper Miocene to Pleistocene clay- and radiolarian-bearing diatom ooze containing numerous discrete ash layers. The presence of discrete ash layers within the Miocene to Pleistocene sedimentary section at both Site 1149 and 1179 provides a unique opportunity to conduct 40Ar/39Ar ash chronology to refine the excellent magnetostratigraphic records (based on the scale of Berggren et al., 1995) obtained shipboard from both sites (Plank, Ludden, Escutia, et al., 2000, doi:10.2973/odp.proc.ir.185.2000; Kanazawa, Sager, Escutia, et al., 2001, doi:10.2973/odp.proc.ir.191.2001).In this data report we present the analytical results from the 40Ar/39Ar incrementally heated analyses and provide a new combined late Miocene to Pleistocene 40Ar/39Ar and magnetostratigraphic chronology for the northwestern Pacific.