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doi:10.2204/iodp.proc.309312.103.2006

Site 1256¹

Expedition 309/312 Scientists²

Preliminary results

Integrated Ocean Drilling Program (IODP) Expeditions 309 and 312 comprise the second and third scientific ocean drilling cruises in a multiphase mission to Site 1256 to recover, for the first time, a complete section of the upper oceanic crust from extrusive lavas down through the dikes and into the uppermost gabbros. Hole 1256D in the eastern equatorial Pacific was started during Ocean Drilling Program (ODP) Leg 206 and is the first basement borehole prepared with the infrastructure desirable for drilling a moderately deep hole into the oceanic crust (~1.5–2 km).

Expedition 309, Superfast Spreading Rate Crust 2, successfully deepened Hole 1256D (6.736°N, 91.934°W) by 503 m to a total depth (TD) of 1255.1 meters below seafloor (mbsf) (1005.1 meters subbasement [msb]). At the end of Expedition 309, Hole 1256D penetrated >800 m of lavas and entered a region dominated by intrusive rocks. Following the completion of a comprehensive wireline logging program, the hole was successfully exited and left clear of equipment with only minor unconsolidated fill at the bottom of the hole.

Expedition 312, Superfast Spreading Rate Crust 3, reoccupied Hole 1256D and deepened the hole by 250.2 m to 1507.1 mbsf (1257.1 msb). The hole passed through the sheeted dikes (345 m thick) and continued 100.5 m into gabbroic rocks. Gabbro was first encountered at 1406.6 mbsf, near the middle of the depth range predicted from geophysical observations. At the end of the expedition, the hole was logged with a full suite of wireline tools and left open and ready for further drilling into the plutonic foundation of the oceanic crust.

Hole 1256D is now the fourth deepest hole drilled into oceanic basement since the inception of scientific ocean drilling in 1968 and the second deepest penetration into in situ ocean crust behind Hole 504B, drilled during the Deep Sea Drilling Project (DSDP) and continued during ODP.

Descriptions of cores recovered during Expedition 309/312 coupled with Leg 206 observations have led to a preliminary subdivision of the upper oceanic crust at Site 1256. The lava pond (250–350 mbsf) includes a massive ponded flow >74 m thick that overlies an interval of massive sheet and pillow flows with flow inflation structures (350–534 mbsf) that require eruption onto a subhorizontal surface. The great thickness of the massive ponded lava required significant topographic relief to pool the lavas, and distinctive textures of the inflated flows suggest that these two lava groups formed off axis, giving a total thickness of off-axis lavas of 284 m. Sheet and massive flows (534–1004 mbsf) make up the majority of the extrusive sequence in Hole 1256D. Cryptocrystalline to microcrystalline sheet flows, tens of centimeters to <3 m thick and commonly with glassy subhorizontal margins, are the dominant lava morphology. Massive fine-grained lavas (defined as >3 m thick), although subordinate, become more abundant with depth. The lithologic transition zone (1004–1061 mbsf) begins with a cataclastic massive unit that comprises subvertically oriented cryptocrystalline basalt clasts hosted by highly altered, incipiently brecciated, fine-grained basalt crosscut by numerous fine veins and cataclastic stringers. At 1018 mbsf, the first subvertical intrusive contact was recovered during Expedition 309. Below this depth, dike chilled margins become more common downhole, although extrusive textures and vesicles are present to 1061 mbsf. Subvertical fracture sets, possibly indicative of nearby diking, are common below ~900 mbsf. Breccias of various styles are common in the transition zone, including a spectacular mineralized volcanic breccia (~1028 mbsf) with hyaloclastite and basaltic clasts cemented by subgreenschist-facies minerals.

The upper boundary to the sheeted dikes is defined by common subvertical, intrusive dike contacts and the absence of sheet flows or massive basalts below 1611 mbsf. Massive basalts of the sheeted dikes are commonly aphyric and nonvesicular. Where they are fine grained (the coarsest basalts), they have holocrystalline, doleritic groundmass textures.

The basal 50 m portion of the sheeted dikes, from 1348.3 to 1406.6 mbsf (Units 1256D-78 through 80b), is designated as the granoblastic dikes. The rocks are highly to completely altered and are locally recrystallized to granoblastic textures containing secondary clinopyroxene.

Gabbroic rocks were first encountered at 1406.6 mbsf, and the plutonic section extends from 1406.6 mbsf to the bottom of the hole at 1507.1 mbsf. This section consists of a 52.3 m thick upper gabbro unit (Gabbro 1; 1406.6–1458.9 mbsf) and a ~24 m thick lower gabbro unit (Gabbro 2) separated by a metamorphosed dike screen with granoblastic textures. Gabbroic rocks range from gabbro to disseminated oxide gabbro, oxide gabbro, orthopyroxene-bearing gabbro, and trondhjemite and to quartz-rich oxide diorite. The base of the section includes gabbronoritic rocks of uncertain origin that are perhaps intrusive gabbronorite or metamorphosed dikes. They are cut by a late, relatively unaltered basalt dike.

The overall mineralogical characteristics of upper basement drilled during Expedition 309/312 are similar to those of cores from the upper part of Hole 1256D recovered during Leg 206. There are, however, some important differences. Phenocrysts are much less abundant in the Expedition 309/312 cores with >80% aphyric basalts (Leg 206 = <40%). Also, olivine is the dominant phenocryst phase in the upper 500 m of Hole 1256D, but phenocrysts, when present, are most commonly plagioclase in the lower portion of the hole. Basalts show evidence for repetitive fractionation and replenishment cycles downhole. Trace element concentrations are within one standard deviation of average East Pacific Rise (EPR) mid-ocean-ridge basalt (MORB), albeit on the relatively trace element–depleted side. Bulk compositions of the two gabbroic bodies fall at the primitive end of the range of compositions for lavas and dikes but are too evolved to be in equilibrium with mantle olivine. This means that cumulates must have formed deeper in the crust and the lower crust cannot have formed solely by subsidence of such high-level evolved melt lenses as penetrated in Hole 1256D.

Hole 1256D is only the second penetration of the transition from low-temperature alteration to high-temperature hydrothermal alteration in a continuous section of oceanic crust. Prior to Expedition 309, this transition had only been described in Hole 504B. Rocks that reacted with seawater at low temperatures and were altered under conditions similar to those encountered by Leg 206 cores are present to ~965 mbsf. Black, brown, and mixed alteration halos and dark patches are common throughout the rocks from 752 to 918 mbsf and are related to veins filled by saponite, celadonite, and iron oxyhydroxides. Black halos, formed in very young ocean crust, tend to be overprinted by brown halos that result from subsequent interaction with cold oxidizing seawater. From 918 to 964 mbsf, black, brown, and mixed halos are absent and dark gray background alteration related to abundant saponite and pyrite is ubiquitous. The interval from 964 to 1028 mbsf marks an alteration transition characterized by the presence of pyrite-rich alteration halos and mixed-layer chlorite-smectite instead of pure saponite. Anhydrite is common from this depth to the bottom of the hole.

The alteration transition to hydrothermal conditions is complete by ~1028 mbsf with the occurrence of the mineralized volcanic breccia. The first occurrences of actinolite, prehnite, titanite, and epidote—minerals indicative of hydrothermal alteration under subgreenschist- to greenschist-facies conditions—are recorded at 1027, 1032, 1051, and 1095 mbsf, respectively. In this part of the crust, green-gray vein halos and patches are common, with 10%–100% chlorite, actinolite, titanite, albite, pyrite ± minor quartz, chalcopyrite, and prehnite replacing plagioclase and clinopyroxene and filling interstitial spaces. Actinolite becomes abundant below ~1300 mbsf in the dikes, and hornblende and secondary plagioclase are present below ~1350 mbsf, reflecting a steep thermal gradient. Granoblastic dikes in the basal 50 m of the sheeted dikes contain irregularly distributed localized granoblastic patches, where the rock is completely recrystallized to secondary plagioclase and equant secondary clinopyroxene, magnetite, ilmenite, and rare orthopyroxene. The mineralogy and textures indicate recrystallization at high temperatures related to underlying gabbros. Gabbros are highly altered to amphibole, chlorite, plagioclase, titanite, and minor laumontite and epidote, with chlorite and epidote more abundant in the lower gabbro. The intensity of gabbro alteration is strongly dependent on the grain size of the rock, with coarser material being more altered.

Volcanic rocks at Site 1256 are less altered compared to most other basement sites (e.g., ODP Sites 417 and 418 and Holes 504B and 896A). Within the extrusive lavas, Hole 1256D contains a much smaller proportion of black, brown, and mixed alteration halos compared to Holes 504B and 896A, and this alteration style is not systematically observed within the uppermost region of the crust. Instead, such alteration by relatively oxidizing fluids occurs irregularly with depth and is most commonly associated with well-developed steeply dipping vein networks. As observed during Leg 206, the amount of calcite within Hole 1256D is very low compared to other basement penetrations. Although pyrite is abundant in Expedition 309 cores, black smoker fluid–derived quartz-epidote-chalcopyrite stockwork mineralization as present in Hole 504B was not encountered in the transition from extrusive to intrusive rocks or the change from low-temperature to hydrothermal alteration. Anhydrite, which is sparse in Hole 504B, is more common at Site 1256. Although hydrothermal alteration is generally similar in the dikes of Holes 1256D and 504B, the thermal gradient was much steeper at Site 1256 because of differences in the thicknesses of the dike sections.

Basalts recovered during Expedition 309 exhibit brittle structures and minor brittle-ductile structures. The main structural features are represented by veins, vein networks, cataclastic zones, shear veins, microfaults, and breccia. In the sheet and massive flows, structures and fracturing are heterogeneously partitioned and are most intensely developed at the top of the massive flows. Vertical sets of veins, cataclastic zones, and shear veins are present in massive units, whereas breccias are more common in sheet flows. Vertical vein sets become more common below ~900 mbsf. Most structures are related to lava cooling and are represented by curved, radial, Y-shaped, and irregular veins filled with secondary minerals. Chilled margins are common in the sheeted dikes, and overall structures are generally steeply dipping. Gabbroic rocks contain fabrics and structures related to melt transport in addition to brittle fractures.

The intense drilling overprint and uncertainty about how completely the overprint has been removed by demagnetization necessitates caution when interpreting paleomagnetic results from Hole 1256D. Because of the equatorial paleolatitude of the site, polarity remains ambiguous until absolute declinations can be obtained. The generally positive inclinations measured on many samples were not expected for the low paleolatitude, and the most likely explanation is that a significant portion of the drilling overprint remains on these samples.

Magnetic intensities in Expedition 309 samples show a recurrent concave pattern with relatively high intensities at the upper and lower boundaries of igneous cooling units and lower intensities in the unit interiors. About 70% of the igneous units and subunits show the repeated concave patterns, suggesting the presence of multiple cooling units, ~1.0 ± 0.5 m thick, within many of the igneous units. Downhole variations in magnetic patterns for Expedition 312 samples are minor, with demagnetization behavior of dikes indistinguishable from that of gabbros.

P-wave velocities (V_P) of Expedition 309/312 basalts range from 4.8 to 6.1 km/s, with an average of 5.5 ± 0.3 km/s, similar to velocities estimated from regional seismic reflection data. From 752 mbsf, average V_P increases ~0.05 km/s for each 50 m downhole to ~6.1 km/s at 1240 mbsf. Average V_P is slightly higher below 1061 mbsf (5.8 ± 0.1 km/s) than above (5.4 ± 0.3 km/s). Porosities range from 2% to 14%, with an average of 4%. There is a decrease in porosity of the massive units from above 1061 mbsf to those below this level of 4% ± 1% to 2% ± 1%. Average thermal conductivity in the sheet and massive flows is 1.8 ± 0.2 W/(m·K), but there is a significant increase in thermal conductivity starting in the transition zone and a distinct steplike increase to 2.1 ± 0.1 W/(m·K) at the top of the sheeted dikes. Both bulk and grain densities vary more in the gabbro than in the lower dikes, which is consistent with the observed variability in petrology and alteration. For all Expedition 312 samples, average bulk density is 2.97 ± 0.09 g/cm³, grain density is 2.99 ± 0.08 g/cm³, and porosity is 1.2% ± 1.4%. Physical properties change downhole from dikes into gabbros; velocity and density decrease as porosity increases.

Seismic results place the Layer 2–3 transition at 1450–1750 mbsf at Site 1256. Expedition 312 results show that the transition from Layer 2 to Layer 3 at Site 1256 does not correlate with the transition from dikes to gabbro. Because the Layer 2–3 transition occurs deeper than the bottom of Hole 1256D, however, we cannot draw any conclusions about what this seismic transition corresponds to in the crust.

Following the completion of drilling in Hole 1256D during Expedition 312, a complete suite of geophysical wireline logs confirmed that the hole is in good condition. Caliper readings from both the triple combination (triple combo) and Formation MicroScanner (FMS)-sonic tool strings show generally good borehole conditions with a diameter typically between 11 and 14 inches and with the smaller diameters prevailing in the lower 150 m of the hole. However, comparison of the pre- and postdrilling hole caliper of the upper 500 m of basement shows an enlargement of Hole 1256D due to drilling, with a number of intervals quite strongly eroded. Borehole deviation reaches ~5° at 1507.1 mbsf. A vertical seismic profile (VSP) experiment was conducted during Expedition 312, and velocities generally follow those of the sonic log and discrete samples. Preliminary analysis of downhole geophysical measurements and images recorded in Hole 1256D show a high degree of variation, reflecting the basement lithologies, and petrophysical intervals can be distinguished that closely match the lithologic subdivisions developed from core observations.

¹Expedition 309/312 Scientists, 2006. Site 1256. In Teagle, D.A.H., Alt, J.C., Umino, S., Miyashita, S., Banerjee, N.R., Wilson, D.S., and the Expedition 309/312 Scientists. Proc. IODP, 309/312: Washington, DC (Integrated Ocean Drilling Program Management International, Inc.), 1–549. doi:10.2204/​iodp.proc.309312.103.2006

²Expedition 309/312 Scientists’ addresses.

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