KEY OUTCOMES OF THE DRILLING LEG

To achieve its objectives the initial strategy for Leg 193 was to drill at three contrasting sites (Fig. F4): (1) the Snowcap field of altered outcrops with low-temperature diffuse venting (Site 1188), (2) the Roman Ruins field of chimneys with high-temperature focused venting (Site 1189) located some 1000 m northeast of Snowcap, and (3) "background" Site 1190, lying remote from known hydrothermal deposits and 240 m east-southeast of Roman Ruins. With these, we expected to establish both lateral and vertical variations in alteration and mineralization patterns in relation to the contrasted seafloor vent styles and a reference section of unaltered volcanic rocks. Only shallow penetration to 17.2 meters below seafloor (mbsf) in fresh rhyodacite was achieved with three holes at "background" Site 1190 (Holes 1190A, 1190B, and 1190C). During a brief time window made available by operations, Hole 1191A was drilled at an additional high-temperature site, the Satanic Mills chimney field (Site 1191; ~300 m north-northeast of Snowcap), but limited penetration (20.1 mbsf) into fresh dacite-rhyodacite with sulfide films and alteration along fractures did not provide the anticipated comparison with Roman Ruins.

The major achievements of Leg 193 were deep-cored penetrations at Sites 1188 and 1189, together with wireline logging and deployment of logging-while-drilling (LWD) in separate holes for each of these sites. At Snowcap (Site 1188), Hole 1188A was cored to 211.6 mbsf, whereas Hole 1188F, located 23 m away, was cored from 218.0 to 386.7 mbsf below 190 m of casing. In combination, these holes penetrated the upper two-thirds of the Pual Ridge volcanic edifice. Hole 1188A was initially drilled through ~35 m of fresh dacite/rhyodacite, a somewhat unexpected thickness considering its proximity to seabed outcrops of altered rock. Drilling then passed rapidly into pervasively altered volcanic rocks that persisted to the base of Hole 1188A and continued throughout the deeper Hole 1188F. Holes 1188C, 1188D, and 1188E were unsuccessful attempts to spud-in the deeper penetration hole. Hole 1188B in the same vicinity commenced as a successful LWD experiment to 72 mbsf, followed by deployment of the advanced diamond core bit that recovered only two samples of cristobalite-bearing altered dacite from hole fill before failing further penetration.

At Roman Ruins (Site 1189), Hole 1189A commenced between sulfide chimneys, and, after initially intersecting a thin unit of fresh dacite (~0.2 m recovered), it penetrated to 125.8 mbsf in pervasively altered volcanic rock. A 6-cm unit of semimassive sulfide was present at 107.7 mbsf (curated depth). In Hole 1189B, drilled beside an active chimney 35 m from Hole 1189A and 8 m higher on the Roman Ruins mound, both soft and hard rocks were encountered during initial hammer-in casing to 31 mbsf, then coring continued to 206.0 mbsf. Core recovery was exceptionally poor (<1%) within a rapidly penetrated "Stockwork Zone" (31–118 mbsf) of pyritic veins and breccias within altered volcanic rocks. The first sample (31.00 mbsf) was an 8-cm piece of semimassive sulfide. Recovery in Hole 1189B improved (~19%) for a "Lower Sequence" of variably altered volcanic rocks from ~118 to 206.0 mbsf, prominently distinguished from the overlying Stockwork Zone in borehole resistivity imagery by a more widely spaced, blocky fracture pattern. Hole 1189C in the same vicinity was an uncored LWD hole drilled to 166.0 mbsf, also partly logged by wireline methods, a first for ODP.

The extent of alteration intersected beneath a thin cap of fresh lava at both Sites 1188 and 1189 was a major surprise. Prior to drilling, we expected to intersect only localized zones of alteration and mineralization within an overall relatively fresh volcanic sequence. Widespread disseminated pyrite was present in most of the altered rocks encountered, but a second major surprise was the very restricted development of subsurface massive and semimassive sulfides, both overall and especially below seafloor chimneys at Site 1189. Casing to 31 mbsf prevented this assessment in the topmost part of Hole 1189B. No buried deposits of Fe oxyhydroxide were encountered: minor intervals of bright red "jasperoid" at Site 1189 proved to be hematite-bearing quartz veins and matrixes of hydrothermal breccias (Binns, this volume).

Detailed lithologic and structural description of all cores recovered, supplemented by thin section petrology, X-ray diffraction (XRD), and chemical analyses of representative samples, constituted a major shipboard activity for the scientific party (see Binns, Barriga, Miller, et al., 2002) and formed a vital basis for subsequent onshore research. Results of downhole geophysical surveys, including resistivity images, were compiled on board, but there was insufficient time to integrate these with the core data. The highest borehole temperature recorded was 313C at 360 mbsf in Hole 1188F, 8 days after completion of drilling. Early termination of the leg prevented comparable measurements in Hole 1189B. Attempts to collect subsurface hydrothermal fluids were frustrated by hole blockages and inadequate equipment (Binns et al., this volume). With protocols developed to minimize contamination, representative core samples were taken immediately after recovery for successful shipboard and onshore microbiological studies.

Cruises Subsequent to Leg 193

Four research expeditions and a commercial survey have been conducted at PACMANUS since the completion of Leg 193. During the BISMARCK-2002 cruise of the Franklin (Binns et al., 2002a) additional bottom sampling of chimneys and lavas was conducted, including small andesite cones along the crest of Pual Ridge and its adjacent flanks. A new hydrothermal field (North Pual) hosted by andesite at 1800 mbsl was discovered 8 km northeast of PACMANUS.

During the DaeYang02 cruise of the Onnuri (McConachy, 2002; Lee, 2003; Hong et al., 2003) reflection seismic and bottom-tow magnetic surveys were conducted in the Pual Ridge vicinity. Seismic evidence was obtained of a magma body under PACMANUS, ~2 km below the crest of Pual Ridge. Magnetic profiles indicated two intensity lows—centers of subsurface magnetite destruction—under Roman Ruins and from Snowcap to Satanic Mills, respectively (Lee, 2003; Hong et al., 2003), confirming similar indications from deep-tow magnetic surveys conducted during the earlier Binatang-2000 cruise of the Franklin (Binns et al., 2000; Cousens et al., 2003).

Shallow diamond drilling in 2002 from the Sonne (CONDRILL cruise: Herzig et al., 2003; Petersen et al., 2003, 2005) achieved 10 holes in close proximity to Hole 1189B at Roman Ruins, 4 of which penetrated to ~5 mbsf. The results partly fill the information gap to 31 mbsf arising from casing the top of Hole 1189B during Leg 193. Massive sulfides including chimney fragments and reworked sulfide sediments were recovered, along with nodular sulfide breccias with anhydrite and altered wallrock fragments that are comparable with the first core sample in Hole 1189B. Two holes ended in weakly altered dacite.

A commercial survey of mineral deposits in the eastern Manus Basin conducted by a joint venture between Placer Dome Oceania Pty. Ltd. and Nautilus Minerals Inc. in January–March 2005 included deep-tow sidescan sonar, swath bathymetric, and magnetic coverage of Pual Ridge. The results are proprietary but reveal fault structures consistent with tilting of Pual Ridge, as was inferred by Shipboard Scientific Party (2002d) from an 8 difference from the present-day orientation in magnetic inclinations measured on unaltered rhyodacite cores at Site 1191.

In August 2006, during the MGLN06MV cruise of the Melville, the remotely operated vehicle Jason-2 and autonomous underwater vehicle ABE were deployed at PACMANUS and other Manus Basin sites for geophysical surveys, mapping, and collection of vent fluids, rocks, and chimneys (Roman and Ferrini, 2006; Seewald et al., 2006; Tivey et al., 2006). An exceptional outcome was discovery and sampling of boiling vents (356C) at a new site near the southern fringe of the Satanic Mills field.

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