Early diagenetic quartz formation at a deep iron oxidation front in the eastern Equatorial Pacific; a modern analogue for banded iron/chert formations?

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doi: 10.1016/j.gca.2014.03.035
Author(s): Meister, Patrick; Chapligin, Bernhard; Picard, Aude; Meyer, Hanno; Fischer, Cornelius; Rettenwander, Daniel; Amthauer, Georg; Vogt, Christoph; Aiello, Ivano W.
Author Affiliation(s): Primary:
University of Vienna, Department of Geodynamics and Sedimentology, Vienna, Austria
Max Planck Institute for Marine Microbiology, Germany
Alfred Wegener Institute Helmholtz Centre for Polar and Marine Research, Germany
University of Bremen, Germany
University of Salzburg, Austria
Moss Landing Marine Laboratories, United States
Volume Title: Geochimica et Cosmochimica Acta
Source: Geochimica et Cosmochimica Acta, Vol.137, p.188-207. Publisher: Elsevier, New York, NY, International. ISSN: 0016-7037 CODEN: GCACAK
Note: In English. 135 refs.; illus., incl. 2 tables, sketch map
Summary: The mechanisms of early diagenetic quartz formation under low-temperature conditions are still poorly understood. In this study we investigated lithified cherts consisting of microcrystalline quartz recovered near the base of a 420 m thick Miocene-Holocene sequence of nannofossil and diatom ooze at a drill site in the Eastern Equatorial Pacific (Ocean Drilling Program Site 1226). Precipitation seems still ongoing based on a sharp depletion in dissolved silica at the depth of the cherts. Also, palaeo-temperatures reconstructed from δ18O values in the cherts are in the range of adjacent porewater temperatures. Opal-A dissolution appears to control silica concentration throughout the sequence, while the solution remains oversaturated with respect to quartz. However, at the depth of the sharp depletion in dissolved silica, quartz is still saturated while the more soluble silica phases are strongly undersaturated. Hence, precipitation of quartz was initiated by an auxiliary process. A process, previously observed to assist in the nucleation of quartz is the adsorption of silica on freshly precipitated iron oxides. Indeed, a deep iron oxidation front is present at 400m below seafloor, which is caused by upward diffusing nitrate from an oxic seawater aquifer in the underlying oceanic crust. Sequential iron extraction showed a higher content of the adsorbed iron hydroxide fraction in the chert than in the adjacent nannofossil and diatom ooze. X-ray absorption near-edge structure (XANES) spectroscopy revealed that iron in the cherts predominantly occurs in illite and amorphous iron oxide, whereas iron in the nannofossil and diatom ooze occurs mainly in smectite. Mossbauer spectroscopy also indicated the presence of illite that is to 97% oxidized. Two possible mechanisms may be operative during early diagenetic chert formation at iron oxidation fronts: (1) silica precipitation is catalysed by adsorption to freshly precipitated iron oxide surfaces, and (2) porewater silica concentration is locally decreased below opal-A and opal-CT saturation allowing for precipitation of the thermodynamically more stable phase: quartz. This mechanism of chert formation at the iron oxidation front in suboxic zones may explain why early-diagenetic microcrystalline chert only occurs sporadically in modern marine sediments. It may also serve as a modern analogue for the deposition of much more abundant banded iron/chert formations at the time of the great oxidation event around 2.4Ga BP, which was probably the largest iron oxidation front in Earth's history. Abstract Copyright (2014) Elsevier, B.V.
Year of Publication: 2014
Research Program: ODP Ocean Drilling Program
Key Words: 02 Geochemistry; 06 Petrology, Sedimentary; Algae; Atomic absorption spectra; Banded iron formations; Cenozoic; Chemically precipitated rocks; Chert; Clastic sediments; Deep-sea environment; Depositional environment; Diagenesis; Diatoms; EXAFS data; Early diagenesis; East Pacific; Equatorial Pacific; Framework silicates; Holocene; Iron; Iron formations; Isotope ratios; Isotopes; Leg 201; Marine environment; Metals; Microfossils; Miocene; Mossbauer spectra; Nannofossils; Neogene; O-18/O-16; ODP Site 1226; Ocean Drilling Program; Ooze; Oxidation; Oxygen; Pacific Ocean; Paleoenvironment; Paleotemperature; Plantae; Quartz; Quaternary; SEM data; Sedimentary rocks; Sediments; Sequential extraction; Silica minerals; Silicates; South Pacific; Southeast Pacific; Spectra; Stable isotopes; Tertiary; X-ray fluorescence spectra; X-ray spectra; XANES spectra
Coordinates: S030600 S030600 W0904900 W0904900
Record ID: 2014060421
Copyright Information: GeoRef, Copyright 2019 American Geosciences Institute. Reference includes data from CAPCAS, Elsevier Scientific Publishers, Amsterdam, Netherlands