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Benthic processes

Geochemistry of oceanic crust
Project Manager/Lead Scientist: Debra Stakes

This is the third year of this continuing project. All analyses have been completed on the in-hand suites from Oman and Indian Ocean.  We have completed one field season on the Morro Ridge, Mendocino Transform and the Blanco/Juan de Fuca Ridge. All samples from these three areas have been analyzed for rock chemistry and petrological and mineral chemical analyses are underway. A description of the rock chemistry and petrology of the Morro Ridge and Mendocino Transform suite of rocks is the topic for intern Jenni Kela. Age dates on mineral separates have been obtained from the Morro Ridge (30 my) and the basalts from the Mendocino Transform (8-12 my). The single dive at Morro Ridge provided some excellent samples of alkaline compositions and also convinced us that useful outcrops are few. Our second dive series to the Mendocino Transform was completed in August 2001. The existing data from the Mendocino Ridge and Gorda Escarpment indicate that intact slabs of relatively evolved oceanic crust from the Gorda plate have been transferred to the Pacific plate. The compositions, however, include alkaline compositions and volcaniclastic textures reminiscent of the Morro Ridge and other California Borderland seamounts of similar age.

During July 2000, a series of 9 ROV Tiburon dives (on- and off-axis) were successfully completed, using the EM300 data as a real-time basemap to collect lavas (137 samples), examine the volcanic stratigraphy and its relationship to faults and volcanic features, and to make magnetic measurements along the axis, up to 5 km off-axis and down the walls of the adjacent Blanco Fracture Zone. During the cruise, Tiburon successfully utilized newly developed ROV rock cores, the Tivey three-axis fluxgate magnetometer and magnetic compass, as well as the drillsled to collect oriented drillcores for magnetic measurements. These dives represent one of the most detailed tectonic, volcanologic, petrologic, magnetic and hydrologic studies of a medium spreading-rate ridge segment; particularly with respect to off-axis features.

The initial field season was highlighted by the discovery of off-axis hydrothermal venting (3 km from the axis) and evidence of small volumes of off-axis volcanism along eruptive fissures and from point-sources; in some cases these seem related to the formation of abyssal hills and rift-bounding faults. Some of the off-axis flows are highly evolved ferro-andesites and FeTi-basalts that are spatially associated with faults that form the inward-facing rift valley walls. Other flows seem to be related to the formation of the first series of abyssal hills, consistent with "volcanic growth faults" draped with syntectonic lava flows. The low-T venting, up to 7 C as measured by the CTD, is expressed as low, ridge-parallel mounds composed of uplifted sheet flows draped with bright green precipitates of amorphous silica, iron, manganese and bacterial filaments. These linear mounds appear to be controlled by the location of the toes or edges of the uppermost volcanic pillow mounds which serve to hydrologically cap subsurface fluid flow and direct this laterally to the flanking margins off-axis. The fluids from these vigorously venting sites provide a unique window into the subsurface biogeochemical environment without the complication of intervening sediments and sedimentary pore fluids.

The target area that we propose for dives in 2002 is the southernmost Juan de Fuca Ridge, to take advantage of the Western Flyer operations in this area. Four dives will be used to complete a series of long transects for both geology and exploration of additional off-axis vent sites.