ISSN: 2642-1283
Authors: Cavalcanti JAD* and Santos RV
Ferromanganese crusts (or Fe-Mn crust) on the Rio Grande Rise (RGR) in the South Atlantic were analyzed on the view point their structures and chemical composition. The main structures of the ferromanganese crusts are wave-layered, columnar, botryoidal and oolite, which are very common in stromatolites. The microstratigraphic succession of the ferromanganese crusts of RGR comprises an older phosphatized crust on the base, and a younger non-phosphatized crust on the top. Mn and Fe are the main compositional elements of the young non-phosphatized crust, with an average concentration (wt%) of 19.04% and 16.07%, respectively. The Fe/Mn ratios average 0.85 indicate the hydrogenous origin of these deposits. In these ferromanganese crusts the average concentrations (wt%) of Co, Ni, Pb, Ti, Cu and Ce are 0.92 %, 0.34 %, 0.25 %, 0.03 %, 0.92 % and 0.15 %, respectively. By comparison, in the old phosphatized crust, the main compositional elements are CaO and P2O5, with an average concentration (wt%) of 34.77% and 15.93%, respectively. In this latter type of crust the ore forming elements content is less and only the Ni concentration is noticeable (0.41%), while the average concentrations (wt%) of Co, Ti, Cu, Pb and Ce are 0.18%, 0.04%, 0.2%, 0.02% and 0.02%, respectively. Graphs of CeSN/CeSN* ratio vs YSN/HoSN ratio indicate that old phosphatized crusts is diagenetic and young non phosphatized crust is hydrogenous. In spider diagrams of REY, old phosphatized crust show positive Y anomaly and young non phosphatized crust show positive anomaly of Ce and negative anomaly of Y. Based on available literature, the comparison between the microbial diversity in deep-sea ferromanganese crusts and in stromatolites confirms this similarity. Microbial assemblages in samples from the RGR showed the dominance of the classes gammaproteobacteria, alphaproteobacteria, and deltaproteobacteria, as described for others ferromanganese crusts deposits around the world (Western Pacific Ocean, Clarion-Clipperton zone of the Pacific Ocean, South China Sea, Central Atlantic Ocean and Central Indian Ocean Basin). The results led us to the interpretation that these crusts can be originated from biogenic-hydrogenous processes and they can represent a type of black stromatolites on the seabed, deposited between 17.6 and 14.6 Ma ago.
Keywords: Stromatolites; Fe-Mn Crusts; Rio Grande Rise; South Atlantic Ocean
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