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51.
On the basis of seismic refraction investigations and gravimetric data we have modelled the crustal structure of the southern Central Andes (21–23°S). A pronounced variation in crustal parameters is seen in N-S- and W-E-crossing seismic profiles over the entire Andean orogene, characterized by a crustal thickness of up to 70 km under the magmatic arc and backarc, strongly reduced seismic velocities and a Bouguer minimum of −450 mGal. Anomalously low velocities of 5.9–6.0 km/s in the deeper crust of the Western Cordillera and Altiplano regions lead to an over-compensation of the Bouguer minima resulting in values of crustal densities higher than estimates based purely on seismic velocity measurements. In an attempt to reconcile these differences, the behavior of crystalline rocks based on published laboratory data was studied under varying pressure and temperature conditions up to the range of partial melting. If the temperature is increased above the melting point, a rapid decrease in seismic velocity is accompanied by a slow decrease in density. For the Central Andes, a good fit of the observed and calculated Bouguer anomalies is obtained if the densities of the rocks from the low-velocity zone (LVZ) beneath the Western Cordillera and the Altiplano are varied. Model calculations lead to a velocity-density relation for partial molten rocks that allows the melt proportions of rocks to be estimated. Model calculations indicate that 15–20 vol.% of basaltic to andesitic melt at depth is necessary to explain the LVZ and Bouguer anomaly beneath the arc and parts of the backarc. High heat flow values (100 mW/m2) support the idea that large areas of the deeper Andean crust are strongly weakened by the presence of partially molten rocks, resulting in reduced seismic velocities, with the Western Cordillera, the active volcanic arc of the Andean mountain range, acting as a ductile buffer between the two more rigid crustal blocks of the forearc and backarc regions. 相似文献
52.
Allen K. Kennedy Jörn-Frederik Wotzlaw James L. Crowley Mark Schmitz Urs Schaltegger Benjamin Wade Laure Martin Cristina Talavera Bryant Ware Thi Hao Bui 《Geostandards and Geoanalytical Research》2023,47(2):373-402
Twelve apatite samples have been tested as secondary ion mass spectrometry (SIMS) reference materials. Laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) analysis shows that the SLAP, NUAN and GR40 apatite gems are internally homogeneous, with most trace element mass fractions having 2 standard deviations (2s) ≤ 2.0%. BR2, BR5, OL2, AFG2 and AFB1, which have U > 63 μg g-1, 206Pb/204Pb > 283, and homogeneous SIMS U-Pb data, have respective isotope dilution thermal ionisation mass spectrometry (ID-TIMS) ages of 2053.83 ± 0.21 Ma, 2040.34 ± 0.09 Ma, 868.87 ± 0.25 Ma, 478.71 ± 0.22 Ma and 473.25 ± 0.09 Ma. Minor U-Pb heterogeneity exists and accurate SIMS results require correction with the 3D Concordia-constrained common Pb composition. Among the studied samples, AFG2 and BR5 are the most homogeneous U-Pb reference materials. The SIMS sulfur isotopic compositions of eight of the apatites shows they are homogeneous, with 2s for both 103δ34S and 103δ33S < 0.55‰. One apatite, BR96, has Δ33S = -0.36 ± 0.2‰. The apatite samples have ID-TIMS 87Sr/86Sr between 0.704214 ± 0.000030 and 0.723134 ± 0.000035. 相似文献
53.
Maximiliano J. Bezada Michael Schmitz María Ins Jcome Josmat Rodríguez Franck Audemard Carlos Izarra the BOLIVAR Active Seismic Working Group 《Journal of Geodynamics》2008,45(4-5):191-200
The Falcón Basin in northwestern Venezuela has a complex geological history driven by the interactions between the South American and Caribbean plates. Igneous intrusive bodies that outcrop along the axis of the basin have been associated with crustal thinning, and gravity modeling has shown evidence for a significantly thinned crust beneath the basin. In this study, crustal scale seismic refraction/wide-angle reflection data derived from onshore/offshore active seismic experiments are interpreted and forward-modeled to generate a P-wave velocity model for a 450 km long profile. The final model shows thinning of the crust beneath the Falcón Basin where depth to Moho decreases to 27 km from a value of 40 km about 100 km to the south. A deeper reflected phase on the offshore section is interpreted to be derived from the downgoing Caribbean slab. Velocity values were converted to density and the resulting gravimetric response was shown to be consistent with the regional gravity anomaly. The crustal thinning proposed here supports a rift origin for the Falcón Basin. 相似文献
54.
Davide Lenaz Vanni Lughi Diego Perugini Maurizio Petrelli Gianluca Turco Birger Schmitz 《Meteoritics & planetary science》2019,54(12):3089-3100
MgAl2O4 spinels from Allende and NWA 763 carbonaceous chondrites were studied by X‐ray single crystal diffraction, SEM, electron microprobe, LA‐ICP‐MS, and Raman spectroscopy. Those from Allende are almost pure, but, in one case, we found a strong FeOtot zonation. Spinels from NWA 763 show Mg‐Fe2+ substitutions. Almost pure MgAl2O4 spinels from both meteorites underwent slow cooling and reached their intracrystalline closure temperature (Tc) in the range 460–520 °C. The NWA 763 spinel with higher FeO content shows a Tc of about 720 °C. X‐ray single crystal diffraction and Raman spectroscopy suggest a slow cooling and an ordered structure with trivalent cations in M site and divalent in T site. Among the trace elements, Ti and Co are enriched with respect to the terrestrial analogs, while Mn, Ni, and Sn show intermediate values between different terrestrial occurrences. Vanadium cannot be used as a tracer of extraterrestrial origin as for Cr‐spinels, because its content is similar in extraterrestrial and terrestrial spinels. In the zoned crystal from Allende, Co show a strong zonation similar to that of FeO. 相似文献
55.
Shocked chromites in fossil L chondrites: A Raman spectroscopy and transmission electron microscopy study 下载免费PDF全文
Surya S. Rout Philipp R. Heck Nestor J. Zaluzec Takayuki Ishii Jianguo Wen Dean J. Miller Birger Schmitz 《Meteoritics & planetary science》2017,52(9):1776-1796
Chromites from Middle Ordovician fossil L chondrites and from matrix and shock‐melt veins in Catherwood, Tenham, and Coorara L chondrites were studied using Raman spectroscopy and TEM. Raman spectra of chromites from fossil L chondrites showed similarities with chromites from matrix and shock‐melt veins in the studied L chondrite falls and finds. Chromites from shock‐melt veins of L chondrites show polycrystallinity, while the chromite grains in fossil L chondrites are single crystals. In addition, chromites from shock‐melt veins in the studied L chondrites have high densities of planar fractures within the subgrains and many subgrains show intergrowths of chromite and xieite. Matrix chromite of Tenham has similar dislocation densities and planar fractures as a chromite from the fossil meteorite Golvsten 001 and higher dislocation densities than in chromite from the fossil meteorite Sextummen 003. Using this observation and knowing that the matrix of Tenham experienced 20–22 GPa and <1000° C, an upper limit for the P,T conditions of chromite from Golvsten 001 and Sextummen 003 can be estimated to be 20–22 GPa and 1000° C (shock stage S3–S6) and 20 GPa and 1000° C (S3–S5), respectively, and we conclude that the studied fossil meteorite chromites are from matrix. 相似文献
56.
Uwe Ulbrich Gerd Bürger Dierk Schriever Hans von Storch Susanne L Weber Gerhard Schmitz 《Climate Dynamics》1993,8(6):277-285
The sensitivity of the atmospheric circulation to an increase in ocean surface roughness in the Southern Hemisphere storm track is investigated in a paired general circulation model experiment. Such a change in sea roughness could be induced by ocean waves generated by storms. Two extended permanent-July runs are made. One with standard sea surface roughness, the other with ten times as a large surface roughness over open sea poleward of 40° S. The regional increase in ocean surface roughness significantly modifies the tropospheric circulation in the Southern Hemisphere. The strongest effect is the reduction of tropospheric winds (by 2 m/s or 10%) above the area with increased roughness. The poleward eddy momentum flux is reduced in the upper troposphere and the meridional eddy sensible heat flux is reduced in the lower troposphere. Zonal mean and eddy kinetic energy are consistently reduced. 相似文献
57.
Simon Scheider Jiong Wang Maarten Mol Oliver Schmitz Derek Karssenberg 《International journal of geographical information science》2020,34(7):1398-1427
ABSTRACT Spatial point tracks are of concern for an increasing number of analysts studying spatial behaviour patterns and environmental effects. Take an epidemiologist studying the behaviour of cyclists and how their health is affected by the city’s air quality. The accuracy of such analyses critically depends on the positional accuracy of the tracked points. This poses a serious privacy risk. Tracks easily reveal a person’s identity since the places visited function as fingerprints. Current obfuscation-based privacy protection methods, however, mostly rely on point quality reduction, such as spatial cloaking, grid masking or random noise, and thus render an obfuscated track less useful for exposure assessment. We introduce simulated crowding as a point quality preserving obfuscation principle that is based on adding fake points. We suggest two crowding strategies based on extending and masking a track to defend against inference attacks. We test them across various attack strategies and compare them to state-of-the-art obfuscation techniques both in terms of information loss and attack resilience. Results indicate that simulated crowding provides high resilience against home attacks under constantly low information loss. 相似文献
58.
Time-Series Reconstruction from Natural Archive Data with the Averaging Effect Taken into Account 总被引:2,自引:2,他引:0
Veerle Beelaerts Fjo De Ridder Nele Schmitz Maite Bauwens Rik Pintelon 《Mathematical Geosciences》2010,42(6):705-722
Environmental information of the past can be obtained by processing and analyzing proxies recorded by environmental archives.
Natural archives are sampled at a distance grid along their accretion axis. Starting from these distance series, a time series
needs to be constructed, because comparison of different data records is only meaningful on a time grid. However, distance–time
relationships are nonlinear as the accretion rate of natural archives is dependent on environmental and physiological factors.
Furthermore, in environmental archives, samples are taken over a volume in distance, rather than over a point in distance.
This implies that the sample-values will be averaged over the volume of the sample. In this paper a method is proposed, which
establishes the nonlinear distance–time relationship and corrects for the averaging effects. The method is built upon the
assumption that the proxy record on a time axis is harmonic. If this is not the case, then a harmonic approximation is made.
As a consequence of the nonlinear distance–time relationship, this harmonic proxy signal is nonlinearly distorted on a distance
axis. As such, a harmonic signal model with a nonlinear phase distortion and an averaging effect is fitted on the data. Since
environmental records are short data records, the statistical performance of the estimator on noisy data is verified by means
of Monte Carlo simulations. The applicability of the method is demonstrated on the measurement of the vessel density, in a
mangrove tree, Rhizophora mucronata, which is an indicator of the rainfall in tropical coastal ecosystems. 相似文献
59.
Sylvia Schmitz Frank E. Brenker Tom Schoonjans Bart Vekemans Geert Silversmit Laszlo Vincze Manfred Burghammer Christian Riekel 《Geochimica et cosmochimica acta》2009,73(18):5483-5492
We detected additional CAI-like material in STARDUST mission samples of comet 81P/Wild 2. Two highly refractory cometary dust fragments were identified in the impact track 110 [C2012, 0, 110, 0, 0] by applying high resolution synchrotron induced confocal and conventional XRF analysis (HR SR-XRF). The use of a polycapillary lens in front of the detector for confocal spectroscopy dramatically improves the fidelity of particle measurements by removing contribution from the surrounding aerogel. The high spatial resolution (300 × 300 nm2; 300 × 1000 nm2) obtained allowed the detailed non-destructive in situ (trapped in aerogel) study of impacted grains at the sub-μm level.For the two largest particles of the track, the terminal particle and a second particle along the impact track, Ca concentration is up to 30 times higher than CI and Ti is enriched by a factor of 2 compared to CI. High resolution (HR) SR-XRF mapping also reveals that the highest concentrations of Ca, Ti, Fe (and Ni) measured within each grain belongs to different areas of the respective maps which indicate that the particles are composed of several chemically diverse mineral phases. This is in agreement with the finding of a complex phase assemblage of highly refractory minerals in the first ever detected Stardust mission CAI grain “Inti” of Track 25.Principle component analysis (PCA) is a powerful tool for extracting the dominant mineral components and was applied to the two grains indicating that regions in the terminal particle and the second particle are consistent with anorthite or grossite and gehlenite, monticellite or Dmitryivanovite (CaAl2O4), respectively.Our new findings demonstrate that the HR SR-XRF with confocal geometry and PCA analysis is capable of identifying CAI-like fragments without the need to extract particles from the aerogel matrix which is a time-consuming, complex and destructive process.Furthermore, the detection of new CAI-like fragments in the coma dust of comet 81P/Wild 2 strengthens the observation that strong mixing effects and, therefore, mass transport before or during comet formation must have occurred at least up to the region where Kuiper Belt comets formed (30 AU). 相似文献
60.
On the Elimination of Bias Averaging-Errors in Proxy Records 总被引:2,自引:2,他引:0
Veerle Beelaerts Fjo De Ridder Nele Schmitz Maite Bauwens Frank Dehairs Johan Schoukens Rik Pintelon 《Mathematical Geosciences》2009,41(2):129-144
Knowledge of and insight into past environmental conditions can be obtained by processing and analyzing proxies. The proxies
need to be processed as precisely and accurately as possible, otherwise the conclusion of the analysis will be biased. A calibration
method which reduces bias errors in the proxy measurements due to averaging is presented. Sampling with nonzero sample sizes
causes an averaging of the true proxy signal over the volume of the sample. The method is applied on a linear synthetic record
which results in an optimal correction for frequency components ranging from the dc-frequency (DC) to one half of the sample
frequency (f
s
/2). Next, the method is tested on non-linear synthetic data where the signal is reconstructed reasonably well. Finally, the
method is applied to a real vessel density record of R.
mucronata from Makongeni, Kenya, and to a real delta deuterium record of ice core EDC from dome C, Antarctica. The method discussed
in this paper is a valuable tool for the calibration of proxy measurements; it can be applied as a correction for low resolution
measurements and expanded to other types of samples and proxies. Working with small sample sizes (high resolution) amounts
to working near the detection limit, where the signal-to-noise-ratio is low. This correction method provides an alternative
in which low resolution measurements can be upgraded to minimize the loss of information due to larger sample sizes. 相似文献