We define lithologic units based on core analyses and logging results.
The total sediment thickness at Site 1224 is ~28–30 m (Fig. F7). Paleontological analysis of calcareous nannofossils suggests that the whole sedimentary sequence was deposited within a few million years of the crustal age of ~46 Ma (Eocene) or contains reworked Eocene radiolarians. Basement at Site 1224 is separated into three lithologic units: Unit L-1 (massive flows; 26–62.7 mbsf), Unit L-2 (thin flows and pillow basalts; 62.7–133.5 mbsf), and Unit L-3 (intermediate to thin flows and pillow basalts; 133.5 mbsf). Unit L-1 is divided into upper and lower flow units based on grain size of groundmass and distribution of alteration effects.
The lithologic unit boundaries were also identified by logging and physical property measurements. Logging Units LG-I and LG-II correspond to lithologic Unit L-1, logging Units LG-III and LG-IV correspond to lithologic Unit L-2, and logging Unit LG-V corresponds to lithologic Unit L-3.
The massive basalt flows of lithologic Unit L-1 are subdivided into upper and lower subunits. This unit includes all basalt cores of Holes 1224A, 1224D, and 1224E. The base of the unit in Hole 1224F is curated at 62.7 mbsf, and its thickness in Hole 1224F is curated at 34.7 m. Recovery in Unit L-1 was 52.6%. Logging data show changes in porosity and density at the curated depth of the base of Unit L-1.
Changes in logging porosity and density corresponding to the base of Unit L-2 occur at ~140 mbsf, a few meters deeper than the curated depth. This lithologic unit comprises thin flows and two major pillow zones. The base of Unit L-2 is curated at 133.5 mbsf, and its thickness as curated is 70.8 m. Recovery in Unit L-2 was 14.6%. Two hyaloclastites were recovered in this unit.
Lithologic Unit L-3 comprises basalt flows of intermediate thickness alternating with thin flows and pillows. The base of the unit is at 161.7 mbsf, and its curated thickness is 28.2 m. Recovery in Unit L-3 was 21.4%.The changes in physical properties and downhole logging data correspond to the lithologic and chemostratigraphic changes that define the units and subunits.
Geochemical analysis shows important results (Haraguchi and Ishii, this volume; Lustrino, this volume) (Fig. F8A). SiO2 is distributed at 48–53 wt% and MgO at 5–7 wt%. FeO/MgO ratio characteristics of the rocks from all three units are 1.3–3.0, higher than the typical MORB ratio (normally 1–1.5). Some lavas in the core have the highest iron enrichment differentiation among abyssal tholeiites in the eastern Pacific. FeO and TiO2 are as high as ~14 wt% and 3.5 wt%, respectively (Fig. F8A, F8B) (Haraguchi and Ishii, this volume). Those basalts are altered differentiated tholeiitic ferrobasalts with the depleted geochemical characteristics of MORB. High FeOT and TiO2 were found in Hole 735B in the southwestern Indian Ocean (e.g., Dick et al., 2002), at Juan de Fuca Ridge, and recently in Atlantis massif during Integrated Ocean Drilling Program (IOPD) Expedition 304/305 (Ohara et al., 2005). These materials are classified as ferrobasalts (e.g., Natland, 1980) and/or ferrogabbro. Incompatible element analysis of three basement rocks show high contents, a factor of 2 or more of high field strength element (HFSE; Y and Zr) content than N-MORB, and some element contents are similar to ocean island basalt (OIB) as complied by Sun and McDonough (1989) (Haraguchi and Ishii, this volume). The depth dependences of Nb, Y, and Zr are very similar to the TiO2 distribution (see Fig. F8C) (Lustrino, this volume). Y is a factor of 2 greater than OIB, and the Y/Zr ratio is similar to that of normal and enriched MORB in contrast to that of OIB (Fig. F9). Differences in HFSE and rare earth element compositions among the three units are remarkable. Lithologic Unit L-2 is subdivided into two characteristic layers by similar incompatible elements. Lithologic Unit L-3 has high contents of these elements and Unit L-2 has low. Unit L-3 pillows show almost the greatest TiO2 enrichment (Fig. F8B) (Lustrino, this volume), as observed elsewhere along the East Pacific Rise (Natland, 1991) and corresponds to the highest end of the differentiation process.
Paul et al. (2006) analyzed the alteration at this site and found that most of the basalts are <5% altered to secondary mineral assemblages of Fe oxyhydroxides, celeadonite, saponite, Ca carbonate, trace pyrite, and rare phillipsite and quartz. They also found that Ca carbonate formed within 20 m.y. of crustal formation and occurred at low temperatures (4°–11°C).
Logging by two wireline tool strings was carried out in Hole 1224F. Using both drilling and logging data, we can make an excellent comparison of the petrological and geophysical characteristics of upper basaltic layers in fast-spreading Pacific crust at an age of ~46 Ma.
The triple combination (triple combo) tool string (Accelerator Porosity Sonde [APS], Hostile Environmental Litho-Density Tool [HLDT], and Dual Induction Tool [DIT]) was used for the first logging run. A tool string consisting of the Formation MicroScanner [FMS], General Purpose Inclinometer [GPIT], and Dipole Sonic Imager [DSI] was used for the second logging run. Five physical parameters (density, porosity, VP, VS, and magnetic susceptibility) were obtained during logging (Figs. F10, F11). For the layer unit classification based on logging, we use LG. In Hole 1224F, we can identify five distinct basaltic logging units in the basement below the ~28-m-thick sediment layer (Fig. F10):
The temperature in the hole also increases at 135–137 mbsf, suggesting that seawater flowing down the hole entered the formation in a zone of high permeability at this depth.
On the other hand, P-wave velocities from core samples acquired by the shipboard scientific party (Stephen, Kasahara, Acton, et al., 2003) identified seven layers (Table T1) at 38, 41, 65, 100, 132, 143, and below 143 mbsf, shown in Figures F11 and F12. Except for two thin layers, the rest of the layer boundaries are nearly the same as the logging unit classification.
The comparison of logging data and physical properties for porosity, density, VP, and VS is shown in Figure F13. The two data sets fit well except for some porosity data in logging Units LG-III and LG-IV.
VP and VS measurements of 11 samples taken from Holes 1224D and 1224F under pressure up to 100 MPa do not show much change: 2.1% (5.6–5.72 km/s) for VP and 3.6% (3.08–3.19 km/s) for VS (Fig. F14) (Sun et al., this volume). The porosity of these samples is <5%. The shallow part of the oceanic crust usually shows sharp increases from 4.5 to 6.5 km/s, corresponding to the transition from Layer 2B to 2C or 3. Because velocity measurements under high pressure do not show a large velocity change, the velocity increase observed in the shallow oceanic crust may be caused by a change from the fracture-altered basalts to massive, less fractured basalts. Macroscopic porosities can be produced by fractures because basalt specimens do not show extremely high porosities.
A seismic model down to 170 mbsf was obtained based on velocities from physical properties measurements and logging (Fig. F15). It will aid the analysis of the broadband, downhole seismometer data. Sun et al. (this volume) produced synthetic seismograms using logging VP values (Fig. F16). They compared the synthetic waveforms with the seismic reflection records and obtained good agreement (Fig. F17). Based on the reflection records (Stephen, Kasahara, Acton, et al., 2003) (Fig. F18), there is a deeper reflector at 60 ms TWT. This deeper reflector corresponds to the unit boundary at ~137–143 mbsf between logging Units LG-IV and LG-V or synthesis Unit S-6. In Leg 148 Hole 504B in the Panama Basin, the highly fractured zone was found at ~600–800 mbsf (Alt, Kinoshita, Stokking, et al., 1993). Average porosities in synthesis Unit S-5 are ~20%–60%. Porosities in synthesis Unit S-6 are 50%. Units S-1 through S-3 also show porosities of 40%–70%.
Recently, very intensive seismic surveys using a large number of ocean bottom seismometers and >8000-in3 air guns have been carried out (e.g., Nishizawa et al., 2005a, 2005b; Kaneda et al., 2005). The tomographic and forward analyses of traveltimes and waveforms show that the velocity profiles in the upper crust are not well resolved because ray paths are masked by water waves, in particular, when the high-velocity layer is near the ocean floor. Therefore, the present result is one of a few good models for the uppermost part of oceanic crust. The present results can be used to make a receiver function model for the downhole seismometers.
Logging data and physical property data may mainly relate to the mechanical properties, but lithological and geochemical measurements may mainly relate to the results of chemical reaction and alteration. Table T1 summarizes synthesis units (S-0 through S-7), distinguished mainly based on physical properties. There is excellent agreement among physical properties, logging, lithological, and geochemical data. Physical property measurements (Stephen, Kasahara, Acton, et al., 2003) suggest that synthesis Units S-3 through S-6 (Table T1) are largely fractured. Rock samples from synthesis Units S-1 through S-7 contain large amounts of altered minerals in halos (Paul et al., 2006). VP in synthesis Units S-2 and S-6 is lower than other units (4.0–4.5 km/s). For these two units, alteration and microfractures penetrate into small rock bodies (Fig. F11). Especially, VP in synthesis Unit S-6 is the lowest among the measured VP . The percentage in halos in synthesis Unit-S6 is higher than other synthesis units (Paul et al., 2006). The abrupt change of water temperature in synthesis Unit S-6 is related to water flow through fractures suggested by this high percentage.
Recently, a huge amount of biomass has been found living in the extreme conditions found under the seafloor (e.g., Isozaki et al., 2003). Prokaryotes significantly contribute to the overall biomass in marine ecosystems and play a major role in biogeochemical processes (Nealson, 1997; Parkes et al., 2000; Newman and Banfield, 2002). To date, prokaryotes have dominated the discoveries of microbial life in deeply buried oceanic sediments (Parkes et al., 1994) and sedimentary rocks (Lovley and Chapelle, 1995; Zhang et al., 1997; Liu et al., 1997). Microorganisms have been recovered from depths as deep as 800 mbsf in sediment (Taylor, Huchon, Klaus, et al., 1999) and have been found fossilized within continental basalts of the Columbia River basalts group (McKinley et al., 2000). Based on previous studies, it is thought that eukaryotes are less likely to be found in extreme environments such as subseafloor sediment and rocks.
Using standard optical microscopic techniques and field emission scanning electron microscopy (FE-SEM) with an X-ray energy dispersed (EDX) spectrometer system, dark brownish filamentous structures within carbonate-filled vesicles (0.3–3 mm in diameter) were found in Hole 1224D. These were analyzed and were interpreted to be fossilized fungi based on morphological traits including branching, septa, and central pores (Schumann et al., 2004) (Fig. F19). Schumann et al. (2004) proposed that these represent higher anaerobic fungi because they have septated hyphae. Chemical analysis shows that the chemical composition of the fungal structure differs from the surrounding crystalline carbonate matrix. The discovery of fungal eukaryotes in abyssal basalts is surprising because micro-eukaryotes are thought to live in the subsurface only in aquifers with relatively young groundwater (Balkwill, 1989).