SUB-SEAFLOOR HYDROTHERMAL ACTIVITY AT THE DEEP-SEA PACMANUS HYDROTHERMAL FIELD, MANUS BASIN, PAPUA NEW GUINEA: EVIDENCE FROM FLUID INCLUSIONS IN QUARTZ

The PACMANUS hydrothermal field is at a water depth of about 1650 to 1700 m on the summit of Pual Ridge, a young volcanic feature in the eastern part of the Manus backarc basin. Two sites of active venting are Roman Ruins, with high-temperature (220-276°C) sulfide chimneys, and Snowcap, which is an area of lower-temperature (6-65°C) diffuse flow. Both of these sites were sampled by deep-sea drilling during Ocean Drilling Program Leg 193. Drill cores reached sub-seafloor depths of 387 m at Snowcap and 206 m at Roman Ruins.

At Snowcap, the upper 35 m of core is fresh rhyodacite, while the underlying rocks are highly to completely altered rhyodacite. The hydrothermal style is variable with complex overprinting of acid sulfate (silica-pyrophyllite-illite-anhydrite) and argillic (silica-chlorite-smectite) alteration. At shallow depths the silica is cristobalite, whereas quartz is the silica polymorph at depth. Secondary quartz occurs in amygdules, alone or with accessory anhydrite and pyrite; in cm-scale nodules; and as approximately 100µm grains forming an open mosaic with interstitial clays and pore space. Late stage crosscutting fractures are common, filled by anhydrite with accessory pyrite.

Fluid inclusions (FI) in secondary quartz are small (10-20µm diameter is typical), irregular in shape, and contain liquid (L) plus vapor. They are evenly distributed, and only a few are arrayed along healed fractures. For this reason most FI are interpreted as primary. FI homogenize to L between about 290°C and 390°C (trapping temperatures are inferred to be about 10°C higher). Ice melting temperatures between about -10°C and –0.4°C show that while most of the FI have salinities near that of seawater (with ice melting at –2.0°C), a significant number are much more saline than seawater, while others are much less saline than seawater, approaching fresh water. These data are best explained if the hydrothermal solution periodically phase-separated. Indeed, many FI indicate temperatures essentially on the boiling curve for seawater.

Data from FI in quartz from Snowcap match those reported previously from anhydrite veins. The high alteration temperatures exceed the temperature of present-day diffuse seafloor venting at Snowcap, but are closer to the 313°C temperature measured at a depth of 360 m during the drilling leg.