735 research outputs found
New insights into the provenance of Saudi Arabian Palaeozoic sandstones from heavy mineral analysis and single-grain geochemistry
Saudi Arabian Palaeozoic siliciclastics cover a stratigraphic range from the Cambrian to the Permian. They crop out along the eastern margin of the Arabian Shield and are comprised of highly mature sandstones. Their heavy mineral assemblage reflects their mineralogical maturity and is dominated by the ultra-stable phases zircon, tourmaline and rutile. Less stable accessories are apatite, staurolite and garnet. Standard heavy mineral analysis of samples from two study areas in central/northern (Tabuk area) and southern (Wajid area) Saudi Arabia reveals distinct changes in provenance. Cambrian–Ordovician sandstones are first-cycle sediments, probably sourced from the ‘Pan-African’ basement. The overlying Hirnantian glaciogenic deposits consist of recycled Cambrian–Ordovician material. Devonian–Permian sandstones show a significant influx of fresh basement material, as attested by an increase of meta-stable heavy minerals. Single-grain geochemical analysis of rutile and garnet has proven to be a powerful supplementary technique. Rutile varietal studies reveal distinct differences in host rock lithologies between the two study areas: the Tabuk area contains predominantly felsic rutiles, whereas the Wajid area has more mafic input. Zr-in-rutile thermometry identified granulite-facies detritus in the lower Palaeozoic of the Tabuk area and has the potential to further define source areas. The distribution patterns of garnet host rock lithologies are remarkably similar in both study areas. They are dominated by amphibolite-facies metasediments and intermediate to felsic igneous rocks. Garnets derived from granulite-facies metasediments, which are scarce in the Arabian–Nubian Shield, also occur. Possible source rocks for high-grade garnets can be found in Yemen or farther south in the Mozambique Belt
The potential of ground gravity measurements to validate GRACE data
New satellite missions are returning high precision, time-varying, satellite measurements of the Earth’s gravity field. The GRACE mission is now in its calibration/- validation phase and first results of the gravity field solutions are imminent. We consider here the possibility of external validation using data from the superconducting gravimeters in the European sub-array of the Global Geodynamics Project (GGP) as ‘ground truth’ for comparison with GRACE. This is a pilot study in which we use 14 months of 1-hour data from the beginning of GGP (1 July 1997) to 30 August 1998, when the Potsdam instrument was relocated to South Africa. There are 7 stations clustered in west central Europe, and one station, Metsahovi in Finland. We remove local tides, polar motion, local and global air pressure, and instrument drift and then decimate to 6-hour samples. We see large variations in the time series of 5–10<i>µ</i>gal between even some neighboring stations, but there are also common features that correlate well over the 427-day period. The 8 stations are used to interpolate a minimum curvature (gridded) surface that extends over the geographical region. This surface shows time and spatial coherency at the level of 2– 4<i>µ</i>gal over the first half of the data and 1–2<i>µ</i>gal over the latter half. The mean value of the surface clearly shows a rise in European gravity of about 3µgal over the first 150 days and a fairly constant value for the rest of the data. The accuracy of this mean is estimated at 1<i>µ</i>gal, which compares favorably with GRACE predictions for wavelengths of 500 km or less. Preliminary studies of hydrology loading over Western Europe shows the difficulty of correlating the local hydrology, which can be highly variable, with large-scale gravity variations.<br><br><b>Key words. </b>GRACE, satellite gravity, superconducting gravimeter, GGP, ground trut
Localized versus itinerant magnetic moments in Na0.72CoO2
Based on experimental 59Co-NMR data in the temperature range between 0.1 and
300 K, we address the problem of the character of the Co 3d-electron based
magnetism in Na0.7CoO2. Temperature dependent 59Co-NMR spectra reveal different
Co environments below 300 K and their differentiation increases with decreasing
temperature. We show that the 23Na- and 59Co-NMR data may consistently be
interpreted by assuming that below room temperature the Co 3d-electrons are
itinerant. Their magnetic interaction appears to favor an antiferromagnetic
coupling, and we identify a substantial orbital contribution corb to the
d-electron susceptibility. At low temperatures corb seems to acquire some
temperature dependence, suggesting an increasing influence of spin-orbit
coupling. The temperature dependence of the spin-lattice relaxation rate
T1-1(T) confirms significant variations in the dynamics of this electronic
subsystem between 200 and 300K, as previously suggested. Below 200 K, Na0.7CoO2
may be viewed as a weak antiferromagnet with TN below 1 K but this scenario
still leaves a number of open questions.Comment: 8.7 pages, 6 Figures, submitted to Phys. Rev.
L'expérience Ghyraf au Bénin : première comparaison entre suivi gravimétrique absolu et variation de stock hydrique
Sur le site soudanien du SO Amma-Catch, le projet ANR GHYRAF effectue des mesures trimestrielles de gravimétrie absolue depuis 2008. Ces mesures sont comparées à des variations gravimétriques simulées à partir des données hydrologiques (piézométrie, sonde à neutrons) appliquées à un modèle homogène du milieu souterrain. Les mesures gravimétriques sont cohérentes avec les observations hydrologiques et suffisamment précises pour assurer un suivi de la variabilité interannuelle des stocks ou une estimation de l'évapotranspiration. (Résumé d'auteur
Detection of Phase Jumps of Free Core Nutation of the Earth and their Concurrence with Geomagnetic Jerks
We detected phase jumps of the Free Core Nutation (FCN) of the Earth directly
from the analysis of the Very Long Baseline Interferometer (VLBI) observation
of the Earth rotation for the period 1984-2003 by applying the Weighted Wavelet
Z-Transform (WWZ) method and the Short-time Periodogram with the Gabor function
(SPG) method. During the period, the FCN had two significant phase jumps in
1992 and 1998. These epochs coincide with the reported occurrence of
geomagnetic jerks.Comment: 8 pages, 4 figure
Semianalytical estimates of scattering thresholds and gravitational radiation in ultrarelativistic black hole encounters
Ultrarelativistic collisions of black holes are ideal gedanken experiments to
study the nonlinearities of general relativity. In this paper we use
semianalytical tools to better understand the nature of these collisions and
the emitted gravitational radiation. We explain many features of the energy
spectra extracted from numerical relativity simulations using two complementary
semianalytical calculations. In the first calculation we estimate the radiation
by a "zero-frequency limit" analysis of the collision of two point particles
with finite impact parameter. In the second calculation we replace one of the
black holes by a point particle plunging with arbitrary energy and impact
parameter into a Schwarzschild black hole, and we explore the multipolar
structure of the radiation paying particular attention to the near-critical
regime. We also use a geodesic analogy to provide qualitative estimates of the
dependence of the scattering threshold on the black hole spin and on the
dimensionality of the spacetime.Comment: 29 pages, 19 figure, 6 tables, minor changes to match version in
press in Phys.Rev.
Reconstruction of nuclear charged fragment trajectories from a large gap sweeper magnet
A new method to reconstruct charged fragment four-momentum vectors from
measured trajectories behind an open, large gap, magnetic dispersion element (a
sweeper magnet) has been developed. In addition to the position and angle
behind the magnet it includes the position measurement in the dispersive
direction at the target. The method improves the energy and angle resolution of
the reconstruction significantly for experiments with fast rare isotopes, where
the beam size at the target position is large.Comment: 13 pages, 5 figures, accepted for publication in Nucl. Instrum. Meth.
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