Gorda Ridge Cruise
August 5 - 21, 2002

August 11, 2000: Day #7

Log Entry: We returned to the "Seacliff" hydrothermal vents today to collect fluidsfrom more vents and to measure temperatures at the vents, with secondary objectives to collect additional vent fauna and volcanic samples.

We had done some minor modifications to the hydraulic ram on the Raptor manipulator and on the water bottles to make triggering the bottles more reliable.

On Dive 191, we relocated the vents we had found previously and then quickly collected 2 major water samples from the vent where we had obtained good gas-tight samples 3 days ago. The temperature was measured and seemed too low, indicating that the temperature probe was not functioning properly. At this point, the Raptor manipulator failed and we terminated the dive in order to repair it and replace the temperature probe. After rapid repairs during lunch, we redeployed Tiburon on Dive 192

We again found the vents, re-measured the temperatures and got 300-305 degrees centigrade at three different vents. During this operation the pan-and-tilt on the starboard camera failed and we had to collect the water samples without being able to fully utilize the camera. Despite this handicap, the pilots collected two major and two gas-tight water samples from a different vent. We then collected a fragment of the chimney structure and the underlying hydrothermal mound in a push core, collected additional samples of the blue mat that grows on low-temperature hydrothermal crust, and cored the material beneath the crust. During the coring operation, the Raptor manipulator failed as it had on the Dive 191. We terminated the dive for repairs at this point.

Despite a day when things did not go well, we completed the primary and secondary dive objectives and several lower priorities as well. With the work done in the hydrothermal field we have now collected good fluid samples from three vents in the hydrothermal field, determined that the fluids have low salinities and temperatures of about 300 degrees centigrade. The hydrothermal precipitates at the high temperature vents are dominantly anhydrite, with minor clays and sulfides, whereas those at the lower temperature vents are dominantly barite. The mounds around the vents consist dominantly of clays and silica, after the anhydrite dissolves. We also mapped the extent the hydrothermal field and collected a wide range of vent fauna. We have found some evidence that an off-axis eruption occurred in the area, since some of the thin crusts near the margins of the hydrothermal field appear to be hyaloclastite and some thin sheet-like flows drape part of the upper slope. Rock coring continues in the axis of the rift valley at night. When we head south tonight, we will have completed 17 rock cores, of which 16 collected glass. Tomorrow we will dive in the transform at the north end of the Escanaba Trough.

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