Bruce M. Moskowitz

University of Minnesota Duluth, Duluth, Minnesota, United States

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Publications (96)233.35 Total impact

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    ABSTRACT: The exposure of mantle peridotite to water at crustal levels leads to a cascade of interconnected dissolution-precipitation and reduction-oxidation reactions - a process referred to as serpentinization. These reactions have major implications for microbial life through the provision of hydrogen (H2). To simulate incipient serpentinization under well-constrained conditions, we reacted cm-sized pieces of uncrushed harzburgite with chemically modified seawater at 300°C and 35 MPa for ca. 1.5 years (13441 hours), monitored changes in fluid chemistry over time, and examined the secondary mineralogy at the termination of the experiment. Approximately 4 mol % of the protolith underwent alteration forming serpentine, accessory magnetite, chlorite, and traces of calcite and heazlewoodite. Alteration textures bear remarkable similarities to those found in partially serpentinized abyssal peridotites. Neither brucite nor talc precipitated during the experiment. Given that the starting material contained ~4 times more olivine than orthopyroxene on a molar basis, mass balance requires that dissolution of orthopyroxene was significantly faster than dissolution of olivine. Coupled mass transfer of dissolved Si, Mg, and H+ between olivine and orthopyroxene reaction fronts was driven by steep activity gradients and facilitated the precipitation of serpentine. Hydrogen was released in significant amounts throughout the entire experiment; however, the H2 release rate decreased with time. Serpentinization consumed water but did not release significant amounts of dissolved species (other than H2) suggesting that incipient hydration reactions involved a volume increase of ~40%. The reduced access of water to fresh olivine surfaces due to filling of fractures and coating of primary minerals with alteration products led to decreased rates of serpentinization and H2 release. While this concept might seem at odds with completely serpentinized seafloor peridotites, reaction-driven fracturing offers an intriguing solution to the seemingly self-limiting nature of serpentinization. Indeed, the reacted sample revealed a number of textural features diagnostic of incipient reaction-driven fracturing. We conclude that fracturing must have far reaching impacts on the rates of serpentinization and H2 release in peridotite-hosted hydrothermal systems.
    American Mineralogist 10/2014; · 2.20 Impact Factor
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    Jessica L. Till, Bruce M. Moskowitz
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    ABSTRACT: We present observations of deformation features in magnetite from synthetic magnetite-bearing silicate aggregates deformed between 1000 °C and 1200 °C in transpressional shear experiments with strains of up to 300%. Anisotropy of magnetic susceptibility and shape preferred orientation (SPO) analysis were combined with electron backscatter diffraction (EBSD) to characterize the magnetite deformation fabrics and intragrain microstructures. Crystallographic preferred orientation (CPO) in magnetite is very weak in all deformed samples and does not vary as a function of either temperature or shear strain. Magnetic anisotropy and SPO increase strongly with both strain and deformation temperature and indicate that strain partitioning between magnetite and the plagioclase matrix decreases at higher temperatures. EBSD orientation mapping of individual magnetite particles revealed substantial dispersions in intragrain orientation, analogous to undulose extinction, after deformation at 1000 and 1100 °C, indicating that dislocation creep processes were active in magnetite despite the lack of a well-developed CPO. Geometrical analysis of crystallographic orientation dispersions from grain map data indicates that low-angle grain boundary formation in magnetite could have been accommodated by slip on {110} or {100} planes, but no evidence for dominant slip on the expected {111} planes was found. Evidence for activation of multiple slip systems was seen in some magnetite grains and could be partially responsible for the lack of CPO in magnetite. These results suggest that, at least in polyphase rocks, crystallographic textures in magnetite may be inherently weak or slow to develop andCPO alone is not an adequate indicator of magnetite deformation mechanisms. These results may aid in the interpretation of deformation textures in other spinel-structured phases such as chromite and ringwoodite.
    Tectonophysics 08/2014; · 2.68 Impact Factor
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    ABSTRACT: Iron oxide minerals typically compose only a few weight percent of bulk atmospheric dust but are important for potential roles in forcing climate, affecting cloud properties, influencing rates of snow and ice melt, and fertilizing marine phytoplankton. Dust samples collected from locations across eastern Australia (Lake Cowal, Orange, Hornsby, and Sydney) following the spectacular “Red Dawn” dust storm on 23 September 2009 enabled study of the dust iron oxide assemblage using a combination of magnetic measurements, Mössbauer spectroscopy, reflectance spectroscopy, and scanning electron microscopy. Red Dawn was the worst dust storm to have hit the city of Sydney in more than 60 years, and it also deposited dust into the Tasman Sea and onto snow cover in New Zealand. Magnetization measurements from 20 to 400 K reveal that hematite, goethite, and trace amounts of magnetite are present in all samples. Magnetite concentrations (as much as 0.29 wt%) were much higher in eastern, urban sites than in western, agricultural sites in central New South Wales (0.01 wt%), strongly suggesting addition of magnetite from local urban sources. Variable temperature Mössbauer spectroscopy (300 and 4.2 K) indicates that goethite and hematite compose approximately 25–45% of the Fe-bearing phases in samples from the inland sites of Orange and Lake Cowal. Hematite was observed at both temperatures but goethite only at 4.2 K, thereby revealing the presence of nanogoethite (less than about 20 nm). Similarly, hematite particulate matter is very small (some of it d < 100 nm) on the basis of magnetic results and Mössbauer spectra. The degree to which ferric oxide in these samples might absorb solar radiation is estimated by comparing reflectance values with a magnetic parameter (hard isothermal remanent magnetization, HIRM) for ferric oxide abundance. Average visible reflectance and HIRM are correlated as a group (r2 = 0.24), indicating that Red Dawn ferric oxides have capacity to absorb solar radiation. Much of this ferric oxide occurs as nanohematite and nanogoethite particles on surfaces of other particulate matter, thereby providing high surface area to enhance absorption of solar radiation. Leaching of the sample from Orange in simulated human-lung fluid revealed low bioaccessibility for most metals.
    Aeolian Research 03/2014; · 2.52 Impact Factor
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    Geology 01/2014; 42(2). · 4.64 Impact Factor
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    ABSTRACT: Of the two nanocrystal (magnetosome) compositions biosynthesized by magnetotactic bacteria (MTB), the magnetic properties of magnetite magnetosomes have been extensively studied using widely available cultures, while those of greigite magnetosomes remain poorly known. Here we have collected uncultivated magnetite- and greigite-producing MTB to determine their magnetic coercivity distribution and ferromagnetic resonance (FMR) spectra and to assess the MTB-associated iron flux. We find that compared with magnetite-producing MTB cultures, FMR spectra of uncultivated MTB are characterized by a wider empirical parameter range, thus complicating the use of FMR for fossilized magnetosome (magnetofossil) detection. Furthermore, in stark contrast to putative Neogene greigite magnetofossil records, the coercivity distributions for greigite-producing MTB are fundamentally left-skewed with a lower median. Lastly, a comparison between the MTB-associated iron flux in the investigated estuary and the pyritic-Fe flux in the Black Sea suggests MTB play an important, but heretofore overlooked role in euxinic marine system iron cycle.
    Nature Communications 01/2014; 5:4797. · 10.74 Impact Factor
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    ABSTRACT: Dust layers deposited to snow cover of the Wasatch Range (northern Utah) in 2009 and 2010 provide rare samples to determine the relations between their compositions and radiative properties. These studies are required to comprehend and model how such dust-on-snow (DOS) layers affect rates of snow melt through changes in the albedo of snow surfaces. We evaluated several constituents as potential contributors to the absorption of solar radiation indicated by values of absolute reflectance determined from bi-conical reflectance spectroscopy. Ferric oxide minerals and carbonaceous matter appear to be the primary influences on lowering snow-cover albedo. Techniques of reflectance and Mössbauer spectroscopy as well as rock magnetism provide information about the types, amounts, and grain sizes of ferric oxide minerals. Relatively high amounts of ferric oxide, indicated by hard isothermal remanent magnetization (HIRM), are associated with relatively low average reflectance (<0.25) across the visible wavelengths of the electromagnetic spectrum. Mössbauer spectroscopy indicates roughly equal amounts of hematite and goethite, representing about 35% of the total Fe-bearing phases. Nevertheless, goethite (α-FeOOH) is the dominant ferric oxide found by reflectance spectroscopy and thus appears to be the main iron oxide control on absorption of solar radiation. At least some goethite occurs as nano-phase grain coatings less than about 50 nm thick. Relatively high amounts of organic carbon, indicating as much as about 10% organic matter, are also associated with lower reflectance values. The organic matter, although not fully characterized by type, correlates strongly with metals (e.g., Cu, Pb, As, Cd, Mo, Zn) derived from distal urban and industrial settings, probably including mining and smelting sites. This relation suggests anthropogenic sources for at least some of the carbonaceous matter, such as emissions from transportation and industrial activities. The composition of the DOS samples can be compared with sediments in a likely dust-source setting at the Milford Flat Fire (MFF) area about 225 km southwest of Salt Lake City. The MFF area represents geologically and physiographically similar and widespread dust sources west-southwest of the Wasatch Range and heavily populated Wasatch Front. The DOS layers and MFF sediments are similar in some textural, chemical, and magnetic properties, as well as in the common presence of goethite, hematite, magnetite-bearing basalt fragments, quartz, plagioclase, illite, and kaolinite. Textural and some chemical differences among these deposits can be explained by atmospheric sorting as well as by inputs from other settings, such as salt-crusted playas and contaminant sources.
    Aeolian Research 09/2013; · 2.52 Impact Factor
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    ABSTRACT: 1] Acid-sulfate alteration of basalt by SO 2 -bearing volcanic vapors has been proposed as one possible origin for sulfate-rich deposits on Mars. To better define mineralogical signatures of acid-sulfate alteration, laboratory experiments were performed to investigate alteration pathways and geochemical processes during reaction of basalt with sulfuric acid. Pyroclastic cinders composed of phenocrysts including plagioclase, olivine, and augite embedded in glass were reacted with sulfuric acid at 145 C for up to 137 days at a range of fluid : rock ratios. During the experiments, the phenocrysts reacted rapidly to form secondary products, while the glass was unreactive. Major products included amorphous silica, anhydrite, and Fe-rich natroalunite, along with minor iron oxides/oxyhydroxides (probably hematite) and trace levels of other sulfates. At the lowest fluid : rock ratio, hexahydrite and an unidentified Fe-silicate phase also occurred as major products. Reaction-path models indicated that formation of the products required both slow dissolution of glass and kinetic inhibitions to precipitation of a number of minerals including phyllosilicates and other aluminosilicates as well as Al-and Fe-oxides/oxyhydroxides. Similar models performed for Martian basalt compositions predict that the initial stages of acid-sulfate alteration of pyroclastic deposits on Mars should result in formation of amorphous silica, anhydrite, Fe-bearing natroalunite, and kieserite, along with relict basaltic glass. In addition, analysis of the experimental products indicates that Fe-bearing natroalunite produces a Mössbauer spectrum closely resembling that of jarosite, suggesting that it should be considered an alternative to the component in sulfate-rich bedrocks at Meridiani Planum that has previously been identified as jarosite.
    Journal of Geophysical Research Atmospheres 04/2013; 118:577-614. · 3.44 Impact Factor
  • Jessica Till, Mike Jackson, Bruce Moskowitz
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    ABSTRACT: Experimental investigations of deformation-induced remagnetization demonstrate that a primary remanence can survive conditions equivalent to moderate metamorphism in certain cases and that pre-deformation magnetic fabric can play an important role in determining the remanence stability. Results from shear experiments at elevated temperatures on magnetite-bearing rock-analogues demonstrate that complex interactions between temperature, applied field, stress, and anisotropy are responsible for determining the robustness of a pre-deformation magnetization. Syndeformational remagnetization is partly attributed to a stress-softening mechanism resulting in a piezoremanent magnetization, but the extent of remagnetization was largely dependent on the initial anisotropy, and generally unrelated to the deformation fabric. Similarly, the initial anisotropy of deformed samples is found to strongly influence the development of deformation fabrics and overprinting causes the deformation geometry to be obscured at low strains. These results raise several questions concerning paleomagnetic study in tectonized terranes and highlight a research area with much potential for future study.
    04/2013;
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    J L Till, Bruce Moskowitz
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    ABSTRACT: A meta-analysis of existing experimental deformation data for magnetite and other spinel-structured ferrites reveals that previously published flow laws are inadequate to describe the general deformation behavior of magnetite. Using updated rate equations for oxygen diffusion in magnetite, we present new flow laws that closely predict creep rates similar to those found in deformation experiments and that can be used to predict strain partitioning between cubic Fe oxides and other phases in the Earth's crust. New deformation mechanism maps for magnetite have been constructed as functions of temperature and grain size. Using the revised creep parameters, estimates of strain partitioning between magnetite, ilmenite, and plagioclase indicate that concentrated zones of Fe-Ti oxides in oceanic crust near slow-spreading ridges could accommodate significant amounts of strain at moderate temperatures and may contribute to aseismic creep along spreading-segment faults.
    Geophysical Research Letters 02/2013; 40:697-702. · 3.98 Impact Factor
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    United States Geological Survey Scientific Investigations Report. 01/2013; 2013-5054.
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    ABSTRACT: Acid‐sulfate alteration of pyroclastic basalts in active fumaroles at Cerro Negro volcano, Nicaragua, was studied as a means to infer the mineralogical and chemical consequences of basalt alteration in analogous environments on early Mars. At this site, recently erupted basaltic cinders are undergoing alteration by SO2‐bearing steam. During alteration, silicate phenocrysts, including plagioclase, olivine, and augite, react much more rapidly than basaltic glass. Secondary mineralogy is dominated by a very limited number of phases that include amorphous silica, gypsum, Fe‐bearing natroalunite, and Fe‐oxides/oxyhydroxides, including hematite and magnetite. The major element chemistry of the deposits is controlled by two processes: gradual depletion of the major cations other than Si as the basalt components decompose and elements are mobilized out of the deposits, and enrichment in Ca and S from precipitation of gypsum, with Ca apparently supplied from sources below the surface. Reaction path models constrained by these observations but extrapolated to Martian conditions predict that alteration of pyroclastic deposits in similar environments on Mars should produce a secondary mineral assemblage that includes amorphous silica, Fe‐bearing natroalunite, anhydrite, kieserite, and hematite. Iron‐bearing natroalunite was found to produce a Mössbauer signal similar to that of jarosite, suggesting that this phase should be considered as an alternative to the jarosite component identified at Meridiani Planum. Spheroidal hematite formed in close association with natroalunite suggests a pathway for formation of hematite deposits on Mars.
    Journal of Geophysical Research Atmospheres 01/2013; 118(9):1-33. · 3.44 Impact Factor
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    ABSTRACT: A magnetotactic bacterium, designated strain MV-1T, was isolated from sulfide-rich sediments in a salt marsh near Boston, Massachusetts. Cells of strain MV-1T are Gram-negative, and vibrioid to helicoid in morphology. Cells are motile by means of single polar flagellum. Cells appear to display a transitional state between axial and polar magnetotaxis: cells swim in both directions but generally have longer excursions in one direction than the other. Cells possess a single chain of magnetosomes containing truncated hexa-octahedral crystals of magnetite, positioned along the long axis of the cell. Strain MV-1T is a microaerophile that is also capable of anaerobic growth on some nitrogen oxides. Salinities greater than 10% of seawater are required for growth. Strain MV-1T exhibits chemolithoautotrophic growth on thiosulfate and sulfide with oxygen as the terminal electron acceptor (microaerobic growth), and on thiosulfate using nitrous oxide (N2O) as the terminal electron acceptor (anaerobic growth). Chemoorganoautotrophic and methylotrophic growth is supported by formate under microaerobic conditions. Autotrophic growth occurs via the Calvin-Benson-Bassham cycle. Chemoorganoheterotrophic growth is supported by various organic acids and amino acids, under microaerobic and anaerobic conditions. Optimal growth occurs at pH 7.0 and between 26-28°C. The genome of strain MV-1T consists of a single, circular chromosome about 3.7 Mb in size with a G + C content of 52.9-53.5 mol%. Phylogenetic analysis based on 16S rRNA gene sequences indicates that strain MV-1T belongs to the family Rhodospirillaceae within the Alphaproteobacteria, but is not closely related to members of the genus Magnetospirillum. The name Magnetovibrio blakemorei gen. nov., sp. nov. is proposed for strain MV-1T, which is designated as the type strain (= ATCC BAA-1436 T = DSM 18854T).
    INTERNATIONAL JOURNAL OF SYSTEMATIC AND EVOLUTIONARY MICROBIOLOGY 09/2012; · 2.11 Impact Factor
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    J. L. Till, B. M. Moskowitz, M. J. Jackson
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    ABSTRACT: Magnetic fabric development has been studied in synthetic magnetite-silicate aggregates in a series of high-temperature simple shear deformation experiments. Samples composed of magnetite grains with a nominal size of 20–40 μm dispersed at 3 wt % in a matrix of plagioclase were deformed between 1000 and 1200 °C with a confining pressure of 300 MPa, shear stresses in the range 10–130 MPa, and shear strains up to = 3. We determined that both magnetite and plagioclase were deformed plastically at these conditions. An oblate magnetic fabric ellipsoid develops subparallel to the shear plane and the degree of AMS rapidly increases with strain up to a value of 2.5. Theoretical strain response models of magnetic fabric development were applied to the data to obtain estimated magnetite strains. The shape-preferred orientation of magnetite grains after deformation, determined from reflected light image analysis, was used to calculate independent magnetite strain estimates. These results were then compared with strains estimated from theoretical magnetic anisotropy development. Both strain estimation methods indicate strain partitioning between magnetite and the plagioclase matrix and the results are used to determine approximate viscosity ratios between the two phases at the experimental conditions.
    Geophysical Journal International 03/2012; 189(1):229 - 239. · 2.85 Impact Factor
  • T. McCollom, B. Moskowitz, T. Berquo, B. Hynek
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    ABSTRACT: A combined field, experimental, and model study of acid-sulfate alteration indicates that the initial stage of alteration of martian basalt in fumarole environments should be dominated by amorphous silica, gypsum, Fe-rich natroalunite, and kieserite.
    03/2012;
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    ABSTRACT: Atmospheric dust contains iron oxide minerals that can play important roles in various physical and biological processes affecting atmospheric and surface temperatures, marine phytoplankton productivity, and human health. Iron oxide minerals in dust deposited on mountain snow cover are especially important because these minerals absorb solar and IR radiation leading to changes in albedo and affecting the timing and rate of spring and summer snowpack melting. As part of an ongoing project to study physical and chemical properties of dust from sources to sinks in the western US, we will describe one approach to characterize iron oxide mineralogy using magnetic property measurements and Mossbauer spectroscopy. Magnetic property measurements over a wide range of temperatures (2-300 K) and magnetic fields (0-5 T) are particularly sensitive to composition, particle size (from nanometer to micrometer), and concentration of iron oxide and oxyhydroxide minerals. The high sensitivity of magnetic measurements to target minerals allows the measurement of bulk samples preventing any aliasing of composition or grain size resulting from attempts at prior magnetic separation. In addition, different magnetic measurement protocols can isolate different particle-size assemblages and different compositions in multicomponent mixtures and help to identify dust-source areas. These techniques have been applied to dust deposited on snow (DOS) cover of the San Juan Mountains, Colorado (collected 2005-2010) and Wasatch Mountains, Utah (collected 2010) and possible dust-source sediments from the North American Great Basin and Colorado Plateau deserts. Results show that all samples contain a high coercivity phase consistent with hematite and/or goethite as the dominate ferric oxide mineral plus minor amounts of magnetite (<0.5 wt%). The presence of magnetite was determined from the detection of the characteristic Verwey transition (T=121 K) on low-temperature (< 300 K) remanence and susceptibility curves. Room temperature remanence parameters for the San Juan Mountains DOS fall into two discrete populations of hematite concentration ( x2 difference) but with similar spreads in magnetite concentrations (0.05-0.2%) within each group. Preliminary Mössbauer spectroscopy at 300 K for San Juan Mountains DOS indicates hematite as the sole magnetic phase with magnetite below the detection limits. However, spectra taken at 4.2 K show an increase in the hematite component and the appearance of goethite indicating superparamagnetism and nanoparticle size distribution for both phases. The lack of the Morin transition (T=263 K) for hematite on low-temperature remanence curves is also consistent with nanohematite as the main iron oxide phase in DOS from the San Juan Mountains.
    AGU Fall Meeting Abstracts; 12/2011
  • T. S. Berquo, B. M. Moskowitz
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    ABSTRACT: Nontronite is an Fe-rich smectite clay that occurs widely in terrestrial soils, sediments and weathering formations and may also be present in the Martian regolith. The thermal decomposition of nontronite is known to form various magnetic iron oxides but their compositions, magnetic properties, and formation pathways remain poorly understood. The magnetic alteration products of nontronite have been proposed as a source for the magnetic phases in the surface layers and dust on Mars as well as in some archeological fired-bricks and ceramic pottery. One alteration product of nontronite is ɛ-Fe2O3 which is ferrimagnetic with a Curie temperature of ~ 500 K and extremely large coercivity (HC ~ 1-2 T) at 300 K. In this work nontronite samples from eight source localities were heated to 1000°C in air for one hour. The magnetic properties of the alteration products were investigated with low-temperature (LT) magnetization and AC susceptibility curves, hysteresis loops, Mossbauer spectroscopy, and X-ray diffraction. The thermal treatment was effective in converting the nontronite to a combination of different polymorphs of ferric oxide depending on source locality and included: hematite (α-Fe2O3), ɛ-Fe2O3, and a cubic spinel phase that suggest the presence of maghemite (γ-Fe2O3). Mossbauer spectra at 300 K identified hematite and ɛ-Fe2O3 as the main phases in 7 samples with amounts ranging from 26-100% for hematite 0-69% for ɛ-Fe2O3. One sample showed a paramagnetic Fe3+ doublet and a broad sextet characteristic of magnetic relaxation effects. Upon cooling to 4.2 K, the Mossbauer spectrum was consistent with maghemite. In all samples except one, the magnetic hyperfine fields for the hematite phase are slightly reduced as compared with its stoichiometric form indicating some iron substitution with ions such as Al. This is consistent with the observation that all but one sample lacked the characteristic Morin transition for pure hematite on LT-remanence warming curves. Samples with ɛ-Fe2O3 also had a main Curie temperature near 500 K and weaker one near 800 K plus constricted hysteresis loops with HC ~ 0.5-1.2 T at 300 K. However, the high coercivities persisted only until 500 K above which the hysteresis loops for a sample containing the largest amount of ɛ-Fe2O3 became normal in shape with HC < 10 mT. Finally, most samples exhibited frequency dependent susceptibility (5-300 K) with blocking temperatures of 200-300 K even for samples with HC > 1 T. This feature may represent the signal from superparamagnetic particles of γ-Fe2O3. The magnetic behavior of the heat-treated nontronite shows that the resulting mineralogy is complex and likely dependent on the initial compositions of the starting material. The initial nontronite composition can play a role in the composition of the final products after thermal treatment and the presence of foreign ions may have strong influence in the formation of hematite, maghemite, and ɛ-Fe2O3.
    AGU Fall Meeting Abstracts. 12/2011;
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    ABSTRACT: Ferric oxide minerals in atmospheric dust can influence atmospheric temperatures, accelerate melting of snow and ice, stimulate marine phytoplankton productivity, and impact human health. Such effects vary depending on iron mineral type, size, surface area, and solubility. Generally, the presence of ferric oxides in dust is seen in the red, orange, or yellow hues of plumes that originate in North Africa, central and southwest Asia, South America, western North America, and Australia. Despite their global importance, these minerals in source sediments, atmospheric dust, and downwind aeolian deposits remain poorly described with respect to specific mineralogy, particle size and surface area, or presence in far-traveled aerosol compounds. The types and properties of iron minerals in atmospheric dust can be better understood using techniques of rock magnetism (measurements at 5-300 K), Mössbauer and high-resolution visible and near-infrared reflectance spectroscopy; chemical reactivity of iron oxide phases; and electron microscopy for observing directly the ferric oxide coatings and particles. These studies can elucidate the diverse environmental effects of iron oxides in dust and can help to identify dust-source areas. Dust-source sediments from the North American Great Basin and Colorado Plateau deserts and the Kalahari Desert, southern Africa, were used to compare average reflectance values with a magnetic parameter (hard isothermal remanent magnetization, HIRM) for ferric oxide abundance. Lower reflectance values correspond strongly with higher HIRM values, indicating that ferric oxides (hematite or goethite, or both) contribute to absorption of solar radiation in these sediments. Dust deposited to snow cover of the San Juan Mountains (Colorado) and Wasatch Mountains (Utah) was used to characterize dust composition compared with properties of sediments exposed in source-areas identified from satellite retrievals. Results from multiple methods indicate that nanohematite is the dominant ferric oxide in most dust-on-snow (DOS) samples (collected 2005-2010) from the San Juan Mountains and in many redbed-derived dust sources upwind in Colorado Plateau drylands. Goethite in some San Juan Mountain DOS is probably derived from source-area sediments from Cretaceous marine deposits. In contrast, goethite appears to be the dominant ferric oxide in Wasatch Mountains DOS collected in 2010. Goethite-bearing Late Pleistocene lake sediments in west-central Utah have been identified as the dominant dust sources for these DOS layers from satellite retrievals. The distinction between hematite and goethite is useful for modeling radiative forcing by dust in mountain snow and ice and in the atmosphere.
    AGU Fall Meeting Abstracts; 12/2011
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    J L Till, M J Jackson, B M Moskowitz
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    ABSTRACT: Shear experiments were performed on magnetite‐bearing calcite aggregates to examine magnetic fabric development and remanence stability in a deforming system using elevated temperature and pressure to encourage deformation by crystal‐plastic processes. Samples composed of 1 wt % pseudo‐single‐domain magnetite (1–2 mm) in a calcite matrix were created with either strong or weak initial fabrics and deformed in coaxial simple shear to strains up to g = 1.5 at constant strain rates between 6 × 10 −5 and 1 × 10 −4 s −1 at 500°C and confining pressure of 300 MPa. Samples were given weak field thermal remanent magnetiza-tions prior to deformation. Demagnetization of postdeformation remanence reveals that a primary remanent magnetization can withstand deformation at pressures and temperatures approximately equivalent to greenschist facies metamorphic conditions on laboratory time scales, but this stability is found to depend on the character of the predeformation fabric. The origin of secondary remanence components acquired during deformation is uncertain but is likely to partially result from thermal viscous remagnetization. Complete postdeformation remagnetization in initially anisotropic samples appears to involve a stress‐softening or piezoremanent magnetization mechanism. Postdeformation anisotropy measurements show progressive changes in magnetic fabric strength with strain. In the absence of a strong initial magnetic anisotropy, mag-netic fabric intensity increases linearly as a function of strain; however, deformation that overprinted an existing fabric results in an apparent decrease of the initial anisotropy at low strains followed by rapid increases in magnetic fabric strength with increasing strain. Our results underscore the important role that initial fabric can play in determining the character of deformation fabrics.
    Geochemistry Geophysics Geosystems 12/2010; 11(12). · 2.94 Impact Factor
  • J. L. Till, M. J. Jackson, B. M. Moskowitz
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    ABSTRACT: We present results from a series of deformation experiments designed to study development of magnetic anisotropy under simulated high-grade metamorphic conditions. Synthetic plagioclase aggregates containing a low concentration of magnetite (1.5 vol%) were deformed in coaxial simple shear to strains of up to 300% at high temperature (T=1000-1200°C). Magnetic anisotropy increases rapidly and monotonically as a function of bulk shear strain, suggesting that changes in anisotropy can be attributed to plastically deforming magnetic grains. Increases in magnetic grain shape anisotropy were verified with microscopy. Magnetic fabric strength was also found to be distinctly temperature-dependent, indicating varying degrees of strain partitioning at different temperatures. Our experimental data were compared with previously-published strain-response models and were found to fit the ``ductile'' model of magnetic fabric development. These models can be used to make preliminary estimates of viscosity contrasts between magnetite and plagioclase at our experimental conditions, and place constraints on effective strain accumulation in the magnetite. Disagreement between published magnetite flow laws and the evidence for magnetite deformation at our experimental conditions points to the need for better data on magnetite rheology. Our results are also compared with previous work on magnetic fabrics in natural magnetite-bearing mylonitic zones. If the ``ductile'' strain-response model can be experimentally verified, further work on strain-partitioning between magnetic minerals and silicates could lead to the development of potential strain proxies from magnetic fabrics for high-strain zones.
    AGU Fall Meeting Abstracts; 12/2010

Publication Stats

2k Citations
233.35 Total Impact Points

Institutions

  • 1992–2014
    • University of Minnesota Duluth
      Duluth, Minnesota, United States
  • 1994–2013
    • University of Minnesota Twin Cities
      • • Institute for Rock Magnetism (IRM)
      • • School of Physics and Astronomy
      Minneapolis, Minnesota, United States
  • 2009
    • University of Colorado
      Denver, Colorado, United States
  • 2002
    • University of Toronto
      • Department of Physics
      Toronto, Ontario, Canada
  • 1994–2002
    • National Institutes of Health
      • Laboratory of Research Technologies
      Maryland, United States
  • 1993
    • Northeastern University
      • Marine Science Center
      Boston, MA, United States
  • 1989
    • University of California, Davis
      • Department of Geology
      Davis, CA, United States