Previous   |    Next

doi:10.2204/iodp.proc.309312.103.2006

Ocean crust formed at a superfast spreading rate: deep drilling of ocean basement in Hole 1256D

Drilling in Hole 1256D was undertaken on two expeditions during 2005, Expedition 309 (July–August 2005), closely followed by Expedition 312 (November–December 2005). From the beginning of planning, these expeditions were regarded as a single scientific program with combined shipboard scientific and sampling parties. This section provides a brief overview of the coring accomplished on these expeditions and a summary of the lithostratigraphy of Hole 1256D as understood at the end of operations on Expedition 312, building on results of Leg 206 (Wilson, Teagle, Acton, et al., 2003). Following a description of water sampling and preliminary logging undertaken before drilling during Expedition 309 (see “Predrilling experiments”), we present the core descriptions, shipboard analyses, and logging accomplished during Expedition 309 (see “Expedition 309”). This is followed by core descriptions, shipboard analyses, and wireline logging accomplished during Expedition 312 (see “Expedition 312”). Both sections build upon and include observations from previous deep drilling at Site 1256, providing overviews of prior results, but we have not attempted to integrate the discipline-specific observations of each expedition. These are specific to the scientific expertise of the shipboard parties, and postcruise integration could result in a loss of scientific information. An integrated summary of results is presented in the “Expedition 309/312 summary” chapter.

During Expedition 309, Hole 1256D was reentered with an RCB assembly and C-9 coring bit at 2325 h on 18 July 2005 and ~27 m of loose fill was cleared from the bottom of the hole so that it was open to the full depth achieved during Leg 206 (752 mbsf; 502 msb). Rotary coring of the basement continued until 20 August (~33 days), when the hole was conditioned for wireline logging operations (see “Expedition 309” in “Operations”). A total of nine C-9 RCB hard formation coring bits were used, and Hole 1256D was deepened by ~503 m to 1255.1 mbsf (~1005.1 msb) with an average recovery rate of 36.3%.

Hole 1256D was reentered at 2030 h on 15 November 2005 during Expedition 312. Following ~5.5 days of remedial washing and reaming, the hole was opened and cleared of debris to the depth reached during Expedition 309. Rotary coring of basement proceeded for ~23 days, interrupted by ~5 days of fishing and milling when the cones were lost from a C-7 RCB coring bit. Eight RCB coring bits (seven C-9 and one C-7) were used, and the hole was deepened by 250.2 m to 1507.1 mbsf (1257.1 msb; average recovery = 18.5%), reached at 0250 h on 19 December. The hole was then prepared for a full suite of wireline logging. Logging tools did not penetrate the hole past 1432 mbsf (1182 msb), indicating an obstruction or fill reaching ~75 m above the bottom of the hole. The hole was exited cleanly and remains open for further drilling.

At 1257.1 msb, Hole 1256D is the fourth deepest hole drilled into oceanic basement since the launch of scientific ocean drilling in 1968 (Fig. F32). ODP Hole 735B and IODP Hole U1309D were initiated in tectonically exposed lower oceanic crust near ridge-transform fault intersections on the Southwest Indian Ridge (Robinson, von Herzen, et al., 1989; Dick, Natland, Miller, et al., 1999) and the Mid-Atlantic Ridge (Blackman, Ildefonse, John, Ohara, Miller, MacLeod, et al., 2006), respectively. Although these sites provide critical windows into lower crustal processes at slow-spreading ridges, the relevance of these penetrations to intact ocean crust far removed from fracture zones remains uncertain. Hole 504B, deepened during seven DSDP and ODP legs, was drilled into 6.9 Ma crust on the southern flank of the intermediate-spreading-rate Costa Rica Rift and remains the deepest penetration of in situ ocean crust. Prior to Expedition 309, Hole 504B was the only hole to sample the transition from extrusive rocks to sheeted dikes (Alt, Kinoshita, Stokking, et al., 1993). At the end of Expedition 309, Hole 1256D had penetrated >750 m of extrusive lavas and entered a region dominated by intrusive rocks. Following Expedition 312 operations, the hole extends through the 345.7 m thick sheeted dike complex and 100.5 m into gabbroic rocks with basalt dike screens. Gabbroic rocks were first encountered at 1406.6 mbsf (1156.6 msb), within the depth range predicted from geophysical models (1025–1300 msb). Coring in the very hard rock of the dikes increased in difficulty downhole, with average ROPs and recovery in the sheeted dikes of 1 m/h and 33% (weighted averages of Expedition 309/312 data), respectively. ROPs in gabbroic rocks near the bottom of the hole, however, were greater than in the lowermost dikes and dike screens. Hole 1256D is open and ready for further drilling into the plutonic foundation of the oceanic crust.

Preliminary subdivision of upper oceanic crust at Site 1256

To facilitate description and discussion of the crustal stratigraphy at Site 1256 and to assist in the interpretation of cores recovered during Expedition 309/312, we present a preliminary subdivision of the rocks whose detailed descriptions are presented in the following sections (Table T10; Fig. F33).

Lava pond

The lava pond caps the uppermost crust at Site 1256. This domain includes Units 1256C-1 through 18 and 1256D-1 (~250–350.3 mbsf). The uppermost lavas were not recovered in Hole 1256D because 16 inch casing was set 17 m into basement and the cased interval was not cored. In Hole 1256C, the rocks immediately below the sediments comprise thin basaltic sheet flows a few tens of centimeters to ~3 m thick separated by chilled margins and rare intervals of recrystallized sediment (Units 1256C-1 through 17). The massive ponded flow, senso stricto (Units 1256C-18 and 1256D-1), is defined at its top by a ~75 cm rind of glassy to cryptocrystalline aphyric basalt that overlies ~30 and ~74 m of fine-grained basalt in Holes 1256C and 1256D, respectively. The massive ponded flow becomes abruptly cryptocrystalline ~1.5 m from the base of the flow. Although the massive flow is much thicker in Hole 1256D than in Hole 1256C, it is interpreted as a single lava body whose interior was liquid at the same time in both locations. The dramatic increase in thickness over 30 m of lateral distance and a total thickness >74 m requires at least this much paleotopography in order to pool the lava. On fast-spreading-rate ridges, such topography does not normally develop until ~5–10 km from the axis (e.g., Macdonald et al., 1989), and we suspect that these lavas flowed a significant distance off axis before ponding in a faulted depression.

Inflated flows

Immediately underlying the lava pond is a sequence of massive flows, pillow lavas, and sheet flows (Units 1256D-2 through 15; 350.3–533.9 mbsf) grouped together as the inflated flows. Although rocks exhibiting a number of eruptive styles are included here, the critical criterion for subdivision is the occurrence of subvertical elongate fractures filled with quenched glass and hyaloclastite (e.g., Sections 206-1256D-21R-1 and 40R-1) at the top of the lava flows. These features are interpreted to indicate flow-lobe inflation that requires eruption onto a subhorizontal surface less than a few degrees (Umino et al., 2000, 2002). This implies that these lavas formed off axis, suggesting a total thickness of off-axis lavas of 284 m.

Sheet and massive flows

The bulk of the extrusive lavas at Site 1256 are included in the sheet and massive flows (Units 1256D-16 through 39b; 533.9–1004.1 mbsf). This sequence consists of sheet flows tens of centimeters to ~3 m thick with subordinate massive flows >3 to 26 m thick (defined as >3 m), and uncommon breccias. The flows are aphyric to sparsely phyric, generally cryptocrystalline to microcrystalline basalts. Units are defined by the presence of chilled margins or by grain size or lithological variations. Throughout this interval, glassy chilled margins are common. Fresh glass is rare below 941 mbsf, with only a few occurrences strongly altered to saponite/​chlorite below this level.

Transition zone

It is the very essence of a transitional sequence that its boundaries are loosely defined and subjective. In Hole 1256D, the transition zone, from Unit 1256D-40 to 44a (1004.1–1060.9 mbsf), is identified by a number of criteria and different rock types as opposed to one specific feature. Shore-based analysis of wireline logs or further petrographic and geochemical investigations may help refine the boundaries of this zone. Most of the rocks within the transition zone are aphyric, cryptocrystalline sheet flows. The top of the transition zone is defined by a cataclastic massive unit at Section 309-1256D-117R-1, 85 cm. This unit includes subvertically oriented cryptocrystalline basalt clasts within a very highly altered fine-grained basalt that has been incipiently brecciated and deformed along numerous fine veins and cataclastic stringers. Core 309-1256D-120R includes the first subvertical intrusive contact; dike chilled margins become more common downhole, although extrusive textures and vesicles are still present. Breccias of various styles are common in the transition zone, including a spectacular mineralized volcanic breccia that comprises Unit 1256D-42a (interval 309-1256D-122R-1, 20 cm, through 122R-2, 30 cm). The transition zone contains secondary mineral assemblages (chlorite-smectite, albite, chlorite, actinolite, anhydrite ± minor prehnite, epidote, and laumonite) indicative of hydrothermal alteration at subgreenschist- to greenschist-facies temperatures. It should be noted that subvertical fracture sets possibly indicative of diking into the host rocks near Hole 1256D are common from ~900 mbsf.

Sheeted dike complex

Hole 1256D penetrates a 345.7 m thick sheeted dike complex from 1060.9 to 1406.6 mbsf (Units 1256D-44a through 80b). The upper boundary is defined by a change from sheet flows to massive basalts (Unit 1256D-44a; Core 309-1256D-129R). Below that level, subvertical intrusive contacts are increasingly common. These can be sharp or irregular and lobate, the latter style indicating the intrusion of magma into hypersolidus rocks (e.g., interval 309-1256D-149R-1, 30–97 cm; 1156 mbsf). No unambiguous indicators of eruption were observed below 1061 mbsf. Groundmass grain sizes vary from glassy to microcrystalline to fine grained with holocrystalline doleritic texture in the coarser rocks. No fresh glass was found in the sheeted dikes, but altered glass is present along some dike chilled margins and associated breccias in the upper half of the dike section. There is a step change in physical properties downhole into the sheeted dikes, with significant increases in average thermal conductivity (from 1.8 ± 0.2 to 2.1 ± 0.1 W/[m·K]) and seismic velocity (5.4 ± 0.3 to 5.8 ± 0.1 km/s). The average porosity of massive units decreases from 4% ± 1% to 2% ± 1% across the 1060.9 mbsf boundary. Subvertical contacts that grade inward from glassy chilled margins to microcrystalline to fine-grained massive basalt were not recovered from the upper half of the sheeted dikes (cored during Expedition 309) but are common in the lower portion (cored during Expedition 312). Greenschist and subgreenschist-facies minerals occur in the upper dikes, and alteration intensity and grade generally increase downhole in the dikes below ~1300 mbsf. The amount of recrystallization and the abundance of actinolite increase below this depth, and secondary plagioclase and hornblende first appear in small amounts below ~1350 mbsf.

Granoblastic dikes

In the lower portion of the sheeted dikes, from 1348.3 to 1406.6 mbsf (Units 1256D-78 through 80b), the rocks are highly to completely altered and locally recrystallized to granoblastic textures, leading to their designation as the granoblastic dikes. These rocks contain irregularly distributed granoblastic patches, within which 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.

Plutonic section

The first gabbroic rock was encountered at 1406.6 mbsf (Unit 1256D-81) in an intrusive contact with the overlying sheeted dikes. The plutonic section extends from 1406.6 mbsf to the bottom of the hole at 1507.1 mbsf (Units 1256D-81 through 95). This section consists of a 52.3 m thick upper gabbro unit (Gabbro 1; 1406.6–1458.9 mbsf; Units 1256D-81 through 89b) and a 24 m thick lower gabbro unit (Gabbro 2; 1483.1–1507.1 mbsf; Units 1256D-91 through 95) that are intrusive into basaltic dikes. The gabbroic rocks are fine to coarse grained (mostly medium grained) and range from gabbro to disseminated oxide gabbro, oxide gabbro, orthopyroxene-bearing gabbro, and quartz diorite and to quartz-rich oxide diorite. The rocks are highly altered to amphibole, chlorite, plagioclase, titanite, and minor laumontite and epidote, with chlorite and epidote more abundant in Gabbro 2. Stoped basalt fragments are common in the margins of Gabbro 2.

The two gabbro units are separated by a 24.2 m thick dike screen (Unit 1256D-90a), consisting of fine-grained, highly to completely altered cryptocrystalline basalt dikes. These rocks commonly display granoblastic textures like those in the basal sheeted dikes. At the base of the drilled section, a gabbronorite (Unit 1256D-94) may be an intrusive unit or a metamorphosed dike. This lowermost section contains a late dike (Unit 1256D-95; 1502.6 mbsf). This rock is highly altered but is interpreted as late because it does not exhibit granoblastic texture and hence must postdate the high-temperature metamorphism associated with intrusion of the gabbros.

Top of page   |    Previous   |    Next