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Ingunn H. Thorseth - One of the best experts on this subject based on the ideXlab platform.

  • Abiotic and candidate biotic micro-alteration textures in subseafloor basaltic glass: A high-resolution in-situ textural and geochemical investigation
    Chemical Geology, 2015
    Co-Authors: Leif-erik Rydland Pedersen, Nicola Mcloughlin, Per Erik Vullum, Ingunn H. Thorseth
    Abstract:

    Abstract The oceanic crust provides one of the largest habitats for subsurface microbial life on earth, where lithoautotrophs utilize redox gradients between reduced elements in volcanic rocks and oxygenated seawater to form the basis of a deep microbial biosphere. Progressive alteration of the oceanic crust is argued to be “in part” microbially mediated, but identifying robust textural and geochemical biosignatures with good fossilization potential is challenging. This study investigates pillow basalts from the Antarctic Australian Discordance (AAD) at the South East Indian Ridge (SEIR) containing candidate textural biosignatures in alteration products of the glassy margins (Thorseth et al., 2003). Samples include 2.5 Ma dredged seafloor basalts, and 18–28 Ma drill core samples from the Ocean Drilling Program (ODP) Leg 187. The focused ion beam (FIB) technique was used to prepare electron transparent foils across spherical microtextures in zeolite filled fractures and altered glass (Palagonite), and across microtunnels at the interface of fresh and altered glass. Transmission electron microscopy (TEM) was used to map chemical and ultrastructural variations and to evaluate both biotic and abiotic origins of the candidate textural biosignatures in the FIB prepared foils. Three foils were cut from zeolite hosted, hollow microspheres, which comprise purely Fe-oxyhydroxides, or mixed Mn–Mg, and Fe–Mn oxyhydroxides. The microspheres are 1 to 4 μm across, with a radiating ultrastructure, and have a denser inner surface and a more porous outer surface, suggesting outwards growth from a spherical initial surface. Amorphous organic carbon is associated with some of the microtextures both on the inner and outer walls. These microtextures are interpreted as mineral encrusted microbial cells. A FIB-foil was also cut from Palagonite-hosted microspheres, which are more irregular in shape and partially infilled by Palagonite. Amorphous organic carbon is abundant in the vicinity of the microtextures but is spatially unrelated, and may be derived from several sources. The results indicate that maturation of the Palagonite, involving dehydration and recrystallization, overprints and destroys potential biosignatures in this alteration phase. In contrast, the zeolite-hosted microtextures appear to have a higher preservation potential. Tubular microtextures in the glass at the alteration front comparable to argued “bioalteration” textures are also abundant in the AAD basalts. However, their angular cross-sectional shape and lack of “bio-elements” in the Palagonite infill, mean that an abiotic origin cannot be excluded. In summary, FIB-TEM provides multiple high-resolution lines of information to characterize alteration textures in ocean floor basalts. Comparing the evidence obtained from glass hosted microtunnels, zeolite and Palagonite hosted microspheres we conclude that the zeolite hosted microtextures are the strongest candidate biosignature. The combination of the size, rim ultrastructure, and elemental composition is consistent with an origin as cell encrustations, resulting from the biologically induced mineralization of microbial cells that inhabited fractures in pillow lavas both at the seafloor and the subseafloor stage.

  • A combined TEM and NanoSIMS study of endolithic microfossils in altered seafloor basalt
    Chemical Geology, 2011
    Co-Authors: Nicola Mcloughlin, Claudia Kruber, Ingunn H. Thorseth, David Wacey, Matt R. Kilburn, Rolf B. Pedersen
    Abstract:

    Abstract The incipient alteration of basaltic glass to Palagonite in recent seafloor lavas from the arctic Mohns Ridge was studied by complimentary high-spatial-resolution geochemical techniques: TEM-EDS (transmission electron microscopy energy dispersive X-ray spectroscopy) and NanoSIMS (nano-scale secondary ion mass spectrometry). Rounded to elongated pores 0.5–2 μm across were found embedded in compact Palagonite that have sizes and shapes comparable to microbial cells. In-situ elemental mapping revealed that the micropore rims are comparable in composition to the bulk Palagonite and that some are enriched in manganese. Elevated concentrations of carbon and nitrogen were also found in some of the micropores. Hence these structures are interpreted as fossilised bacteriomorphs of endolithic microorganisms that inhabited fractures in the basaltic glass. The preferential accumulation of Mn in some of the cell encrustations suggests the mineralisation of Mn-oxidising bacteria. These data provide further evidence for the involvement of microorganisms in the colonisation and chemical alteration of recent seafloor volcanic glass and identify micro-scale Mn enrichments associated with micropores as a promising biosignature in such rocks.

  • Seafloor alteration of basaltic glass: Textures, geochemistry, and endolithic microorganisms
    Geochemistry Geophysics Geosystems, 2008
    Co-Authors: Claudia Kruber, Ingunn H. Thorseth, Rolf B. Pedersen
    Abstract:

    [1] The incipient low-temperature alteration of glassy margins of recent seafloor lavas from the Mohns Ridge is characterized by scanning electron microscopy and bulk chemical analyses of major and trace elements. These investigations are designed to evaluate the endolithic microbial biomass, to identify relations between alteration textures and microorganisms, and to identify the chemical processes that take place during alteration. The basaltic glass along intersecting fractures and around vesicles is typically altered to concentrically zoned, yellow-brown amorphous gel-Palagonite. In most fractures microorganisms are observed at the outer surfaces of Palagonite rims, within porous zones of the rims, and frequently also at the glass-Palagonite interfaces. The cells act as nucleation sites for precipitation and become encrusted and embedded in Palagonite with time. Zones of porous Palagonite containing numerous hollow, fossilized cells alternate with zones of compact Palagonite lacking distinct cell structures, which together indicate that the microbial growth is discontinuous. The Palagonite has an average organic carbon content of 0.9 wt% (δ13Corg: −22‰), which derive from both living and fossilized biomass. The microbial growth and biomineralization are major controls on the porosity and texture of the Palagonite and thus likely on the chemical exchange between glass and seawater. Pit marks in the glass in fractures both with and without microbes indicate that microbial as well as abiotic processes mediate pitting. Elemental and isotope data show that the transformation of glass to gel-Palagonite at seafloor conditions with high water/rock ratios results in near complete loss of Si and alkali elements to seawater; formation of Fe- and Al-oxyhydroxides; complete exchange of alkaline earth elements like Sr with seawater; retention of V, Cu, Y, Pb, Th, U, and trivalent REEs, accompanied by a strong gain of these and other elements from seawater. Higher Fe/Ti ratios in the gel-Palagonite compared to the parental glass suggest that the Palagonite represents a mixture of Fe-Ti oxyhydroxides derived from both the glass and seawater.

  • The importance of microbiological activity in the alteration of natural basaltic glass
    Geochimica et Cosmochimica Acta, 1992
    Co-Authors: Ingunn H. Thorseth, Harald Furnes, Mikal Heldal
    Abstract:

    Abstract The textural development of Palagonite may differ profoundly depending on whether alteration occurred in the outermost 6–7 mm thick light-exposed surface zone of deposits, or elsewhere. In the former case, a pit-textured development of the parent basaltic glass develops as a consequence of local establishment of a highly alkaline micro-environment (pH > 9) for which the light-dependent cryptoendolithic cyanobacteria are considered most likely to be responsible. A highly porous, sponge-textured variety of Palagonite, frequently defining zoned layers, contains abundant examples of bacteria. The shape and size of the pores combined with the geochemical development strongly suggest that bacteria have played an important role in the development of this texture.

  • A textural and chemical study of Icelandic Palagonite of varied composition and its bearing on the mechanism of the glass-Palagonite transformation
    Geochimica et Cosmochimica Acta, 1991
    Co-Authors: Ingunn H. Thorseth, Harald Furnes, Ole Tumyr
    Abstract:

    Palagonite of basalt and basaltic andesite parentages from hyaloclastite deposits in Iceland has been investigated. SEM studies indicate a sharp to diffuse alteration front which may propagate along microfractures in the glass, resulting in a progressive partial dissolution yielding Palagonite of variable, but generally increasing porosity towards grain surfaces. The Palagonite has a granular texture. Incipient alteration is indicated by the development of individual globules (ca. 0.01 μm in diameter), whereas at an advanced stage chains of globules, defining a sponge-like texture, characterize the Palagonite. In the basaltic andesite, the precursor to brown Ti- and Fe-rich Palagonite is a white variety, for which an evolutionary model is presented. EDS line-scans across the fresh glass-Palagonite boundary show the existence of a 2–4 μm thick zone of glass-like material in which all elements have been depleted, except Si, and in some cases Al, which have been relatively enriched. The white Palagonite is characterized by strong depletion of Ti, Fe, Na, Mg, and to lesser extent, Ca, Al (in order of decreasing loss). Regardless of the degree of porosity development (ca. 1–43 vol%), the extent of element depletion relative to SiO2 is constant. This gives evidence for selective element mobility prior to a variable degree of congruent network dissolution of the Si-rich residue, yielding zoned Palagonite with a different porosity. In order for Fe, Ti, and Al to dissolve, a pH 3, Fe, Ti, and Al will precipitate in the pores of the white Palagonite as oxides/hydroxides, thus creating the brown variety, which is characterized by highly variable contents of the above-mentioned elements. The applicability of this model to Palagonite derived from basalt parentages at different pH conditions is discussed.

F. Guyot - One of the best experts on this subject based on the ideXlab platform.

  • Alteration of submarine basaltic glass from the Ontong Java Plateau: A STXM and TEM study
    Earth and Planetary Science Letters, 2007
    Co-Authors: K. Benzerara, N. Menguy, N.r. Banerjee, Tolek Tyliszczak, G.e. Brown Jr, F. Guyot
    Abstract:

    Frequent observations of tubular to vermicular microchannels in altered basalt glass have led to increasing appreciation of a possible significant role of microbes in the low-temperature alteration of seafloor basalt. We have examined such microchannel alteration features at the nanoscale in basalt glass shards from the Ontong Java Plateau using a combination of focused ion beam milling, transmission electron microscopy and scanning transmission X-ray microscopy. Three types of materials were found in ultrathin cross-sections cut through the microchannels by FIB milling: fresh basalt glass, amorphous Si-rich rims surrounding the microchannels, and Palagonite within the microchannels. X-ray absorption spectroscopy at the C K-edge and Fe L2,3-edges showed the presence of organic carbon in association with carbonates within the microchannels and partial oxidation of iron in Palagonite compared with basalt glass. Although these observations alone cannot discriminate between a biotic or abiotic origin for the microchannels, they provide new information on their mineralogical and chemical composition and thus better constrain the physical and chemical conditions prevailing during the alteration process.

  • Alteration of submarine basaltic glass from the Ontong Java Plateau: A STXM and TEM study
    Earth and Planetary Science Letters, 2007
    Co-Authors: K. Benzerara, N. Menguy, N.r. Banerjee, Tolek Tyliszczak, G.e. Brown, F. Guyot
    Abstract:

    International audienceFrequent observations of tubular to vermicular microchannels in altered basalt glass have led to increasing appreciation of a possible significant role of microbes in the low-temperature alteration of seafloor basalt. We have examined such microchannel alteration features at the nanoscale in basalt glass shards from the Ontong Java Plateau using a combination of focused ion beam milling, transmission electron microscopy and scanning transmission X-ray microscopy. Three types of materials were found in ultrathin cross-sections cut through the microchannels by FIB milling: fresh basalt glass, amorphous Si-rich rims surrounding the microchannels, and Palagonite within the microchannels. X-ray absorption spectroscopy at the C K-edge and Fe L2,3-edges showed the presence of organic carbon in association with carbonates within the microchannels and partial oxidation of iron in Palagonite compared with basalt glass. Although these observations alone cannot discriminate between a biotic or abiotic origin for the microchannels, they provide new information on their mineralogical and chemical composition and thus better constrain the physical and chemical conditions prevailing during the alteration process

William M. Grundy - One of the best experts on this subject based on the ideXlab platform.

  • Visible/near-infrared spectra and two-layer modeling of Palagonite-coated Basalts
    Geophysical Research Letters, 2001
    Co-Authors: Jeffrey R. Johnson, William M. Grundy
    Abstract:

    Fine-grained dust coatings on Martian rocks and soils obscure underlying surfaces and hinder mineralogic interpretations of both remote sensing and in-situ observations, We investigate laboratory visible/near-infrared spectra of various thicknesses of Palagonite coatings on basalt substrates. We develop a two-layer Hapke scattering model incorporating porosity, grain size, and derived absorption coefficients of Palagonite and basalt that reproduces the observed spectra only when the single scattering particle phase function is varied with wavelength.

  • visible near infrared spectra and two layer modeling of Palagonite coated basalts
    Geophysical Research Letters, 2001
    Co-Authors: Jeffrey R. Johnson, William M. Grundy
    Abstract:

    Fine-grained dust coatings on Martian rocks and soils obscure underlying surfaces and hinder mineralogic interpretations of both remote sensing and in-situ observations, We investigate laboratory visible/near-infrared spectra of various thicknesses of Palagonite coatings on basalt substrates. We develop a two-layer Hapke scattering model incorporating porosity, grain size, and derived absorption coefficients of Palagonite and basalt that reproduces the observed spectra only when the single scattering particle phase function is varied with wavelength.

Jack D. Farmer - One of the best experts on this subject based on the ideXlab platform.

  • Subglacial hydrothermal alteration minerals in Jökulhlaup deposits of Southern Iceland, with implications for detecting past or present habitable environments on Mars.
    Astrobiology, 2010
    Co-Authors: Nicholas H Warner, Jack D. Farmer
    Abstract:

    Abstract Jokulhlaups are terrestrial catastrophic outfloods, often triggered by subglacial volcanic eruptions. Similar volcano-ice interactions were likely important on Mars where magma/lava may have interacted with the planet's cryosphere to produce catastrophic floods. As a potential analogue to sediments deposited during martian floods, the Holocene sandurs of Iceland are dominated by basaltic clasts derived from the subglacial environment and deposited during jokulhlaups. Palagonite tuffs and breccias, present within the deposits, represent the primary alteration lithology. The surface abundance of Palagonite on the sandurs is 1–20%. X-ray diffraction (XRD) analysis of Palagonite breccias confirms a mineral assemblage of zeolites, smectites, low-quartz, and kaolinite. Oriented powder X-ray diffractograms (

  • subglacial hydrothermal alteration minerals in jokulhlaup deposits of southern iceland with implications for detecting past or present habitable environments on mars
    Astrobiology, 2010
    Co-Authors: Nicholas H Warner, Jack D. Farmer
    Abstract:

    Abstract Jokulhlaups are terrestrial catastrophic outfloods, often triggered by subglacial volcanic eruptions. Similar volcano-ice interactions were likely important on Mars where magma/lava may have interacted with the planet's cryosphere to produce catastrophic floods. As a potential analogue to sediments deposited during martian floods, the Holocene sandurs of Iceland are dominated by basaltic clasts derived from the subglacial environment and deposited during jokulhlaups. Palagonite tuffs and breccias, present within the deposits, represent the primary alteration lithology. The surface abundance of Palagonite on the sandurs is 1–20%. X-ray diffraction (XRD) analysis of Palagonite breccias confirms a mineral assemblage of zeolites, smectites, low-quartz, and kaolinite. Oriented powder X-ray diffractograms (<2 μm fraction) for Palagonite breccia clasts and coatings reveal randomly ordered smectite, mixed layer smectite/illite, zeolites, and quartz. Visible light–near infrared (VNIR) and shortwave infrar...

Hans-ulrich Schmincke - One of the best experts on this subject based on the ideXlab platform.

  • Palagonite - a review
    International Journal of Earth Sciences, 2002
    Co-Authors: Nicole A. Stroncik, Hans-ulrich Schmincke
    Abstract:

    Palagonite is the first stable product of volcanic glass alteration. It is a heterogeneous material, usually with highly variable optical and structural properties, ranging from a clear, transparent, isotropic, smooth and commonly concentrically banded material, commonly called "gel-Palagonite", to a translucent, anisotropic, slightly to strongly birefringent material of fibrous, lath-like or granular structure, commonly called "fibro-Palagonite". The color of Palagonite ranges from shades of yellow to shades of brown. Palagonite forms rinds of variable thickness on every mafic glass surface exposed for some time to aquatic fluids. It is formed by either incongruent dissolution or by congruent dissolution of glass with contemporaneous precipitation of insoluble material at the glass–fluid interface. The process of palagonitization is accompanied by extensive mobilization of all elements involved in the alteration process, resulting in the depletion or enrichment of certain elements. The extent and direction of element mobility and the palagonitization process itself (including the rate of palagonitization) depend on a number of different, complex interacting properties: e.g. (1) temperature, (2) the structure of the primary material, (3) the reactive surface area of the primary material, (4) the structure of the precipitating secondary phases, (5) the growth rates of the secondary phases, (6) time, and (7) fluid properties such as fluid flow rates, pH, Eh, ionic strength, and oxygen fugacity. The fluid properties themselves are affected by different hydrogeological properties such as porosity, permeability, and pressure gradients.

  • Evolution of Palagonite: Crystallization, chemical changes, and element budget
    Geochemistry Geophysics Geosystems, 2001
    Co-Authors: Nicole A. Stroncik, Hans-ulrich Schmincke
    Abstract:

    [1] The structural and chemical evolution of Palagonite was studied as a function of glass composition, alteration environment, and time by applying a range of analytical methods (electron microprobe, infrared photometry, atomic force microscopy, X-ray fluorescence, and X-ray diffraction). Palagonitization of volcanic glass is a continuous process of glass dissolution, Palagonite formation, and Palagonite evolution, which can be subdivided into two different reaction stages with changing element mobilities. The first stage is characterized by congruent dissolution of glass and contemporaneous precipitation of “fresh,” gel-like, amorphous, optically isotropic, mainly yellowish Palagonite. This stage is accompanied by loss of Si, Al, Mg, Ca, Na, and K, active enrichment of H2O, and the passive enrichment of Ti and Fe. The second stage is an aging process during which the thermodynamically unstable Palagonite reacts with the surrounding fluid and crystallizes to smectite. This stage is accompanied by uptake of Si, Al, Mg, and K from solution and the loss of Ti and H2O. Ca and Na are still showing losses, whereas Fe reacts less consistently, remaining either unchanged or showing losses. The degree and direction of element mobility during palagonitization was found to vary mainly with Palagonite aging, as soon as the first precipitation of Palagonite occurs. This is indicated by the contrasting major element signatures of Palagonites of different aging steps, by the changes in the direction of element mobility with Palagonite aging, and by the general decrease of element loss with increasing formation of crystalline substances in the Palagonite. Considering the overall element budget of a water-rock system, the conversion of glass to Palagonite is accompanied by much larger element losses than the overall alteration process, which includes the formation of secondary phases and Palagonite aging. The least evolved palagonitized mafic glass studied has undergone as much as 65 wt% loss of elements during Palagonite formation, compared to ∼28 wt% element loss during bulk alteration. ABout 33 wt% element loss was calculated for one of the more evolved, in terms of the aging degree, rocks studied, compared to almost no loss for bulk alteration.