Hole 1256C, our single-bit pilot hole, was cored 88.5 m into basement, and Hole 1256D, the cased reentry hole, was cored 502 m into basement during Leg 206. Hole 1256D is located ~30 m due south of Hole 1256C (see Fig. F17). The basement/sediment interface was encountered at 251.8 and ~250 mbsf in Holes 1256C and 1256D, respectively, in a water depth of 3634.7 m. Recovery from both holes was good, and excellent in places, with an average recovery of 61.3% and 47.8% in Holes 1256C and 1256D, respectively.
A summary of the igneous stratigraphy is presented in Figure F29. We divided the basement into 22 units in Hole 1256C and 26 units in Hole 1256D (Tables T9, T10). The igneous basement is dominated by thin (tens of centimeters to ~3 m) basaltic sheet flows separated by chilled margins, which make up ~60% of the cored interval in both holes. Massive flows (>3 m thick) are the second most common rock type in both holes, including the thick ponded flow near the top of each hole. Minor intervals of pillow lavas (~20 m) and hyaloclastite (a few meters) and a single dike were recovered in Hole 1256D. We drilled a massive ponded flow (~35–75 m thick) in both Holes 1256C and 1256D, which is a clear marker unit for correlation of the igneous stratigraphy between holes. So far we have been unable to determine the transition from axial eruptions to lavas that flowed out onto the ridge flanks; however, the thickness of some of the massive ponded flows requires significant basement relief in order to pool the magmas.
The upper 27 m of basement in Hole 1256C (251.8–278.7 mbsf) is composed of thin basaltic sheet flows a few tens of centimeters to ~3 m thick, separated by chilled margins and containing rare intervals of recrystallized sediment (Fig. F29). The basalt is aphyric to sparsely phyric, with plagioclase, olivine, and clinopyroxene phenocrysts (in order of decreasing abundance). The groundmass is cryptocrystalline to microcrystalline in flow interiors, decreasing in average grain size toward the margins (Fig. F30), and preserving glassy margins in some cases (Fig. F29; Table T11).
Units 1256C-18 and 1256D-1 each consist of a single cooling unit of cryptocrystalline to fine-grained basalt, which we interpret as a ponded lava flow. A total of 32 m of this unit was cored in Hole 1256C, of which 29 m was recovered. In Hole 1256C the deformed upper surface of the flow was encountered at 280.27 mbsf and consists of ~75 cm of cryptocrystalline to glassy aphyric basalt. The groundmass of the interior of the flow is fine grained but abruptly becomes cryptocrystalline ~1.5 m from the base of the flow. The base of the flow has been deformed and recrystallized, probably during and shortly after emplacement (Fig. F31).
This ponded flow is much thicker in Hole 1256D than in Hole 1256C, and although an exact thickness of the flow in Hole 1256D cannot be calculated because the top was not cored, the flow has a minimum thickness of 74.2 m. Although the top of Unit 1256D-1 is somewhat shallower than the top of Unit 1256C-18 (<276.1 mbsf, compared to 280.27 mbsf in Hole 1256C), we interpret the two units to be parts of a single ponded lava flow where the interior at the locations of both holes was liquid at the same time. The dramatic increase in thickness over 30 m of lateral distance suggests steep paleotopography, with Hole 1256D deeper in the depression that was filled in by the flow.
The remainder of the section in Hole 1256D (with the exception of Units 1256D-3, 4a, 4c, 8c, 16d, and 21) consists of sheet flows tens of centimeters to ~3 m thick with uncommon massive flows 3.5–16 m thick (see Table T10). These flows are aphyric to sparsely phyric, cryptocrystalline to microcrystalline basalt and are distinguished by chilled margins or by increasing grain size toward the interiors of flows where the margins were not recovered (Fig. F32). Chilled margins are common and the locations of glass and altered glass in the hole are compiled in Table T11.
We distinguished pillow basalt from thin sheet flows on the presence of curved glassy margins oblique to the sides of the core and radial pipe vesicles oriented perpendicular to the chilled margins. We cored one ~20-m-thick interval of aphyric to sparsely phyric cryptocrystalline pillow basalt with glassy chilled margins near the top of the section (Unit 1256D-3).
We recovered two hyaloclastite intervals in the basement (Units 1256D-4c and 1256D-21) (Fig. F33). These intervals consist of angular to rounded clasts of basaltic glass several centimeters to <10 cm in diameter and smaller (<1 cm) curved shards of glass within a matrix of altered glass. We also recovered an interval of volcanic breccia composed of angular fragments of cryptocrystalline basalt embedded in a matrix of altered glass (Unit 1256D-4a) (Fig. F34).
The basalt shows a large variation in grain size and textures from holohyaline in the outermost chilled margins of lava flows and hyaloclastite clasts through aphanitic groundmass, consisting of devitrified cryptocrystalline varioles, to the coarser intergranular textures in the lava pond. The basaltic lava is dominantly aphyric to sparsely phyric, with 72% of the examined thin sections having <5 vol% phenocrysts and 18% being aphyric (Fig. F35). The modal peak for Site 1256 is slightly shifted toward higher abundance of phenocrysts when compared to sheeted dikes from Hole 504B, which formed at an intermediate spreading rate but which are also dominantly aphyric (Dick, Erzinger, Stokking, et al., 1992) with a sharp peak at <2 vol% phenocrysts. In contrast, flows and dikes from the slow-spreading Mid-Atlantic Ridge (MAR) show bimodal phenocryst abundance with peaks at <10 vol% and >18 vol% (Bryan and Moore, 1977; Hekinian, 1982; Hodges, 1978; O'Donell and Presnall, 1980; Sato et al., 1978; Shipboard Scientific Party, 1988).
Phenocrysts are dominantly olivine (average = 68% among all phenocrysts for Hole 1256C and 70% for Hole 1256D) with subordinate amounts of plagioclase (average = 31% among all phenocrysts for Hole 1256C and 25% for Hole 1256D) and clinopyroxene phenocrysts (average = 5.4% among all phenocrysts for Hole 1256C and 4.7% for Hole 1256D). Most clinopyroxene is augite. Rare spinel is present in a few samples as tiny inclusions in completely altered olivine phenocrysts.
The ratio of three phenocrystic phases (clinopyroxene, plagioclase, and olivine) of the basalt is shown in Figure F36 compared to the sheeted dikes from Hole 504B and lava flows and dikes from the MAR. Nearly 50% of the basalt lava from Site 1256 plots on the plagioclase-olivine join. The 504B dikes are mostly plotted within the triangle and have slightly higher proportion of clinopyroxene among the three phases than the Site 1256 lavas. In contrast, the majority of the MAR lavas and dikes plot on the plagioclase-olivine join. In respect to both the phenocrystic abundance and proportions, the Site 1256 lavas have intermediate characteristics between these two extremes.
Olivine is the most common phenocryst phase (0.1–11 vol%), but fresh olivine was found only in fresh glass of some chilled margins of lava flows and hyaloclastite. Plagioclase is the second most abundant phenocryst phase and is mostly subhedral to euhedral crystals clotted together with either other plagioclase crystals or with clinopyroxene, though euhedral platy to stubby discrete crystals are also present. Most plagioclase phenocrysts from Hole 1256C and from the upper units of Hole 1256D are unzoned. Zoning is more common in lower units from Hole 1256D and may be either normally or reversely zoned or both. Normally zoned plagioclases commonly have clear cores with euhedral outlines, surrounded by thin and less calcic rims. Inclusions are uncommon except for some glass blebs and clinopyroxene aligned along twin planes of the host plagioclase (Fig. F37). Much less common is plagioclase with resorbed cores, mottled with bleblike inclusions of clinopyroxene, magnetite, and glass, and enclosed in less calcic mantle. In contrast, reversely zoned plagioclase has dusty resorbed cores with euhedral, more calcic mantles, which are further enclosed by sodic rims. The dusty inclusions are small skeletal to dendritic magnetite, tiny acicular clinopyroxene, and thin plagioclase laths and pale brown glass mostly altered to clay minerals. Although clinopyroxene is the least common phenocryst phase, it is present in >40% of the thin sections examined. Augite phenocrysts typically occur as subhedral to euhedral, stubby to short prismatic crystals that commonly form crystal clots with platy plagioclase. The most common variety is black to dark green augite, but a pale yellowish green prismatic pyroxene that resembles the groundmass pigeonite in the coarse basalt lava from the massive ponded lava flow is also observed.
The groundmass is composed of plagioclase, augite, and magnetite, with or without minor amounts of glass, pigeonite, green clinopyroxene, quartz, apatite, and granophyric intergrowths of quartz and sodic plagioclase. Dark green clinopyroxene occurs as the outermost rims surrounding augite and pigeonite and as a discrete prismatic crystal in the coarser fine-grained basalt. Groundmass clinopyroxene and plagioclase show a variety of textures that correspond to different cooling rates or the degree of undercooling but are mostly radially arranged to form spheroidal or fan-shaped crystal aggregates. Groundmass plagioclase occurs as very tiny acicular crystals, thin planar laths, curved very thin planar crystals, skeletal fan-shaped or bowtie-shaped crystals in finer-grained samples, and platy and more stubby crystals in coarser-grained samples. Pigeonite is much less abundant than augite and commonly forms intergrowths with augite, where prismatic pigeonite is sandwiched in between pale brown-green augite lamellae. Dendritic chains to equant skeletal crystals of Fe-Ti oxide minerals are a ubiquitous groundmass phase. In coarser-grained samples, host magnetite has exsolved ilmenite lamellae during cooling. Fine-grained and some microcrystalline basalt contains interstitial mesostasis of quartz-albite granophyric to vermicular intergrowths, quartz, granular to prismatic clinopyroxene (mostly altered to secondary clay minerals), acicular apatite, and dendritic to skeletal magnetite
Unit 18 of Hole 1256C is a >30-m-thick massive lava body that begins at 280.3 mbsf with a holohyaline-cryptocrystalline lava surface, which develops downhole into a intergranular to coarse variolitic fine-grained massive basalt and to cryptocrystalline, recrystallized basalt at the bottom (312 mbsf). The uppermost 0.7 m is composed of aphanitic lava with folded glassy chilled margins and volcanic rubble, including glassy clasts, and is interpreted as the folded and jumbled surface crust of a lava flow. The basal 1.6-m-thick lava is unusual aphanitic basalt consisting of recrystallized variolitic groundmass and late magmatic veins, which shows synmetamorphic ductile deformation textures. In Hole 1256D we encountered a similar fine-grained massive lava (Unit 1) from the first core at 276.1 mbsf, ~4 m above the top of Unit 18 of Hole 1256C, which continued downhole to 350 mbsf. This unit is lithologically correlated to the thick massive lava unit in Hole 1256C, but is much thicker (>75 m thick) and lacks both the quenched upper surface and the basal recrystallized basalt lava. Such a thick lava flow could potentially have formed as a lava pond, where rapidly delivered lava accumulates in a depression, or as an inflated sheet flow, with slowly delivered lava confined by its own chilled margin. We interpret these massive basaltic units to be a thick ponded lava and not an inflated sheet flow on the following grounds:
The ponding of such a massive lava flow (>75 m thick) requires a significant basement topography near the axis to pool the magma. Small basement faults with throws of ~100 m are apparent in the site survey seismic sections of the region surrounding Site 1256, and faults of such magnitude are commonly observed 5–10 km from the axis of the East Pacific Rise (Macdonald et al., 1996). The very smooth basement topography present at Site 1256 may result from the infill of small ridge flank half-grabens by large lava flows. The topography required to accumulate such a thick lava suggests that magma may have flowed a significant distance off axis (>5 km) before ponding. Assuming that the ponded lava drilled at Site 1256 has not merely filled an isolated small depression but has pooled against some form of buttress, it would have a significant volume (5 x 104 to 1 x 106 m3 per meter of ridge crest). Such an eruption would drain a very significant fraction of the lava in an axial melt lens, if in fact these geophysically imaged features are the source of the magmas (10 x 104 to 2 x 105 m3 per meter of ridge crest). Very large lava flows, with much larger volumes than needed to supply the massive ponded flow at Site 1256, have been discovered on modern fast spreading ridges (e.g., 8°S EPR) (Macdonald et al., 1989).
Petrographically fresh samples were selected from the basement cores of Hole 1256C and 1256D and analyzed for their major and trace element concentrations using the shipboard inductively coupled plasma–atomic emission spectrophotometer (ICP-AES). There are general downhole variations with Mg#, Cr, Ni, and Ca/Al ratios broadly increasing with depth, while TiO2, Fe2O3, Zr, Y, Nb, V, and Sr broadly decrease with depth (Fig. F38). Superimposed on these broad trends are smaller-scale variations, for example, near-constant Mg# in lavas of Units 1256D-2 to 1256D-6, which is higher than those in the units immediately above and below. On a Zr-Y-Nb ternary diagram (Fig. F39), all lavas from Site 1256 plot in the N-MORB field.
TiO2 and Zr behave similarly, as demonstrated by a relatively coherent pattern of increasing TiO2 with increasing Zr (Fig. F40A). Two groups are apparent on this diagram, one more evolved group with high Zr and TiO2, which includes all samples from Hole 1256C and Unit 1 in Hole 1256D, and a single sample from deeper in Hole 1256D. The deeper samples (>365 mbsf) from Hole 1256D form the low-Ti, low-Zr group. Relatively high Ti and Zr in the shallow samples is consistent with their lower Mg# and higher concentrations of incompatible elements and suggests that they are more evolved than deeper lavas. Two anomalous groups of lavas are notable compared to this general trend of decreasing evolution with depth: one group of samples within Unit 1256C-18 that have very high K2O (Fig. F41) and one group of four lavas with anomalously high Zr for a given TiO2 value.
The massive ponded flow forms the majority of the evolved group distinguishable on the Zr vs. TiO2 diagram, but samples from 294 to 306 mbsf in Hole 1256C (approximately the middle to lower two-thirds of Unit 1256C-18) have exceptionally high K2O (0.53–0.74 wt%, compared with 0.05–0.20 wt% for other Hole 1256C and Unit 1256D-1 samples). This order of magnitude increase in K2O coincides with an increase in natural gamma radiation (NGR) measurements, but neither the chemical nor the NGR anomaly are apparent in Hole 1256D. This large increase in K2O is not matched by variations in Mg# or other measures of fractionation, and some other explanation must be invoked, such as an along-rift geochemical zonation in source composition or tapping a small pod of more evolved magma or local assimilation of an unknown high-K sediment or altered lava.
In the lavas directly below the large massive flow in there is a sharp increase in Mg# accompanied by an increase in incompatible element concentrations (Fig. F38). The combination of high Mg# and high incompatible element concentrations argues against differentiation as the cause of the enrichments and suggests that there is variation in the primitive magma composition.
Rocks from throughout Holes 1256C and 1256D exhibit a dark gray background alteration, where the rocks are slightly to moderately altered and olivine is replaced and pore spaces are filled by saponite and minor pyrite. This background alteration is reflected in the distribution of dark gray rocks (Fig. F42) and of pyrite and saponite (e.g., Fig. F43) and is the result of low-temperature seawater interaction at low cumulative seawater/rock ratios.
The local effects of late magmatic/hydrothermal fluids are restricted to within the massive ponded lava near the top of the section (Units 1256C-18 and 1256D-1, i.e., mainly at 276–330 mbsf). These effects include granophyric intergrowths of plagioclase and quartz in veins and interstitial areas, secondary green clinopyroxene reaction rims on primary augite, trace interstitial brown mica and blue-green phyllosilicate (chlorite?), and partial replacement of primary calcic plagioclase by albite.
Vein-related alteration is manifested as different-colored alteration halos along veins. The black halos contain celadonite and have been interpreted to result from the presence of upwelling distal low-temperature hydrothermal fluids enriched in iron, silica, and alkalis (Edmond et al., 1979; see summary in Alt, 1999). The iron oxyhydroxide–rich mixed halos are later features, which formed by circulation of oxidizing seawater. The brown halos have a similar origin and formed along fractures that were not bordered by previously formed black halos.
This vein-related alteration occurs irregularly throughout Hole 1256D below the massive Unit 1 but is concentrated in two zones, at 350–450 and 635–750 mbsf (Fig. F44). These zones correspond to peaks in frequency and proportion of celadonite and iron oxyhydroxide veins and minima in the abundance of pyrite veins (Figs. F45, F46). These were likely zones of greater permeability and, consequently, increased fluid flow.
Vein carbonate occurs more commonly above ~530 mbsf, but despite lower frequency of occurrence, overall higher abundance occurs at greater depths (Figs. F44, F46). The absolute amount of CaCO3 in the basement as Site 1256 is very low relative to other basement sites (Alt and Teagle, 1999).
Three peaks in glass abundance are present at 400, 460, and 600 mbsf (Fig. F42), corresponding to the presence of hyaloclastites in the core. These are important because of the substantial degree of glass alteration with the presence of saponite cementing the breccia resulting in corresponding peaks in the abundance of secondary minerals (saponite) at these depths (Fig. F44).
The appearance of albite and saponite partially replacing plagioclase below 625 mbsf indicates a change in alteration conditions (Figs. F43, F44). This change may result in part because of slightly higher temperatures at depth or more evolved fluid compositions (e.g., decreased K/Na and elevated silica).
Overall, the basalt recovered from Site 1256 does not exhibit a general decrease in seawater interaction with depth and there is no simple decrease in the amount of alteration halos or iron oxyhydroxide with depth. In contrast, alteration appears to have been concentrated into different zones that may be related to the architecture of the basement such as lava morphology, distribution of breccia and fracturing, and the influence of these on porosity and permeability.
Alteration of the basement section of Hole 1256D is compared with other sites in Figure F47. Compared with most of these sites, Hole 1256D contains a much smaller amount of brown, mixed, and black alteration halos. The abundance of carbonate veins in Hole 1256D is also lower than at many other sites. Site 1256 is, however, quite similar to another section of crust generated at a fast spreading ridge, Site 801. The latter site, however, contains two low-temperature hydrothermal deposits and associated intense hydrothermal alteration. One important feature is the lack of any oxidation gradient with depth in Hole 1256D, in contrast to the stepwise disappearance of iron oxyhydroxide and celadonite in Hole 504B and the general downward decrease in seawater effects at Sites 417 and 418.
Both primary magmatic and postmagmatic structures were described in the basement rocks of Holes 1256C and 1256D. Primary igneous features included magmatic fabrics, laminations and flattened vesicles, folds and shear-related structures, late magmatic veins, and fracturing. Postmagmatic structures include veins, shear veins, microfaults, joints, and breccia. The distribution of these features is shown in Figure F48.
Late magmatic features are mainly restricted to within the massive ponded lava near the top of the basement and include felsic veins (usually >0.5 mm wide) and glassy veins (usually <0.5 mm wide). Felsic veins are characterized by quartz + alkali feldspar (or albite) symplectites showing a granophyric texture clinopyroxene grows with face-controlled geometries, and they are commonly associated with vesicles and amygdules. Late magmatic veins are present either as planar features or fill tension gashes or sigmoidal pull-aparts. Some late magmatic veinlets show evidence of multiple episodes of folding. Shear bands and tension gashes that cut these folds indicate a progressive transition from predominantly ductile to brittle-ductile deformation.
Veins are the most prominent structural features observed in rocks recovered from Holes 1256C and 1256D. Veins mostly have a planar or slightly curved morphology, commonly with irregular margins. Individual veins commonly branch into a number of diverging splays at their ends. Multiple veins commonly occur in anastomosing geometries, and, where veining is pervasive, develop into vein networks. In many cases veins are oriented in en echelon, Riedel-shear arrays. Stepped veins are common in both basement holes and are locally characterized by millimeter-scale pull-aparts filled with secondary minerals. Shear veins are mostly present in massive coarser-grained lithologic units (e.g., Units 1256C-18 and 1256D-1) and are filled with fibrous clay minerals. Microfaults are restricted to the interval 289.9–331.90 mbsf and have thin bands of cataclasite and fibrous minerals. Shear veins and microfaults indicate both strike-slip and oblique apparent senses of shear. In Hole 1256D shear veins show a change in the sense of shear, from reverse to normal, from ~645 mbsf to the bottom of the hole.
Five different types of breccia were described from the basement at Site 1256: hyaloclastite, talus breccia, breccia with interflow sediment, incipient brecciation, and hydrothermal/tectonic breccia. Some core intervals show evidence of incipient brecciation associated with the progressive development of anastomosing vein networks.
True dip data obtained by measurement of structure orientation in Holes 1256C and 1256D (Fig. F49) show that, in general, structures of Hole 1256D are mostly gently dipping, having most common frequency dip angles of ~15° of basement. Other dip angles are represented nearly by the same frequency throughout the hole. In Hole 1256C, true dip angles show a maximum in frequency between 10° and 20°; however, dip values around 50°–55° and 90° are common as well.
Late magmatic veins are mostly gently dipping in Holes 1256C and 1256D, showing the highest frequency at 15° and 5°, respectively. By contrast, shear veins are moderately to steeply dipping in the two holes (maximum frequency ranges from 45° to 75°).
In Hole 1256C, the distribution of true dips per lithologic unit shows that, in the upper units, the dip values are bimodally distributed in sets making an angle of 50°–60°. This is linked to the presence of conjugate systems of veins in the upper part of the hole, whereas in the middle and lower parts, true dips are mostly clustered in one group. In the lower three igneous units, structures mainly have gentle orientations. In Hole 1256D, the distribution of true dip angles with depth does not show any systematic variation.
The variation in dips of the veins and in their density can be related mainly to the physical properties and morphology of the lithologic units rather than to the depth of their occurrence.
Basalt samples from Site 1256 show a strong tendency to have been partially or fully remagnetized during drilling, much more so than for most other DSDP and ODP sites. In several of the massive basalt units, a downward and radially inward magnetization is the only component that can be recognized. In most cases, a pre-overprint component can be discerned, if not always measured accurately with the shipboard equipment. For Hole 1256D, most samples from igneous Units 3–8a and 14–26 demagnetize to a shallow inclination, as expected for the equatorial paleolatitude (Fig. F50). For Hole 1256C, all samples have steep inclinations and most are dominated by overprint, but a few samples from Units 3, 7, 18c, 18h, and 22 show evidence for a stable, steep component distinct from the overprint (Fig. F51). The steep inclination may reflect eruption during the magnetic polarity transition between Chrons 5Br and 5Bn, which would imply transport of these lavas at least ~5 km from the ridge axis. The apparent shared direction for multiple units from Hole 1256C, if confirmed by shore-based studies, suggests a maximum time interval on the order of centuries for erupting these geochemically similar, but not identical, lavas.
The drilling overprint is sufficiently strong for most of the recovered samples that it is not yet possible to make a quantitative assessment of the contribution of the cored section to the magnetic anomalies measured at the sea surface. Careful integration of the sample measurements with downhole measurements of the magnetic field will offer the best opportunity to test the common interpretation that the extrusive layer contributes most of source of marine magnetic anomalies.
The basalt of Site 1256D has bulk densities ranging from 2.55 to 2.98 g/cm3 (average = 2.83 g/cm3). The basalt has generally low porosities, ranging from ~2% to 6%, with higher porosities exhibited by the pillow basalt. The physical property parameters of density, porosity, velocity, natural gamma radiation, magnetic susceptibility, and thermal conductivity vary systematically downhole and correspond to the igneous units and eruptive style. Increasing bulk density is well correlated with increasing velocity, but bulk density and velocity are inversely related to porosity. There is no significant anisotropy in P-wave velocity in the Site 1256 lavas.
Hole 1256D provides a rare opportunity for determining if microbial life is present in crust formed at a fast spreading rate. Igneous samples were collected immediately after core retrieval for shore-based microbiological studies (petrological observation, scanning electron microscope and microprobe analysis, deoxyribonucleic acid (DNA) extraction, in situ hybridization, and cultivation).
Shipboard analyses focused on examination of thin sections for extant and fossil microbial activity. Of the thin sections examined so far from hyaloclastites and glassy flow margins, none contained unequivocal textures characteristic of the style of alteration previously attributed to microbial alteration. Instead of the irregular alteration textures attributed to alteration by microbes, smooth alteration fronts with clay minerals replacing the isotropic glass are most common, suggesting chemical alteration as the dominant process.
One interesting discovery is filamentous textures preserved in a 6.2-mm vein cutting cryptocrystalline basalt in Sample 206-1256C-8R-3, 136–148 cm. The vein is filled with chalcedony, iron oxyhydroxide, saponite, celadonite, and minor aragonite. Within the iron oxyhydroxide in the vein are curved filaments 5–10 x 50–100 size and morphology to iron oxidizing bacteria and could represent their fossilized remains. These textures will be the focus of future shore-based study.
To estimate the amount of fluid intrusion into the recovered cores, chemical and particulate tracers were deployed in Hole 1256. Perfluoro(methylcyclohexane) was used as the chemical perfluorcarbon tracer (PFT) and 0.5-were conducted while coring Cores 206-1256C-7R, 9R, and 11R with the RCB. The PFT tests indicated drilling fluid intrusion into the center of the core in amounts below the detection limit. Fluorescent microspheres were rare in the crushed interior of the rock even though they were detected on the outside of the cores and in the drill water. The low abundance of microspheres detected in the interior of the cores indicates very low levels of drill water intrusion, which is consistent with the PFT tests.
A full suite of logging tools were run in Hole 1256D following the suspension of coring operations (Fig. F52). The tools utilized, in order of deployment, were the triple combo tool string, the FMS/sonic, the BGR gyromagnetometer, the UBI, and the WST. This is the first time the UBI has been used in a hard rock hole. Because of tool failure during the first attempts to run the BGR magnetometer, this tool was rerun following the completion of the WST experiment. Unfortunately, despite lowering the BGR tool at a very slow rate (600 m/hr), rapid rotational and vertical accelerations resulted in an unusually high current demand by the gyro and failure of the instrument before it entered the open hole.
Hole 1256D was in excellent condition, and no constrictions or ledges impeded the passage of the various wireline strings throughout the logging schedule. Caliper readings from both the triple combo and the FMS tool string show the borehole diameter to be mostly between 11 and 14 in, with only four short intervals >16 in. The hole conditions were ideal for those tools that require contact with the wall of the borehole. Despite the limitations of deploying the UBI tool in a larger than optimal borehole, many fractures were imaged by the tool.
The downhole measurements and images recorded show a large amount of variation, reflecting the massive units, lava flows, pillow lavas, and hyaloclastites recovered in Hole 1256D. Combined measurements of FMS and UBI coupled with other measured parameters will allow the stratigraphy of different rock types and flow thicknesses to be determined and structural features to be measured (Fig. F52). Multiple passes by the GPIT allow the cored basement to be subdivided into a number of magnetic subunits that will be compared with observed variations in rock type postcruise.
The exterior of all whole-round core pieces that could be rotated smoothly through 360° were imaged on the DMT Digital Color CoreScan system. Correlations between the whole-round core images and electrical or acoustic representations of the borehole wall, derived from downhole log measurements, allow determination of the true core depth (as opposed to ODP curated depth) in intervals with greater or less than 100% recovery. Distinctive features such as lithologic boundaries, fractures, veins, and breccia, can be depth-matched to allow repositioning of core pieces. The certainty of the correlation will vary with recovery, the continuity of the pieces, and the number of distinctive features within a particular interval. Individual pieces or sequences of pieces that can be correlated can then be shifted to the appropriate depths with respect to the log data. This enables direct comparison between structural, physical, and chemical properties measured on the core and those recorded downhole.
Individual core pieces (and associated structural data) that can be confidently depth-matched can then ultimately be reoriented or rotated so they are oriented with respect to true geographic north using data from the GPIT, which is included in the tool string with both the FMS and the UBI. Preliminary attempts at correlation and reorientation of core pieces, using the methods described in Haggas et al. (2001), show potentially good matches between the unrolled core images and the FMS and UBI data from Hole 1256D (Fig. F53). Because the procedures involved in correlating the whole-round core images to FMS and UBI images are very time intensive, further depth matching and core reorientation will be completed postcruise.