1. Leg 193 Summary¹

Shipboard Scientific Party²

ABSTRACT

Ocean Drilling Program Leg 193 undertook an exploration of the lateral, vertical, and temporal variability of a seafloor hydrothermal system hosted by felsic volcanic rocks in a convergent plate margin setting. In planning for this expedition, we knew that we would encounter a myriad of technical challenges, so we brought with us an arsenal of tools, some still unproven, to help us achieve our objectives. Three sites on the crest of Pual Ridge in the Eastern Manus Basin were proposed as primary targets, one each in an area of high-temperature venting (a black smoker chimney field), a field of lower temperature diffuse venting, and at a reference site that, although proximal to a high-temperature venting area, showed no surficial evidence of hydrothermal activity. From the first core recovered to the last, and throughout our coring, wireline logging, and logging while drilling operations, the prophetic watchwords of the cruise—to "expect the unexpected"—rang true. With the material recovered, we expect to assess the interplay between magmatic-derived fluids and seawater and to examine fluid pathways with an eye toward establishing a comprehensive chemical and hydrologic model for this system. The intensity, degree, and distribution of alteration facies and details regarding the abundance of clay minerals and anhydrite chronicled in the Leg 193 Initial Reports volume directly address these objectives. We have intersected an actively forming base metal sulfide system. However, poor core recovery in the sulfide-rich interval precludes adequate assessment of the dimension of such mineralization. Detailed consideration of extensive downhole logging data, still being processed, will throw additional light on this issue. Our initial perception is that despite the pervasive alteration indicative of a long-lived hydrothermal system, we have glimpsed the prenatal development of a massive sulfide deposit, which, given continued maturation, could develop into a deposit on the order of those exploited for centuries in ancient ore environments.

One of our aims is to evaluate the relative importance of magmatic-vs. seawater-derived components in subsurface hydrothermal fluids. Our initial perspective is that, in contrast with obvious acid-sulfate parageneses found within the overlying chimneys, subseabed mineralization and alteration may be largely dominated by a seawater-derived component. Magmatically derived fluids possibly passed through footwall rocks with only minor alteration, through large, open fractures. Additionally, we seek to develop a petrogenetic model for the volcanic rocks at Pual Ridge and evaluate the volcanic architecture of this edifice. Low recovery notwithstanding, the descriptions and analyses of the cores from all three types of sites drilled during this expedition define a fresh volcanic cap underlain by pervasively altered volcanic flows and breccias, albeit with short intervals of less intense alteration, which reveal the nature of the growth and evolution of this edifice. Finally, we set out to determine the nature, extent, and habitat controls of microbial activity in this hydrothermal system. Direct counting and biological tracer analyses indicate the presence of a biomass to >100 m below seafloor. Cultivation experiments indicate that potential microbiological activity persists to much deeper and more extreme environments. We recognize that all of these scientific objectives rely on postcruise research to be fully realized. We are confident, however, that by integrating the results of that research with the detailed descriptions we have recorded and the array of continuous records provided by the logging tools, we will be able to make direct comparisons with not only other active seafloor hydrothermal areas, but with ancient ore deposition environments as well, improving our understanding of these complex systems.