Other seafloor deposits
Low temperature vent fluid samples, defined as those deposited at
temperatures of less than 50°C, have been gathered from numerous
seafloor sites such as Loihi Seamount, Hawai;i  Galapagos Rift, Southeast Pacific; Axial Seamount, Northeast Pacific;[47,61] Kasuga Seamount, Northern Mariana Arc;  Guaymas Basin, Gulf of California and 21°N, East Pacific Rise.  Fig. 9 presents the measured pH of the vent fluids at each of these sites in comparison to the calculated pH of the triple point of Fe2+, Fe(OH)3 and Fe3(OH)8 for the temperature and iron content of the vent fluids at each of the sites.
At sites dominated by the active deposition of amorphous iron
oxyhydroxides, such as at Franklin Seamount, Loihi Seamount and
Galapagos, the agreement between the measured and calculated pH is
good, suggesting that the iron in the vent fluid is in equilibrium with
the triple point separating the Fe2+ and the solid phases.
In contrast, there is little coincidence between the measured and
calculated pH at Kasuga Seamount, Guaymas Basin and one of the sites at
21°N suggesting that the iron is not in equilibrium here. For example,
the deposits at Kasuga Seamount have considerable elemental sulfur and
the fluids contain H2S (ref. ); at Guaymas Basin the deposits contain voluminous hydrothermal carbonate and the fluids are very alkaline.  The Fe–O–H buffer of the pH and f(O2) at the vent site appears to be operative only in the absence of H2S and CO2. Fig. 9
suggests that the processes described for Franklin Seamount and in
soils are occurring at those other seafloor deposits that are dominated
by iron oxyhydroxides.
Comparisons with ancient oxide-facies iron formation are made by
modeling geochemically the mineral equilibria of phases that are
postulated to be stable analogues of those found on the modern
seafloor. Klein and Bricker
conducted a detailed thermodynamic study of Proterozoic iron formation
using minerals considered to be early diagenetic in origin. These
mineral equilibria were recalculated from the Klein and Bricker free
energies using the iron and silicon contents in the vent fluid from
Franklin Seamount in Table 1 and plotted in Fig. 10.
The results suggest that the mixed valence phase magnetite would
predominate if the seafloor oxyhydroxides were preserved unoxidized,
providing Fe(OH)3 is in equilibrium with magnetite. Magnetite is commonly a major constituent in Precambrian iron-formation.  Primary non-tronite could be replaced by stilpnomelane or greenalite. [64,66]
Considerable amounts of stilpnomelane are found in the Key Tuffite, a
silica-iron exhalative horizon lateral to the Archean Matagami Lake
massive sulfide deposit in north-western Quebec (Boyd, unpublished
Hematite forms from the slow dehydration or thermal transformation of ferrihydrite.  Carlson and Schwertmann
found that this transformation is inhibited by the incorporation of
silicon into the material. The amorphous iron oxyhydroxides from
Franklin Seamount contain considerable amounts of silicon intimately
associated with the iron,
so it would be expected that the transformation to magnetite from
ferrosic hydroxides would occur much more rapidly than the formation of
hematite from ferrihydrite. This is supported by the near absence of
preserved ferrosic hydroxide in nature in contrast with the widespread
occurrence of ferrihydrite and magnetite.
Schwab and Lindsay determined that Fe(OH)3 (ferrihydrite) and magnetite together are more stable relative to Fe3(OH)8,
so the former two phases should replace the latter. The result is that
magnetite in iron formation is probably not in equilibrium with
hematite, but rather with its precursor Fe(OH)3. The phase boundary between hematite and magnetite (dashed line in Fig. 10) is at such a low f(O2)
that the latter's stability field is usurped by iron silicates. The
boundary between goethite and magnetite is similarly at a very low f(O2). .
Therefore, goethite or hematite can precipitate with or replace
magnetite only at very low Si concentration in an extremely reduced
environment, so it would be expected that the occurrence of magnetite
would be rare in iron formation if this were its means of formation.
However, as magnetite is a major constituent of banded iron formation
these conditions for replacement must not have prevailed.
As shown in Fig. 8,
hematite originates from ferrihydrite formed as a by-product to the
crystallization of magnetite, and/or due to the microbial or
hydrothermal oxidation of ferrosic hydroxides and/or precipitated
directly from hydrothermal solutions under aerobic conditions. Klein
noted that the calculated Eh–pH equilibria for magnetite could be too
wide due to a lack of thermodynamic data on its precursor. However, the
results of this investigation suggest that the authors successfully
modeled the early diagenetic mineral assemblages of iron formation with
the ferric hydroxides forming in equilibrium with magnetite.
The evidence that ferrosic hydroxide and ferrous trioctahedral
smectite are precipitating directly on the seafloor provides an insight
into the mineral assemblages of Algoma-type Archean banded iron
formation (BIF). Magnetite in BIF could be derived from ferric iron
however, a suboxic to anoxic seafloor environment, combined with a
paucity of microbial activity would also preserve magnetite and iron
silicates such as greenalite. An increase in f(O2),
and/or microbial activity, would result in the dominance of hematite
consistent with that found in ironstones of Cenozoic and Mesozoic age
and iron exhalites associated with massive sulfides such as
"tetsusekiei" flanking and overlying Kuroko massive sulfide deposits in
The above explanation is consistent with both the gradual oxygenation
of the atmosphere and the evolution of the earth's biosphere since the
Archean. According to Klein
the average chemical compositions of BIF from 3.8 to 1.8 Ga (billion
years) are very similar with a large amount of the iron occurring as Fe2+ while 95% of the total iron in Phanerozoic BIF, deposited between 0.8 to 0.6 Ga, is Fe3+.
An understanding of the role of iron bacteria in modern vents has
direct bearing on the interpretation of their fossils in ancient rocks.
Duhig et al. 
describe the preservation of abundant microbial fossils and iron
agglomerates in unmetamorphosed Cambrian silica–iron exhalites lateral
to a massive sulfide deposit. The authors suggest that iron oxidizing
bacteria mediated the deposition of the iron by catalyzing the
oxidation of the Fe2+ in the fluid to Fe3+. Iron
mineralization bound within and on bacterial cell fossils have been
detected in 2.0 Ga red chert from the Gunflint Formation. 
Sub-greenschist facies ironstone pods and BIF in the Archean Barberton
greenstone belt are dominated by ferric oxides, but also contain early
magnetite and indications that hematite and goethite pseudomorphed
euhedral magnetite. 
Delicate features suggestive of relict hydrothermal discharge such as
chimney structures, honeycomb-like cavities and fluid-flow channels
were identified in the well preserved Barberton rocks, but petrographic
studies found no indication of microfossils. Fossils of
filament-forming microbes up to 3.5 billion years old have been
reported, although little is known how common their occurrence was during the Archean.
Previous studies have proposed that the amorphous iron oxyhydroxides
were precipitated, directly or indirectly, due to microbial activity
and suggest this may have played an important role in the formation of
BIF. However, our investigation cautions that microbial micro-textures
in ancient BIF cannnot be assumed to be primary. Instead, they could
have formed by early diagenetic reactions similar to those in the
oxyhydroxide deposits at Franklin Seamount.