Kooperativer Bibliotheksverbund

Berlin Brandenburg

and
and

Your email was sent successfully. Check your inbox.

An error occurred while sending the email. Please try again.

Proceed reservation?

Export
  • 1
    Language: English
    In: Geology, Feb, 2005, Vol.33(2), p.117(4)
    Description: Considerable mobilization of Fe without mobilization of Al in Precambrian paleosols has been documented and attributed to either anoxic- or ligand-promoted dissolution. To elucidate these mechanisms, basalt was dissolved under oxic and anoxic conditions with and without citrate, and the mobility of several elements was analyzed. The extent of release of Fe and P was minor (in citrate-free conditions) or considerable (with citrate) regardless of oxygen pressure. Release of AI was minor in all cases, whereas release of Cu was minor (in anoxic conditions) or considerable (in oxic conditions). Release of Cu was enhanced by citrate. In comparison, in the weathered surface of two of the oldest-known basalt-derived paleosols--the Mount Roe (2.76 Ga) and the Hekpoort (2.25 Ga)--Fe and P were considerably depleted and Al retained, consistent with the presence of organic ligands. Cu, retained in the Mount Roe paleosol but considerably mobilized in the Hekpoort paleosol, documents formation under an anoxic atmosphere and an oxic atmosphere, respectively, as inferred by others on the basis of Fe mobility. The immobility of AI in both paleosols is consistent with formation under conditions in which the annual volume of rainwater was lower than the topsoil pore volume. Mobilization of P in such paleosols developed under low-rainfall conditions provides a new proxy for identification of ligands secreted by terrestrial organisms on early Earth. Keywords: Precambrian paleosols, organic ligands, basalt, element mobility, dissolution, early Earth.
    Keywords: Iron Compounds -- Research ; Precambrian Era -- Natural History ; Organic Compounds -- Research ; Organic Compounds -- Environmental Aspects
    ISSN: 0091-7613
    E-ISSN: 19432682
    Library Location Call Number Volume/Issue/Year Availability
    BibTip Others were also interested in ...
  • 2
    In: Geology, June, 2001, Vol.29(6), p.535
    Description: Small natural variations in Fe isotopes have been attributed to biological cycling. However, without understanding the mechanisms of fractionation, it is impossible to interpret such variations. Here we show that the [Delta][sup.56] Fe of Fe dissolved from a silicate soil mineral by siderophore-producing bacteria is as much as 0.8 [salinity] lighter than bulk Fe in the mineral. A smaller isotopic shift is observed for Fe released abiotically by two chelates, and the magnitude of the shift increases with affinity of the ligand for Fe, consistent with a kinetic isotope effect during hydrolysis of Fe at the mineral surface. Fe dissolved abiotically without chelates shows no isotopic shift. The [Delta][sup.56]Fe of the exchange fraction on soil grains is also lighter by ~0.6 [salinity]-1 [salinity] than Fe from both hornblende and iron oxyhydroxides. The kinetic isotope effect is therefore preserved in open systems such as soils. When recorded in the rock record, Fe isotopic fractionation could document Fe transport by organic molecules or by microbes where such entities were present in the geologic past.
    Keywords: Fractional Distillation -- Research ; Soil Microbiology -- Research ; Organic Acids -- Research ; Hydrolysis -- Research ; Chelates -- Research ; Soil Science -- Research ; Iron (Metal) ; Isotopes
    ISSN: 0091-7613
    E-ISSN: 19432682
    Library Location Call Number Volume/Issue/Year Availability
    BibTip Others were also interested in ...
  • 3
    In: Geology, Oct, 1995, Vol.23(10), p.933(4)
    Description: Airborne measurements of CO (sub 2) released from Oldoinyo Lengai, the only carbonatite-erupting volcano in the world, reveal a CO (sub 2) flux of 0.055 X 10 (super 12) mol/yr. This flux is substantially smaller than that of Mount Etna (1X10 (super 12) mol/yr), which accounts for over half of the global carbon flux attributed to subaerial volcanoes (1-2X10 (super 12) mol/yr). We propose that the subaerial flux distribution may be a power-law distribution (fractal) rather than Gaussian. Fitting the limited available volcanic flux data to a fractal distribution yields a power-law exponent of 〈1. Although rigorous testing of the power-law nature of the flux distribution is impossible, the skewed nature of the distribution and low value of the power-law exponent suggest that simultaneous measurement of the 20 largest-flux volcanoes could yield an accurate assessment of the volcanic CO (sub 2) flux. Summation over the power-law distribution predicts a maximum global subaerial passive volcanic flux of 2-3X10 (super 12) mol/yr and 2-3X10 (super 11) mol/yr for CO (sub 2) and SO (sub 2) , respectively. Normalizing the emission flux by scaling per unit crater area (instead of per volcano) to investigate the extension of the power-law behavior to geothermal areas with lower gas fluxes yields a power-law exponent of approximately 1 and predicts a subaerial volcanic-metamorphic CO (sub 2) flux of 6X10 (super 12) mol/yr.
    Keywords: Volcanoes -- Research ; Volcanism -- Research ; Metamorphism (Geology) -- Research ; Carbon Dioxide -- Environmental Aspects
    ISSN: 0091-7613
    Library Location Call Number Volume/Issue/Year Availability
    BibTip Others were also interested in ...
  • 4
    Language: English
    In: Geology, Jan, 2008, Vol.36(1), p.67(4)
    Description: Where Martian rocks have been exposed to liquid water, chemistry versus depth profiles could elucidate both Martian climate history and potential for life. The persistence of primary minerals in weathered profiles constrains the exposure time to liquid water: on Earth, mineral persistence times range from ~10 k.y. (olivine) to ~250 k.y. (glass) to ~1 m.y. (pyroxene) to ~5 m.y. (plagioclase). Such persistence times suggest mineral persistence minima on Mars. However, Martian solutions may have been more acidic than on Earth. Relative mineral weathering rates observed for basalt in Svalbard (Norway) and Costa Rica demonstrate that laboratory pH trends can be used to estimate exposure to liquid water both qualitatively (mineral absence or presence) and quantitatively (using reactive transport models). Qualitatively, if the Martian solution pH 〉~2, glass should persist longer than olivine; therefore, persistence of glass may be a pH indicator. With evidence for the pH of weathering, the reactive transport code CrunchFlow can quantitatively calculate the minimum duration of exposure to liquid water consistent with a chemical profile. For the profile measured on the surface of the exposed Martian rock known as Humphrey in Gusev Crater, the calculated exposure time is 22 k.y., which is a minimum due to physical erosion. If correct, this estimate is consistent with short-term, episodic alteration accompanied by ongoing surface erosion. More of these depth profiles should be measured to illuminate the weathering history of Mars. Keywords: Mars, basalt, weathering reactive transport modeling, weathering rind, pH.
    Keywords: Mars (Planet) -- Natural History ; Basalt -- Properties ; Weathering -- Evaluation ; Impact Craters -- Properties
    ISSN: 0091-7613
    E-ISSN: 19432682
    Library Location Call Number Volume/Issue/Year Availability
    BibTip Others were also interested in ...
  • 5
    In: Geology, April, 1992, Vol.20(4), p.303(4)
    Description: Petrographic analysis of sandstones from the vicinity of a pressure seal (transition from normal to overpressure) at 5.5-km depth in the lower Tuscaloosa Formation in Louisiana documents local, high porosity above and below the seal. Packing analysis shows that compaction is greater in normally pressured, high-porosity sandstones than in overpressured, high-porosity sandstones; compaction in overpressured, high-porosity sandstones is similar to that in normally pressured, well-cemented sandstones. We propose that focused corrosive fluids created a zone of high secondary porosity, allowing further compaction that we call "secondary compaction." Secondary compaction is greater above the seal than below, suggesting that high-pressure fluid below the seal has preserved porosity and that the pressure seal became effective soon after dissolution of cement. Cuttings from the pressure-seal zone reveal an unusual texture of fragmented, pressure-solved grains and matrix, which may be a result of extensive secondary compaction.
    Keywords: Sandstone -- Analysis ; Porosity -- Analysis
    ISSN: 0091-7613
    Library Location Call Number Volume/Issue/Year Availability
    BibTip Others were also interested in ...
  • 6
    Language: English
    In: Geology, 1995, Vol.23(11), p.975
    Description: The thicknesses of veins of a single generation from the Kodiak accretionary complex show a power-law distribution with an exponent of D = 1.33. This value (D 〉 1) indicates that a significant portion of the vein-related extension is accommodated by the thinnest veins. Thick vein segments are typically confined to southeast-dipping brittle-ductile shear zones. Textural observations of veins within these zones indicate a progressive evolution from crack-seal to euhedral growth of quartz with an increase over time in the duration that fractures remained open. Model simulations of vein-thickness distributions show that a constant growth model results in a negative-exponential distribution and that it is necessary to assume size-proportional growth to produce a power-law distribution. Textures and power-law thickness distributions for the Kodiak veins are thus consistent with an increasing time-averaged growth rate as vein thickness increased, comprising a positive feedback. The positive feedback resulted in the progressive evolution from fractures to brittle-ductile shear zones and ultimately to faults.
    Keywords: Structural Geology ; Accretionary Wedges ; Alaska ; Continental Margin ; Cracks ; Cretaceous ; Deformation ; Extension ; Feedback Mechanism ; Geometry ; Kodiak Formation ; Mesozoic ; Power Law ; Quartz Veins ; Size Distribution ; Statistical Distribution ; Stochastic Processes ; Textures ; Thickness ; United States ; Upper Cretaceous ; Veins;
    ISSN: 0091-7613
    Source: CrossRef
    Library Location Call Number Volume/Issue/Year Availability
    BibTip Others were also interested in ...
  • 7
    In: Geology, Feb, 1990, Vol.18(2), p.136(4)
    Description: Microcracks in quartz approximately 100 mu m in length and 〈 approximately 10 mu m in width heal in 4h at 600 degrees C and water pressure of 200 MPa (fluid pressure [P (sub f) ] = confining pressure [P (sub c) ]). Healing is thermally activated; the activation energy is estimated to be between 80 and 35 kJ/mol, depending on the model assumed. Rates also show dependence on fluid pressure, chemistry, and crack dimensions. Faster healing rates are observed in smaller cracks. Thus, when new cracks are not being produced in rocks at elevated temperatures and pressures, fractures will have a vast range of lifetimes: macrofractures transport most of the fluid volume and seal relatively slowly, whereas microcracks allow pervasive penetration of fluid into the rock mass but heal quickly.
    Keywords: Porosity -- Research ; Quartz -- Research ; Petrofabric Analysis -- Research ; Metamorphic Rocks -- Research ; Rock Permeability
    ISSN: 0091-7613
    Library Location Call Number Volume/Issue/Year Availability
    BibTip Others were also interested in ...
Close ⊗
This website uses cookies and the analysis tool Matomo. Further information can be found on the KOBV privacy pages