Amorphous iron oxyhydroxides combined with variable amounts of manganese oxyhydroxides and silica are common in the oceans. [1-3]
but have not been as extensively studied as sulfide deposits. They are
typically found in areas of volcanic activity such as at mid-ocean and
back-arc spreading centers and on intraplate seamounts. Some are
spatially associated with sulfide deposits, usually around the margins
and, from their high base metal content, are clearly oxidation products
of the sulfides. Others, the subject of this paper, were precipitated
as oxides directly from hydrothermal fluids. These can form large
deposits up to hundreds of meters across. They are found both in
proximity to sulfide deposits and completely isolated from them. A
distinctive characteristic is that, regardless of their location, these
primary oxide deposits have a much lower content of base (except iron)
and precious metals than those that have formed by oxidation of
sulfides [3].
Primary oxyhydroxide deposits cover extensive > 100 m2 areas of Franklin Seamount at 2143–2366 m water depth in Western Woodlark Basin, Papua New Guinea (Fig. 1). [4,5]
The basaltic andesite volcano straddles an active spreading center
propagating westward into the Papua New Guinea continental margin. [6,7]
During the Soviet Union–Papua New Guinea–Australia–Canada (SUPACLARK)
expedition in 1990, numerous actively venting oxyhydroxide chimneys and
mounds were observed and sampled using the submersible Mir.
Analysis of samples gathered from the Franklin Seamount deposits
indicate they consist dominantly of amorphous iron oxyhydroxide
(two-XRD-line ferrihydrite), locally contain major amounts of
birnessite and nontronite, variable amounts of hydrothermal opal-A,
variable amounts of incorporated volcanic and biologically-derived
detritus, and minor vernadite and todorokite. [8] No authigenic mixed-valence or ferrous iron is preserved in the samples.
The
observation of the active creation of these chimneys together with
their distinctive chemical composition characterized by a paucity of
hydrothermally immobile elements such as Al, Ti and Zr (ref. [3])
suggests a primary abiotic hydrothermal control on the formation of
these types of deposits. Conversely, close examination of similar
materials by Juniper and Fouquet,[9] Alt[10] and Fortin[11]
found they possess complex filamentous micro-textures of apparent
bacterial origin. The central role of filamentous bacteria in the
precipitation of iron in acid mine drainage environments,[12] subterranean environments[13] and acidic surface hot springs[14] is well known. Fortin and Ferris[15,16]
discussed the capability of bacteria to provide nucleation sites
favorable for iron and manganese deposition. However, the importance of
such sites, whether or not due to active metabolic processes, are apt
to diminish under the near-neutral pH conditions of warm seafloor
springs because kinetic barriers to the chemical precipitation of iron
are greatly reduced at higher pH. [17]
In general, both the microbial and abiotic chemical processes that
occur at seafloor vent sites need to be considered in relation to the
genesis of large amorphous iron oxyhydroxide deposits because the free
energy of crystal nucleation (and thus precipitation) is constrained
thermo-dynamically by the bulk free energy of the solution and the
interfacial free energy of available surfaces. [16,17]
Their study is important because such marine deposits are similar in
their chemistry and geological setting to many ancient iron formations
found on land. [8,18]
Iron, because of its multi-valent nature, is a sensitive indicator
of its redox environment thus its behavior at the seafloor
sediment/water interface can serve as a tool to understanding the
physicochemical conditions for the precipitation of the oxyhydroxides
at Franklin Seamount. This paper presents mineralogical, chemical and
morphological information on the oxyhydroxides from Franklin Seamount
that is interpreted in light of the thermodynamics and kinetics of low
temperature hydrothermal iron systems, systematics of vent fluid
chemistry and previous research on bacterial processes and habitats.
These interpretations are used to explain the precipitation of iron
locally around the vent site and to make comparisons with analogous
environments associated with the precipitation of iron such as in other
seafloor locations, soils and ancient iron formations.
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