PROPOSED SITESThe above scientific objectives will be tackled with a program of as many as four sites, the strategy for which is illustrated in reference to expected subsurface geology in Figure 8. These localities will explore (1) two outflow sites under a zone of low-temperature diffuse venting (Site PCM-2A) and as close as possible to a site of focused high-temperature venting (PCM-3A), (2) a background position (PCM-1A) away from known activity, and (3) a likely inflow site (PCM-4A) where faulting should facilitate entry of seawater to the system. Figure 8 shows that even though major advances in knowledge will be made during Leg 193, we will not have penetrated to the enigmatic source regions for metals and fluids. Hence, answers to the major scientific questions will require much subsequent laboratory analysis and interpretation of data.
Rough volcanic topography over most of the PACMANUS field and closely packed chimneys within the high-temperature vent fields severely constrain the number of drilling sites suitable for use of the hard-rock guide base (HRGB). Alternate drilling locations within < 150 m (< 10% water depth) of primary targets were identified from submersible video footage but will be occupied only if drilling conditions at primary targets become intractable and sufficient time remains in the operational schedule to drill additional targets.
The priority order in which the sites are described below, and their target depths, may change at sea, depending on results progressively obtained and technical factors such as whether the hammer-drill system, drill in-casing, or diamond drilling capabilities are available. The two outflow sites will provide a comparison between alteration/mineralization and fluid pathways beneath a zone of focused high-temperature venting (PCM-3A, Roman Ruins chimney field) and a zone of diffuse venting (PCM-2A, Snowcap field). Nearby, the third site on the crest of Pual Ridge (PCM-1A) will provide an unaltered "reference" volcanic section as well as indications of possible shallow seawater influxes in the upper sections and a variety of possible outcomes in the lower sections. In effect, this drilling strategy achieves the assessment of vertical and lateral heterogeneity of the ore-forming hydrothermal system.
Proposed Site PCM-2A (water depth 1655 m) is located on the thoroughly explored crest of the Snowcap field (Fig. 7), at a site where there are few rocky outcrops and no obvious shimmering water. The anticipated immediate substrate is altered dacite. The site will establish whether the Snowcap field is underlain and perhaps inflated at relatively shallow depth by one or more zones of "subhalative" mineralization. Investigation of deeper levels will identify alteration and mineralization pattern vertical zonality and should reveal the subsurface structural conditions that govern diffuse rather than focused venting. The site depth is targeted nominally at 500 m to fit within our operational schedule, but high temperatures may preclude achieving this dpeth. However if drilling proves easy and new exciting phenomena are still being observed, we may consider deeper penetration here at the risk of deleting a lower priority site from the program.
Proposed Site PCM-3A (water depth 1696 m) is located within the Roman Ruins hydrothermal field (Fig. 7), as defined by both submersible observations and by differential global positioning system (DGPS) navigated camera tow. The site is surrounded by chimneys, including black smokers, and will provide comparative data on alteration/mineralization patterns and fluid pathways beneath this focused high-temperature field. High temperatures are expected to limit the depth of penetration, making 300 mbsf a nominal though geologically desirable goal to allow assessment of telescoped alteration patterns. Again, if drilling conditions prove ideal and novel observations are being made, coring deeper than the nominal limit may be considered.
Proposed Site PCM-1A (water depth 1720 m) is located on a mostly smooth-surfaced dacite sheet flow forming a very low rise 160 m from the Roman Ruins field (Fig. 7). The sediment cover is estimated to be a maximum of 10 cm. Presuming this site will be least affected by high temperatures, it is designated for the deepest hole (700 m). It would penetrate some 250 m below the base of Pual Ridge (i.e., below the collar of the 350-m-deep inflow zone at Site PCM-4A). Possible intersections at depth, relevant to science objectives, include
If operations at proposed Site PCM-2A do not yield sufficient material to adequately decipher the volcanic architecture of Pual Ridge, the priority of drilling Site PCM-1A may be enhanced.
Proposed Site PCM-4A (water depth 2139 m) is located among the basaltic andesite sheet and lobate flows flooring the valley southeast from PACMANUS, in a lightly sedimented (20-50 cm) depression or collapse pit on the track of a submersible dive. While investigation of structural and mineralogical features at the inflow zone is a priority, this site is placed to intersect the following features with minimal penetration but reasonable confidence:
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