RRS James Cook JC021
Cruise summary report
| Cruise Info. | |
| Ship name (ship code) | RRS James Cook (740H) |
| Cruise identifier | JC021 |
| Cruise period | 2008-01-04 — 2008-02-10 |
| Status | Completed |
| Port of departure | Panama, Panama |
| Port of return | Caldera, Costa Rica |
| Purpose | Research |
| Objectives | RRS James Cook Cruise JC21 (6th January to10th February 2008) investigated exposures of lower oceanic crust that crop out at Hess Deep (~2°15'N, 101°30'W); a rifted depression formed by the westward propagation of the Cocos-Nazca plate boundary towards the East Pacific Rise. Hess Deep provides unique exposures of the deeper levels of ocean crust formed at a fast spreading mid-ocean ridge, one of the fundamental processes in the solid Earth cycle. Our principal aims were to provide detailed site survey for future Integrated Ocean Drilling Program (IODP) drilling, and to collect a suite of deep crustal rocks with good geological context to test hypotheses of mid-ocean ridge accretion.
The specific objectives were as follows: 1. To find suitable locations for the drill sites proposed in the highly-ranked IODP proposal #551. This requires use of a remotely-operated vehicle or autonomous underwater vehicle to collect accurately navigated high-resolution bathymetry and seafloor photographs in order to identify areas of outcrop and/or flat benches suitable for spud-in by a non-riser drillship. 2. To characterise the lithologies present at these potential sites, and provide geological context on a broader scale, by surveying and sampling in as much detail as possible the lower crustal section exposed on the southern slope of the intra-rift ridge of Hess Deep. This we intend to do by using the British Geological Survey's 'BRIDGE' seabed rock drill to collect geographically orientated core material, supplemented if appropriate with sampling using an ROV and/or dredging. The broader scientific objectives were defined as follows: 3. To quantify as far as possible the heat and mass flux between mantle and crust and crust and ocean by constraining the structure, composition and alteration history of gabbroic rocks from the Hess Deep section. 4. To test the hypothesis that the Oman ophiolite is an appropriate analogue for fastspread ocean lithosphere. 5. To test competing models for the accretion of the lower ocean crust at fast-spreading ridges. |
| Chief scientist | Christopher J MacLeod (University of Cardiff School of Earth and Ocean Sciences) |
| Cruise report |  (3.70 MB) |
| Ocean/sea areas | |
| General | North East Pacific Ocean (limit 180W) |
| Specific | Hess Deep |
| Measurements | |
| Geology and geophysics | |
| Grab | Quantity: number of dives = 12 Description: ISIS ROV dives over an aggregate of 14.5 days from depths between 3000 and 5400m to do swath mapping and sample collection |
| Grab | Quantity: number of samples = 145 Description: 760kg of sample collected, rock types includedharzburgite, dunite, troctolite, olivine and other gabbro, dolorites and basalts |
| Core - rock | Quantity: number of deployments = 3 Description: BRIDGE drill deployment, no cores recovered |
| Bottom photography | Quantity: number of dives = 8 Description: ISIS ROV deployed near bottom to acquire digital imagery of basement exposures and to collect outcrop samples using the vehicle's manipulator arms |
| Multi-beam echosounding | Quantity: survey square kilometres = 11 Description: Bathymetric survey of the slop north of the nadir of Hess Deep and the intra-rift ridge horst block further north within the rift valley |
| Multi-beam echosounding | Quantity: number of dives = 12 Description: ISIS ROV dives over an aggregate of 14.5 days from depths between 3000 and 5400m to do swath mapping and sample collection |
