Generation of banded iron formations by internal dynamics and leaching of oceanic crust Export

@article{citeulike:6097053, abstract = {The chemical signatures and mineralogy of banded iron formations have the potential to provide information about the ocean environment on early Earth1, 2, 3, 4, 5, 6, 7. Their formation requires iron- and silicon-rich fluids, but the mechanisms by which the alternating layers of Si- and Fe-rich rock formed remain controversial8, 9, 10, 11. Here we use thermodynamic calculations to show that Fe- and Si-rich fluids can be generated by hydrothermal leaching of low-Al oceanic crustal rocks such as komatiites. We find that positive feedbacks occur among the chemical reactions when hydrothermal fluids mix with ambient sea water. These feedbacks lead to alternating precipitation of Fe and Si minerals, owing to the formation of complexes between Fe(II) and silicic acid. We suggest that the small-scale (<1 cm) banding was produced by internal dynamics of the geochemical system, rather than any external forcing. As the Archaean eon progressed, the oceanic crust produced was rich in Al12. When Al-rich crust undergoes hydrothermal alteration, Fe is locked in Al–Fe silicate minerals. This results in iron-depleted hydrothermal fluids, and thus prevents the deposition of Fe-rich minerals. We therefore conclude that the widespread cessation of banded iron formation deposition 1.7 billion years ago reflects the changing composition of the oceanic crust.}, author = {Wang, Yifeng and Xu, Huifang and Merino, Enrique and Konishi, Hiromi}, citeulike-article-id = {6097053}, citeulike-linkout-0 = {http://dx.doi.org/10.1038/ngeo652}, citeulike-linkout-1 = {http://dx.doi.org/10.1038/ngeo652}, day = {11}, doi = {10.1038/ngeo652}, issn = {1752-0894}, journal = {Nature Geoscience}, keywords = {archaean, bif, iron}, month = {October}, number = {11}, pages = {781--784}, posted-at = {2009-11-11 07:44:04}, priority = {2}, publisher = {Nature Publishing Group}, title = {Generation of banded iron formations by internal dynamics and leaching of oceanic crust}, url = {http://dx.doi.org/10.1038/ngeo652}, volume = {2}, year = {2009} }

TY - JOUR ID - citeulike:6097053 L3 - citeulike-article-id:6097053 N2 - The chemical signatures and mineralogy of banded iron formations have the potential to provide information about the ocean environment on early Earth1, 2, 3, 4, 5, 6, 7. Their formation requires iron- and silicon-rich fluids, but the mechanisms by which the alternating layers of Si- and Fe-rich rock formed remain controversial8, 9, 10, 11. Here we use thermodynamic calculations to show that Fe- and Si-rich fluids can be generated by hydrothermal leaching of low-Al oceanic crustal rocks such as komatiites. We find that positive feedbacks occur among the chemical reactions when hydrothermal fluids mix with ambient sea water. These feedbacks lead to alternating precipitation of Fe and Si minerals, owing to the formation of complexes between Fe(II) and silicic acid. We suggest that the small-scale (<1 cm) banding was produced by internal dynamics of the geochemical system, rather than any external forcing. As the Archaean eon progressed, the oceanic crust produced was rich in Al12. When Al-rich crust undergoes hydrothermal alteration, Fe is locked in Al–Fe silicate minerals. This results in iron-depleted hydrothermal fluids, and thus prevents the deposition of Fe-rich minerals. We therefore conclude that the widespread cessation of banded iron formation deposition 1.7 billion years ago reflects the changing composition of the oceanic crust. IS - 11 JF - Nature Geoscience SN - 1752-0894 EP - 784 TI - Generation of banded iron formations by internal dynamics and leaching of oceanic crust PB - Nature Publishing Group VL - 2 SP - 781 KW - archaean KW - bif KW - iron AU - Wang, Yifeng AU - Xu, Huifang AU - Merino, Enrique AU - Konishi, Hiromi PY - 2009/10/11/ UR - http://dx.doi.org/10.1038/ngeo652 ER -