Sojourn Leg 2 Aboard the R/V Melville

Departed Papeete, Tahiti - October 28, 1996
Arrived Hanga Roa, Easter Island - December 3, 1996

The primary purpose of the cruise was to survey the narrow axial zone of the ultrafast-spreading EPR at 17*18'-42'S using the fiber-optic ARGO II near-bottom optical/acoustic system and the AMS-120 sonar system. Our goal was to test the hypothesis (based on ARGO data from EPR 9-10*N) that along-strike thermal gradients set up by the segmented pattern of magma supply to fast-spreading MOR's exert primary control on the distribution and types of hydrothermal vents and vent biota, as well as on variations in fine-scale volcanotectonic characteristics along the axial zone. On the 4th order scale at EPR 9-10N, this magmatic control of hydrothermal discharge is manifested by the concentration of high-temperature vents along eruptive fissures. EPR 17*18'-42'S is a superb area for further investigation of relationships between magmatic processes and other axial processes. Along a segment of ridge only 45 kms long, seismic data show that the axial magma chamber (AMC) changes along strike from a flat-topped body at relatively constant depth to a peaked cupola ("spike") that intrudes to within 0.8 km of the seafloor at 17*27'S. This represents the most extreme along-srike variations in thermal gradients that we know of on the MOR, and contrasts with the flat-topped AMC at EPR 9-10N. The survey was designed (and will be interpreted) within the context of seismic reflection/refraction data, SeaMARC II and MR1 imaging, SeaBeam bathymetry, gravity and magnetic data, submersible observations, and extensive petrologic/geochemical data that exist already for the study area and adjacent ridge flanks. These data show that this apparently unsegmented portion of the EPR is actually partitioned into at least six 4th-order segments (our survey may reveal more), and that the axial zone exhibits along-strike changes in morphology and some extreme along-strike changes in axial lava compositions. Ridge morphology and some visual observations indicate recent eruptive activity in part of the survey area. Beyond testing ideas about coupled magmatic/hydrothermal segmentation along the MOR, we also observed how hydrothermal and other axial zone processes are affected by ultrafast spreading rates and extreme along-strike thermal/magmatic gradients. We determined the nature of the axial troughs found along portions of the axial zone in the study area (axial summit caldera or graben?) and investigated the development of these important axial features. To this end, we carried out a secondary Argo survey of a hydrothermally-active portion of the axial summit trough on the segment south of the main survey area (at approx. 18.5S). Finally, we provided a baseline survey of the fine-scale segmentation and distribution of vents and biota along a ridge segment destined for future seismic and submersible studies.

We also collected CTD and transmissometer data using instruments mounted on the ARGO and AMS 120 vehicles and on the towing cable.

ARGO II watches required AT LEAST 5 people. Two of the watchstanders were designated as a dataloggers. Both people watched the real-time video and logged observations in real time, one digitally and the other handwritten. In this way we were able to manage the huge visual dataset. The datalogger files were subsequently edited by verifying the logged observations with the handwritten backup. By this means the classification of features was standardized and erroneous data were deleted from the files. The end product was a set of digitized and categorized GIS/ArcInfo coverages that could be plotted in any combination (for example, black smokers and fissures; vent communities and Age 1 lavas; etc.). This was a very powerful approach to data management that had also worked beautifully for the EPR 9-10N ARGO I dataset.