BACKGROUND

On the now-inactive Manus Trench or its antecedent, volcanism above Eocene-Oligocene subduction of the Pacific Plate under the Indo-Australian Plate formed an island arc represented by exposures on New Ireland, New Hanover, Manus, and parts of New Britain (e.g. Hohnen, 1978; Stewart and Sandy, 1988). Paleomagnetic measurements (Falvey and Pritchard, 1985) indicate that these islands have been relocated to their present positions by an imperfectly understood sequence of backarc developments (Exon and Marlow, 1988). In the late Miocene or Pliocene, when arrival of the Ontong Java Plateau blocked subduction at the Manus Trench, convergence switched to the New Britain Trench. Here the Cretaceous oceanic Solomon microplate is moving under what is now the South Bismarck microplate (a unit separated from the Pacific plate by more recent backarc processes). Above the north-dipping Wadati-Benioff zone associated with the New Britain Trench, a chain of young arc volcanoes has formed along the concave northern side of New Britain (Bismarck or New Britain arc; Johnson, 1976).

The present-day configuration of spreading segments and obliquely oriented transform faults in the Manus Basin (Fig. 2) (Fig. 3) is well established by bathymetric, sidescan, seismic reflection, gravity, and magnetics surveys (Taylor, 1979; Taylor et al., 1991) and by microseismicity (Eguchi et al., 1989), which defines left-lateral movement on the transform faults. In contrast to the wedge shaped Manus spreading center, where new backarc oceanic crust has been forming since the 0.78 Ma Brunhes/Matuyama boundary (Martinez and Taylor, 1996), the rift zone of the eastern Manus Basin, which lies between the islands of New Ireland and New Britain and between two major transform faults (Fig. 2) (Fig. 3), is a pull-apart zone of distributed extension on mostly low-angle faults approximately normal to the transforms. Martinez and Taylor (1996) infer ~80 km of extension across a 150-km-wide rift zone, concentrated mostly in the bathymetrically deeper portion of thinned crust that is coincident with an isostatic gravity high (Fig. 4). They argue that this extension is equivalent to that accomplished by a combination of backarc spreading and microplate rotation in the central portion of the Manus Basin (Fig. 3). Bathymetry, gravity modeling, and reverse magnetization indicate that basement of the eastern Manus Basin (called the Southeastern Rifts by Martinez and Taylor, [1996]) is arc crust equivalent to the Eocene-Oligocene exposures on New Britain and New Ireland. Reflection seismic traverses (B. Taylor and K.A.W. Crook, unpubl. data) across the eastern Manus Basin show essentially undeformed graben and half-graben fills up to 0.3 s, equivalent to about 1 m.y. at current sedimentation rates. This is consistent with rifting in the eastern Manus Basin covering a similar duration to spreading on the central Manus spreading center. The sediment fill is commonly tilted, denoting block rotation on listric master faults. Dredging of fault scarps where seismic profiles indicate exposure of lower, more deformed sequences has yielded fossiliferous calcareous mudstones and volcaniclastic sandstones ranging in age from early Miocene to the Pliocene-Pleistocene boundary. Although mainly of deeper marine origin, these are contemporaneous with the Miocene Lelet Limestone and Pliocene Rataman Formation that overlie the Eocene-Oligocene Jaulu volcanics of New Ireland (Stewart and Sandy, 1988) and with equivalent sequences on New Britain. Undated, mildly metamorphosed basalts dredged from inner nodal scarps near the active ends of the two transform faults (Fig. 5) may represent the presumed arc volcanic basement.

Built on this nascent continental crust, and probably controlled by subtle, relatively recent changes in the extensional stress field, a series of high-standing neovolcanic edifices (eastern Manus volcanic zone; Binns and Scott [1993]) extends enechelon across the trend of the rift faults (Fig. 5). Because these edifices do not significantly disturb the negative regional magnetization derived from basement, they are considered to be superficial features (Martinez and Taylor, 1996). The neovolcanic edifices range from central eruptions of more mafic lavas (basalt and basaltic andesite) to linear ridges formed by fissure eruption of andesite, dacite, and rhyodacite. The westernmost volcanic feature in Figure 5 is a low axial ridge with midocean ridge basalt (MORB) affinity set within a deep trough (Fig. 4). This is probably a failed spreading center; however, the other edifices are distinctly but variably potassic and have trace element and isotopic affinities comparable to subaerial arc volcanoes of New Britain (Binns et al., 1996a; Woodhead and Johnson, 1993), rather than to the MORBs at the Manus spreading center (Woodhead et al., 1998) or the adjacent East Sherburne volcanic zone (Fig. 3). The eastern Manus volcanic zone appears to be a submarine segment of the New Britain arc displaced from the main subaerial chain and erupted in the rifted backarc region.

The PACMANUS hydrothermal field targeted by Leg 193 is located near the crest of Pual Ridge, a 500- to 700-m-high felsic neovolcanic ridge with negligible sediment cover (Fig. 4) (Fig. 5). This ridge is externally constructed of stacked, subhorizontal lava flows 5-30 m thick, with negligible to minor sediment cover along the crest. Whether this "layer cake" character persists internally is an open question. Dacite and some rhyodacite block lavas with rough surface topographies predominate, but there are also some smoother sheet flows and lobate flows of dacite (Waters et al., 1996). Consanguineous lobate flows of andesite occupy the lower reaches of Pual Ridge, whereas the 2100-m-deep valley to its east is floored by lobate flows of basaltic andesite (Fig. 5).

PACMANUS Hydrothermal Field
Isolated hydrothermal deposits have been photographed along 13 km of the main crestal zone of Pual Ridge (Binns and Scott, 1993; Binns et al., 1995, 1996b, 1997a, 1997b). The more significant active deposits occur in the center of this zone between two low knolls on the ridge crest (Fig. 5). Lavas in this central area are exclusively dacitic to rhyodacitic (65%-71% SiO₂). Based on extensive bottom-tow photography and manned submersible observation (Fig. 6), four principal fields of hydrothermal activity, including sulfide chimneys, and several smaller sites have been delineated and named (Fig. 7). Much of the information cited below is unpublished and is derived from cruises listed in the caption of Figure 6.

Roman Ruins (1693-1710 m water depth, 150 m across) contains many closely packed simple columnar chimneys as high as 20 m, and some complex multispired chimneys with numerous conduits. Commonly, these coalesce into wall-like constructions with north-south orientation. Many chimneys are broken (seismic effects?) and some show later regrowth. Fallen chimneys form a 10-m-high pediment for the active structures, including black smokers and diffuse venters of clear fluid. A smaller, deeper (1730-1740 m) field to the north, Rogers Ruins, is linked to Roman Ruins by a zone of Fe oxyhydroxide deposits. Numerous small occurrences of Fe oxyhydroxide and Mn oxides are common throughout the PACMANUS field.

Satanic Mills (1708-1720 m water depth, 200 m across) is an equivalent-sized field of more scattered deposits marked by clouds of black smoke from predominant multispired hydrothermal constructions. Both black to gray smokers and vigorous venters of clear fluid are in close proximity. East of this field there are north-south dacite fissures encrusted with fauna that emit clear fluid and are interpreted as juvenile vents soon to become smoker fields. To the south, the area of active venting is linked by a zone of altered dacite with diffuse venting and scattered Fe and Mn oxide deposits to the smaller Marker 14 field, which at 1745 m depth is the deepest hydrothermal site so far recognized at the PACMANUS site. Deflections of bathymetric contours beyond both the Roman-Rogers and Satanic-Marker lines suggest that both fields are located on north-northwest-trending fracture zones.

The Tsukushi field (1680-1686 m water depth) at the southwestern end of the PACMANUS field contains numerous actively venting chimneys up to 30 m high, many very slender, but some as large as 10 m in diameter. No chimneys were sighted when this field—discovered during a 1996 Shinkai-2000 submersible dive—was traversed by a sea-bottom camera in 1993 and by a Shinkai 6500 submersible dive in 1995. Additional large chimneys were present in 1998; hence this field might be very young. Iron oxyhydroxide and Fe and Mn oxide crusts are common in the zone extending northeast from Tsukushi.

Snowcap (1654-1670 m water depth), the other major active hydrothermal site at PACMANUS, is very different in character. It occupies the crest and flanks of a 10- to 15-m-high hill, 100 m x 200 m in size, bounded on its eastern side by a north-northeast-striking fault scarp 60-80 m high. Outcrops of altered dacite-rhyodacite lava and hyaloclastite predominate, locally covered with patches of both sandy sediment and metalliferous hemipelagic ooze (only millimeters thick). Gravity corer and grab operations revealed the sand to be altered lava disaggregated by bioturbation or hydrothermal fragmentation. Typical alteration assemblages at Snowcap are dominated by cristobalite, with lesser natroalunite, diaspore, and illite-montmorillonite with traces of pyrite, marcasite, chalcopyrite, enargite, and native sulfur. These reflect relatively low-temperature interaction between dacites and a highly acid, relatively oxidized hydrothermal fluid (advanced argillic alteration), indicating that SO₂-bearing magmatic components were present in the fluid.

Diffuse low-temperature venting (6°C; compared with 3°C ambient seawater) is extensive across the gently undulating to flat crest of Snowcap knoll. More intense shimmer occurs at the edges of the occasional Mn oxide encrusted outcrop of altered dacite. The diffuse vent sites are marked by white surficial patches, probably including both bacterial mat and methane hydrate deposits. Around the southwestern fringe of the Snowcap knoll, there are several small fields of actively smoking and inactive chimneys, aligned in north-south-trending clusters.

Orifice temperatures measured at black or gray smokers and sulfide chimneys venting clear fluid are comparable for the Satanic Mills, Roman Ruins, and Tsukushi fields, ranging between 220° and 276°C. End-member vent fluids are very acidic (pH 2.5-3.5), show high K/Ca values (reflecting equilibration with dacite wall rocks), are high in Mn and Fe relative to midocean ridge fluids, and have variable salinities (Gamo et al., 1996; Auzende et al., 1996; Charlou et al., 1996). The variable salinities imply subsurface phase separation, meaning hydrothermal temperatures exceed 350°C at indeterminate depths below the chimney fields. This is supported by mineralogical evidence of phase separation (Parr et al., 1996). End-member gas compositions of 20-40 mM CO₂, 20-40 µM CH₄, and R/R_A(He) = 7.4 denote significant contribution to the hydrothermal fluids from arc-type magmatic sources (Ishibashi et al., 1996). Douville et al. (1999) ascribe unusually high fluorine contents in the fluids to magmatic sources. Temperatures of 40° to 73°C have been measured in shimmering clear fluid emitted from Fe oxyhdroxide deposits in the Tsukushi-Snowcap zone.

A very high thermal gradient of 15°C/m was measured at a sediment pocket on Snowcap adjacent to a 6°C shimmering water zone. Fluids collected near this location by a funnel sampler are similar to seawater in composition but are enriched in Mn, Fe, and Al. All outcrops of altered dacite in the vicinity of the shimmering water are heavily encrusted by Fe and Mn oxides.

Chimneys collected from Roman Ruins and Satanic Mills are comparatively rich in precious metals (average = 15 ppm Au and 320 ppm Ag), and are composed predominantly of chalcopyrite and sphalerite, with subsidiary pyrite, bornite, tennantite, galena, and dufreynosite (Scott and Binns, 1995; Parr et al., 1996). Barite is the principal gangue, but anhydrite substitutes in some samples. Chimneys at Roman Ruins typically contain less Cu than those at Satanic Mills. Fewer samples have been recovered from Tsukushi and the southwestern side of Snowcap, but these are virtually devoid of Cu and Au and contain more Pb and Ag. Their gangue includes appreciable amorphous SiO₂ as well as barite.

PACMANUS chimneys have elevated contents of "magmatophile" trace elements (e.g., As, Sb, In, Tl, and Te). Sulfur isotope ratios near zero³⁴S (Gemmell, 1995, Gemmell et al., 1996) indicate a larger magmatic-sourced component than occurs at midocean ridge hydrothermal sites and mature backarc spreading axes. Direct evidence for the importance of magmatic fluids is in Cu+Zn-rich gas-filled cavities within glass melt inclusions in phenocrysts of Pual Ridge andesites (Yang and Scott, 1996), as well as in the gas compositions of collected vent fluids (see above).

The PACMANUS hydrothermal field supports an exceptionally abundant vent macrofauna dependent on chemosynthetic bacteria, broadly similar to those of other southwest Pacific sites (Hashimoto et al., 1999). At Snowcap, dredged samples of altered dacite possess microscopic tube worms (unidentified species) along internal hairline fractures. These, and their presumed symbiotic bacteria, imply the presence of a subsurface biosphere that will also be investigated by the scientists aboard Leg 193. ODP is currently negotiating with the Papua, New Guinea Office of Environment and Conservation regarding permission to undertake investigations in this unique environment.