Showing papers by "IPG Photonics published in 2001"

First-principles modeling of the infrared spectrum of kaolinite

Abstract: The theoretical infrared spectrum of kaolinite [Al2Si2O5(OH)4, triclinic] was computed using ab initio quantum mechanical calculations. Calculations were performed using the Density Functional Theory and the generalized gradient approximation. The low-frequency dielectric tensor of kaolinite was determined as a function of the light frequency using linear response theory. The IR spectrum was then calculated using a model that takes into account the shape and size of kaolinite particles. A remarkable agreement was obtained between theory and experiment, especially on the position of the stretching bands of OH groups. This agreement provides a firm basis for the interpretation of the IR spectrum of kaolinite in terms of structural parameters.

The accretion, composition and early differentiation of Mars

Abstract: The early development of Mars is of enormous interest, not just in its own right, but also because it provides unique insights into the earliest history of the Earth, a planet whose origins have been all but obliterated. Mars is not as depleted in moderately volatile elements as are other terrestrial planets. Judging by the data for Martian meteorites it has Rb/Sr ≈ 0.07 and K/U ≈ 19,000, both of which are roughly twice as high as the values for the Earth. The mantle of Mars is also twice as rich in Fe as the mantle of the Earth, the Martian core being small (~20% by mass). This is thought to be because conditions were more oxidizing during core formation. For the same reason a number of elements that are moderately siderophile on Earth such as P, Mn, Cr and W, are more lithophile on Mars. The very different apparent behavior of high field strength (HFS) elements in Martian magmas compared to terrestrial basalts and eucrites may be related to this higher phosphorus content. The highly siderophile element abundance patterns have been interpreted as reflecting strong partitioning during core formation in a magma ocean environment with little if any late veneer. Oxygen isotope data provide evidence for the relative proportions of chondritic components that were accreted to form Mars. However, the amount of volatile element depletion predicted from these models does not match that observed — Mars would be expected to be more depleted in volatiles than the Earth. The easiest way to reconcile these data is for the Earth to have lost a fraction of its moderately volatile elements during late accretionary events, such as giant impacts. This might also explain the nonchondritic Si/Mg ratio of the silicate portion of the Earth. The lower density of Mars is consistent with this interpretation, as are isotopic data. 87Rb-87Sr, 129I-129Xe, 146Sm-142Nd, 182Hf-182W, 187Re-187Os, 235U-207Pb and 238U-206Pb isotopic data for Martian meteorites all provide evidence that Mars accreted rapidly and at an early stage differentiated into atmosphere, mantle and core. Variations in heavy xenon isotopes have proved complicated to interpret in terms of 244Pu decay and timing because of fractionation thought to be caused by hydrodynamic escape. There are, as yet, no resolvable isotopic heterogeneities identified in Martian meteorites resulting from 92Nb decay to 92Zr, consistent with the paucity of perovskite in the martian interior and its probable absence from any Martian magma ocean. Similarly the longer-lived 176Lu-176Hf system also preserves little record of early differentiation. In contrast W isotope data, Ba/W and time-integrated Re/Os ratios of Martian meteorites provide powerful evidence that the mantle retains remarkably early heterogeneities that are vestiges of core metal segregation processes that occurred within the first 20 Myr of the Solar System. Despite this evidence for rapid accretion and differentiation, there is no evidence that Mars grew more quickly than the Earth at an equivalent size. Mars appears to have just stopped growing earlier because it did not undergo late stage (>20 Myr) impacts on the scale of the Moon-forming Giant Impact that affected the Earth.

Grain reconstruction of porous media: Application to a low-porosity Fontainebleau sandstone

Abstract: The fundamental issue of reconstructing a porous medium is examined anew in this paper, thanks to a sample of low-porosity Fontainebleau sandstone that has been analyzed by computed microtomography. Various geometric properties are determined on the experimental sample. A statistical property, namely, the probability density of the covering radius, is determined. This is used in order to reconstruct a porous medium by means of a Poissonian generation of polydisperse spheres. In a second part, the properties of the real experimental sample and of the reconstructed one are compared. The most important success of the present reconstruction technique is the fact that the numerical sample percolates despite its low porosity. Moreover, other geometrical features and conductivity are found to be in good agreement.

Metamictization and chemical durability of detrital zircon

Abstract: We have investigated the effect that metamictization has on the weathering of zircon in detrital continental sediments and tropical soils of the Amazon basin, Brazil. The degree of radiation damage in the near-surface region of the zircon grains was determined by Raman microprobe. In each of the four series investigated (i.e., sediment, podzol, topsoil, and subsoil horizons of lateritic soil), the degree of radiation damage ranges from less than 1014 to ~3.5 × 1015 α−decay/mg. The maximum degree of radiation damage coincides with the first percolation threshold of the metamictization process at ~3.5 × 1015 α-decay/mg. Below this threshold, amorphous volumes in the structure of damaged zircon are not connected to each other. The ranges of U, Th, and Pb contents (in ppm) measured by proton induced X-ray emission (PIXE) microanalysis are 100 < U < 7000, 100 < Th < 18000, and 100 < Pb < 1300. Chemical ages, assessed from U, Th, and total-Pb, range between 0.15 Ga and 2.8 Ga. This range is roughly consistent with the ages reported for the Precambrian shields of the Amazon basin (0.45–3.5 Ga). Corresponding radiation doses range between <2 × 1015 and 3 × 1016 α-decay/mg. Comparison of calculated doses with the degree of structural damage indicates that most of the zircon grains have experienced significant annealing. However, the degree of annealing differs from one grain to another. Thus, the acute maximum limit observed for the degree of radiation damage of the whole zircon series is better explained by low-temperature alteration or weathering processes than by thermal resetting. Following this interpretation, our results provide evidence for a dramatic decrease in the chemical durability of zircon in natural weathering environments when the radiation dose exceeds 3.5 × 1015 α-decay/mg. Below the first percolation threshold, the zircon population survives the soil formation intact, but more damaged zircons are dissolved during weathering/ alteration processes.

Gravity anomalies, crustal structure and thermo-mechanical support of the Himalaya of Central Nepal

Abstract: SUMMARY We use two gravity profiles that we measured across Central Nepal, in conjunction with existing data, to constrain the mechanical behaviour and the petrological structure of the lithosphere in the Himalayan collision zone. The data show (1) overcompensation of the foreland and undercompensation of the Higher Himalaya, as expected from the flexural support of the range; (2) a steep gravity gradient of the order of 1.3 mgal km x1 beneath the Higher Himalaya, suggesting a locally steeper Moho; and (3) a 10 km wide hinge in southern Tibet. We compare these data with a 2-D mechanical model in which the Indian lithosphere is flexed down by the advancing front of the range and sedimentation in the foreland. The model assumes brittle Coulomb failure and nonlinear ductile flow that depends on local temperature, which is computed from a steadystate thermal model. The computed Moho fits seismological constraints and is consistent with the main trends in the observed Bouguer anomaly. It predicts an equivalent elastic thickness of 40‐50 km in the foreland. The flexural rigidity decreases northwards due to thermal and flexural weakening, resulting in a steeper Moho dip beneath the high range. Residuals at short wavelengths (over distances of 20‐30 km) are interpreted in terms of (1) sediment compaction in the foreland (Dr=150 kg m x3 between the Lower and Middle Siwaliks); (2) the contact between the Tertiary molasse and the meta-sediments of the Lesser Himalaya at the MBT (Dr=220 kg m x3 ); and (3) the Palung granite intrusion in the Lesser Himalaya (Dr=80 kg m x3 ). Finally, if petrological transformations expected from the local (P, T) are assumed, a gravity signature of the order of 250 mgal is predicted north of the Lesser Himalaya, essentially due to eclogitization of the lower crust, which is inconsistent with the gravity data. We conclude that eclogitization of the Indian crust does not take place as expected from a steady-state local equilibrium assumption. We show, however, that eclogitization might actually occur beneath southern Tibet, where it could explain the hinge observed in the gravity data. We suspect that these eclogites are subducted with the Indian lithosphere.

High-pressure phase transformations in the MgFe 2 O 4 and Fe 2 O 3 –MgSiO 3 systems

Abstract: The crystal structure of MgFe2O4 was investigated by in situ X-ray diffraction at high pressure, using YAG laser annealing in a diamond anvil cell. Magnesioferrite undergoes a phase transformation at about 25 GPa, which leads to a CaMn2O4-type polymorph about 8% denser, as determined using Rietveld analysis. The consequences of the occurrence of this dense MgFe2O4 form on the high-pressure phase transformations in the (MgSi)0.75(FeIII)0.5O3 system were investigated. After laser annealing at about 20 GPa, we observe decomposition to two phases: stishovite and a spinel-derived structure with orthorhombic symmetry and probably intermediate composition between MgFe2O4 and Mg2SiO4. At pressures above 35 GPa, we observe recombination of these products to a single phase with Pbnm perovskite structure. We thus conclude for the formation of Mg3Fe2Si3O12 perovskite.

K/Ar dating extended into the last millennium: Application to the youngest effusive episode of the Teide Volcano (Spain)

Abstract: Less than one thousand years dating of the youngest volcanic episode of the Teide volcano (Canary Islands, Spain) has been performed using the K/Ar Cassignol technique. Analyses of about 20 g of pure alkali-feldspar yielded a weighted mean age of 800±300 years. Stratigraphic, historic and archeomagnetic dating validate this age and rule out the hypothesis of the presence of significant excess argon contamination for this flow. Our result shows that the last effusive activity of the Teide volcano took place shortly before European settlement in Tenerife Island. This study confirms that the Cassignol technique can be applied with success for historic dating and demonstrates that it can now even be extended to the last millennium.

Description of new shock‐induced phases in the Shergotty, Zagami, Nakhla and Chassigny meteorites

Abstract: — The Shergottite, Nakhlite and Chassignite (SNC) meteorites, Shergotty, Zagami, Nakhla and Chassigny, have been studied by analytical transmission electron microscopy. New phases, characteristic of strong shock conditions, have been discovered: calcium-rich majorites in Shergottty, wadsleyite with anomalously elevated iron content in Chassigny, and impact melts in Shergotty, Zagami and Nakhla. Cristobalites (α and β polymorphs) observed in Shergotty and Zagami may also be related to shock and are interpreted as back transformation products of post-stishovite silica polymorphs. Shocks corresponding to pressure and temperature conditions characteristic of the Earth's transition zone and lower mantle have occurred in those meteorites. Moreover, impact melts indicate high-temperature conditions in localized areas. On the other hand, no massive impact melting is observed in those meteorites, consistent with previous descriptions. These observations provide evidence of highly heterogeneous shock conditions at the scale of few micrometers in these meteorites. Strongly heterogeneous conditions such as those suggested by the present study may help to explain the preservation in martian meteorites of phases practically unaffected by shock being very close to strongly shock-metamorphized minerals.

Determination of thorium and uranium isotope ratios in low-concentration geological materials using a fixed multi-collector-ICP-MS

Abstract: Techniques used for the measurement of thorium and uranium isotope ratios in low-concentration (<0.1 ppm) geological materials on a Nu Instruments® MC-ICP-MS are described. Using an Aridus® micro-concentric, desolvating nebuliser sample introduction system, the beam current in a Faraday collector is typically 1.0 nA ppm−1232Th (total system efficiency = 0.3%). The abundance sensitivity is less than 100 counts s−1 at 1 mass unit below a large peak with an intensity of 1 × 109 counts s−1, or <0.1 ppm. The Faraday–ion counting gain is stable over a day and the method can provide 230Th/232Th ratio measurements on ng quantities of total dissolved Th to a precision that is better than 1% (95% confidence level). This is comparable to results obtained by thermal ionisation mass spectrometry on samples that are typically 10 to 100 times larger. Results for our internal laboratory standards are presented together with our results for standards used in other laboratories to facilitate inter-laboratory comparison.

Moho topography beneath the Corinth Rift area (Greece) from inversion of gravity data

Abstract: SUMMARY Our aim is to understand better the rifting process by imaging the Moho depth variation beneath Corinth and Evvia. We present here the results of a gravity inversion analysis in the region of the Corinth and Evvia rift system, and compare them to those obtained independently from teleseismic tomography and receiver function analyses. The results of these different studies appear to be consistent and show (1) a 10 km crustal thickening in the western part of the area beneath the Hellenides mountains, (2) NW–SE-trending periodic crustal thinning, and (3) a maximum crustal thinning north of the Gulf of Corinth. This 4 km thinning is unlikely to be the result of the rifting alone, which seems to have been reactivated since only 1 Ma. We propose here a geodynamical scenario in two major steps to explain the evolution of Corinth area. Aegean Miocene extension involving boudinage resulted in periodic crustal thinning, consistent with observations. These lithospheric instabilities could have favoured rupture initiation in particular areas, especially near the city of Corinth. Then, the reactivation of the Corinth Rift extension, 1 Myr ago, led to westward rift propagation. The offset observed between the maximum crustal thinning and the Gulf of Corinth could be accommodated by a lowangle normal fault at about 10–15 km depth. The Corinth Rift is thus asymmetrical and was initiated in places of crustal weakness due to Miocene lithospheric instabilities.

Renormalization of QED with planar binary trees

Abstract: The Dyson relations between renormalized and bare photon and electron propagators \(Z_3 \bar D(q)=D(q)\) and \(Z_2 \bar S(q)=S(q)\) are expanded over planar binary trees. This yields explicit recursive relations for the terms of the expansions. When all the trees corresponding to a given power of the electron charge are summed, recursive relations are obtained for the finite coefficients of the renormalized photon and electron propagators. These relations significantly decrease the number of integrals to carry out, as compared to the standard Feynman diagram technique. In the case of massless quantum electrodynamics (QED), the relation between renormalized and bare coefficients of the perturbative expansion is given in terms of a Hopf algebra structure.

Mineralogy of lead in a soil developed on a Pb-mineralized sandstone (Largentière, France)

Abstract: Lead speciation was determined in a soil developed on a geochemical anomaly arising from a Pb-Zn stratabound deposit in Largentiere (Ardeche, France). This geological setting offers the opportunity to determine the preferred form(s) of Pb following soil formation on this unique anomaly. In the soil profile studied, Pb concentrates in the B-horizon (2055 mg/kg Pb) relative to both the A- (1330 mg/kg Pb) and C- (1874 mg/kg Pb) horizons. Plumbogummite (PbAl 3 (PO 4 ) 2 (OH) 5 ·H 2 O) is the main host of Pb in the soil profile. Pb also appears to be associated with Mn-(hydr)oxides, as shown by micro-analyses (EMPA, SEM-EDS, and μ-SXRF), in the form of inner-sphere Pb 2+ complexes, as suggested by Pb L III -edge EXAFS spectroscopy. Linear least-squares fitting of background-subtracted, k 3 -weighted Pb L III -edge EXAFS functions derived from bulk soil samples was carried out using Pb L III -EXAFS spectra of 22 Pb-containing model compounds. Quantitative assessment of Pb speciation revealed that, whereas plumbogummite is the most abundant Pb phase in the soil profile, Pb 2+ –Mn-(hydr)oxide surface complexes are gradually replaced by Pb 2+ -surface complexes with other phases, possibly Pb 2+ -organic complexes, upward in the soil profile. The presence of large amounts of Pb-phosphate in the Largentiere soil suggests that low solubility phosphates may be important long-term hosts of Pb in Pb-contaminated soils that have sufficiently high phosphorous activities to cause formation of these phases.

Numerical simulation of the December 1997 Debris Avalanche in Montserrat, Lesser Antilles

Abstract: Emplacement of a debris avalanche in the White River valley of southern Montserrat (Lesser Antilles), on 26 December 1997, was caused by sector failure of the south flank of the active Soufriere Hills volcano. Pre- and post-emplacement surveys of the region indicate a debris avalanche deposit volume of about 40–50 × 106 m³. This avalanche is modeled as the gravitational flow of a homogeneous continuum governed by a basal friction law. Mass and momentum equations are depth-averaged over the slide thickness. Numerical results show that the observed distribution of debris and duration of emplacement is simulated well for a Coulomb-type friction law with a dynamic friction coefficient dependent upon the thickness and the velocity of the flowing mass.

Environment of Ni, Co and Zn in low alkali borate glasses: information from EXAFS and XANES spectra

Abstract: XANES spectroscopy confirms that transition elements such as nickel, cobalt and zinc are octahedrally co-ordinated in low-alkali borate glasses, a co-ordination state which is unusual in most oxide glasses. EXAFS spectroscopy indicates that, despite their diluted character, transition elements are inhomogeneously distributed, with a medium range order extending up to 6 A with multiple scattering features characteristic of the presence of collinear cations. This peculiar structure is attributed to the presence of rigid units in these low-alkali borate glasses. The presence of these ordered domains in 0.1Li2O–0.9B2O3 glasses with NiO contents ranging from 0.5 to 2 wt% shows their independence relative to the concentration of the transition element.

The mechanism of solution of aluminum oxide in MgSiO3 perovskite

Abstract: We report 27Al and 29Si Nuclear Magnetic Resonance (NMR) spectra, collected at magnetic fields of 14.1 and 18.8 Tesla on samples as small as 1 mg, for Al-bearing MgSiO3 perovskite synthesized at 26–28 GPa. For Al, we find a 1∶1 ratio of two types of sites: a symmetrical, octahedral site and a low symmetry, distorted site that is most likely to be Al in modifications of normally eight-coordinated Mg sites. A charge coupled substitution of 2 Al for one Si and one Mg cation is strongly supported as the predominant mechanism in this pressure range.

Fault propagation and climatic control of sedimentation on the Ghoubbet Rift Floor: insights from the Tadjouraden cruise in the western Gulf of Aden

Abstract: Summary A detailed geophysical survey of the Ghoubbet Al Kharab (Djibouti) clarifies the small-scale morphology of the last submerged rift segment of the propagating Aden ridge before it enters the Afar depression. The bathymetry reveals a system of antithetic normal faults striking N130°E, roughly aligned with those active along the Asal rift. The 3.5 kHz sub-bottom profiler shows how the faults cut distinct layers within the recent, up to 60 m thick, sediment cover on the floor of the basin. A large volcanic structure, in the centre of the basin, the ‘Ghoubbet’ volcano, separates two sedimentary flats. The organization of volcanism and the planform of faulting, with en echelon subrifts along the entire Asal–Ghoubbet rift, appear to confirm the westward propagation of this segment of the plate boundary. Faults throughout the rift have been active continuously for the last 8400 yr, but certain sediment layers show different offsets. The varying offsets of these layers, dated from cores previously retrieved in the southern basin, imply Holocene vertical slip rates of 0.3–1.4 mm yr−1 and indicate a major decrease in sedimentation rate after about 6000 yr BP, and a redistribution of sediments in the deepest troughs during the period that preceded that change.

Behavior of paramagnetic iron during the thermal transformations of kaolinite

Abstract: Kaolinites with various degrees of structural order and iron content were heated and subsequently analyzed via electron paramagnetic resonance. Iron was present in two different states in the heated materials, either as dilute structural Fe 3+ ions or in concentrated Fe 3+ phases. During metakaolinization, the environment of dilute Fe 3+ ions changed, following modifications of the Al 3+ coordination, and the Fe 3+ concentration increased. With the breakdown of metakaolinite, the diffusion of Fe 3+ ions induced their exsolution in superparamagnetic iron-rich domains (Fe 3+ clusters in γ-Al 2 O 3 and/or Fe 3+ oxide nanophases), which produced a decrease in the dilute Fe 3+ concentration. The subsequent breakdown of γ-Al 2 O 3 and the formation of mullite made the dilute Fe 3+ concentration increase again, because of the incorporation of Fe 3+ ions in the mullite structure.

Medium range structure of borosilicate glasses from Si K-edge XANES: a combined approach based on multiple scattering and molecular dynamics calculations

Abstract: In order to better understand the influence of noble metals precipitated in a borosilicate glass structure, X-ray absorption near-edge structure (XANES) spectra at the silicon K-edge were recorded. The presence of noble metals, although their concentration does not exceed 2%, significantly modifies the Si K-edge spectrum. A shoulder on the high-energy side of the white line disappears when noble metals are present in the glassy matrix. Analysis of the noble metal free spectrum was performed by combining molecular dynamics simulations and multiple scattering calculations. The use of both formalisms allows the determination of the atomic environment up to 4.5 A around silicon atoms. Multiple scattering calculations permit an elucidation of the origin of this peculiar XANES feature, which is a relevant signature of the intermediate range structure. The structural changes within the borosilicate network caused by the incorporation of noble metals are interpreted in terms of modification of the [3]B/[4]B ratio and of the distribution of alkali and alkaline-earth ions within the glassy network.

Permeability of self-affine fractures

Abstract: The permeability of self-affine fractures with various roughness exponents H is investigated by direct three-dimensional numerical simulations. A scaling behavior with an exponent H is exhibited in the self-affine scale range. Permeability can be related to the fractional open area and to the percolation probability by simple models.

Zinc-iron sulphide mineralization in tubes of hydrothermal vent worms

Abstract: Nanocrystalline zinc-iron sulphide minerals were found within tubes of Alvinella pompejana , collected at 9°N on the East Pacific Rise, in a preliminary study carried out by conventional analytical transmission electron microscopy. Their structure is consistent with either the sphalerite or wurtzite polymorphs of ZnS. In contrast with sulphides usually observed in deep-sea hydrothermal environments, the unique sulphide observed within the exoskeleton of the pompeii worms has a (Zn 0.88 Fe 0.12 )S composition. The zinc-iron sulphide nanocrystals are grouped in submicrometer-sized clusters which form layers, concentrically to the proteinaceous tube axis. This is the first characterization of minerals precipitated within a biological matrix from hydrothermal vents organisms. These minerals represent a new example of zinc-iron biologically induced mineralization. Such layers of nanocrystalline zinc-iron sulphide minerals could be used as valuable markers of annelid tubes, in the growing field of studies focusing on fossilized paleo-hydrothermal vent systems.

Palaeomagnetic dating of widespread remagnetization on the southeastern border of the French Massif Central and implications for fluid flow and Mississippi Valley-type mineralization

Abstract: Summary Palaeomagnetic dating techniques have been applied to determine the age of fluid migration that produced the Mississippi Valley-type (MVT) Pb–Zn–Ba–F deposits in the Cevennes region of southern France. 15 sampling sites in two gently deformed areas around the Largentiere and Croix-de-Pallieres mines on the Cevennes border were selected for palaeomagnetic study. They yielded a very well-defined direction of remagnetization corresponding to an Early-Middle Eocene age. This remagnetization cannot be related to the formation of magnetite as a result of the transformation of smectite to illite because the latter has been well dated as a Mesozoic event. The magnetic overprint in this area is related to a chemical phenomenon during fluid migration. The age of remagnetization corresponds to a major uplift in the Pyrenees mountains, located to the south of the Cevennes. This implies that fluid migration occurred from the south to the north as a result of hydraulic head established in the Pyrenees orogenic belt during orogenesis and suggests that the MVT deposits in the Cevennes region formed from a gravity-driven fluid system as described by Garven & Freeze (1984a,b).

Interactions Between Ridges and Plumes

Abstract: In the last 30 years, geologists have made much progress on understanding slabs, lithosphere, and mid-ocean ridges but mantle plumes remain enigmatic. In their Perspective, [Montagner and Ritsema][1] chart recent progress, largely based on seismic anisotropy, toward understanding how such plumes interact with mid-ocean ridges. Existing data are insufficient, however, to determine whether mantle plumes act independently from plate tectonics. The authors call for ocean bottom seismic stations to settle this question. [1]: http://www.sciencemag.org/cgi/content/full/294/5546/1472