Monterey Bay Aquarium Research Institute

Juan de Fuca Ridge Cruise
July 20 - August 1, 2000
Cruise Summary

The Chief Scientist on this dive series was Debra Stakes from MBARI. She was joined by Michael Perfit of the University of Florida (basalt geochemisty) and Maurice Tivey of the Woods Hole Oceanographic Institute (magnetics). Thomas Chapin from MBARI was the scientist in charge of the instrument deployment/recovery dive at Axial Caldera. Other members of the shipboard scientific party were all from MBARI. These included Karen Salamy who was responsible for the Tiburon rock coring sled and for the archival of all geological samples. Tony Ramirez was responsible for navigation and post-dive data. Greg Moretti (summer intern) was responsible for sample photography and cruise documentation for a website. Josh Plant worked with Chapin to prepare the instruments and analyze fluids from Axial Caldera. Robby Young was the shipboard biologist who collected both vent and non-vent specimens.

Prior to the dive series, Maurice Tivey’s geocompass and magnetometer were successfully interfaced to the vehicle to permit both instruments to be used on a single dive. Salamy also worked with Perfit to develop a set of 12 wax cores at MBARI for basalt glass sampling from Tiburon. The rock coring sled performed flawlessly, with increasing quality and quantity of rock core with each dive. This tool should prove to be a valuable asset for many MBARI efforts. The rock wax cores worked very well and were particularly useful when samples were too friable and glassy to sample or when it was impossible to break pieces of older rocks with the Tiburon manipulators. A navigation computer was added to the control room to permit the use of the ArcView-based real-time navigation. This was invaluable in planning and interpreting dive results. It was also the primary reason that the work requiring relocation of old vent sites proceeded without enormous loss of bottom time. The vent fluid thermometer was completed for this leg but the electronics housing flooded on the first dive. Testing this equipment on our dive series has probably paved the way for success on the critical NURP dives scheduled for the next leg.

Nine dives were successfully completed on the Cleft Segment and Blanco Fracture Zone for geology and geochemistry. Only one dive required an early recovery due to vehicle problems. This was estimated as a maximum of only 4 hours lost on this dive. All other dives were a minimum of 12 hours. One dive was successfully completed at Axial caldera for instrument deployment and recovery. This was the first opportunity for the Tiburon pilots to work at a high-temperature site. The instrumentation was deployed in an impressively short time. The vehicle and pilots performed superbly. The geological transects required intense sampling efforts and the acquisition of detailed structural information. The pilots cooperated and anticipated our requests, resulting in a smooth and efficient operation. A total of 137 geological samples, and approximately 700 biological samples were recovered using the ROV Tiburon. This was supplemented by a night program that provided 41 glass samples from 49 stations occupied. The non-recovery stations were used to identify heavily sedimented areas.

Dive accomplishments:

  1. The Tiburon rock coring sled recovered four rock cores in basalt in the Blanco Fracture Zone and the southern Cleft Segment. Two of these will be used for detailed magnetics studies.
  2. A geocompass and magnetometer were successfully used on Tiburon. The geocompass was used to measure the magnetic orientation of boreholes resulting from the drilling operation. The results will allow the recovered rock core to be oriented relative to north and its magnetic inclination providing constraints on the initial orientation of the rock units. A three-axis fluxgate magnetometer was mounted to the Tiburon and collected magnetic field data on all of the Cleft ridge dives. These magnetic field data will be analyzed and interpreted in terms of the recent volcanic and tectonic history of the ridge axis.
  3. Low-temperature vent sites inhabited by abundant microbial mats were discovered 2-3 km off axis on the eastern side of the Cleft Segment. The sites are located at the contact between thick coherent pillowed lava flows that emanate from near axis and the adjacent older fissured seafloor. We hypothesize that the thick coherent flows units provide a hydrologic cap that channels the low-temperature fluids to these sites. Temperatures up to 6 C were measured.
  4. Axis-parallel faults were traced into the Blanco Fracture Zone. These appear to leak low-temperature fluids that result in local abundances of pelagic communities.
  5. Three high-temperature sites were visited and sampled for both geology and biology: Vent 1 on S. Cleft, Monolith on N. Cleft and ASHES within the Axial Caldera. Unreported extinct sulfides were discovered near the western boundary fault within S. Cleft. These are in the same geological setting as the N. Cleft sulfides and might have formed during the same hydrothermal episode.
  6. Critical field observations include the contact relationships between different generations of axial flows in both South and North Cleft. Near ridge initiation of abyssal hill formation was deduced for the magmatically starved segment center. This area was characterized by pillowed highs rather than linear fissure eruptions.
  7. The volcanically active portion of the southern Cleft Segment was traced into the curved ridges that bound the Blanco Fracture Zone. These were the source of cascades of pillowed basalts that drape the entire western end of the RTI.
  8. 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. Both styles of volcanism were documented and sampled to geochemically determine there petrogenetic lineages and origins.

  9. We began to synthesize the systematics of regionally defined flow units. Now the Cleft Segment is the only section of mid-ocean ridge, except possibly for 9-10 N on the EPR where any systematic, detailed geologic mapping and sampling have been undertaken.

-Debra Stakes, Chief Scientist