Teagle, Damon A. H. et al. (2010): Integrated Ocean Drilling Program Expedition 335 scientific prospectus; superfast spreading rate crust 4

Leg/Site/Hole:
IODP 309 U1256
IODP 312 U1256
Identifier:
2011-001035
georefid

1932-9415
issn

10.2204/iodp.sp.335.2010
doi

Creator:
Teagle, Damon A. H.
University of Southampton, National Oceanography Centre, Southampton, United Kingdom
author

Ildefonse, Benoit
Universite de Montpellier II, France
author

Blum, Peter
Integrated Ocean Drilling Program, United States
author

Identification:
Integrated Ocean Drilling Program Expedition 335 scientific prospectus; superfast spreading rate crust 4
2010
Scientific Prospectus (Integrated Ocean Drilling Program)
IODP Management International, College Station, TX, United States
335
55 pp.
Integrated Ocean Drilling Program (IODP) Expedition 335 (14 April-4 June 2011) will be the fourth ocean cruise of the "Superfast" campaign to drill a deep hole into intact oceanic basement and will return to Ocean Drilling Program (ODP) Hole 1256D to deepen this scientific reference penetration a significant distance into cumulate gabbros. Cores and data recovered during the Superfast 4 expedition will provide hitherto unavailable observations that will test models of the accretion and evolution of the oceanic crust. Site 1256 was specifically located on oceanic crust that formed at a superfast spreading rate (>200 mm/y) to exploit the observed relationship between spreading rate and depth to axial low velocity zones, thought to be magma chambers, seismically imaged at active mid-ocean ridges. This was a deliberate strategy to reduce the drilling distance to gabbroic rocks because thick sequences of lavas and dikes have proved difficult to penetrate in past. ODP Leg 206 (2002) initiated operations at Site 1256, including the installation in Hole 1256D of a reentry cone with 16 inch casing inserted through the 250 m thick sedimentary cover and cemented into basement to facilitate deep drilling. The hole was then cored approximately 500 m into basement. IODP Expeditions 309 and 312 (2005) successfully completed the first sampling of an intact section of upper oceanic crust from lavas, through the sheeted dikes, and into the upper gabbros. Hole 1256D now penetrates >1500 meters below seafloor (mbsf) and >1250 m subbasement and currently resides in the dike-gabbro transition zone. The first gabbroic rocks were encountered at 1407 mbsf. Below this lies a approximately 100 m complex zone of fractionated gabbros intruded into contact metamorphosed dikes. Although previous cruises achieved the benchmark objective of reaching gabbro in intact ocean crust, critical scientific questions remain. These include the following: 1. Does the lower crust form by the recrystallization and subsidence of a high-level magma chamber (gabbro glacier), crustal accretion by intrusion of sills throughout the lower crust, or some other mechanism? 2. Is the plutonic crust cooled by conduction or hydrothermal circulation? 3. What is the geological nature of Layer 3 and the Layer 2/3 boundary at Site 1256? 4.What is the magnetic contribution of the lower crust to marine magnetic anomalies? Hole 1256D is poised at a depth where samples that should conclusively address these questions can be obtained, possibly with only a few hundred meters of drilling. Importantly, as of the end of Expedition 312, the hole was clear of debris and open to its full depth. Increased rates of penetration (1.2 m/h) and enhanced core recovery (>35%) in the gabbros indicate that this return to Hole 1256D could deepen the hole >300 m into plutonic rocks, past the transition from dikes to gabbro, and into a region of solely cumulate gabbroic rocks
English
Coverage:Geographic coordinates:
North:6.4400
West:-91.5600East: -91.5600
South:6.4400

Solid-earth geophysics; Igneous and metamorphic petrology; boreholes; cores; cumulates; downhole methods; drilling; East Pacific; East Pacific Rise; Equatorial Pacific; Expedition 335; gabbros; geochemistry; geophysical methods; heat flow; igneous rocks; instruments; Integrated Ocean Drilling Program; lava; lithostratigraphy; marine drilling; North Pacific; Northeast Pacific; Ocean Drilling Program; ODP Site 1256; Pacific Ocean; planning; plutonic rocks; programs; sea-floor spreading; seismic methods; velocity structure;

.