Bayreuth, Germany
Bayreuth, Germany

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Ruskov T.,Bulgarian Academy of Science | Spirov I.,Bulgarian Academy of Science | Georgieva M.,Sofia University | Yamamoto S.,Tokyo Institute of Technology | And 3 more authors.
Journal of Metamorphic Geology | Year: 2010

Mössbauer spectroscopy was applied to study the valence state of iron in chromite from massive, nodular and disseminated podiform chromitite ores of the Luobasa ophiolite massif of Tibet. The results show that Fe3+/ΣFe = 0.42 in chromite from massive ore, and Fe3+/ΣFe = 0.22 in chromite from nodular and disseminated ores. The massive ore records traces of ultra high pressure mineralogical assemblages, such as diamond inclusions in OsIr alloys, exsolution lamellae of coesite and diopside in chromite, inclusions of metal-nitrides, native iron and others, which suggests a strongly reducing environment. In contrast, chromite from nodular and disseminated ore contains abundant low-pressure OH-bearing mineral inclusions whose formation requires a more oxidizing environment. The high value of Fe3+/ΣFe in the 'reduced' massive ore is explained by crystallographic stabilization of Fe3+ in a high-pressure polymorph of chromite deep in the upper mantle despite low ambient fO2 conditions. The presence of high-pressure phases within the massive chromitite ore requires that the latter, together with its host peridotite, was transported in the solid state from a highly reduced deep mantle environment to shallow depths beneath an ocean spreading centre. It is suggested that in the low-pressure environment of the spreading centre, the deep-seated, reduced, massive chromitites partially reacted with their host peridotite in the presence of hydrous melt, yielding the nodular and disseminated chromitite ores. The preponderance of evidence suggests that the latter interaction involved boninitic melts in a supra-subduction zone environment as proposed previously. © 2010 Blackwell Publishing Ltd.

Domeneghetti M.C.,University of Pavia | Fioretti A.M.,CNR Institute of Geosciences and Earth Resources | Camara F.,University of Turin | McCammon C.,Bayerisches Geoinstitut | Alvaro M.,University of Chieti Pescara
Geochimica et Cosmochimica Acta | Year: 2013

High resolution single-crystal X-ray diffraction (HR-SCXRD) and Mössbauer spectroscopy of the intracrystalline cation distribution have been performed on augitic core-crystals from a Miller Range nakhlite (sample MIL 03346,13) with approximate composition of En36Fs24Wo40. The Mössbauer data on the single-crystal yielded a very low Fe3+ content [Fe3+/Fetotal - 0.033(23)a.p.f.u.] that, together with the Electron microprobe analysis (EMPA) and the X-ray structural data allowed us to obtain the accurate cation site distribution and the Fe2+-Mg degree of order. This leads to a closure temperature (Tc) of 500 with a standard deviation of ±100°C that would correspond to a slow cooling rate, which is in disagreement with petrologic evidence that indicates that this sample originates from a fast cooled (~3-6°C/h) lava flow.In order to clarify this discrepancy we undertook (i) a SC-XRD study of an augite (~En49Fs9 Wo42) from a pyroxenite (TS7) of Theo's flow, a 120-m-thick lava flow regarded as a terrestrial analogue of MIL 03346; (ii) an annealing experiment at 600°C on a crystal from exactly the same fragment of MIL 03346. SC-XRD data from TS7 augite yields a Tc=600(20)°C, consistent with the cooling rate expected at 85m below the surface. This Tc is higher, although similar within error, to the Tc=500(100)°C obtained for MIL 03346; thus suggesting relatively slower cooling for MIL 03346 with respect to TS7. The annealing experiment on the MIL 03346 crystal clearly showed that the degree of order remained unchanged, further confirming that the actual Tc is close to 600°C.This result appears inconsistent with the shallow depth of origin (~<2m) assumed for MIL 03346, further supporting the discrepancy between MIL 03346 textural and petrologic evidence of fast cooling and the abovementioned Tc results obtained for augite. Therefore, a tentative scenario is that, soon after eruption and initial quench and while still at relatively high-T (~600°C), MIL 03346 was blanketed with subsequent lava flows that slowed down the cooling rate and allowed the augite Fe2+-Mg exchange reaction to proceed. © 2013 Elsevier Ltd.

Asker C.,Linköping University | Kargen U.,Linköping University | Dubrovinsky L.,Bayerisches Geoinstitut | Abrikosov I.A.,Linköping University
Earth and Planetary Science Letters | Year: 2010

We have calculated the equation of state and elastic properties of face-centered cubic Fe and Fe-rich Fe-Mg alloy at ultrahigh pressures from first principles using the Exact Muffin-Tin Orbitals method. The results show that adding Mg into Fe influences strongly the equation of state, and cause a large degree of softening of the elastic constants, even at concentrations as small as 1-2. at.%. Moreover, the elastic anisotropy increases, and the effect is higher at higher pressures. © 2010 Elsevier B.V.

Rubie D.C.,Bayerisches Geoinstitut | Laurenz V.,Bayerisches Geoinstitut | Jacobson S.A.,Bayerisches Geoinstitut | Morbidelli A.,Observatoire de la Cote dAzur | And 3 more authors.
Science | Year: 2016

Highly siderophile elements (HSEs) are strongly depleted in the bulk silicate Earth (BSE) but are present in near-chondritic relative abundances. The conventional explanation is that the HSEs were stripped fromthe mantle by the segregation of metal during core formation but were added back in near-chondritic proportions by late accretion, after core formation had ceased. Here we show that metal-silicate equilibration and segregation during Earth's core formation actually increased HSE mantle concentrations because HSE partition coefficients are relatively low at the high pressures of core formation within Earth. The pervasive exsolution and segregation of iron sulfide liquid from silicate liquid (the "Hadean matte") stripped magma oceans of HSEs during cooling and crystallization, before late accretion, and resulted in slightly suprachondritic palladium/iridium and ruthenium/iridium ratios. Copyright © 2016 by the American Association for the Advancement of Science; all rights reserved.

Robinson P.,Geological Survey of Norway | Fabian K.,Geological Survey of Norway | McEnroe S.A.,Norwegian University of Science and Technology | Heidelbach F.,Bayerisches Geoinstitut
Geophysical Journal International | Year: 2013

New experimental and computational approaches to interpret orientation and intensity of natural remanent magnetization (NRM) carried by lamellar magnetism are applied to historic magnetic measurements on a collection of 82 massive hemo-ilmenite samples from the Allard Lake District in the Grenville Province, Quebec. The anisotropy of magnetic susceptibility (AMS), together with declination and inclination ofNRM, indicate a systematic deflection β of the NRM vector away fromthe unit vector v that represents the Mesoproterozoic magnetizing field direction. The deflection β is caused by a statistical lattice-preferred orientation (LPO) of the individual (0001) basalplanes, to which the NRM is confined in hemo-ilmenite crystals. Here, we study a second deflection Ψ that is the angle the NRM makes with the statistical (0001) basal plane of the crystal assemblage, in relation to the angle α between the statistical (0001) basal plane and v. The relatio between these two angles depends on the scatter of thedistribution of crystal platelets, which also influences the AMS of theassemblage. For a Fisher distribution of basal planes, the distribution parameterK can be determined from Ψ and a. It is then furtherpossible to infer the single-crystal anisotropy of individual platelets. Typical crystals of hemo-ilmenite turn out to have a relatively weak AMS so that samples with a narrow Fisher distribution of plateletsnevertheless can have a weak AMS. This has been confirmed in two samplesby measurement of the (0001) basal plane distribution of crystalsusing electron backscatter diffraction, and in one of these two samples by measuring AMS and NRM of a single hemo-ilmenite crystal. Based on our estimated K values for selected samples, we calculate values of β, NRM intensity and Ψ for any value of α. These data provide striking examples of the influence of the orientation of the crystal LPO on the intensity of lamellar magnetism, and explain the large variation of observedNRM intensities by varying orientation with respect to the magnetizing field, without requiring large variations ofthe paleomagnetic field intensity. This relation between NRM and LPO is also important for anomaly interpretation in areas with strong foliation. © The Authors 2012. Published by Oxford University Press on behalf of The Royal Astronomical Society.

Mullner M.,University of Bayreuth | Lunkenbein T.,University of Bayreuth | Schieder M.,University of Bayreuth | Groschel A.H.,University of Bayreuth | And 5 more authors.
Macromolecules | Year: 2012

We demonstrate the synthesis of uniform one-dimensional (1D) titania hybrid nanotubes using core-shell-corona cylindrical polymer brushes (CPBs) as soft templates. The CPBs consist of a polymethacrylate backbone with densely grafted poly(ε-caprolactone) (PCL) as the core, poly(2-(dimethlamino)ethyl methacrylate) (PDMAEMA) as the cationic shell, and poly(oligo(ethylene glycol) methyl ether methacrylate) (POEGMA) as the corona. The weak polyelectrolyte shell complexed an oppositely charged titania precursor, namely titanium(IV) bis(ammonium lactate) dihydroxide (TALH), and then acted as a nanoreactor for the hydrolysis and condensation of TALH, resulting in crystalline TiO 2. The POEGMA shell provides solubility in aqueous and organic solvents. The hybrid titania nanotubes containing anatase nanoparticles were characterized by atomic force microscopy (AFM), transmission electron microscopy (TEM), and scanning electrion microscopy (SEM). The phase purity of the crystalline nanostructures was verified by powder X-ray diffractometry (PXRD). © 2012 American Chemical Society.

Stagno V.,Carnegie Institution of Washington | Stagno V.,Bayerisches Geoinstitut | Frost D.J.,Bayerisches Geoinstitut | McCammon C.A.,Bayerisches Geoinstitut | And 2 more authors.
Contributions to Mineralogy and Petrology | Year: 2015

The oxygen fugacity (fO2) at which carbonate-bearing melts are reduced to either graphite or diamond in synthetic eclogite compositions has been measured in multi-anvil experiments performed at pressures between 3 and 7 GPa and temperatures between 800 and 1,300 °C using iron–iridium and iron–platinum alloys as sliding redox sensors. The determined oxygen fugacities buffered by the coexistence of elemental carbon and carbonate-bearing melt are approximately 1 log unit below thermodynamic calculations for a similar redox buffering equilibrium involving only solid phases. The measured oxygen fugacities normalized to the fayalite–magnetite–quartz oxygen buffer decrease with temperature from ~−0.8 to ~−1.7 log units at 3 GPa, most likely as a result of increasing dilution of the carbonate liquid with silicate. The normalized fO2 values also decrease with pressure and show a similar decrease with temperature at 6 GPa from ~−1.5 log units at 1,100 °C to ~−2.4 log units at 1,300 °C. In contrast to previous arguments, the stability field of the carbonate-bearing melt extends to lower oxygen fugacity in eclogite rocks than in peridotite rocks, which implies a wider range of conditions over which carbon remains mobile in natural eclogites. The raised prevalence of diamonds in eclogites compared to peridotites may, therefore, reflect more effective scavenging of carbon by melts in these rocks. The ferric iron contents of monomineralic layers of clinopyroxene and garnet contained in the same experiments were also measured using Mössbauer spectroscopy. A preliminary model was derived for determining the fO2 of eclogitic rocks from the compositions of garnet and clinopyroxene, including the Fe3+/ΣFe ratio of garnet, using the equilibrium, (Formula Presneted.) (Formula Presented.) The model, which reproduces the independently determined fO2 of the experimental data to within 0.5 log units, can be used to estimate the fO2 of ultrahigh-pressure metamorphic eclogites and cratonic eclogitic xenoliths. Although there are very few analyses of garnet Fe3+/ΣFe ratios from eclogite samples, the range in fO2 recorded by available eclogitic xenoliths is similar to that reported for peridotitic xenoliths and generally within the graphite/diamond stability field. Estimates for the average bulk Fe3+/ΣFe ratio of modern basaltic oceanic crust, however, are higher than the values for most of these xenoliths, and upon subduction, crustal carbon is likely to remain in the carbonate stability field to depths of at least 250 km. If eclogite xenoliths originated from subducted oceanic crust, then their generally lower fO2 most likely reflects either lower initial basaltic Fe3+/ΣFe ratios, loss of Fe2O3 through partial melting or the initial presence of organic carbon. © 2015, Springer-Verlag Berlin Heidelberg.

Stagno V.,Bayerisches Geoinstitut | Stagno V.,Carnegie Institution of Washington | Tange Y.,Ehime University | Miyajima N.,Bayerisches Geoinstitut | And 3 more authors.
Geophysical Research Letters | Year: 2011

The oxygen fugacity at which magnesite (MgCO3) is reduced to diamond in a typical mantle assemblage has been determined between 16 and 45 GPa and 1500-1700C in experiments employing a multianvil device. This oxygen fugacity for carbonate stability, measured using a sliding redox sensor that employs IrFe alloy, was found to be greater than 2 log units above the iron-wstite oxygen buffer (IW+2). Reversal experiments employing FeNi alloy confirmed complete oxidation of Ni in the presence of magnesite and diamond even at 45 GPa. As the oxygen fugacity of the transition zone and lower mantle is most likely at or below the IW buffer, mantle carbon, if distributed relatively homogeneously, is unlikely to be hosted in carbonates throughout most of the mantle but is more likely present as diamond, methane, Fe-rich carbide or as a carbon-component dissolved in Fe-Ni metal. The existence of carbonate at these depths would imply the presence of unusually oxidized regions of the deeper mantle. Such regions could form in the deeper mantle from an influx of subduction related carbonate melt, which would reduce by causing oxidation of the surrounding silicates. Due to changes in the degree of oxidation of the surrounding mantle such melts could potentially travel further in the transition zone mantle than in the lower mantle. The results do not exclude the possibility that carbonate could coexist with Fe-Ni metal or carbide at the very base of the lower mantle. Copyright 2011 by the American Geophysical Union.

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