Publication Date:
2022-05-26
Description:
Cruise Report
R/V Melville
MAGELLAN-06, Rabaul, Papua New Guinea to Suva, Fiji
July 21st 2006 to September 1st 2006
Description:
The hydrothermal systems in the Manus Basin of Papua New Guinea (PNG) were
comprehensively investigated through a combination of sampling and mapping using the
Remotely-Operated Vehicle (ROV) Jason, the autonomous underwater vehicle (AUV) ABE
(Autonomous Benthic Explorer) and ship-based CTD work and multi-beam bathymetric mapping
using the RV Melville. The objectives of the cruise (July 21st to Sept. 1st, 2006) were to identify
the tectonic/geologic settings of the vent systems, examine the interactions of seawater with
felsic rocks that constitute the high silica end-member range of seafloor basement
compositions, determine the extent of volatile magmatic inputs into these systems and to
examine the evolution of hydrothermal activity through time. The first 10-day portion of the
cruise was funded by Nautilus Minerals in a collaborative research effort to examine the Manus
Spreading Center and the Vienna Woods basalt-hosted hydrothermal vent systems. The
second 32-day portion of the cruise, funded by the National Science Foundation (NSF), focused
on the felsic-hosted hydrothermal systems of the PACMANUS (Papua New Guinea – Australia
– Canada Manus) vents drilled by the Ocean Drilling Program (ODP) in 2000 and the nearby
seafloor volcano vent systems of Desmos and SuSu Knolls. Nautilus Minerals generously
funded the add-on use of ABE throughout the NSF program allowing for high resolution
mapping to be completed on all the major vent sites within the eastern Manus Basin. A total of
30 ROV dives (497 operational hours) were completed collecting 198 vent sulfides, 83 altered
substrate and 43 fresh lava samples along with 104 black, gray and clear fluid samples using
gastight and major samplers. ABE successfully completed 14 high resolution bathymetric, CTD
and magnetic field mapping dives covering a total of 364 line km of seafloor.
We located and mapped in detail the Vienna Woods and nearby Tufar-2 and -3 vent areas on
Manus Spreading Center documenting the strong tectonic control on the distribution of the vent
systems and the presence of reduced magnetization i.e. “magnetic burnholes”, that help define
the lateral extent of the vent fields. The Vienna Woods vent systems (273°-285°C) form treetrunk-
like chimneys 5-15 m tall, that emit black to gray fluids with pH and compositions similar to
other documented midocean ridge (MOR) systems like the East Pacific Rise. At PACMANUS,
high-resolution mapping by ABE reveals a distinctive seafloor morphology associated with
dacitic lava flows along with discrete magnetic burnholes associated with the active venting
systems of Roman Ruins, Satanic Mills, Snowcap, Tsukushi and a new vigorous vent system
discovered southeast of the Satanic Mills area named Fenway. Another vent field in its waning
stages was also discovered ~8 km northeast of PACMANUS on the Northeast Pual Ridge. At
PACMANUS, the 40 m diameter Fenway mound hosts outcrops of massive anhydrite on the
seafloor beneath the sulfide chimneys, a rare occurrence as anhydrite is unstable at ambient
seafloor conditions. Fenway is also boiling (356°C, 172 bar) with two-phase fluid producing a
”flashing” phenomenon when the Jason lights illuminated the vent orifices. The five
PACMANUS vents (271° – 356°C) have ubiquitous low pH (2.3 to 2.8) relative to Vienna Woods
and typical MOR fluids, presumably reflecting water-rock reaction with the felsic hosted lava,
input of magmatic volatiles and the subsurface deposition of metal sulfides.
We investigated two strongly magmatically influenced vent systems associated with seafloor
volcanoes. Desmos is a breached caldera with white smokers (70°-115°C) that are highly acidic
(pH 1 – 1.5) and sulfur lava flows. SuSu Knolls and the adjacent Suzette mound (Solwara-1 of
Nautilus Minerals) were mapped in detail and sampled intensively. Hydrothermal activity at
SuSu Knolls showed a remarkable range from boiling black smokers to white sulfur-rich fluids,
native sulfur flows and massive anhydrite outcrops. Vent fluids from North Su (48° – 325°C) are
2
characterized by a measured pH of 0.87, more than an order of magnitude more acidic than any
deep-sea vent fluid sampled to date. Many of the low pH fluids sampled at North Su and
Desmos were actively precipitating native sulfur creating thick plumes of dense white smoke. In
general, sampled fluids show a considerable range in pH and gas contents, sometimes within
individual hydrothermal fields. The pronounced variability of fluid chemistry within 10’s to 100’s
of m at North Su is probably unparalleled in systems studied to date. The most plausible
explanation for the observed variability is that different fluid-rock reaction pathways are
expressed in regimes of variable magmatic volatile input and extent of subsurface cooling. This
hypothesis is supported by the distribution of alteration types at the seafloor, where the
occurrence of advanced argillic alteration - that relates to interactions with acid-sulfate waters
such as sampled at Desmos and North Su – is patchy and spatially confined to patches of
active (Desmos, North Su) and past (Snowcap) venting of such fluids.
In relationship to the ODP drilling results at PACMANUS we identified and sampled examples of
advanced argillic rock alteration similar to that seen in the drill core. Good examples came from
Snowcap and from the North Su pillar. We sampled highly clay-altered basement from just
underneath extinct chimney complexes at two locations in the Satanic Mills hydrothermal field.
Both samples have dense networks of sulfide veins and may represent the stockwork or feeder
zone through which hydrothermal fluids rise up to the seafloor. These samples, in addition to
the other altered rock types recovered, will provide useful stepping stones in bridging the
knowledge gap between the extensive surface sampling now accomplished and the basement
rocks recovered by ODP, where coring was almost nil shallower than 40 m subseafloor depth.
Overall, the quality and quantity of solid and fluid samples that can be put in a direct
geochemical context is remarkably high. This unique dataset encompasses a broad range of
geological environments that includes hydrothermal activity in basalt-hosted oceanic style
spreading centers to hydrothermal systems associated with arc-style volcanism. For the first
time, alteration assemblages that are commonly observed in drillcore and outcrop on land have
been observed in the aqueous environment responsible for their formation.
Description:
NSF Grant – OCE0327448; NSF Grant – OCE0425591
Keywords:
Hydrothermal vents
;
Pacific Ocean
Repository Name:
Woods Hole Open Access Server
Type:
Working Paper
Format:
application/pdf
Format:
application/zip
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