Numerical study of the variations of magnetic resonance signals caused by surface slope

When performing forward modelling and inversion of Magnetic Resonance Sounding (MRS) data, the water-content distribution is typically assumed to be horizontal (1D case). This assumption is fully justified because MRS is often used for characterizing continuous aquifers in a nearly flat environment. However, MRS can also be used in areas with sharp topographical variations. Following a review of the standard MRS equations when using a coincident transmitter/receiver loop, the mathematical terms potentially affected by tilting of the loop are discussed. We present the results of a numerical modelling exercise, studying a case where the surface is not horizontal and the loop cannot be considered to be parallel to the top of the aquifer. This shows that maximum variations in the MRS-signal amplitude are caused mainly by north- or south-dipping slopes. Slope effects depend on the loop size (a larger loop produces a larger error) especially in the presence of shallow water. With a geomagnetic-field inclination of 65° and a slope angle ≤ 10°, the topography causes a maximum variation in amplitude of less than 10%. Near magnetic poles and equator, the slope effect is lower and undetectable in most cases. It was found that within a 10% range of variation in the amplitude, errors introduced into inversions are within the typical uncertainty for MRS inversion and hence no topographic corrections are necessary. Thus, a significant effect from non-horizontal topography might be expected only when data uncertainty is lower than the slope effect (the slope effect is lower than equivalence when data quality is poor). Today, most field data sets are inverted using the modulus of the MRS signal, but some new developments consider the complex signal (both modulus and phase). However, inversion of complex MRS signals, which would provide a higher sensitivity to groundwater distribution, may be affected by slope effect. Thus, the slope orientation and dip angle should be accurately measured in the field when the phase of MRS signals is inverted too.     

Microstructures of metal grains in ordinary chondrites: Implications for their thermal histories

Abstract— This paper reports one of the first attempts to investigate by analytical transmission electron microscopy (ATEM) the microstructures and compositions of Fe‐Ni metal grains in ordinary chondrites. Three ordinary chondrites, Saint Séverin (LL6), Agen (H5), and Tsarev (L6) were selected because they display contrasting microstructures, which reflects different thermal histories. In Saint Séverin, the microstructure of the Ni‐rich metal grains is due to slow cooling. It consists of a two‐phase assemblage with a honeycomb structure resulting from spinodal decomposition similar to the cloudy zone of iron meteorites. Microanalyses show that the Ni‐rich phase is tetrataenite (Ni = 47 wt%) and the Ni‐poor phase, with a composition of ～25% Ni, is either martensite or taenite, these two occurring adjacent to each other. The observation that the Ni‐poor phase is partly fcc resolves the disagreement between previous transmission electron microscopy (TEM) and Mössbauer studies on iron meteorites and ordinary chondrite metal. The Ni content of the honeycomb phase is much higher than in mesosiderites, confirming that mesosiderites cooled much more slowly. The high‐Ni tetrataenite rim in contact with the cloudy zone displays high‐Ni compositional variability on a very fine scale, which suggests that the corresponding area was destabilized and partially decomposed at low temperature. Both Agen and Tsarev display evidence of reheating and subsequent fast cooling obviously related to shock events. Their metallic particles mostly consist of martensite, the microstructure of which depends on local Ni content. Microstructures are controlled by both the temperature at which martensite forms and that at which it possibly decomposes. In high‐Ni zones (>15 wt%), martensitic transformation started at low temperature (<300 °C). Because no further recovery occurred, these zones contain a high density of lattice defects. In low‐Ni zones (<15 wt%), martensite grains formed at higher temperature and their lattice defects recovered. These martensite grains present a lath texture with numerous tiny precipitates of Ni‐rich taenite (Ni = 50 wt%) at lath boundaries. Nickel composition profiles across precipitate‐matrix interfaces show that the growth of these precipitates was controlled by preferential diffusion of Ni along lattice defects. The cooling rates deduced from Ni concentration profiles and precipitate sizes are within the range 1–10 °C/year for Tsarev and 10–100 °C/year for Agen.     

Devonian–Carboniferous slivers within the basement area of north-east Oscar II Land, Spitsbergen

Formed during an early compressional period in the opening of North Atlantic Ocean, a Tertiary fold-thrust belt extends along the mid-to- southern part of the western coast of Spitsbergen. Complex thrust structures involve the basement (Caledonian and older) and many shallow dipping thrust faults dissect the overlying cover rocks (Devonian and younger) in Oscar II Land in the northern part of the belt. Some of these faults occur within the basement rocks with slivers or fault blocks of the cover rocks from south-western Brøggerhalvøya to innermost St. Jonsfjorden in north-eastern Oscar II Land. Six of the slivers contain Carboniferous rocks and one is a fault-bounded block with Devonian rocks. These steeply west-dipping faults form a complex fault system- EOFC (Engelskbukta-Osbornbreen Fault Complex) - within the basement area. The lithological units of the basement are separated by faults within the EOFC, which is structurally continuous with the Brøggerhalvøya fold-thrust zone to the north and is thought to continue to the fold-thrust zone on the south-eastern coast of St. Jonsfjorden. Some previous authors considered that the two lithologically contrasting Vendian diamictites and intervening Moefjellet Formation are stratigraphically continuous and defined two separate tilloid successions in the present area. This interpretation has been extended over the whole of western Spitsbergen. However, the present study indicates that these two tilloid formations and the Moefjellet Formation are separated by the faults, probably thrusts, within the EOFC and are not in a continuous stratigraphic relation. Therefore, the two-stage history of Vendian glaciation seems questionable.     

Design and implementation of an image stabilization device at the THEMIS solar telescope

An image stabilizer has been inserted into the optical path of the THEMIS solar telescope. THEMIS is a Ritchey–Chretien reflector telescope using an altazimuthal mount and closely tied to its spectrograph. The optical and mechanical design, implementation and system tests are described, and emphasis is put on the complexity of situations that this stabilizer must accept, including the scanning of the solar surface while stabilizing. The current closed-loop crossover frequency of the device is 65 Hz at ??3 dB on all typical solar scenes.     

Bioconcentration of the anionic surfactant linear alkylbenzene sulfonate (LAS) in the marine shrimp Palaemonetes varians: A radiotracer study

Uptake and depuration kinetics of dissolved [¹⁴C]C₁₂-6-linear alkylbenzene sulfonate (LAS) were determined in the shrimp Palaemonetes varians using environmentally relevant exposure concentration. The shrimp concentrated LAS from seawater with a mean BCF value of 120 L kg⁻¹ after a 7-day exposure. Uptake biokinetics were best described by a saturation model, with an estimated BCF_ss, of 159 ± 34 L kg⁻¹, reached after 11.5 days. Shrimp weight influenced significantly BCF value with smaller individuals presenting higher affinity to LAS. To the light of a whole body autoradiography, major accumulation of LAS occurred in the cephalothorax circulatory system (gills, heart, hepatopancreas) and ocular peduncle, but not in the flesh, limiting potential transfer to human consumers. LAS depuration rate constant value of the shrimp was 1.18 ± 0.08 d⁻¹ leading to less than 1% of remaining LAS in its tissues after 8 days of depuration.     

SIMS analyses on trace and rare earth elements in coexisting clinopyroxene and mica from minette mafic enclaves in potassic syenites crystallized under high pressures

Trace and rare earth element contents were determined by SIMS technique in clinopyroxene and mica crystals from minette lamprophyric enclaves in a potassic syenite host. This co-mingled system was crystallized at high pressures, which varied about 3–5 GPa, as indicated by the presence of K-clinopyroxenes and pyrope-rich garnet with measurable amounts of K₂O and Na₂O, among the near-liquidus phases. Major and trace element composition of these lamprophyric enclaves is quite similar to those observed in silica-rich lamproites, suggesting that similar sources were involved in their origin. In a general view, the concentrations of most trace and rare earth elements in clinopyroxene of the studied enclaves are higher than those referred to by other authors. Clinopyroxene/melt partition coefficient for most trace elements are close to determinations in alkali-basalts and lamproites from Leucite Hills, with considerable differences relative to Gaussberg lamproites. Furthermore, these partition data are completely different from those determined for potassic lavas crystallized under crustal pressures. Spidergrams for clinopyroxenes exhibit negative-Sr anomalies relative to LREE, which have been associated by most authors to crystallization under low-pressures, out of garnet stability field. The presence of pyrope together with K-clinopyroxene excludes such hypotheses for the studied enclaves. Y and HREE are concentrated in clinopyroxene, whilst the other trace elements have Kd<1. LIL elements, except Rb, have incompatible (Kd<1) behavior in phlogopite. The high partition coefficient for Nb (Kd>3) determined in the studied phlogopite is unusual in lamproites, lamprophyres, and basalts, but frequently observed in phlogopite from metasomatic mantle samples, as well as in acid magmas. This partition value may indicate the lack of other mineral phase with high partition for this element during crystallization, and may be enhanced by the liquid composition progressively closer to alkali feldspar, an unsuitable structure for six-coordinated cations. Ce/Yb, Rb/Sr, and Zr/Hf ratios in clinopyroxene and mica suggest that the minettic magma could produce the host Piquiri potassic syenite by fractional crystallization. This hypothesis is not consistent with Ba concentrations in clinopyroxene and mica, which suggest that a Ba-bearing phase (e.g. alkali feldspar) should be among the fractionated phases in order to produce the potassic syenites.     

Frequency analysis of the stability of asteroids in the framework of the restricted three-body problem

The stability of some asteroids, in the framework of the restricted three-body problem, has been recently proved in (Celletti and Chierchia, 2003), by developing an isoenergetic KAM theorem. More precisely, having fixed a level of energy related to the motion of the asteroid, the stability can be obtained by showing the existence of nearby trapping invariant tori existing at the same energy level. The analytical results are compatible with the astronomical observations, since the theorem is valid for the realistic mass-ratio of the primaries. The model adopted in (Celletti and Chierchia, 2003), is the planar, circular, restricted three-body model, in which only the most significant contributions of the Fourier development of the perturbation are retained. In this paper we investigate numerically the stability of the same asteroids considered in (Celletti and Chierchia, 2003), (namely, Iris, Victoria and Renzia). In particular, we implement the nowadays standard method of frequency-map analysis and we compare our investigation with the analytical results on the planar, circular model with the truncated perturbing function. By means of frequency analysis, we study the behaviour of the bounding tori and henceforth we infer stability properties on the dynamics of the asteroids. In order to test the validity of the truncated Hamiltonian, we consider also the complete expression of the perturbing function on which we perform again frequency analysis. We investigate also more realistic models, taking into account the eccentricity of the trajectory of Jupiter (planar-elliptic problem) or the relative inclination of the orbits (circular-inclined model). We did not find a relevant discrepancy among the different models.     

Mafic pyroclastic flows at Santa Maria (Gaua) Volcano, Vanuatu: the caldera formation problem in mainly mafic island arc volcanoes

At Santa Maria Volcano (New Hebrides island arc), extensive ash and scoria flow deposits overlie the mainly effusive, pre-caldera cone. Hydromagmatic features characterize these deposits, the composition of juvenile clasts ranges from basalt to acid andesite/dacite (SiO₂= 51–63.6%) with a dominant basaltic composition. The stratigraphic position of this pyroclastic series and its spatial distribution around a 8.5 km × 6 km wide caldera provide evidence of a relationship between this series and the caldera formation. In addition, these pyroclastic deposits are co-genetic to parasitic cones and lava flows developed along faults concentric to the caldera. Both series result from a compositionally layered magma reservoir, the subordinate differentiated magmas being the result of fractional crystallization from the basalts. A model of caldera formation which implies a large hydromagmatic eruption at the central vent and minor magma withdrawal by flank eruptions is proposed. This model emphasizes the importance of mafic hydroclastic eruptions in the caldera forming event and contradicts a model implying only quiet subsidence, a process often proposed for the formation of calderas in island are volcanoes of mainly mafic composition.