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An authigenic origin for Precambrian greenalite; implications for iron formation and the chemistry of ancient seawater

Nicholas J. Tosca, Stephen Guggenheim and Peir K. Pufahl
An authigenic origin for Precambrian greenalite; implications for iron formation and the chemistry of ancient seawater
Geological Society of America Bulletin (2015) 128 (3-4): 511-530

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 Fe (super 2+) and SiO (sub 2) (aq) in anoxic seawater-derived solutions promote rapid nucleation of a hydrous Fe(II)-silicate gel at 25 degrees C. By merging experimental data with crystallographic constraints, we observe that structural rearrangement and dehydration produce Fe-rich serpentine nanoparticles within the gel, which eventually 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 Fe (super 2+) concentrations in nonsulfidic seawater. The primary precipitation of greenalite and/or siderite would set a ceiling for dissolved Fe (super 2+) 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.


ISSN: 0016-7606
EISSN: 1943-2674
Coden: BUGMAF
Serial Title: Geological Society of America Bulletin
Serial Volume: 128
Serial Issue: 3-4
Title: An authigenic origin for Precambrian greenalite; implications for iron formation and the chemistry of ancient seawater
Affiliation: University of Oxford, Department of Earth Sciences, Oxford, United Kingdom
Pages: 511-530
Published: 2015
Text Language: English
Publisher: Geological Society of America (GSA), Boulder, CO, United States
References: 187
Accession Number: 2016-014755
Categories: Mineralogy of silicatesStratigraphy
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus.
Secondary Affiliation: University of Illinois-Chicago, USA, United StatesAcadia University, CAN, Canada
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2017, American Geosciences Institute.
Update Code: 201608
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