GEOLOGICAL SETTING

The Manus backarc basin, constituting two microplates (North Bismarck and South Bismarck) set between opposed fossil and active subduction zones, lies within the complex zone of oblique convergence between the Australian and Pacific plates (Fig. F1). Northward subduction of the oceanic Solomon microplate beneath the South Bismarck microplate, where basement formed by earlier Tertiary arc volcanism and backarc spreading, currently occurs along the New Britain Trench with associated eruption of young arc volcanoes along the concave northern side of the island of New Britain. Present-day structure of the basin is dominated by seafloor spreading in the center and extensional rifting in the east, the various segments delineated by sinistral transform faults (Martinez and Taylor, 1996). According to Macpherson et al. (1998, 2000) isotopic evidence in submarine lavas indicates presence of an active mantle plume under the central and western portions of the Manus Basin, but it is unclear to what extent this influenced the eastern portion where Leg 193 was conducted.

The eastern Manus Basin is a pull-apart rift zone of distributed extension on low-angle normal faults between two transform or transfer faults. An east-west–trending belt of mostly high standing neovolcanic edifices overlying early Tertiary arc volcanic crust and younger Tertiary–Holocene sediment-filled half-grabens links the active ends of the bounding Djual and Weitin transforms (Fig. F1). Individual edifices are variously dominated by lavas ranging from picritic basalt to dacite-rhyodacite with similar isotopic and trace element geochemistry (Binns et al., 1996, 2002b; Kamenetsky et al., 2001) to subaerial arc volcanoes on the island of New Britain to the south (Woodhead and Johnson, 1993).

Pual Ridge, a linear edifice trending northeast at about the center of this neovolcanic belt, stands some 500–600 m above basaltic andesite- and sediment-floored valleys to the east and west, respectively (Fig. F2). Seafloor outcrops are predominantly dacitic to rhyodacitic in composition near PACMANUS, although consanguineous basaltic andesites and andesites occur as small, isolated cones along the crest and flanks and as more extensive flows on the northwestern flank and southern end of Pual Ridge. The eastern side of Pual Ridge has a terraced morphology resulting from a sequence of subhorizontal dacite flows ~30 m thick that here forms the hanging wall of a major normal fault (Gennerich, 2001). Glassy flows along the crest have negligible to minor sediment cover and vary in structure from lobate or tube flows and sheet flows to block lava with increasingly siliceous character (Waters et al., 1996). Local pockets of hyaloclastite are present, including spalled surficial tube pumice. The lavas of Pual Ridge are vesicular, and conspicuously aphyric or sparsely porphyritic compared to most other eastern Manus volcanic edifices. In the vicinity of PACMANUS, lava compositions tend to fall into three compositional groups whose distributions and topographic relationships suggest individual flows or lava fields from 200 m to 1 km in extent (Fig. F3).

Isolated hydrothermal deposits occur for some 13 km along the crest of Pual Ridge (Binns and Scott, 1993; Scott and Binns, 1995; Binns et al., 1995; Shipboard Scientific Party, 2002a). The more significant active deposits extend for 2 km between two elongate highs on the ridge crest (Fig. F2). These lie between 1650 and 1750 meters below sea level (mbsl) and are collectively named the PACMANUS hydrothermal field after the 1991 discovery expedition (Binns and Scott, 1993).

Three of the five main hydrothermal centers within the PACMANUS field were examined during Leg 193 (Fig. F4). Roman Ruins (1680–1690 mbsl; Site 1189) and Satanic Mills (1685–1695 mbsl; Site 1191) are 100- to 200-m-wide sites of focused high-temperature activity expelling acid fluids (pH = 2.3–3.5) at temperatures commonly above 250C (Douville et al., 1999). Chimneys venting boiling fluid (356C) were recently discovered at a site on the southern extension of Satanic Mills (Tivey et al., 2006; Seewald et al., 2006). Sulfide chimneys range to as tall as 20 m but are generally 1–3 m high. They rise directly above largely unaltered dacite-rhyodacite lavas or surmount mounds of fallen chimney fragments and Fe oxyhydroxide deposit. Tsukushi (~1665 mbsl) and Rogers Ruins (~1700 mbsl) are additional chimney fields at PACMANUS not examined during Leg 193 (Fig. F4).

By contrast, Snowcap (1635–1645 mbsl; Site 1188) is a broad knoll (10–15 m high x 150 m across) where both fresh and altered volcanic outcrops are interspersed with flat patches of gravely to sandy sediment (containing both altered and fresh volcanic clasts), metalliferous hemipelagic ooze, and dark surficial Mn-Fe oxide crusts. Some active sulfide chimneys occur on the lower western flank of Snowcap. Shimmering low-temperature venting is extensive across the crest, especially at the edges of altered outcrops (measured at 6C but likely higher locally) and within patches of white to cream-colored microbial mat. Submersible and deep-tow video observations at Snowcap include several instances where younger glassy dacite lava has flowed over altered dacite-rhyodacite outcrops. Dredge samples of the glassy lava fall into the 67.5 wt% SiO2 youngest category of Figure F3, and one contains a 5-cm xenolith of pyrite-bearing, totally altered dacite (Binns et al., 2002b). Samples of altered rock collected from Snowcap by dredge, grab, and submersible exhibit lapilli-like structures, some with remnant glassy rhyodacite kernels whose vesicle and microlite orientations establish that the parent rock was a coarse hyaloclastite. Alteration is dominated by cristobalite, natroalunite, diaspore, illite-smectite, and native sulfur (Yeats et al., 2000; Binns et al., 2002b). Some samples contain fine networks of pyrite veins. The "acid sulfate" alteration assemblage, including globules and layers of formerly molten sulfur implying disproportionation of SO2, suggests involvement of magmatic components in the responsible fluids and higher temperatures than those measured at shimmering vents. Minor occurrences of similar alteration have been dredged along with dominant fresh lavas at Satanic Mills and Roman Ruins (Binns and Scott, 1993; Binns et al., 2002b; Giorgetti et al., 2006).

PACMANUS chimneys are particularly rich in chalcopyrite and sphalerite and contain high levels of gold and silver (Parr et al., 1995, 2003; Moss and Scott, 2001; Binns et al., 2002b; Binns, 2004). Barite is the principal gangue mineral, although anhydrite is present in some chimneys. Chimneys at Tsukushi and the western flank of Snowcap tend to be richer in Pb and poorer in Cu and Au than those from Roman Ruins, Satanic Mills, or Rogers Ruins. Elevated contents of "magmatophile" trace elements (As, Sb, Bi, Te, In, Tl, and Mo) unlikely to be sufficiently available by leaching processes suggest involvement of an exsolved magmatic fluid component in the PACMANUS hydrothermal system, and this is supported by low sulfur isotope ratios in chimney sulfides (Binns et al., 2002b). On both criteria the magmatic component is greatest at Satanic Mills and the chimneys fringing Snowcap. Douville et al. (1999) ascribe exceptionally high SO4, H2S, and F contents of PACMANUS vent fluids to incorporation of magmatic volatiles into the hydrothermal system. Ishibashi et al. (1996) also infer significant magmatic input from gas compositions in vent fluids.

In addition to occurrences within or linking the chimney fields (Fig. F4), independent patches and spires of Fe-Mn-Si oxide deposit meters to tens of meters across are scattered along the crest of Pual Ridge (Heath et al., 2000; Binns et al., 2002b). Some actively vent shimmering clear fluid measured at temperatures as high as 73C (BIOACCESS'98 cruise; C.J. Yeats, pers. comm., 1998). The principal constituents are Fe oxyhydroxide and opaline silica, with Mn oxides at the outer surface of some samples. Filamentous microfabrics denote microbial activity during formation.

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