IODP

doi:10.14379/iodp.pr.352.2015

Principal results

Site U1439 summary

Operations

After a 457 nmi transit from Yokohama, Japan, the vessel arrived at Site U1439 (proposed Site BON-2A; Figure F8). The vessel stabilized over the site at 0324 h (all times reported are ship local time, UTC + 9 h) on 6 August 2014. Because of the short initial period planned at this site, no seafloor-positioning beacon was deployed, and GPS was used for positioning the ship. A seafloor beacon was subsequently deployed once the vessel returned to Site U1439 on 26 August.

Site U1439 consists of three holes. Hole U1439A was cored using the advanced piston corer (APC)/extended core barrel (XCB) system to 199.4 m below seafloor (mbsf) (Table T1). Nonmagnetic core barrels were used for Cores 352-U1439A-1H to 10H. Core orientation was performed using the FlexIT tool on Cores 2H through 9H. Temperature measurements were taken with the advanced piston corer temperature tool (APCT-3) on Cores 4H, 6H, 8H, and 10H. Basement was tagged with the XCB system for the purpose of identifying where in the volcanic stratigraphy this section belongs. Ten APC cores were taken over a 92.3 m interval and recovered 84.3 m (91%). Thirteen XCB cores were taken over a 107.1 m interval and recovered 86.4 m (81%). Overall recovery in Hole U1439A was 86%. The total time spent on Hole U1439A was 59.75 h.

The vessel was offset 20 m east on 8 August, and Hole U1439B was drilled without coring to 42.2 mbsf for a jet-in test in advance of deploying casing beneath a reentry cone in Hole U1439C. After the completion of the jet-in test, the drill string was raised to 100 m above the seafloor, and at 2030 h on 8 August the vessel started the move to Site U1440 using the dynamic positioning system.

After completion of operations at Site U1440, the vessel moved back to Site U1439 on 26 August. A reentry system was prepared, and 178.5 m of 10.75 inch casing was assembled and landed in the reentry cone in the moonpool. A drilling bottom-hole assembly (BHA), including a mud motor, underreamer, and bit, was picked up and installed. The casing with the reentry system attached was lowered to the bottom, drilled into the seafloor, and released on 27 August. Hole U1439C was cored with the rotary core barrel (RCB) system to 544.3 mbsf (Table T1). Coring was terminated on 8 September as a result of poor hole conditions. A total of 45 rotary cores were taken over a 362.3 m interval and recovered 107.8 m (30%). An additional 1.5 m of material was recovered during hole cleaning operations. The hole was logged to ~400 mbsf with the triple combination–Magnetic Susceptibility Sonde (MSS) and Formation MicroScanner (FMS)-sonic tool strings. The total time spent in Hole U1439C was 382.75 h. The total time spent at Site U1439 was 447.75 h, or 18.66 days. The vessel moved to Site U1441 on 11 September.

Sedimentology

Sediment and sedimentary rocks were recovered from the seafloor to 176.47 mbsf in Hole U1439A, beneath which a thin interval of basic volcanic and volcaniclastic rocks was recovered within the igneous basement. The sediment represents the late Eocene–recent deep-sea sedimentary cover of the Izu-Bonin fore-arc basement. The underlying volcanogenic rocks are interpreted as the fore-arc basement. The sedimentary succession is divided into 5 lithologically distinct units (Figure F11). Lithologic Units I and II are each further divided into 2 subunits. The main criteria for the recognition of the lithologic units and subunits are a combination of primary lithology, grain size, color, and diagenesis. Within the overall succession, 44 ash or tuff layers were observed.

  • Unit I (0–50.43 mbsf) is recognized mainly on the basis of a relatively high abundance of calcareous nannofossils compared to the sediment beneath. Unit I is divided into an upper, relatively nannofossil-poor subunit (0–5.54 mbsf) and a lower, relatively nannofossil-rich subunit (5.54–50.43 mbsf).
  • Unit II (50.43–100.50 mbsf) is recognized on the basis of a downward change to silty mud and fine to coarse sand, in which the upper subunit (50.43–82.80) is relatively fine grained and the lower subunit (82.80–100.50 mbsf) relatively coarse grained.
  • Unit III (100.50–110.93 mbsf) is easily recognizable because of a predominance of pale nannofossil ooze.
  • Unit IV (110.93–129.76 mbsf) is marked by a distinct downward change to more clastic sediment dominated by clay, with minor silt, sand, and nannofossil-bearing sediment.
  • Unit V (129.76–178.50 mbsf) is characterized by a diverse mixture of fine- to coarse-grained clastic sediment interbedded with fine-grained, nannofossil-rich sediment and sedimentary rock. The base of Unit V is defined as a thin (<3 cm) layer of dark gray to black, weakly consolidated manganese oxide–rich sediment.

Biostratigraphy

Calcareous nannofossils were present in 19 of 22 Hole U1439A core catcher samples and Sample 352-U1439A-20X-2, 0–2 cm. Preservation of calcareous nannofossils is variable, ranging from “good” in the most recent samples to “poor” in certain taxa and intervals. The oldest samples in the hole exhibit more diagenesis than younger samples. Reworking may be common throughout the section, making initial age constraints somewhat difficult. Close examination reveals somewhat continuous recovery from the Upper Pleistocene to the upper Eocene with a few gaps, especially in Miocene-aged sediment (Figure F11). The youngest age obtained was Late Pleistocene (Subzone CN14a; ~0.44–1.04 Ma), whereas the oldest age was late Eocene/early Oligocene (Zones NP19/20 or NP21; ~34.44–35.92 Ma).

Fluid geochemistry

Twenty-one samples were collected in Hole U1439A for headspace hydrocarbon gas analysis as part of the standard shipboard safety monitoring procedure; 1 sample per core was collected from Cores 352-U1439A-1H through 23X, except for Cores 21X and 22X in which no sediment was recovered. Thirteen whole-round samples were collected for interstitial water analyses in Hole U1439A; 1 sample per core from Cores 1H through 10H and 1 sample every 3 cores from Cores 13X through 19X. No headspace gas or interstitial water samples were collected from the basement rocks in Hole U1439C. All interstitial water samples were analyzed for salinity, alkalinity, pH, Cl, Br, SO42–, Na+, K+, Ca2+, Mg2+, and PO43–.

Methane concentrations range from 2.49 to 12.44 ppmv in Hole U1439A, with the highest methane concentration measured in Core 1H at 5.9 mbsf. This high value is attributed to decomposition of organic matter in the uppermost layers of the sedimentary column. No ethane or propane was detected in Hole U1439A samples. The major result of the interstitial water analyzes from Hole U1439A is a broad correlation with the described lithologic units, with the exception of Mg2+ and Ca2+. The distribution of these elements in the sedimentary column is invariant of lithology and shows a downhole increase in Ca2+ to 40.5 mM and a decrease in Mg2+ to 39.2 mM. These variations can probably be attributed to metasomatism by interaction with fluids released from the basaltic basement.

Petrology

Igneous rocks were recovered in Holes U1439A and U1439C. Hole U1439A tagged basement during XCB coring (Cores 352-U1439A-20X through 23X; 3.7 m recovery), whereas Hole U1439C penetrated 362.3 m of igneous basement (Cores 352-U1439C-2R through 45R; 108.5 m recovery). The uppermost part of the section comprises heterolithic breccia, which represents seafloor colluvium. The lowermost part of the section is composed of mafic dikes or sills. The volcanic rocks in between are dominated by pillow lava with intercalations of massive sheet flows, igneous breccias especially hyaloclastites, and subaqueous pyroclastic flow deposits. This site is notable for the variety of boninites cored and for the intercalation of more and less differentiated boninites at several levels throughout the section. In one unit, these magmas appear to have erupted simultaneously, forming complex magma mingling textures. Phenocrysts are common in the basement boninitic rocks throughout Holes U1439A and U1439C. However, the variations in phase assemblages and abundances are not always diagnostic. As a result, chemical distinctions based on portable X-ray fluorescence (pXRF) spectrometry were also used to assess changes in rock composition and to track the occurrence of different magma series.

Ten igneous units were identified in the basement at Site U1439 (Figure F12). Unit boundaries represent an abrupt change in chemical characteristics, phenocrysts, and groundmass assemblages. Subunits typically represent changes in the eruptive nature of a unit (e.g., from hyaloclastite to pillow lava or massive lava), although minor changes in chemical composition occur at some subunit boundaries. Unit 10 is doleritic and was interpreted to represent sheeted shallow dikes or sills.

Fresh igneous rocks at Site U1439 are dominantly boninites characterized by phenocrysts of olivine and orthopyroxene and are typically set in a groundmass of pale glass and acicular to tabular pyroxene prisms. Acicular plagioclase is commonly present in the groundmasses of lower-silica boninites, but is only in the groundmasses of the most highly differentiated high-Si boninite series lavas. Phenocryst and groundmass assemblages document a range in boninite compositions:

  1. Orthopyroxene > olivine phenocrysts with an orthopyroxene-dominated groundmass (Units 1–4),
  2. Olivine + augite ± orthopyroxene phenocrysts with an augite ± plagioclase–bearing groundmass (Unit 5), and
  3. Olivine > orthopyroxene ± augite phenocrysts with augite ± orthopyroxene ± plagioclase in the groundmass (Units 6–9).

Based on using Niton handheld pXRF analyses, these units are also distinguished by their Ti/Zr ratios (see “Preliminary scientific assessment”): 1 has very low Ti/Zr (<60), 2 has significantly higher Ti/Zr (90–120), and 3 has intermediate Ti/Zr (65–90).

Boninites in the first group are canonical boninites, whereas those in the second and third groups have lower silica concentrations and were assigned a shipboard classification of “basaltic boninites” and “low-Si boninites,” respectively (see discussion of nomenclature under “Preliminary scientific assessment”).

The basaltic boninites in Hole U1439 have abundant modal plagioclase in the groundmass and high magnetic susceptibilities. These basaltic boninites also are notable, however, for their low TiO2 contents and Ti/Zr compared with those of the fore-arc basalts of Site U1440.

Alteration in Hole U1439C basement units is highly variable. Fresh boninite glass is relatively common, but most samples have calcite, zeolite, and/or smectite clays partially to replacing groundmass, olivine and more rarely, orthopyroxene phenocrysts. Palagonite, clays, and more rarely, zeolites replace glass. Calcite and zeolite-filled veins and vesicles are common throughout the core, rarely associated with pyrite. Quartz is a rare component in some veins. The alteration generally becomes more intense with depth. Talc is present from Core 350-U1439C-22R to 26R. A green secondary phyllosilicate, perhaps chlorite, first appears in Core 26R and becomes common below Core 29R. In fresh glass from Cores 28R to 33R, biogenic microtubes are common. Veins, principally of calcite, zeolite, and smectite, are abundant in Hole U1439C downhole to the boundary between boninite pillow lavas and dolerites.

Rock geochemistry

Whole-rock chemical analyzes were performed on 48 igneous rocks and 22 sediment samples representative of the different lithologic units recovered from Site U1439. The 22 sediment samples were collected in Hole U1439A (1 per core) and analyzed for major and trace element concentrations and volatile contents. Hole U1439A sediments show a broad range of compositions, mainly marking the downhole changes in lithology from the carbonate-rich calcareous ooze (CaO > 50 wt%, Sr up to 2000 ppm, total C up to 11 wt%, and Zr of ~30–40 ppm) to the clay- and volcaniclastic-rich silty muds (CaO < 2 wt%, Sr up to ~200 ppm, and Zr up to 150 ppm). The downhole transition to igneous basement is marked by a thin, muddy, manganese-rich layer (MnO = 2–5 wt%) and enrichments in Cu (>500 ppm), V (>200 ppm), and Zr (>150 ppm).

At the bottom of Hole U1439A, 1 orthopyroxene-phyric volcanic rock and 1 volcanic glass were sampled. In addition, 46 igneous rocks were selected by the Shipboard Science Party as representative of the different lithologies recovered from Hole U1439C. The rocks were grouped as olivine-pyroxene-phyric or plagioclase-bearing volcanic rocks, the latter being observed mainly at the bottom of the hole. The 48 igneous rocks were analyzed for major and trace element concentrations by inductively coupled plasma–atomic emission spectroscopy (ICP-AES) and for CO2 and H2O contents by gas chromatography for samples with loss on ignition (LOI) >2 wt%. An aliquot of the powder used for ICP-AES analyzes was subsequently used for XRF analyses, which were carried out with the pXRF. In addition, pXRF “chemostratigraphic” analyses were conducted on 350 archive-half pieces from Hole U1439C cores. The results of these chemical analyses, in conjunction with observations on core material and thin sections carried out by the petrology team, contributed to the lithologic division of the lavas into different units.

Site U1439 igneous rocks range from slightly to highly altered with LOI values from 2.5 to 16.2 wt%. LOI values primarily vary with H2O contents (0.5–8.8 wt%) and, thus, the amount and type of secondary hydrous minerals. However, several samples from the upper Units 1–8 also show high CO2 values (up to 6.4 wt%) together with higher Ca content, the result mostly of late carbonate addition. This shows that the primary compositions of several samples were modified significantly by alteration. For this reason, the igneous rocks of Hole U1439C were screened for alteration based on petrology and selected chemical criteria before further description and interpretation of their primary geochemical signature.

Igneous samples recovered from Hole U1439C are boninites with SiO2 concentrations ranging from 50.5 to 60.2 wt% at total alkali contents of 1.60–4.70 wt%. Olivine-pyroxene-bearing igneous rocks are characterized by high Cr concentrations (221–1562 ppm), high Mg# (cationic Mg/[Mg + Fe], with all Fe as Fe2+) of 64–80, and CaO/Al2O3 ratios of 0.49–0.93. The lowermost lavas and dolerites (Units 9 and 10) have lower Cr concentrations (51–750 ppm) and Mg# (59–76) than overlying units.

A characteristic feature of Site U1439 samples is the progressive decrease in TiO2 concentrations that characterize the transition from the plagioclase-bearing igneous rocks sampled deep in Hole U1439C to the shallower, higher Si boninite samples. Another characteristic feature is the enrichments in highly incompatible and mobile elements (e.g., Ba) in igneous rocks sampled at Site U1439 compared to those from Site U1440. These enrichments are not correlated with indices of alteration and appear to be of magmatic origin.

Structural geology

Structures observed in Site U1439 cores originated from drilling-induced, sedimentary, igneous, and tectonic processes. Drilling-induced deformation in the sediment, including dragging-down, rotational shear, and postretrieval core dilation, prevented observation of sedimentary structures between ~92 and 155 mbsf. Sedimentary structures, such as bedding planes, stylolites, dewatering structures, and cross-bedding, point to an overall nearly horizontal bedding attitude. Igneous structures, although rarely observed, consist of local magmatic foliation marked by alignment of primary minerals, and a few centimeter-long enclaves in zones of magmatic mingling.

Tectonic structures, present mostly in igneous rocks, comprise tension fractures (veins), shear fractures, breccias, cataclasites, and fault zones. Veins are generally filled with (Mg-) calcite, zeolite, and clay. These veins typically dip steeply and do not correlate with the presence of faults. Vein thickness varies with depth and decreases from 350 to 500 mbsf. Three major fault zones occur at 348–401, 420–446, and 475–535 mbsf. The dominant sense of slip determined on slickensides is normal. Calcite microstructures in the deepest intervals include Type I and II twins, as well as subgrain boundaries, which suggests a relatively high differential stress.

Physical properties

Many of the physical property measurements display variability at similar depths, suggesting a few major boundaries. There is a distinct increase in natural gamma ray (NGR) values at 100–130 mbsf in lithologic Units III–IV. At 135–180 mbsf in Unit V, magnetic susceptibility and P-wave velocity increases. The reflectance parameters L*, a*, and b* decrease with depth from 0 to 130 mbsf in Units I–IV and display an abrupt increase in values at 128–130 mbsf at the boundary between Units IV and V. Bulk, dry, and grain densities show no systematic variation with depth. Porosity increases with depth from 0 to 130 mbsf in Units I–IV and decreases in Unit V.

Magnetic susceptibility distinctly increases and density distinctly decreases at 478–540 mbsf in igneous Units 9–10. NGR values decrease with depth from 180 to 390 mbsf in Units 1–6 and are low from 390 to 540 mbsf in Units 8–10, with some peaks correlated to magnetic susceptibility peaks. At 200–240 mbsf in Subunit 3a, density and P-wave velocity increase and porosity decreases. The reflectance parameters a* and b* have a small peak at 330 mbsf in Unit 3.

Paleomagnetism

Sediment cored in Hole U1439A is relatively strongly magnetic and has low coercivities, so it acquired a strong drill string overprint. This overprint was easily removed by alternating field demagnetization, revealing a Pliocene–Pleistocene magnetic stratigraphy in Cores 352-U1439A-1H through 10H (0–85 mbsf). Magnetic chrons downhole to the Gilbert Chron (~4.5 Ma) were clearly identified. The identification of older chrons downhole to Chron 3B is less certain. Cores 15X through 19X show clear magnetic polarity zones tentatively correlated with Chrons 8 through 13 (~25–34 Ma).

Igneous rock samples from Hole U1439C have mostly low inclinations with absolute values less than ~30° and an average of ~5°. This is consistent with the hypothesis that the Izu-Bonin arc formed near the paleoequator. Several zones of outlier paleoinclinations occur near observed fault zones. These anomalous values may be explained by remagnetization or tectonic rotation.

Downhole logging

A ~220 m interval of basement rocks in Hole U1439C was logged over a ~18 h period with 2 tool strings, the triple combo-MSS and FMS-sonic tool strings. Borehole conditions were relatively stable during logging operations, but weather conditions and sea state deteriorated. NGR, density, resistivity, magnetic susceptibility, sonic velocity, and microresistivity images were successfully acquired. Changes in the character and trend of these logs are used to define 7 logging units in this hole.

Logging Unit 1 (~180–189 mbsf) is characterized by increasing values in NGR, resistivity, density, and velocity, in combination with decreasing magnetic susceptibility downhole. Unit 2 (~189–202 mbsf) shows decreases in density, resistivity, and magnetic susceptibility, whereas NGR is high relative to the units above and below. Unit 3 (~202–213 mbsf) exhibits overall decreases in resistivity, magnetic susceptibility, and NGR values with coincident discrete peaks in NGR and density. High-frequency variations in both resistivity and magnetic susceptibility, in combination with anticorrelated profiles of density and NGR, characterize logging Unit 4 (~213–246 mbsf). Unit 5 (~246–314 mbsf), the thickest of the logging units, is characterized by a wide range in magnetic susceptibility values, with a significant high in the uppermost 6 m of the unit. Unit 6 (~314–365 mbsf) is delineated from Unit 5 by a major washed-out zone. The resistivity, magnetic susceptibility, and velocity profiles through this interval are very variable, which can, in part, be attributed to increased borehole rugosity. The deepest unit, Unit 7 (~365–402 mbsf, the bottom of the logged interval), has limited data available, but is differentiated from the overlying unit by higher values of resistivity, magnetic susceptibility, and NGR. Overall, density, velocity, and resistivity increase with depth. NGR and magnetic susceptibility values do not show such systematic changes with depth. The oriented microresistivity images show a wide range of features and textures in the walls of the borehole, including fracture networks, vesicles, and through-going planar features.

Although the logging unit boundaries do not correspond perfectly with the petrologic unit boundaries, there are clear relationships between the logging data and the physical properties and geochemistry of the core. Ongoing integration of the core and logging data sets will be essential in filling in some of the gaps in core recovery in the volcanic extrusive sequence of this hole.

Site U1440 summary

Operations

After an 8.2 nmi transit from Site U1439, the vessel arrived at Site U1440 (proposed Site BON-1A; Figure F8) and a positioning beacon was deployed at 0548 h on 9 August 2014. Site U1440 consists of 2 holes. Hole U1440A was cored with the APC to 103.5 mbsf and then cored with the XCB to a final depth of 106.1 mbsf (Table T1). Nonmagnetic core barrels were used with all APC cores. Cores 352-U1440A-1H through 6H were oriented using the FlexIT tool, which was removed with Core 7H as a result of the high heave conditions experienced by the vessel. APCT-3 temperature measurements were taken with Cores 4H, 6H, 8H, and 11H. Basement contact was recorded at ~101 mbsf. The APC coring system was deployed 12 times, with 103.5 m cored and 96.4 m recovered (93%). The XCB coring system was deployed twice, with 2.6 m cored and 0.2 m recovered (8%). The total time spent in Hole U1440A was 49.25 h.

A reentry system with a reentry cone and 99 m of 10.75 inch casing was drilled into the seafloor in Hole U1440B using a mud motor, underreamer, and drilling bit assembly. Coring with the RCB began at 102.3 mbsf in Hole U1440B and was terminated after bit failure at a final depth of 383.6 mbsf (Table T1). Basement contact was estimated at ~125 mbsf. The RCB coring system was deployed 36 times, with 281.3 m cored and 34.7 m recovered (12%). Following coring, 2 logging runs were made with the triple combo–Ultrasonic Borehole Imager (UBI) and FMS-sonic tool strings. As a result of deteriorating hole conditions, the triple combo-UBI tool string collected data downhole to 254 mbsf, and the FMS-sonic tool string collected data to ~243 mbsf. The total time spent in Hole U1440B was 364.75 h. The acoustic beacon was recovered at 0612 h on 26 August, and the vessel returned to Site U1439. The total time spent at Site U1440 was 414 h or 17.25 days.

Sedimentology

Sediment and sedimentary rocks were recovered from the seafloor to 103.5 mbsf in Hole U1440A, beneath which a thin interval of basic volcanic rocks was recovered. The sediment represents a section through the early Oligocene to recent deep-sea sedimentary cover of the IBM fore-arc basement. The underlying basaltic rocks recovered here are interpreted as representing the fore-arc basement. The sedimentary succession in Hole U1440A is divided into 3 lithologically distinct units (Figure F13). Unit I is further divided into 3 subunits, and Unit II is divided into 2 subunits. The main criteria used to define the lithologic units and subunits are a combination of primary lithology, grain size, color, and diagenesis.

  • Unit I (0–32.98 mbsf) is recognized mainly on the basis of a relatively high abundance of poorly consolidated brown mud. Subunit IA (0–13.33 mbsf) is composed of mud with calcareous nannofossil and ash layers. Subunit IB (13.33–21.61 mbsf) is composed of mud with foraminifers and minor ash layers. Subunit IC contains mud with diatoms, together with minor tuffaceous sandstone and ash layers (21.61–32.98 mbsf).
  • Unit II (32.98–77.50 mbsf) is recognized on the basis of a downward increase in grain size to more clastic and volcanogenic sediment. Subunit IIA (32.98–58.50 mbsf) is relatively coarse grained and volcanogenic. Subunit IIB (58.50–77.50 mbsf) is even coarser grained and includes muddy volcanogenic breccia/conglomerate with gravel.
  • Unit III (77.50–103.52 mbsf) exhibits a return to finer grained silty mud with subordinate volcanogenic gravel. The basalt beneath forms the top of the basement.

The proportions of the main sediment types recovered are

  • Ash/tuff = 2.89 m or 2.9% of the total recovered sediments,
  • Coarse-grained sediment (sand to conglomerate) = 16.5 m or 17.1%,
  • Fine-grained mud, silt/mudstone, and siltstone = 75.51 m or 78.5%, and
  • Nannofossil ooze = 1.24 m or 1.2%.

In addition, sediment was recovered in 3 cores immediately below the drilled interval in Hole U1440B (Cores 2R through 4R; 102.3 to ~115.3 mbsf). These cores correspond to Unit III in Hole U1440A.

Biostratigraphy

Calcareous nannofossils were recovered intermittently in Hole U1440A, where productive intervals are interspersed with barren intervals dominated by siliceous microfossils (especially radiolarians) and volcaniclastic material. There is a long barren interval from Sample 352-U1440A-6H-CC to 10H-CC. The youngest age obtained is Late Pleistocene (Subzone CN14a; ~0.44–1.04 Ma), and the oldest age obtained is early Oligocene (Zone NP23; ~29.62–32.02 Ma) (Figure F13). Three samples were examined from Hole U1440B. Samples 352-U1440B-2R-CC and 4R-1, 14–15 cm, contained calcareous nannofossils sufficient for age diagnostics, whereas Sample 3R-CC was barren. Preservation was moderate to poor in each sample with many taxa showing strong dissolution and overgrowth. Both of the Hole U1440B samples have an early Oligocene age (Zones NP22 and NP21, respectively), with a range that is difficult to constrain better than ~32.02–34.44 Ma given the lack of reliable marker taxa for the equatorial Pacific. Absolute age determinations were more difficult to make at Site U1440 compared to Site U1439 as a result of increased dissolution and a number of barren intervals.

Fluid geochemistry

Twelve samples (1 per core) were collected in Hole U1440A for headspace hydrocarbon gas analysis as part of the standard shipboard safety monitoring procedure, and 12 whole-round samples were collected for interstitial water analyses (1 per core). No headspace gas or interstitial water samples were collected in Hole U1440B. All interstitial water samples were analyzed for salinity, alkalinity, pH, Cl, Br, SO42–, Na+, K+, Ca2+, Mg2+, and PO43–.

Only minor methane was detected in the headspace gas samples. The highest methane concentration (5.84 ppmv) was measured in Core 352-U1440A-1H at 1.5 mbsf and may be attributed to the decomposition of organic matter in the uppermost layers of the sediment.

The major result of the interstitial water analyses from Hole U1440A is the distinctive behavior of Mg2+ and Ca2+. Both elements have seawater concentrations at the top of the hole, but Ca2+ concentrations then decrease with depth to 41.2 mM at the bottom of the hole, whereas Mg2+ concentrations increase to 36.6 mM. These variations are independent of lithologic units and are attributed to pervasive fluid input from the underlying hydrothermally altered basaltic basement and alteration of volcanic ash in the sediment.

Petrology

Igneous rocks were recovered in both Holes U1440A and U1440B. Hole U1440A tagged basement during XCB coring with low recovery (Cores 352-U1440A-13X and 14X; 0.2 m recovered), whereas Hole U1440B penetrated 268.3 m of igneous basement, again with low recovery (Cores 352-U1440B-4R through 36R; 33.9 m recovered). The basement/sediment contact is marked in both holes by a Mn-rich sediment layer or coating. The uppermost igneous unit in both holes comprises a mixture of volcanic rock fragments in a sediment matrix and likely represents talus or volcaniclastic breccia. This unit is ~35 cm thick and underlain by over 175 m of volcanic rock, which transitions over ~70 m into dikes at 329.0 mbsf. The dikes are interpreted as part of a sheeted dike complex. The igneous basement is divided into 15 igneous units numbered in order of increasing depth (including the uppermost breccia) based largely on the physical nature of recovered lithologies, which were interpreted to be hyaloclastites, pillow lavas, sheet flows, and dikes (Fig. F14). The lowermost unit (15) is a dike complex further divided into 5 chemically distinct subunits (15a–15e).

Igneous rocks at Site U1440 are typically aphyric to sparsely phyric, plagioclase-pyroxene-magnetite phyric basalts with intergranular to intersertal textures. The coarser grained units in the dike complex and transition zone are dolerites with subophitic textures. All of the igneous rocks are petrographically similar to IBM FAB collected elsewhere and have chemical compositions consistent with this classification. They are distinct petrographically and chemically from the boninite-suite lavas, which are typically orthopyroxene and olivine phyric.

The degree of alteration of the igneous rocks at Site U1440 is low in the volcanic section where the secondary mineralogy is dominated by calcite, smectite-group clays, and zeolites including phillipsite. These minerals form abundant veins in Cores 352-U1440B-12R through 24R, some of which are associated with pyrite and native copper. Alteration becomes more intense in the transition zone and dike complex where the secondary mineralogy includes chlorite. Secondary minerals typically only replace the groundmass phases, leaving the silicate framework minerals (plagioclase and pyroxene) unaffected, except in the lower part of the dike complex where pyroxene and plagioclase may be partially replaced. Glass is commonly devitrified and, less commonly, replaced by palagonite, clays, and zeolite.

Rock geochemistry

Whole-rock ICP-AES chemical analyses were performed on 33 igneous rocks and 16 sediment samples representative of the different lithologic units recovered at Site U1440. Twelve sediment samples were collected from Hole U1440A (1 per core), and 3 samples were collected in the deepest part of the sediment sequence in Hole U1440B from 104.4 to 115.1 mbsf. Additionally, 1 sandstone piece was recovered within the igneous sequence in Section 352-U1440B-15R-1 (192.8 mbsf). One aphyric basalt sample was collected at the bottom of Hole U1440A, and 32 samples, mostly basaltic, were collected in Hole U1440B. The 16 sediment samples were analyzed for major and trace element concentrations and volatile contents. The 33 igneous rocks were analyzed for major and trace element concentrations. An aliquot of the powder used for ICP-AES analyses was subsequently used for pXRF analyses.

The sediment sampled at Site U1440 is dominantly silty mud, and its compositional variations reflect sedimentary unit changes. The range of compositions is more restricted than in the sediment in Hole U1439A. The sediment has, on average, low CaO contents (<2 wt%), high SiO2 contents (>55 wt%), and variable Cu concentrations (120–240 ppm). A few samples contain slightly higher carbonate contents with total C contents >0.9 wt%, CaO of 5–22 wt%, and lower Cu concentrations (70–90 ppm). Hole U1440B sediment has the same composition as that of lithologic Unit III sediment in Hole U1440A. Similarly, the sandstone recovered within igneous Unit 4 rocks overlaps in composition with lithologic Unit III sediment. This sandstone could represent an accidental fragment displaced by drilling or an accumulation of sand in an open fracture.

Site U1440 igneous rocks are mostly basalts with one andesite unit (igneous Unit 6). SiO2 ranges from 48 to 57 wt%, and total alkali (Na2O + K2O) contents vary from 2.1 to 3.2 wt%. These rocks are relatively depleted in incompatible trace elements (e.g., TiO2 = 0.6–1.4 wt%) and have highly variable Cr concentrations (15–380 ppm), indicating different degrees of differentiation. Downhole profiles of major element compositions exhibit a distinct increase in SiO2 concentrations and Mg# at ~260 mbsf. This depth marks the transition between igneous Units 7 and 8, which is interpreted as the boundary between the volcanic series and the lava/dike transition. The sampled igneous rocks have major element compositions similar to those of FAB collected by diving in the Bonin forearc (cf. Ishizuka et al., 2011).

XRF chemostratigraphic analyses were conducted on archive-half pieces of cores and on thin section billets and powders. The results of these chemical analyses, in conjunction with observations on core material and thin sections carried out by the petrology team, contributed to the 15 unit lithologic division of the lavas and dikes, and are discussed in the “Preliminary scientific assessment” section below. Briefly, TiO2 and Zr concentrations in basalts generally decrease downhole from Unit 4. Although there is some cyclicity, Cr concentrations and Sr/Zr ratios increase over the same interval. Above Unit 4, the basalts have low Zr, TiO2, and Cr concentrations, and relatively high Sr/Zr ratios. Unit 6 andesites are characterized by low TiO2 and Sr concentrations, and the highest Zr concentrations of any lavas from Sites U1440 and U1441.

Structural geology

Bedding planes in the sediment are marked by dark pyroclastic beds and thin sandy layers and are generally subhorizontal. Drilling-induced deformation of core features precluded meaningful structural measurements in the sediment between ~57 and 102 mbsf. In the igneous rocks, magmatic fabrics are rare and limited to a few centimeter-wide domains of grain alignment. Steep, metamorphic, chlorite-based foliation overprints primary fabrics at ~145–146, 281–291, and 358–369 mbsf. Tension veins filled with (Mg-) calcite, zeolite, chlorite, and clays are common at ~164–166, 202–264, and 319–369 mbsf. These veins typically form two sets at a high angle from each other with average dips of ~40° ± 10° and 80° ± 10°. The basalts and dolerites are overall free of plastic and cataclastic deformation features such as slickensides.

Physical properties

Changes in the trends of physical properties are encountered at similar depths, and these changes tend to be associated with different units. At ~10 mbsf in lithologic Unit I, there is a positive spike in P-wave velocity and NGR accompanied by a slight increase of gamma ray attenuation (GRA) density. This is an interval rich in tephra layers. At 35–40 mbsf in Unit II, P-wave velocity and GRA density increase sharply, whereas NGR decreases. Color reflectance parameters L*, a*, and b* decrease in the same interval. Physical properties show significant variability in Unit III. At 83–87 mbsf, magnetic susceptibility increases suddenly and color reflectance parameter L* decreases. At ~87–100 mbsf, P-wave velocity increases; magnetic susceptibility and NGR are variable but generally decrease with depth; color reflectance parameters decrease with depth; GRA, dry, and bulk density increase with depth; and porosity decreases with depth. At 100–102 mbsf, P-wave velocity, magnetic susceptibility, NGR and GRA, and dry and bulk density suddenly decrease accompanied by a sudden increase in porosity. Physical property parameters change in igneous Units 7 and 8. At ~230 mbsf in Unit 7, porosity decreases sharply, P-wave velocity increases, and bulk and dry density increase. At 270–280 mbsf in Unit 8, NGR decreases and magnetic susceptibility increases, with high values observed between 280 mbsf and the bottom of the hole.

Paleomagnetism

Remanent magnetization measurements reveal that sediment cored at Site U1440 is highly magnetic (~0.1–2 A/m natural remanent magnetization [NRM]), apparently as a result of input of volcaniclastic material from nearby sources. A normal Pliocene–Pleistocene magnetic stratigraphy has been established for the upper sedimentary section and includes the period from the upper Gilbert Chron (~4 Ma) to the Brunhes Chron at the surface. Paleomagnetic samples from the igneous basement section reveal a probable magnetic reversal sequence. The upper ~50 and lower ~120 m of the section have normal polarity, whereas the intervening ~70 m has reversed polarity. Until radiometric dates are available for the basement section, the pattern cannot be correlated with the geomagnetic polarity timescale.

Downhole logging

A ~130 m open hole interval of Hole U1440B was logged over a ~24 h period with 2 tool strings, the triple combo-UBI and the FMS-sonic. Although borehole conditions deteriorated while downhole logging was in progress, NGR, density, resistivity, sonic velocity, and microresistivity images were successfully acquired.

Seven logging units are defined on the basis of the character and trend of the various logs. Logging Unit 1 (~99–116 mbsf) is characterized by relatively consistent resistivity and velocity with depth whereas the underlying Unit 2 (~116–122 mbsf) shows sharp increases in NGR, resistivity, and density downhole. Units 3 (~122–164 mbsf) and 5 (~170–211 mbsf) exhibit similarities in their log responses, steadily increasing in resistivity with depth and with no net change in NGR. However, Unit 3 does show much greater variability in bulk density compared to the range of densities measured in Unit 5. Units 4 (~164–170 mbsf) and 6 (~211–222 mbsf) are relatively thin by comparison to Units 3 and 5, and are characterized by high resistivity, high velocity, and increasing density with depth. Unit 7 has limited data available but is differentiated from the overlying unit by a marked changed in the character of the resistivity log. Overall, there are downward increases in density, resistivity, and sonic velocity, whereas NGR and porosity (as derived from resistivity) exhibit decreasing downhole trends. Microresistivity images overall echo the increasing resistivity with depth and also elucidate a range of textures and features through the logged interval.

Preliminary analysis of the data shows a reasonable agreement between the logging unit boundaries and the lithologic unit boundaries that were defined on the basis of core description and geochemical analyses. It is anticipated that the logging data, although only available for the lowermost sedimentary interval and upper volcanic extrusive section, will be useful in filling in some of the gaps in core recovery.

Site U1441 summary

Operations

The JOIDES Resolution completed the 6.2 nmi transit from Site U1439 in dynamic positioning mode while the drill string was being lowered to the seafloor. The vessel arrived at Site U1441 (proposed Site BON-6A) at 1512 h on 11 September 2014, and a seafloor positioning beacon was deployed.

Site U1441 consists of one hole. An RCB BHA was assembled with a C-4 bit. Hole U1440A was spudded at 2245 h on 11 September. The RCB coring system with nonmagnetic core barrels was deployed 22 times (Cores 352-U1441A-1R through 22R) and the hole was advanced to 205.7 mbsf (Table T1). Cores 11R and 12R had no recovery as a result of a plugged bit. Hole U1441A was terminated as a result of poor core recovery, the rubbly nature of the formation, and high risk of getting stuck. The RCB cores recovered 50.7 m over the 205.7 m cored interval (25%). The total time spent in Hole U1441A was 75.75 h. The seafloor positioning beacon was recovered at 0914 h on 14 September, and the vessel started the slow transit to Site U1442 while continuing to pull the drill string to the surface.

Sedimentology

Pelagic and volcaniclastic sediment was recovered from the seafloor to 83.00 mbsf, beneath which igneous rocks were recovered. The sedimentary succession is divided into five lithologically distinct units (Fig. F15). Lithologic Unit I is further divided into 2 subunits. The volcanic rocks beneath are interpreted as the fore-arc basement. The main criterion for the recognition of the lithologic units and subunits is a combination of primary lithology, grain size, color, and diagenesis. Within the overall succession, 16 ash or tuff layers were observed. The bedding planes are generally oriented subhorizontally, with dip angles <10°.

  • Unit I (0–15.02 mbsf) is divided into 2 subunits. Subunit IA (0–0.17 mbsf) is recognized by the occurrence of brownish mud with medium to coarse sand. Subunit IB (0.17–15.02 mbsf) is a relatively nannofossil-rich interval of silty calcareous ooze with nannofossils and sparse planktonic foraminifers.
  • Unit II (15.02–24.50 mbsf) is recognized on the basis of a downward change to more clastic-rich sediment composed of muddy volcanic breccia/conglomerate and volcaniclastic sand layers.
  • Unit III (24.50–58.64 mbsf) is characterized by a return to finer grained silty mud with relatively abundant radiolarians.
  • Unit IV (58.64–70.38 mbsf) is distinguished by a distinct downward change to greener, relatively fine-grained sediment dominated by greenish gray silty clay.
  • Unit V (70.38–83.00 mbsf) is a much coarser, mud-supported conglomerate with sandy and silty clay and also clay.

Biostratigraphy

Calcareous nannofossils were present in 3 of 10 core catcher samples. Samples 352-U1441A-1R-CC and 2R-CC are nannofossil oozes, whereas siliceous fossils dominate Sample 5R-CC. Preservation was “moderate” to “good” in each sample. Samples 1R-CC and 2R-CC yield an approximately Late Pleistocene age, whereas Sample 5R-CC yields an approximately late Miocene age (5.59–8.12 Ma). The widespread presence of radiolarians in the lower part of the sediments will help us improve the biostratigraphy postcruise.

Fluid geochemistry

Ten samples were collected from Hole U1441A for headspace hydrocarbon gas analysis as part of the standard shipboard safety monitoring procedure. Methane concentrations range from 1.08 to 1.29 ppmv, and neither ethane nor propane were detected.

Petrology

All of the igneous rocks at Site U1441 are FAB similar to those drilled at Site U1440. These basalts are also similar texturally and chemically to FAB recovered in diving expeditions in the region. They are dominated by modal plagioclase, clinopyroxene, and magnetite in the groundmass, and most are aphyric. Four units were identified based on hand specimen and thin section description and XRF data (Figure F16). Microphenocrysts of plagioclase are rare, but igneous Unit 3 contains 2%–3% clinopyroxene phenocrysts. Not surprisingly, Unit 3 is also the unit with the highest CaO content. Three chemical varieties of basalt were found. The upper basalts (Units 1 and 2) are depleted in TiO2 and Zr and have low Cr concentrations. The lowest basalts, which comprise Unit 4, are normal FAB very close in composition to those at Site U1440. In contrast, the Unit 3 basalts, which lie stratigraphically between these types, are among the most depleted basalts found along the IBM fore arc, with very low TiO2 and Zr concentrations and high Ti/Zr ratios. With the exception of Unit 3, TiO2, Zr, and Cr all show subtle increases steadily downhole.

Rock geochemistry

Seven igneous rocks from Cores 352-U1441A-10R to 22R were analyzed by ICP-AES for major and trace elements and by CHNS for CO2 and H2O contents. The igneous rocks recovered have LOIs of 2.0–4.6 wt%. They have higher H2O contents in the upper part of the basement (Unit 1) and relatively uniform H2O contents of 2.0–2.5 wt% in the lower units.

The igneous rocks recovered from Hole U1441A are all basalts, with SiO2 concentrations of 49–51 wt% and total alkali contents of 2–4 wt%. Overall, the major element composition of Site U1441 basalts is relatively homogeneous with MgO of 6.4–8.4 wt%, CaO of 10.7–11.6 wt%, and Fe2O3 of 10.8–12 wt%. Site U1441 basalts are very similar in composition with the IBM FABs previously recovered by drilling at Site U1440 and by diving. The single sample analyzed from Unit 3 has high Cr and CaO concentrations, a high Mg#, and low concentrations of TiO2, Zr, and Y. This sample plots as a magnesian end-member composition on trace element variation diagrams but, despite its lower Ti contents, is both geochemically and petrographically distinct from Site U1439 boninites.

Structural geology

In the igneous units, viscous-plastic fabrics related to magmatic flow are rare and limited to millimeter- to centimeter-wide domains, defined primarily at the microscale. These domains are relatively common in the lower parts of Hole U1441A (e.g., in Sections 352-U1441A-19R-1 and 22R-1). The magmatic foliation is mainly defined by the shape-preferred orientation of acicular feldspar crystals embedded within a glassy or microcrystalline matrix.

Extensional fractures without mineral fillings are subvertical and are observed at 85.15 and 180.45 mbsf. Subvertical to inclined, whitish, crystalline, millimeter-thick veins are abundant at 122.22–141.43 and 190.2–190.6 mbsf. In the lower interval the veins form steeply inclined conjugate sets. The vein-filling material consists of (Mg-) calcite and/or zeolite and/or chlorite.

Slickensides are abundant at 84.00–88.25 mbsf and dip steeply to subvertically. The general sense of shear is left-lateral strike-slip to oblique reverse including a left-lateral component as well. One subhorizontal slickenside shows a normal sense of shear. In the lowermost sections of Hole U1441A (interval 20R-1, 15–27 cm), a semiductile to brittle, low-angle shear zone was observed within a highly altered domain. The shear zone was recovered as a single piece with an oblong shape, without a preserved contact with the wall rock. Its position within the lithostratigraphic sequence cannot be defined exactly because of the poor core recovery. The recovered basalt pieces below and above do not show any indication of comparable alteration or deformation. Within the shear zone, shear bands form subparallel sets, indicating a top-down sense of shear.

Physical properties

Many of the physical properties display similar downhole trends in the sedimentary section. P-wave velocities have peaks as high as 1580 m/s at 22–24 mbsf (lithologic Unit II) and 1540 m/s at 56–58 mbsf (Unit III). Magnetic susceptibility values also have peaks to 250–300 IU at the same depths. These peaks in P-wave velocities and magnetic susceptibility values correspond to tephra layers. GRA densities are 1.4–1.5 g/cm3, and NGR values are 10–20 counts/s from 0 to 69 mbsf. All of these parameters have a high peak at 70 mbsf at the bottom of Unit IV. Porosities are 65%–85% from 0 to 78 mbsf. Porosities are higher than 80% in Unit II and have the lowest value of 70% in Unit III.

Physical properties typically exhibit stepwise increases between Units 1 and 2. Magnetic susceptibility values start at >1000 IU at the top of the basement, at ~85 mbsf in Unit 1, and decrease to 500 IU in Units 2–4. GRA density values are 2–2.5 g/cm3 with a peak of 2.7 g/cm3 at 171 mbsf in Unit 3. NGR values increase from 9 to 20 counts/s in Unit 1 and decrease to 5 counts/s in Units 2–4. Although the values of reflectance parameter L* remain steady at 50–55 in Units 1–4, both a* and b* values are high (>10) in Unit 1 and low (<5) in Units 2–4. Bulk densities of the discrete samples are 2.4 g/cm3 in Unit 1 and increase to ~2.8 g/cm3 in Units 2–4. Porosities are ~30% in Unit 1 and decrease to 10% in Units 2–4. P-wave velocities of discrete samples are 3000–4000 m/s in Unit 1 and increase to 5500 m/s in Units 2–4.

Paleomagnetism

The remanent magnetization of archive-half sediment sections from sediment Cores 352-U1441A-3R to 9R was measured with the cryogenic magnetometer. The magnetic inclinations of the sediments define normal and reversed polarity zones. The inclinations are steep, typically >60°, which is probably the result of sediment disturbance by rotary coring. However, there are significant gaps in core recovery that hinder interpretation. In addition, the reversal pattern shows a small number of polarity zones, whereas there should be several on the basis of the biostratigraphic ages and the geomagnetic polarity time scale. Thus, we are unable to interpret the polarity record of the sediments in Hole U1441A.

The remanent magnetization was measured on 14 igneous rock samples from Cores 352-U1441A-10R to 22R. These samples have both normal and reversed polarities with two normal polarity zones at the top and bottom of the section bracketing a reversed polarity zone. The magnetic inclinations are close to that of the current field, implying that these rocks may have been remagnetized in the near geologic past. Most of the samples are from igneous Unit 1, which is thought to consist of talus, so the coherent inclinations are surprising. Therefore, remagnetization could explain the coherency of the jumbled pile of rocks.

Site U1442 summary

Operations

The JOIDES Resolution completed the 5.5 nmi transit from Site U1441 in dynamic positioning mode while the drill string was being raised from the seafloor. The vessel arrived at Site U1442 (proposed Site BON-5A) at 1630 h on 14 September 2014, and a seafloor positioning beacon was deployed. The vessel then offset 500 m at an azimuth of 81°.

A RCB BHA was assembled with a C-4 bit and then lowered to the seafloor. Hole U1442A was spudded at 2320 h on 14 September (28°24.5784′N, 142°37.3368′E; 3162 m water depth). The RCB coring system with nonmagnetic core barrels was deployed 57 times (Cores 352-U1442A-1R through 57R), with 529.8 m cored and 100.7 m recovered (19%) (Table T1). The basement contact was at ~82 mbsf. A free-fall funnel was deployed on 19 September so that the RCB bit could be changed. The bit change occurred at 46.1 h of coring time. Hole U1442A was terminated when the time available for coring expired. The hole was then logged with the triple combo-MSS tool string (to 371 mbsf on the first pass and 305 mbsf on the second pass) and the FMS-sonic (to 287 mbsf on both passes) tool string. The total time spent in Hole U1442A was 235.75 h. The seafloor positioning beacon was recovered at 0940 h on 24 September. After the thrusters were raised at 1030 h on 24 September, the vessel started the transit to Keelung, Taiwan.

Sedimentology

Pelagic and volcaniclastic sediment was recovered from the seafloor to 83.1 mbsf, beneath which igneous rocks were drilled. The sediment represents part of the late Oligocene to recent deep-sea sedimentary cover of the Izu-Bonin fore arc, which is stratigraphically condensed because of its position on a basement high.

The recovered sedimentary succession is divided into 4 lithologically distinct units (Figure F17). Lithologic Unit III is divided into 2 subunits. The main criterion for the recognition of the lithologic units and subunits is a combination of primary lithology, grain size, color, and diagenesis. Within the overall succession, 21 ash or tuff layers were observed.

  • Unit I (0–2.59 mbsf) is mostly silty to sandy nannofossil mud and nannofossil ooze, with additional dark gray “blotches” rich in volcanic glass that probably represent the remains of thin ash-rich layers.
  • Unit II (2.59–33.00 mbsf) is dominantly silty nannofossil ooze with slight color banding (off-white to pale brown), reflecting the presence of muddy and silty/sandy layers. The silty and sandy material is volcaniclastic in origin and is accompanied by several thin, discrete, ash layers, which are dispersed over tens of centimeter–thick intervals within the background sediment.
  • Unit III (33.00–62.40 mbsf) is divided into 2 subunits. Subunit IIIA (33.0–52.60 mbsf) is recognized by the presence of brownish mud and nannofossil-rich mud. Subunit IIIB (52.60–62.40 mbsf) is relatively pure clay with some manganese-stained horizons and also nannofossil ooze intervals.
  • Unit IV (62.40–83.12 mbsf) is distinguished by nannofossil-rich sediment, which becomes more lithified downward, and transition to nannofossil chalk. This sediment contains variable amounts of clay, volcaniclastic silt/siltstone, and volcaniclastic fine sand/sandstone. The most clastic-rich sediment can be classified as nannofossil-bearing fine sand/sandstone. The sedimentary succession is terminated downwards by a thin manganese layer, followed by a profound change to brownish red, noncalcareous volcanogenic sandy and silty clay, interspersed with clasts of mafic extrusive igneous rocks.

Biostratigraphy

Calcareous nannofossils were examined in core catcher Samples 352-U1442A-1R-CC to 9R-CC. An additional sample was taken from Section 10R-2, 22–23 cm, just above the layer containing igneous rocks. Preservation was “moderate” to “good” in each sample. A fairly continuous condensed section was recovered comprising sediments from the Eocene/Oligocene boundary up to as recent as the Late Pleistocene.

Fluid geochemistry

Ten samples were collected from sediment in Hole U1442A for headspace hydrocarbon gas analysis as part of the standard shipboard safety monitoring procedure. Only minor methane was detected (1.08–1.29 ppmv), and the relatively low and uniform methane concentrations imply negligible concentrations of organic matter. No ethane or propane was detected.

Petrology

Igneous rocks were recovered in Hole U1442A, which penetrated more than 440 m of igneous basement (Figure F18). The top of the igneous basement is defined by a Mn-rich sediment layer. The uppermost part of the section comprises breccia that may represent seafloor colluvium. This is underlain by boninitic lavas and hyaloclastites. Hole U1442A contains multiple zones of faulting.

Coring in Hole U1442A recovered igneous units similar to those in Hole U1439C, which lies ~1.3 km away. Nevertheless, there are notable differences between the two holes. Hole U1442A recovered low- and high-silica boninites together with evolved low-silica boninites. However, no dikes and no units dominated by basaltic boninites were encountered. Given the proximity of the 2 sites and the wealth of pXRF data, we correlated like units between the sites (see “Preliminary scientific assessment”). However, faults were encountered in Holes U1439C and U1442A, most indicating normal slip and some with evidence for reverse and oblique strike-slip motion. These faults raise the possibility that their stratigraphic records were disturbed and that the 2 sites were originally further apart. However, with the exception of the igneous Unit 1/2 boundary discussed below, units continued across fault surfaces at both holes suggesting that the stratigraphic record disturbance was minimal. We have no evidence for significant strike-slip motion between the sites, but such motion cannot be ruled out based on present knowledge.

Unit 1 (83–250 mbsf) consists mostly of hyaloclastites and lava flows of high-Si boninitic affinity. Unit 1 in Hole U1439C compositionally correlates with this unit but contains significantly less hyaloclastite. The most distinctive chemical feature of the upper section of Hole U1442A is the variable Cr content, which reaches values in excess of 1000 ppm. Below a fault zone at 240–270 mbsf, which separates Subunit 1e from Subunit 2a, Cr contents drop significantly and, with a few exceptions, remain below 500 ppm. Units 2–4 contain textural and compositional evidence of magma mingling, in which evolved boninite magma was intruded by less-evolved boninite magma. The subtle petrographic and chemical differences seen below Unit 1 were used to define subunits.

Finally, a striking feature of Hole U1442A is its excellent preservation in comparison to Site U1439, which is located only ~1.3 km away. Fresh glass is pervasive throughout the entire igneous interval in Hole U1442A, making it an invaluable resource for postcruise research requiring fresh material.

Rock geochemistry

Seven sediment samples were analyzed from Hole U1442A (1 per core from Cores 352-U1442A-1R through 4R, 7R, 8R, and 10R) for carbonate contents. Carbonate contents range from 50 to 78 wt%, except for 1 sample (interval 7R-5, 49–50 cm) with ~0.6 wt% carbonate.

We selected 21 representative igneous rock samples from Sections 352-U1442A-11R-1 through 43R-1 to be analyzed for major and trace element concentrations by ICP-AES. In addition, H2O and CO2 concentrations were determined for any samples with LOI >2%. The rock surfaces of 167 archive-half pieces were analyzed by pXRF for chemostratigraphic purposes.

The igneous rocks analyzed from Hole U1442A are primarily boninites and their differentiates. The samples have SiO2 contents of 52.5–63.4 wt%, total alkali (Na2O + K2O) contents of 1.44–4.74 wt%, and MgO contents of 2.6–17.0 wt%. Primitive magmas from Sites U1439 and U1442 have wide-ranging major and trace element compositions allowing basaltic boninite, low-Si boninite, and high-Si boninite series with different genetic histories to be distinguished. We were able to track the differentiates for each series based on mineral abundances and variations in the concentrations of SiO2, MgO, and TiO2 (see “Preliminary scientific assessment”). Site U1442 extends to less high-alkali abundances compared to Site U1439, reflecting the greater degree of alteration in the latter.

Structural geology

Bedding planes are subhorizontal in the sedimentary units above 75 mbsf, with dips generally <10×. Between 75 and 155 mbsf, the bedding planes dip ~35× on average. This change in dip angle defines an angular discordance at ~27–32 Ma based on biostratigraphic ages. In the igneous units, magmatic structures include contacts between distinct rock types, laminations, flow banding structures, alignments of elongated vesicles, and magmatic breccias. In general, magmatic minerals exhibit relatively weak to moderate alignment. Tectonic structures in the basement include shear fractures, cataclastic shear bands, cataclastic shear zones, veins, slickensides, and breccias. Three main fault zones occur at 238.2–267.5, 432.8–444.8, and 490.9–502.2 mbsf. The uppermost fault zone comprises fault gouge rich in talc and zeolites, including phillipsite. Slickensides dominantly indicate reverse dip-slip motion, although normal and oblique sense of shear is also observed.

Physical properties

In the sediments, P-wave velocity, magnetic susceptibility, and GRA density values increase across the boundary between lithologic Subunit IIIB and Unit IV at 62 mbsf. This depth corresponds to a strong reflector in the seismic profile across the site. NGR values decrease from the seafloor to the base of Unit IV, with the exception of higher values in Subunit IIIB (clay layer).

In the igneous basement, magnetic susceptibility values are low in igneous Unit 1 (boninite hyaloclastite) and increase abruptly at 260 mbsf near the top of Unit 2 (evolved boninite lavas). NGR values decrease gradually from the top of Unit 1 (83 mbsf) to the base of Unit 4 (523 mbsf). P-wave velocities are high and porosities are low in discrete samples taken from 83–170 and 305–480 mbsf. Thermal conductivity values are relatively constant in Unit 1 and Subunit 2A, increase in Subunit 2B, and decrease in Units 3 and 4.

Paleomagnetism

Sediment Cores 352-U1442A-2R through 9R were measured with the pass-through cryogenic magnetometer. However a discontinuous record, poor recovery, and drilling-related deformation makes it impossible to interpret the magnetic stratigraphy reliably.

Paleomagnetic samples from the igneous units give paleoinclinations mostly near zero. Low negative inclinations predominate in the upper part of Hole U1442A above 400 mbsf, whereas low positive inclinations are seen below 440 mbsf. These shallow inclinations are consistent with the low paleolatitude of the Izu-Bonin arc at the time of its formation. Transitions between positive and negative inclinations above 400 mbsf are most likely the result of secular variation at low latitudes. The shift to positive inclinations in the lower part of the hole may indicate a magnetic reversal or simply eruption of igneous Units 3 and 4 in a short interval, during which there was little secular variation. Interestingly, the Hole U1439C igneous section shows mainly low positive magnetic inclinations, whereas that of Hole U1442A mostly shows low negative inclinations. This difference may represent a change in magnetic polarity.

Downhole logging

Hole U1442A was logged with the triple combo-MSS and FMS-sonic tool strings. The borehole diameter was within the limits needed for the tools to function properly but the borehole conditions deteriorated during the first tool string deployment, with the hole filling in ~84 m by the time the second tool string was deployed. Weather conditions and sea state were excellent, with peak-to-peak heave <1 m. NGR, bulk density, resistivity, magnetic susceptibility, sonic velocity measurements, and microresistivity images were acquired.

Overall, increases in density, resistivity, P-wave velocity, and magnetic susceptibility values are observed with depth, whereas NGR values decrease. Eight logging units are defined on the basis of distinguishing features and trends in the various logs. Logging Unit 1 (~95–120 mbsf) is characterized by decreasing NGR and velocity values, coupled with increasing resistivity and magnetic susceptibility values downhole. Unit 2 (~120–188 mbsf) is differentiated from the overlying unit by elevated NGR and resistivity values and consistently low magnetic susceptibility values. There is increased borehole rugosity in Unit 3 (~188–204 mbsf), which may account for the significant variability across the logging data sets, distinguishing it from the units above and below. Unit 4 (~204–232 mbsf) has lower variability in the NGR and density logs and magnetic susceptibility values decrease with depth. Low magnetic susceptibility values, punctuated by 3 significant peaks, is the defining feature of Unit 5 (~232–258 mbsf) in combination with high variability in NGR values. Unit 6 (~258–282 mbsf) is characterized by decreasing downward trends in both resistivity and density values, which is counter to the overall trend in Hole U1442A. The character of the NGR, density, and resistivity logs is markedly different in Unit 7 (~282–326 mbsf) compared to the overlying unit. Finally, Unit 8 (>326 mbsf) does not have full data coverage but is differentiated from Unit 7 by a relatively constant NGR profile and a less variable resistivity profile. Oriented microresistivity borehole images indicate a range of textures and structural features, including veins, fractures, and vesicles.

The downhole logging data share similarities with the corresponding core physical properties and geochemical data. However, the logging unit boundaries that are defined on the basis of petrophysical properties do not correlate directly with the petrological boundaries. Postcruise core-log interpretation will focus on fully integrating the downhole and core data sets.