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doi:10.2204/iodp.sp.301.2004
Our highest-priority objectives during Expedition 301 are those on Second Ridge, where drilling of new holes into basement, offset 1.0–2.4 km from existing Cork observatories, will allow evaluation of basement alteration, microbiology, solid and fluid chemistry, and crustal hydrogeologic properties (Figs. F3, F4, F5, F6). We are also prepared to drill into an upflow region on the first buried basement ridge (First Ridge), where the extent and significance of alteration and the likely importance of along-strike hydrothermal flow can be evaluated (Figs. F7, F8). Finally, we are prepared to drill into deeply buried basement 125–145 km from the spreading center, where basement temperatures approach and exceed 100°C, to evaluate the continuing influences of hydrothermal circulation on microbiology, fluid chemistry, and crustal evolution (Deep Ridge; DR) (Fig. F9). Site locations are summarized in Table T1. All sites present exciting opportunities, but only SR drilling will eventually allow completion of crustal-scale, cross-hole, multidisciplinary experiments. The other sites provide outstanding second-priority opportunities of various duration (1–24 days).
The operational plan for Expedition 301 is complex but contains considerable flexibility that will allow shipboard personnel to respond to conditions encountered during the expedition (Table T2). In particular, we will have at least two opportunities to drill into the upper oceanic crust and establish new basement observatories. We will also have the opportunity to collect sediment samples at one or more sites, using a combination of advanced piston corer/extended core barrel (APC/XCB) and rotary core barrel (RCB) coring, and to core a long interval into basement. Please see "Drilling Strategy" for more detailed discussion of priorities, options, and contingencies.
Our highest priorities during Expedition 301 are to replace existing Cork systems at Sites 1026 and 1027, drill two new basement holes along the second buried basement ridge, and install new Cork observatories. Primary scientific objectives at the SR sites also include characterization of upper basement rocks (through coring, sampling, and downhole logging) and conduction of short-term packer experiments to determine near-borehole permeability. We will have an opportunity to collect sediment cores on the Second Ridge, although it may be preferable to devote coring time to the First Ridge, where fluids are known to seep upward from basement and basement conditions are similar but the sediment section is thinner. Second Ridge Cork observatories will be used to monitor borehole thermal conditions immediately after installation, sample borehole fluids and inject tracers, and host incubation substrate for microbiological colonization experiments. Seafloor pressure gages, data loggers, and fluid samplers will be attached to the observatories by remotely operated vehicle (ROV) after completion of drilling operations.
The original proposal (545-Full3) planned for all operations at proposed Site SR-1 to occur in a single hole located 1 km south of Hole 1026B, including RCB coring of sediment and basement, reaming, casing, logging, and observatory installation. As a result of precruise discussions with IODP engineering and operations personnel, we have decided to separate shallower and deeper basement operations within two boreholes and install two observatories at Site SR-1. This should provide a better chance for success in achieving drilling, coring, downhole logging, and observatory installation objectives, allowing for both monitoring and penetration through an anticipated unstable ("rubble") zone in upper basement, commonly encountered when drilling young oceanic crust.
The deeper hole will be cased through the uppermost basaltic crust to avoid problems with hole stability and lost circulation and then will be cored to total depth, ideally 500 m or more into basement. The hole will be logged using conventional wireline tools and a borehole televiewer to delineate fine-scale lithostratigraphy, alteration patterns, and fracture distributions. If the hole produces fluids during drilling operations, samples can be collected in the open hole. A conventional vertical seismic profile (VSP) will be run to assess the depth extent of the uppermost extrusive crustal layer. Packer experiments will be run in straddle mode to evaluate near-hole permeability distribution within distinct crustal intervals, and a multilevel Cork-II, housing independent samplers and sampling lines, will be installed to isolate three crustal intervals. We originally hoped for 600 m of basement penetration, but there will be insufficient time to achieve this goal during Expedition 301. Instead, we will have to balance the extent of basement penetration at SR sites against time required to complete downhole measurements, install four Cork observatories, and collect sediment samples. Actual basement penetration at the deeper SR hole will depend on the conditions encountered during the expedition and discussions with shipboard personnel.
The shallower SR basement hole will be drilled a few tens of meters into uppermost basement. Based on consideration of conditions encountered during drilling of the deeper hole and during drilling of nearby Holes 1026B, we may attempt to core uppermost basement in the shallower Site SR-1 hole. However, given expected low recovery in rubbly upper basement rocks, the potential for losing the hole if difficult conditions are encountered, and the drilling depth achieved and conditions encountered in the deeper Site SR-1 hole, we may elect to not attempt basement coring in the shallower Site SR-1 hole. In any case, the open hole interval will be too short and unstable for logging. Once drilling is completed in the shallower Site SR-1 hole, casing will be installed to help hold the hole open, packer tests will be run, and a single-level Cork-II observatory will be installed.
Sediment coring at Site SR-1 is tentatively scheduled to occur after the first half of the expedition, so we can be sure to achieve the primary basement and observatory objectives. However, many basement and observatory operations require a calm sea state, whereas sediment coring can be done when conditions are rougher, so sediment coring may be completed earlier in the expedition depending on weather. It may also be desirable to break up complex Cork-handling activities with sediment coring. The Shipboard Scientific Party may elect to focus sediment coring at First Ridge, rather than at Second Ridge because there is (1) known fluid seepage from basement at First Ridge and (2) thinner sediment cover that would allow a short transect of holes to be completed in the time required to core a single SR hole. However, completing all sediment coring at Site FR-1 would not provide sediment temperature data at Site SR-1, so additional time would be required for this.
We may drill one or more sediment and shallow basement holes along the first buried basement ridge east of the spreading center to evaluate sediment properties, document fluid chemistry and evidence for along-strike fluid flow, and determine the nature of hydrothermal alteration and microbiology in uppermost basement. Sediment thickness is 40–60 m where basement comes closest to the seafloor. This work will require 1.0–5.1 days, depending on the extent of drilling and sampling. This small amount of time may become available if operations in the SR region are completed quickly or if some operations in the SR region are prevented because of weather conditions, drilling problems, or other difficulties. Alternatively, the Shipboard Scientific Party may, based on scientific objectives, elect to focus sediment coring at First Ridge instead of at Second Ridge.
Drilling a short series of shallow holes into First Ridge would allow us to address several questions:
Drilling will provide important information on the end-member composition of reacted fluid, alteration within overlying sediments, the geochemical and physical state of the altered basalt within an upflow zone, and the nature of the biological interactions with fluids, sediments, and basalt.
We may complete APC/XCB and/or RCB coring at two main locations at Site FR-1. APC/XCB Hole FR-1A is located to pass through the center of a narrow acoustic washout in an area where surface cores have detected chemical evidence for slow seepage of hydrothermal fluids (Spinelli et al., 2004; Zühlsdorff and Spiess, 2001). APC/XCB Hole FR-1B is located just west and outside of the acoustic washout. APC/XCB coring will provide excellent recovery above basement and limited basement recovery with a hard-formation XCB shoe. RCB Hole FR-1C would be drilled adjacent to Hole FR-1A, allowing penetration into uppermost basement (two to three cores). Within these holes, sediments will be heavily sampled for fluid chemistry and microbiology. Basement in this upflow area should be highly altered, and since this basement ridge is overpressured (Giambalvo et al., 2000; Spinelli et al., 2004; Stein and Fisher, 2003) and permeable (based on nearby measurements made during Leg 168), a crustal hole may produce fluid once basement is penetrated, potentially allowing fluid and biological sampling in the open hole. Should time allow, additional APC/XCB coring can be done along or across strike of the trend of the First Ridge.
Drilling through deeper sediments and into basement east of the Leg 168 transect would allow an assessment of crustal evolution at greater temperatures, basement ages, and depths of basement burial. Here we can assess the thermal state of young lithosphere and geochemical conditions that control microbial activity far from the spreading axis, where thick sediments and a lack of along-strike outcrops greatly restrict or eliminate continued thermal, fluid, and solute exchange with the overlying ocean. Two sites are located at the tops of buried basement ridges ~125 and ~145 km from the ridge axis: Sites DR-1 and DR-2, respectively (Figs. F1, F2, F9).
Based on experience elsewhere, these buried basement ridges are probably overpressured and should eventually produce samples of altered hydrothermal water and associated microbiological material. Production of uncontaminated crustal fluids could require thermal equilibration of the boreholes below Corks. DR sites are second priority sites and would require considerably more time for successful completion than would FR sites. Work at the DR sites will be considered only if operations at the SR sites cannot be completed and a significant amount of time remains available during the expedition.
Site DR-1 is located where the estimated basement temperature is ~90°–100°C, beneath 650 ms TWT (610 m) of sediment. A single RCB hole drilled in this location could permit core collection, allow measurement of sediment temperatures, and provide access to uppermost basement for coring. Should time allow, a free-fall funnel or a complete reentry cone and casing could permit deeper basement penetration and fluid and microbiological sampling.
Site DR-2 is located where the estimated basement temperature is ~140°C, below 900 ms TWT (890 m) of sediment. As at Site DR-1, a single RCB hole would permit collection of cores and water samples and measurement of sediment temperatures. Use of a free-fall funnel or a complete reentry system are also possible. The best options for recovering pore fluids indicative of uppermost basement at the DR sites are likely to be sampling of the deepest sediments immediately above basement and/or installation of complete reentry and Cork systems, followed by borehole reequilibration and long-term sampling.