8 items with the tag “bif

  • Alternative Formation Mechanisms for Banded Iron Formations
    NAI 2009 University of Wisconsin Annual Report

    Based on field observations during the 2008 BIF filed trip and laboratory investigation, we have proposed a new generation mechanism for the banded iron formations, very important rocks that recorded early earth conditions and environment. Our new model shows how BIFs could have formed when hydrothermal fluids from the interaction between seawater and komatiite (Al-depleted rock) that comes from hot and deep Earth’s mantle, mixed with surface seawater. This mixing triggered the dynamics of oscillation between iron- and silica-rich minerals, which were deposited in layers on the seafloor.

    ROADMAP OBJECTIVES: 4.1
  • New Frontiers in Micro-Analysis of Isotopic Compositions of Natural Materials: Development of O, S, Si, and Li Isotopes
    NAI 2009 University of Wisconsin Annual Report

    The isotope ratios of oxygen and silicon are a sensitive monitor of sedimentary and hydrothermal processes for deposition of banded iron formation. Our focus on banded iron formations reflects the importance these unusual units have in biogeochemical cycling in the early Earth. In particular, we are examining the deposition of microlaminated sections of the Dales Gorge member of the Brockman Iron Formation from the Hamersley basin, which have alternatively been interpreted to represent annual varves in chemical precipitates from seawater, or variations in a sub-surface hydrothermal system. These conflicting models have relevance for interpreting the role of microbial life in precipitation of the Fe-oxides. In addition, δ18O and δ34S values from coexisting minerals can be used to estimate the temperature of the Archean ocean, or of the hydrothermal system. Values of δ7Li and δ18O in zircons allow these tests to be applied to magmas that may have assimilated sedimentary materials, and in the case of pre-4 Ga Jack Hills zircons from SW Australia, provide a record of the earliest Earth before the formation of all known rocks.

    ROADMAP OBJECTIVES: 4.1 7.1
  • Oxygen Isotopes of Apatites in Precambrian Banded Iron Formations
    NAI 2010 University of Hawaii, Manoa Annual Report

    Dominic Papineau from the Carnegie team, received an NAI research scholarship to come work with team member Gary Huss to use the ion microprobe to look at apatite grains in a suite of Precambrian banded iron formations in order to document the ranges of d18O values dot see if this technique could be used for biosignature studies in banded iron formations.

    ROADMAP OBJECTIVES: 4.1
  • Project 3D: Stable Isotope and Mineralogical Studies of Banded Iron Formations: O & Si Isotopes by SIMS
    NAI 2010 University of Wisconsin Annual Report

    The oxygen isotope ratio of modern seawater is 0‰ (δ18O, VSMOW) and the oceans are thought to partly balance high δ18O crustal rocks relative to a primary mantle δ18O value of 5.5‰. Isotope ratios of O and Si from cherts up to 3.5 Ga have been controversially interpreted to reflect either a hot Archean ocean of 50-70°C or a low δ18O Archean ocean of -10 to -13‰. These interpretations assume that cherts record the primary seawater δ18O. We have conducted in situ SIMS analysis of oxygen and silicon isotope ratios in cherts from banded iron formations (BIFs) at Isua, Greenland (3.8 Ga); Hamersley, Western Australia (2.5 Ga); Transvaal, South Africa (2.5 Ga), and Biwabik, Minnesota, USA (1.9 Ga). Correlated values of δ18O and δ30Si are used to test assumptions about the degree to which these isotope ratios record ocean compositions, or exchange during diagenesis or metamorphism. Silicon isotopes may also have the potential to distinguish between continental (δ30Si > -0.4‰) and hydrothermal sources of Si in BIF cherts.

    Oxide facies BIFs are essentially composed of equal parts quartz and iron oxides. Magnetite and hematite are the dominant Fe-oxides and the paragenesis of these minerals is important to understanding the fluid and thermal history of unmetamorphosed or low-temperature (sub-greenschist facies) BIFs. We identified silician magnetite overgrowths by BSE-SEM and used three diffraction techniques to verify that silicon is structural in magnetite. Silician magnetite overgrowths may form in reducing alkaline conditions during BIF diagenesis and metamorphism. We have observed silician magnetite in several low-temperature BIFs from 2.6 to 1.9 Ga and hypothesize that the former presence of organic matter may be required to attain the low oxygen fugacity necessary to stabilize silicon in magnetite. Silician magnetite is thus proposed as a novel biosignature.

    ROADMAP OBJECTIVES: 4.1 7.1
  • Project 4A: Improving Accuracy of in Situ Stable Isotope Analysis by SIMS
    NAI 2010 University of Wisconsin Annual Report

    In situ analysis of isotope ratios of oxygen, sulfur, and iron by SIMS provides a new record of biological, sedimentary, and hydrothermal processes in banded iron formations (BIFs). BIFs formed throughout several broad secular changes in atmospheric and oceanic conditions during the Archean and Proterozoic and provide a non-uniformitarian example of biogeochemical cycling on the early Earth. We have focused on the well-known BIF from the Dales Gorge member of the Brockman Iron Formation, Hamersley Group, Western Australia; alternations of Si- and Fe-rich microlaminae are alternately interpreted as annual varves, formed by oscillations in hydrothermal activity, or due to the internal dynamics of Fe and Si complexes during diagenesis. The question of whether these models are mutually exclusive or act in combination to form the characteristic banding of BIFs is relevant to interpreting the role of microbial life in precipitation of the Fe-oxides. Isotope ratios of oxygen and sulfur from coexisting mineral pairs can provide a temperature estimate of the rock, or the composition of pore fluids during diagenesis and subsequent metamorphism of BIFs. However, many BIF oxides are chemically zoned and/or in cross-cutting relationships. For these textures, oxygen isotope ratios of iron oxides reflect the changing thermal and fluid history of BIFs. Precision and accuracy of in situ stable isotope analysis of ultra-small spots by SIMS have been improved by careful evaluation of sample relief, X-Y effects, crystal orientation, and standardization. Small spot oxygen and sulfur isotope analyses down to 1 μm diameter are pushing the analytical limit for accurate SIMS analysis.

    ROADMAP OBJECTIVES: 4.1 7.1
  • Project 3A: Stable Isotope and Mineralogical Studies of Banded Iron Formations - O and Si Isotopes by SIMS
    NAI 2011 University of Wisconsin Annual Report

    We have studied the mineralogy and microscopic textures of four banded iron formations (BIFs): Isua, Greenland (3.8 Ga); Hamersley, Western Australia (2.5 Ga); Transvaal, South Africa (2.5 Ga), and Biwabik, Minnesota, USA (1.9 Ga). These rocks have been interpreted to preserve records of conditions and chemistry of Precambrian oceans. This hypothesis is evaluated with in situ SIMS analysis of oxygen (δ18O) and silicon isotope ratios (δ30Si) in chert domains within the BIFs. The paragenesis of magnetite and hematite is important to understanding the fluid and thermal history of unmetamorphosed or low-temperature (sub-greenschist facies) oxide-facies BIFs. We identified silician (up to 3 wt% SiO2) magnetite overgrowths in samples of the Dales Gorge BIF from Wittenoom. We hypothesize that silician magnetite is stabilized by organic matter and thus is a biosignature. We investigated the distribution of δ18O values in quartz, magnetite, silician magnetite, and hematite by SIMS and found that while quartz is homogeneous (average δ18O = 22.0 ±1.3‰ 2SD, VSMOW), values of δ18O for magnetite and hematite vary by 13‰ (-5 to +7‰) and are correlated with different generations of magnetite and hematite.

    ROADMAP OBJECTIVES: 4.1 5.2 7.1
  • Project 2D: Establishing Biogenicity and Environmental Setting of Precambrian Kerogen and Microfossils
    NAI 2011 University of Wisconsin Annual Report

    Stable carbon isotope ratios preserved in ancient organic matter provide valuable information about the origin and evolution of metabolic pathways, and the evolution of the carbon cycle in general. In situ carbon isotope analysis allows organic matter to be measured in petrographic context. The microtexture and spatial relationship of sedimentary organic matter and its mineral matrix offers clues about source and diagenetic history – essential elements in the interpretation of isotopic data. We have developed protocols with the IMS-1280 ion microprobe to analyze carbon isotope ratios in organic matter with a spot size of 1–10 μm. A suite of new standards allows correction of matrix effects due to variable chemical composition. Four suites of Proterozoic microfossils of uncontested biogenicity have been analyzed, showing distinct values of δ13C that correlate with taxonomy and metabolism (Williford et al. 2011a in prep). Studies of kerogen and pyrobitumen from 2.7–2.5 Ga sediments support prior conclusions about differential impacts of aerobiosis and methane metabolisms in shallow versus deep-water depositional environments and reveal a degree of isotopic heterogeneity obscured by conventional, bulk analyses (Williford et al, 2011b in prep). Analysis of organic matter from the 3.35 Ga Strelley Pool Chert yield low and consistent values of δ13C, consistent with a biogenic origin (Lepot et al. 2011, in prep).

    ROADMAP OBJECTIVES: 4.1 7.1
  • Project 3A: Stable Isotope and Mineralogical Studies of Banded Iron Formations - O Isotopes by SIMS
    NAI 2012 University of Wisconsin Annual Report

    During the past year, we have made advances in technique development for analysis of mineral chemistry and stable isotope ratios in minerals. Applications to magnetite in Banded Iron Formation (BIF) have lead to the proposal that silician magnetite forms only in low oxygen fugacity conditions and are thus a signature for the former presence of reduced organic matter. Petrography and in situ analysis of δ18O by SIMS has shown that the earliest quartz cements in 1.85 Ga Granular Iron formation (GIF) consistently have high δ18O showing that earlier reports of more variable compositions included altered material.

    ROADMAP OBJECTIVES: 4.1 4.2 7.2