An authigenic origin for Precambrian greenalite: Implications for iron formation and the chemistry of ancient seawater

Abstract

Research Article| March 01, 2016 An authigenic origin for Precambrian greenalite: Implications for iron formation and the chemistry of ancient seawater Nicholas J. Tosca; Nicholas J. Tosca † 1Department of Earth Sciences, University of Oxford, South Parks Road, Oxford OX1 3AN, UK †nickt@earth.ox.ac.uk Search for other works by this author on: GSW Google Scholar Stephen Guggenheim; Stephen Guggenheim 2Department of Earth and Environmental Sciences, University of Illinois at Chicago, Chicago, Illinois 60607, USA Search for other works by this author on: GSW Google Scholar Peir K. Pufahl Peir K. Pufahl 3Department of Earth and Environmental Science, Acadia University, Wolfville, NS B4P 2R6, Canada Search for other works by this author on: GSW Google Scholar Author and Article Information Nicholas J. Tosca † 1Department of Earth Sciences, University of Oxford, South Parks Road, Oxford OX1 3AN, UK Stephen Guggenheim 2Department of Earth and Environmental Sciences, University of Illinois at Chicago, Chicago, Illinois 60607, USA Peir K. Pufahl 3Department of Earth and Environmental Science, Acadia University, Wolfville, NS B4P 2R6, Canada †nickt@earth.ox.ac.uk Publisher: Geological Society of America Received: 20 Apr 2015 Revision Received: 11 Sep 2015 Accepted: 12 Oct 2015 First Online: 09 Mar 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 © 2015 Geological Society of America GSA Bulletin (2016) 128 (3-4): 511–530. https://doi.org/10.1130/B31339.1 Article history Received: 20 Apr 2015 Revision Received: 11 Sep 2015 Accepted: 12 Oct 2015 First Online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn Email Permissions Search Site Citation Nicholas J. Tosca, Stephen Guggenheim, Peir K. Pufahl; An authigenic origin for Precambrian greenalite: Implications for iron formation and the chemistry of ancient seawater. GSA Bulletin 2016;; 128 (3-4): 511–530. doi: https://doi.org/10.1130/B31339.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract Persistent anoxia and the lack of a skeletal silica sink through the Precambrian would have promoted a variety of reactions between iron and dissolved silica through much of Earth's early history. However, although both iron and silica have each left clear fingerprints in the Precambrian record, evidence for their interaction, and the attendant biogeochemical consequences, is cryptic. Here, experimental evidence is presented showing that Fe2+ and SiO2(aq) in anoxic seawater–derived solutions promote rapid nucleation of a hydrous Fe(II)-silicate gel at 25 °C. By merging experimental data with crystallographic constraints, we observe that structural rearrangement and dehydration produce Fe-rich serpentine nanoparticles within the gel, which eventally aggregate to form the mineral greenalite. This nonclassical crystal growth pathway is consistent with the crystal structure of greenalite and with its syndepositional origin in iron formation. A mechanistic underpinning for greenalite precipitation also permits new constraints on the chemistry of ferruginous Precambrian waters. For example, greenalite may have nucleated from waters with a pH as high as 7.7–8.3, implicating alkalinity as a key trigger in coupling and decoupling Fe and Si during the anoxic deposition of several late Archean and Paleoproterozoic iron formations. The common, though not exclusive, association of greenalite with deeper-water iron formation facies (i.e., below the fair-weather wave base) suggests that the upwelling of silica-rich alkaline water masses played an important role in driving precipitation. More broadly, our results prompt a reconsideration of the inorganic reactions that determine the upper limits on water-column Fe2+ concentrations in nonsulfidic seawater. The primary precipitation of greenalite and/or siderite would set a ceiling for dissolved Fe2+ that is sensitive to pH, and higher than previously estimated. These results indicate that a better understanding of greenalite distributions in chemical and siliciclastic sediments will help to disentangle the coevolution of redox and acid-base chemistries through the Precambrian. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.