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This paper presents mineralogical, chemical and morphological information on the oxyhydroxides from …


Biology Articles » Geobiology » Microbial and hydrothermal aspects of ferric oxyhydroxides and ferrosic hydroxides: the example of Franklin Seamount, Western Woodlark Basin, Papua New Guinea » Iron oxides in other settings

Iron oxides in other settings
- Microbial and hydrothermal aspects of ferric oxyhydroxides and ferrosic hydroxides: the example of Franklin Seamount, Western Woodlark Basin, Papua New Guinea

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 [60] Galapagos Rift, Southeast Pacific;[61] Axial Seamount, Northeast Pacific;[47,61] Kasuga Seamount, Northern Mariana Arc; [61] Guaymas Basin, Gulf of California[62] and 21°N, East Pacific Rise. [62] 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. [61]); at Guaymas Basin the deposits contain voluminous hydrothermal carbonate[63] and the fluids are very alkaline. [62] 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.

Iron formations

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[64] 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. [65] 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 data).

Hematite forms from the slow dehydration or thermal transformation of ferrihydrite. [27] Carlson and Schwertmann[67] 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,[8] 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[26] 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). [55]. 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 and Bricker[64] 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 reducing bacteria;[68] 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[65] and iron exhalites associated with massive sulfides such as "tetsusekiei" flanking and overlying Kuroko massive sulfide deposits in Japan. [69] 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[70] 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. [71] 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. [72] 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. [73] 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,[74] 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.

 

 


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