Leg 206 is the first leg of a two-leg program to sample a complete section of upper oceanic crust through the extrusive lavas, sheeted dike complex, and into the upper oceanic gabbros. Site 1256 was drilled into ~15-m.y.-old crust formed during an episode of superfast accretion at the EPR. Seismic observations (e.g., Purdy et al., 1992) suggest an inverse relationship between spreading rate and the depth to axial low-velocity zones at mid-ocean ridges. These low-velocity zones are hypothesized to be partially molten magma chambers that, when frozen, will form the uppermost gabbros. If this is the case, gabbroic rocks should be at their shallowest in superfast spreading crust and drilling crust formed at this rate will provide the best opportunity to sample a complete section of upper oceanic crust. At least in terms of seismic structure, ocean crust formed at fast spreading rates is relatively simple and probably best conforms to the ideal stratigraphy envisioned for Penrose-type ocean crust. Although perhaps only 20% of the present-day mid-ocean ridges are spreading at fast rates (>80 mm/yr), ~50% of the present ocean basins formed at this style of ridge axis. Hence, one deep drill hole through a complete upper crustal section can be reasonably extrapolated to describe a significant portion of the Earth's surface.
Before we embarked on our major aims of drilling operations at Site 1256 it was first necessary to thoroughly characterize the sedimentary overburden above the oceanic crust and determine the stratigraphy, sedimentation and mass accumulation rates, and the role of fluid processes in the sedimentary blanket. Following the sampling of the sedimentary section, operations then concentrated on the underlying basement, and a reentry cone and casing string to basement were installed to enable deep drilling of the upper oceanic crust.
Four holes were drilled at Site 1256 (Table T4). Hole 1256A was a single piston core, shot to establish the mudline at Site 1256. In Hole 1256B, the complete sedimentary sequence was drilled by APC and XCB coring and the uppermost basement was tagged at 250.7 mbsf. Further samples of the sedimentary sequence were recovered by RCB drilling from 220.1 mbsf in Hole 1256C, and this pilot hole was drilled into the upper basement to a depth of 340.3 mbsf (88.5 m subbasement). After the successful installation of the reentry cone and casing strings to basement in Hole 1256D, coring of the lavas resumed and continued until the end of the leg, when the hole was prepared for wireline logging. Hole 1256D was opened to a total depth of 752 mbsf, or 502 m into basement. Hole 1256D was left clear of junk and in excellent condition for deepening during a future scientific drilling expedition.
Drilling in Holes 1256A, 1256B, and 1256C recovered a complete section of the sedimentary stratigraphy at Site 1256 (Fig. F19). The sediments are subdivided into two principal lithologic subdivisions, although more complex structure is revealed by the paleomagnetism, geochemistry, and physical property investigations. Unit I is clay rich with a few carbonate-rich intervals, whereas Unit II is predominantly biogenic carbonate.
Lithologic Unit I (0-40.6 mbsf) comprises clay-rich sediments with Pleistocene dark brown to yellow-brown silty clays (Subunit 1A) (Table T6) with calcareous nannofossils, overlying light olive-gray to yellowish brown Pliocene to upper Miocene sandy clays to silts with calcareous nannofossil-rich intervals (Subunit IB). The Subunit IA/IB transition is taken as the Pleistocene/Pliocene boundary with a change in the relative contribution of clastic and biogenic components apparent in the color reflectance and magnetic susceptibility. Bioturbation is moderate to abundant throughout lithologic Unit I with Planolites, Chondrites, Zoophycos, and Skolithos the most common trace fossils identified. Rare volcanic ash is present in Unit I, and a quartz- and feldspar-rich ash layer was recovered in interval 206-1256B-3H-2, 34-36 cm.
Lithologic Unit II (40.6-250.7 mbsf) (Table T6) comprises upper Miocene to middle Miocene light greenish gray to dark gray calcareous nannofossil ooze with varying amounts of clay and other microfossil groups. Diatoms are a significant minor component that increase in abundance with depth, forming a siliceous biogenic ooze at 85 mbsf. With the exception of two intervals, the remainder of the sedimentary section is dominated by light greeenish gray to white, nearly pure calcareous nannofossil ooze. Bioturbation is common throughout Unit II, and trace fossils include calcified solid and rind burrows as well as Planolites and Skolithos.
The interval from 111 to 115 mbsf consists of a laminated diatom mat comprising abundant tubular diatom tests with minor nannofossils. From 140 to 195 mbsf the nannofossil ooze has significant but variable clay and diatom components, which are most clearly recognized by changes in the color reflectance and density.
Chert nodules are a common feature in the sediments from ~111 mbsf down to the basement. Distinct chert layers at 111 and 158 mbsf were identified by increased resistivity and low porosity in the downhole measurements, and very poor recovery in the lowermost sediment cores indicates that further chert layers are present between 230 mbsf and the basement. Red-brown iron oxide-rich silicified sediments directly overlie the basement (within 1 m), and these may be recrystallized metalliferous sediments.
Rare 5-cm-wide dark greenish gray granular glauconite bands were identified at 76-82 and 217-225 mbsf. At the base of the sedimentary sequence, the nannofossil ooze has a distinct pale bluish hue and X-ray diffraction determined a significant mica component, possibly celadonite or glauconite, in the sediment, perhaps derived from the recrystallization of biogenic opal and hydrothermally derived iron oxides.
A biostratigraphic framework for Site 1256 has been established by the inspection of calcareous microfossil assemblages in core catcher and additional samples from the sedimentary sequence. Calcareous nannofossils are generally abundant and moderately to well preserved, albeit with some fragmentation. Assemblages above ~25 mbsf have been affected by etching and those below ~118 mbsf show the effects of dissolution and overgrowth. There was no evidence for significant reworking of fossils, and more than a dozen nannofossil datums were determined, providing a modest biostratigraphic resolution for the Pleistocene through middle Miocene (Table T7).
The biostratigraphic sequence at Site 1256 is generally similar to those recorded from nearby sites in the Guatemala Basin, Sites 844 and 845 (Mayer, Pisias, Janecek, et al., 1992). Sedimentation rates are discussed in greater detail below, but the drastic decrease from high (>35 m/m.y.) sedimentation rates in the eastern equatorial Pacific in the middle Miocene to much lower (8-14 m/m.y.) rates in the late Miocene, referred to as the "carbonate crash" (Farrell et al., 1995), is also observed at Site 1256.
Alternating-field demagnetization of sedimentary split core sections and discrete samples was used to construct a magnetostratigraphic record of the sedimentary overburden at Site 1256. Nearly all chrons and subchrons are recorded from Chron C1n (Brunhes Chron; 0.0-0.780 Ma) through most of Subchron C5n.2n (9.920-10.949 Ma) (Figs. F20, F21; Table T8). The termination of Subchron C5n.2n (9.920 Ma) is clearly identified at 92.53 mbsf, but the onset of Subchron C5n.2n (10.949 Ma) could not be identified, either due to a coring gap or poor resolution of the characteristic remanent magnetization below 110 mbsf. From 110 mbsf to basement, the sediments have extremely weak intensities, resulting in a paleomagnetic signal dominated by noise. Thus, no polarity stratigraphy could be determined for this part of the sedimentary section.
The age assignments from Site 1256 magnetostratigraphy are in very good agreement with the biostratigraphic constraints from calcareous nannofossil datums (Fig. F22). The magnetostratigraphy provides higher resolution in the late Miocene up to the Brunhes/Matuyama reversal at 0.78 Ma, whereas the biostratigraphy provides constraints within the late Pleistocene, where there is an absence of polarity reversals, and in the middle Miocene, where the paleomagnetic signal was poorly resolved.
Sedimentation rates vary from ~6 to 36 m/m.y., with the rate being about four times faster in the middle Miocene than the average rate during the late Miocene to present (Fig. F23). Linear sedimentation rates were estimated for five intervals (Fig. F22) within which the slope of the age vs. depth data from the magnetostratigraphy and biostratigraphy was constant or nearly so (Fig. F22). The lowermost linear sedimentation rate is 36.4 m/m.y., calculated for interval 9.92-13.6 Ma (95-212.65 mbsf). When this rate is extrapolated to the basement at 250.7 mbsf, the mean age obtained for the basement is 14.6 Ma, consistent with the ~15-Ma age of the oceanic crust estimated from marine magnetic anomalies (Fig. F9).
The high sedimentation rate in the middle Miocene can be attributed to the productivity being very high while the site was near the paleoequator and to complete preservation on young, shallow seafloor above the lysocline. The more recent slower rates can be attributed to lower productivity away from the equator and to partial dissolution as the seafloor subsided toward the carbonate compensation depth (CCD). The rapid decrease in sedimentation rate in the late Miocene, however, requires an influence additional to the northward drift and subsidence of the Site 1256 ocean crust. A similar pattern of sedimentation rate variations was observed at Sites 844 and 845, with the event that occurred at the end of middle Miocene being referred to as a carbonate crash (Farrell et al., 1995; Lyle et al., 1995). The carbonate crash extended from ~11.2 to 7.5 Ma, with the crash nadir at ~9.5 Ma in the equatorial Pacific (Farrell et al., 1995). Presumably, the beginning of the carbonate crash at Site 1256 corresponds to the base of the diatom mat (115 mbsf), which has an age of ~10.9 Ma. The carbonate content in this interval is ~12 wt%, whereas it averages ~79 wt% below. Although the density of carbonate analyses for Site 1256 is relatively low, the crash nadir would presumably occur in the vicinity of the extreme carbonate low (0.25 wt%) at 89.55 mbsf, which has an age of ~9.6 Ma (within Subchron 4Ar.2n). Late Miocene sedimentation rates in the tropical eastern Pacific are complex in detail and must reflect changes in either productivity or the CCD (Farrell et al., 1995; Lyle et al., 1995). Similar carbonate crash events are also observed in the Caribbean, and it has been suggested that the PCO2 of ocean waters in this region may have been influenced by either the onset of North Atlantic Deep Water formation or the partial closing of the Panama Gateway (Lyle et al., 1995; Shipboard Scientific Party, 1997; Roth et al., 2000).
The sediment deposited at Site 1256 is composed of calcium carbonate, terrigenous grains, and biogenic silica with no discernible metalliferous component (Fig. F24). Total organic carbon concentrations are low throughout the sedimentary sequence. Calcium carbonate is dominant below 115 mbsf, although at 187 mbsf the terrigenous component increases to 20%. At ~115 mbsf, biogenic silica is the most abundant phase. Above 115 mbsf, the proportion of terrigenous material increases toward the surface and is the most important component in lithologic Unit I (above 40.6 mbsf).
Ba/Ti ratios are commonly used as a chemical proxy of the biogenic productivity in the overlying waters because sedimentary calcium carbonate, opal, and organic carbon are commonly altered during diagenesis. Ba/Ti ratios are between 2 and 10 in the upper 112 m of Site 1256, and values below 112 mbsf are within the range of 20 to 30, typical of the eastern equatorial Pacific Ocean (Fig. F25). There was a significant decrease in biogenic production following an extended episode of high productivity between 14.6 and 10.8 Ma, with the exception of a brief interval of low productivity at 12.9 Ma. More significantly, productivity decreased significantly at 10.8 Ma and has remained low to modern day. This change in productivity could be a response to the northward movement of the Cocos plate away from the equator, where productivity and, hence, Ba/Ti are commonly higher.
The primary influence on the interstitial water chemistry at Site 1256 is diffusive exchange between seawater and basement fluids (Fig. F26). A chert bed located at 159 mbsf and observed in the downhole logs is continuous enough to form a low-diffusivity barrier that causes abrupt jumps in cation (e.g., magnesium and calcium) concentrations in many of the depth profiles. The concentrations of lithium and potassium mimic those of magnesium in the interstitial waters and are strongly influenced by diffusion between seawater and basement.
The low organic carbon content limits the extent of pore water sulfate reduction (SO42- > 17.5 mM), as seen in decreases in alkalinity in the pore waters with depth. Alkalinity, conversely, reflects the dissolution and reprecipitation of biogenic calcite, and this process is also shown by the increases in the Sr/Ca ratio of the pore waters with depth. Silica concentrations clearly illustrate the dissolution of biogenic organisms during diagenesis and the precipitation of chert.
Changes in physical properties are mostly gradual downhole, with the exception of those properties that are sensitive to the major lithologic and compositional changes at the lithologic Unit I/Unit II boundary (40.6 mbsf), at the diatom mat (111-115 mbsf), and ~40-45 m above basement (at ~205-210 mbsf), below which chert and glauconite increase. Density and porosity are dominated by gradual compaction, with lesser effects of grain composition (Fig. F27). P-wave velocity is nearly uniform and low at ~1540 m/s, with a slight increase below 205 mbsf. Magnetic susceptibility is moderate in the clay-rich parts of lithologic Unit I and drops to very low in the nannofossil oozes.
Downhole temperatures were measured during APC coring of Hole 1256B at depths of 53.6-158.1 mbsf and in the bottom water. Heat flow values average 113 mW/m2, with a slight but possibly significant decrease downhole (Fig. F28). This value is close to the predictions for conductive cooling of oceanic lithosphere, implying that hydrothermal circulation is no longer a major mechanism of heat transport at Site 1256.
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 to >74 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, which would only be developed significantly off axis (>3-5 km) (e.g., Macdonald et al., 1996).
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 (Subunit 1256D-4c and Unit 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 (Subunit 1256D-4a) (Fig. F34).
The basalts show 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 cryptocrystalline varioles, to the coarser intergranular textures in the lava pond. The basaltic lavas are 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 MAR show bimodal phenocryst abundance with peaks at <10 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 phenocryst phases (clinopyroxene, plagioclase, and olivine) 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 lavas from Site 1256 plot on the plagioclase-olivine join. The Hole 504B dikes mostly contain all three major phases and have slightly higher proportions of clinopyroxene than the Site 1256 lavas. In contrast, the majority of the MAR lavas and dikes plot on the plagioclase-olivine join. With respect to both the phenocryst 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 within 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 or with clinopyroxene, though discrete euhedral platy to stubby 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 crystals may be either normally or reversely zoned or both. Normally zoned plagioclases commonly have clear cores with euhedral outlines, surrounded by thin, 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 a less calcic mantle. In contrast, reversely zoned plagioclase has dusty resorbed cores with euhedral, more calcic mantles, which are 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 abundant phenocryst phase, it is present in >40% of the thin sections examined. Augite phenocrysts are typically 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 is present 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 is present 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 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 basalts contain 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 1256C-18 is a >30-m-thick massive lava body that begins at 280.3 mbsf with a holohyaline-cryptocrystalline lava surface, which develops downhole into intergranular to coarse variolitic fine-grained massive basalt and to cryptocrystalline, recrystallized basalt at the base of the flow (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 is unusual aphanitic basalt consisting of deformed and recrystallized variolitic groundmass and late magmatic veins. In Hole 1256D we encountered a fine-grained massive lava (Unit 1256D-1) from the first core at 276.1 mbsf, ~4 m above the top of Unit 1256C-18, which continued downhole to 350 mbsf. Unit 1256D-1 is lithologically correlated to the thick massive lava unit in Hole 1256C but is much thicker (>74 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 accumulated 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 (>74 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 EPR (Macdonald et al., 1996). The very smooth basement topography at Site 1256 may result from the infill of small ridge flank grabens by large lava flows. The topography required to accumulate such 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/m of ridge crest). Such an eruption would drain a 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/m of ridge crest). Very large lava flows, with much larger volumes than those 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 Holes 1256C and 1256D and analyzed for 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, whereas 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 through 1256D-6 that 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. F40). 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 1256D-1, 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 samples 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 there is a sharp increase in Mg# accompanied by a simultaneous increase in some trace elements with widely varying compatibility (e.g., Zr, Sr, Ni, and Cr) in Unit 1256D-2 (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, 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 biotite and blue-green phyllosilicate biotite (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 biotite has 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 1256D-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 is more common above ~530 mbsf, but despite lower frequency of occurrence, overall higher abundances are present 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 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 abundances 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 basalts recovered from Site 1256 do not exhibit a general decrease in seawater interaction with depth and there is no simple decrease in the number of alteration halos or the amount of iron oxyhydroxide with depth. In contrast, alteration appears to have been concentrated in different zones that may be related to the architecture of the basement such as lava morphology, distribution of breccia and fracturing, basement topography, 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 much smaller amounts 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 include magmatic fabrics, laminations, flattened vesicles, folds, 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 ± magnetite ± plagioclase ± apatite. Glassy veins are almost totally replaced by saponite, which grows with face-controlled geometries and is commonly associated with vesicles and amygdules. Late magmatic veins are either planar features, or fill tension gashes or form sigmoidal pull-aparts. Some late magmatic veins 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 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 reversed 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 dip angles of ~15° ± 5°; however, dip values of ~70° are well represented, mainly in the lowermost 100 m of basement. Other dip angles are represented with nearly uniform 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 between lithologic units 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 dips. 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 many 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 1256D-3 through Subunit 8a and Units 14 through 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 1256C-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, because Site 1256 lies ~5 km east of the Anomaly 5Bn-5Br transition. 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 is the primary source of marine magnetic anomalies.
The basalts of Hole 1256D have bulk densities ranging from 2.55 to 2.98 g/cm3 (average = 2.83 g/cm3). The basalts have generally low porosities, ranging from ~2% to 6%, with higher porosities exhibited by the pillow basalts. The physical property parameters of density, porosity, velocity, NGR, magnetic susceptibility, and thermal conductivity vary systematically downhole and correspond to differences in 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 whether 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 analyses, deoxyribonucleic acid [DNA] extractions, 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 inorganic 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 µm wide x 50-100 µm long with curved and irregular morphologies. These filaments are similar in 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 1256C. Perfluoro(methylcyclohexane) was used as the chemical perfluorcarbon tracer (PFT) and 0.5-µm latex fluorescent microspheres were used as the particulate tracer. The tracer tests 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 above 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 tool string, 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 strings 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 large variability, 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 breccias 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.