Ground fissures within the Main Ethiopian Rift: Tectonic,lithological and piping controls

Ground fissures, especially if they open due to a sudden collapse of the surface, is a serious risk for populated areas. Their common occurrence in unconsolidated sediments of the Main Ethiopian Rift was found to be mostly a result of piping. The fissures start by piping in linear sub-horizontal underground voids, which often propagate upwards resulting in ceiling collapse and formation of deep and long ground fissures with vertical walls. In the southern and central Main Ethiopian Rift the fissures pose a serious risk to infrastructure and settlements. The ground fissures are often linear (up to several kilometres long and often tens of metres deep) and accompanied by sinkholes (along the length). A detailed field mapping of the geological (rock composition, orientation and character of lithological boundaries, primary fabrics and brittle structures) and geomorphological features (especially a length, width and depth of fissures, sinkholes and gullies) followed by in situ seismic anisotropy measurements and a laboratory determination of the geomechanical properties of volcanoclastic deposits was carried out to investigate the ground fissures' origin. The conditions and factors leading to the formation of the ground fissures have been linked to: (a) the presence of regional normal faults and the associated extensional joints and (b) the alternation of lithological units with contrasting hydraulic permeability. The latter corresponds to a sequence of less permeable hard rocks (e.g., rhyolitic ignimbrites) overlain by heterogeneous, soft and permeable, unconsolidated volcaniclastic deposits with a low amount of clay (less than 10%). The ground fissures' occurrence has shown affiliation to areas which have a significantly high seismic anisotropy (more than 20% at the study sites), which can be used as a proxy to map out high risk areas prone to piping and ground fissure formation.     

A cloud mask methodology for high resolution remote sensing data combining information from high and medium resolution optical sensors

This study presents a novel cloud masking approach for high resolution remote sensing images in the context of land cover mapping. As an advantage to traditional methods, the approach does not rely on thermal bands and it is applicable to images from most high resolution earth observation remote sensing sensors. The methodology couples pixel-based seed identification and object-based region growing. The seed identification stage relies on pixel value comparison between high resolution images and cloud free composites at lower spatial resolution from almost simultaneously acquired dates. The methodology was tested taking SPOT4-HRVIR, SPOT5-HRG and IRS-LISS III as high resolution images and cloud free MODIS composites as reference images. The selected scenes included a wide range of cloud types and surface features. The resulting cloud masks were evaluated through visual comparison. They were also compared with ad-hoc independently generated cloud masks and with the automatic cloud cover assessment algorithm (ACCA). In general the results showed an agreement in detected clouds higher than 95% for clouds larger than 50 ha. The approach produced consistent results identifying and mapping clouds of different type and size over various land surfaces including natural vegetation, agriculture land, built-up areas, water bodies and snow.     

Contrasting punctuated zircon growth in two syn-erupted rhyolite magmas from Tarawera volcano: Insights to crystal diversity in magmatic systems

Two mineralogically and chemically distinct rhyolite magmas (T1 and T3) were syn-erupted from the same conduit system during the 21.9 ka basalt intrusion-triggered Okareka eruption from Tarawera volcano, New Zealand. High spatial resolution U–Th disequilibrium dating of zircon crystals at the ~ 3–5 μm scale reveals a protracted yet discontinuous zircon crystallization history within the magmatic system. Both magma types contain zircon whose interiors predate the eruption by up to 200 ka. The dominant age peak in the T1 magma is ~ 30 ka with subordinate peaks at ~ 45, ~ 75, and ~ 100 ka, whereas the T3 magma has a dominant zircon interior age peak at ~ 90 ka with smaller modes at ~ 35 and ~ 150 ka. These patterns are consistent with isolated pockets of crystallization throughout the evolution of the system. Crystal rim analyses yield ages ranging from within error of the eruption age to at least ~ 90 ka prior to eruption, highlighting that zircon crystallization frequently stalled long before the eruption. Continuous depth profiling from crystal rims inward demonstrates protracted growth histories for individual crystals (up to ~ 100 ka) that were punctuated by asynchronous hiatuses of up to 30 ka in duration. Disparate zircon growth histories can result from localized thermal perturbations caused by mafic intrusions into a silicic reservoir. The crystal age heterogeneity at hand-sample scale requires considerable crystal transport and mixing. We propose that crystal mixing was achieved through buoyancy instabilities caused by mafic magma flow through crystal mush. A terminal pre-eruptive rejuvenation event was capable of mobilizing voluminous melts that erupted, but was too short (< 10²–10³ years) to result in extensive zircon growth. The contrasting, punctuated zircon histories argue against closed-system fractional crystallization models for silicic magmatism that require protracted cooling times following a mostly liquid starting condition.     

Interaction between sulphide and H2O in silicate melts

Reaction between dissolved water and sulphide was experimentally investigated in soda-lime-silicate (NCS) and sodium trisilicate (NS3) melts at temperatures from 1000 to 1200 °C and pressures of 100 or 200 MPa in internally heated gas pressure vessels. Diffusion couple experiments were conducted at water-undersaturated conditions with one half of the couple being doped with sulphide (added as FeS or Na₂S; 1500-2000 ppm S by weight) and the other with H₂O (∼3.0 wt.%). Additionally, two experiments were performed using a dry NCS glass cylinder and a free H₂O fluid. Here, the melt was water-saturated at least at the melt/fluid interface. Profiling by electron microprobe (sulphur) and infrared microscopy (H₂O) demonstrate that H₂O diffusion in the melts is faster by 1.5-2.3 orders of magnitude than sulphur diffusion and, hence, H₂O can be considered as a rapidly diffusing oxidant while sulphur is quasi immobile in these experiments.In Raman spectra a band at 2576 cm⁻¹ appears in the sulphide - H₂O transition zone which is attributed to fundamental S-H stretching vibrations. Formation of new IR absorption bands at 5025 cm⁻¹ (on expense of the combination band of molecular H₂O at 5225 cm⁻¹) and at 3400 cm⁻¹ was observed at the front of the in-diffusing water in the sulphide bearing melt. The appearance and intensity of these two IR bands is correlated with systematic changes in S K-edge XANES spectra. A pre-edge excitation at 2466.5 eV grows with increasing H₂O concentration while the sulphide peak at 2474.0 eV decreases in intensity relative to the peak at 2477.0 eV and the feature at 2472.3 eV becomes more pronounced (all energies are relative to the sulphate excitation, calibrated to 2482.5 eV). The observations by Raman, IR and XANES spectroscopy indicate a well coordinated S²⁻ - H₂O complex which was probably formed in the glasses during cooling at the glass transition. No oxidation of sulphide was observed in any of the diffusion couple experiments. On the contrary, XANES spectra from experiments conducted with a free H₂O fluid show complete transformation of sulphide to sulphate near the melt surface and coexistence of sulphate and sulphide in the center of the melt. This can be explained by a lower H₂O activity in the diffusion couple experiments or by the need of a sink for hydrogen (e.g., a fluid which can dissolve high concentration of hydrogen) to promote oxidation of sulphide by H₂O via the reaction S²⁻ + 4H₂O = SO₄²⁻ + 4H₂. Sulphite could not be detected in any of the XANES spectra implying that this species, if it exists in the melt, it is a subordinate or transient species only.     

Fe–Mg interdiffusion rates in clinopyroxene: experimental data and implications for Fe–Mg exchange geothermometers

Chemical interdiffusion of Fe–Mg along the c-axis [001] in natural diopside crystals (X _Di = 0.93) was experimentally studied at ambient pressure, at temperatures ranging from 800 to 1,200 °C and oxygen fugacities from 10^?11 to 10^?17 bar. Diffusion couples were prepared by ablating an olivine (X _Fo = 0.3) target to deposit a thin film (20–100 nm) onto a polished surface of a natural, oriented diopside crystal using the pulsed laser deposition technique. After diffusion anneals, compositional depth profiles at the near surface region (~400 nm) were measured using Rutherford backscattering spectroscopy. In the experimental temperature and compositional range, no strong dependence of D ^Fe–Mg on composition of clinopyroxene (Fe/Mg ratio between Di₉₃–Di₆₅) or oxygen fugacity could be detected within the resolution of the study. The lack of fO₂-dependence may be related to the relatively high Al content of the crystals used in this study. Diffusion coefficients, D ^Fe–Mg, can be described by a single Arrhenius relation with $$D^{{{\text{Fe}} - {\text{Mg}}}} = 2. 7 7\pm 4. 2 7\times 10^{ - 7} {\text{exp(}}-3 20. 7\pm 1 6.0{\text{ kJ}}/{\text{mol}}/{\text{RT)m}}^{ 2} /{\text{s}}.$$ D ^Fe–Mg in clinopyroxene appears to be faster than diffusion involving Ca-species (e.g., D ^Ca–Mg) while it is slower than D ^Fe–Mg in other common mafic minerals (spinel, olivine, garnet, and orthopyroxene). As a consequence, diffusion in clinopyroxene may be the rate-limiting process for the freezing of many geothermometers, and compositional zoning in clinopyroxene may preserve records of a higher (compared to that preserved in other coexisting mafic minerals) temperature segment of the thermal history of a rock. In the absence of pervasive recrystallization, clinopyroxene grains will retain compositions from peak temperatures at their cores in most geological and planetary settings where peak temperatures did not exceed ~1,100 °C (e.g., resetting may be expected in slowly cooled mantle rocks, many plutonic mafic rocks, or ultra-high temperature metamorphic rocks).     

Thermodynamic properties and phase stability of wadsleyite II

The thermodynamical stability of a newly observed wadsleyite II phase in the Mg₂SiO₄ system is studied by the density functional theory. The wadsleyite II equation of state has been derived. The phase boundaries of Mg₂SiO₄ polymorphs: wadsleyite, wadsleyite II and ringwoodite are studied using the quasi-harmonic approximation at high external pressures. Clapeyron slopes determined for wadsleyite II–ringwoodite and wadsleyite–wadsleyite II boundaries are 0.0047 and 0.0058 GPa/K, respectively. It is shown that the wadsleyite II phase is not thermodynamically preferred in the pure Mg₂SiO₄ system and will probably not occur between wadsleyite and ringwoodite phases.     

Relation entre fabriques de la sanidine et mise en place des magmas trachytiques (exemple de massif de Hradiště, Bohême du nord)

Résumén

La distribution des fabriques dans un dôme trachytique a été analysée sur la base des orientations préférentielles des cristaux de sanidine. L’orientation préférentielle des plans (010) des cristaux de sanidine a été quantifiée par le calcul du tenseur d’orientation. Le sens de cisaillement a été déterminé à l’aide de relations entre les plans S et C.

Les plans (010) des cristaux de sanidine montrent une fabrique résultant d’un aplatissement dans la partie centrale du dôme et d’un cisaillement simple dans les parties marginales. Les critères de cisaillement montrent le mouvement uni forme de magma trachytique à l’échelle de tout le massif. Nous interprétons la disposition des plans S-C et la répartition des fabriques dans le massif à la superposition de lames successives de magma les unes sur les autres. L’acquisition des fabriques dans le massif est comparée aux expériences en compression uniaxiale le Tullis (1976) et la mise en place des magmas trachytiques est assimilée au mouvement d’un piston dans des conditions de basse pression de confinement et de haute vitesse de déformation pendant le stade d’intrusion précuce. Le gonflement faible a modifié tardivement les fabriques initiales surtout dans la partie de déversement et sommitale.     

Maris Gullying: canonical analysis of erosion and sedimentation sequences (Baronnies - France)

Abstract

The gullying action on Jurassic and Cretaceous marls in the Baronnies is observed, warm season after cold season, thanks to a network of measuring stakes arranged in 12 gullies. The climate, characterized by summer heath, rather cold but luminous winters, and quite heavy rain, can be classified as mountainous-mediterranean. Seasonal ablation/sedimentation measures have been carried out upon 119 stakes five times within two years (768 days): two in automn, two in early spring (april), and one at end-of-spring.

Ablation and sedimentation sequences are described among the stakes population by Correspondance Factor Analysis (CFA), and their covariations in space and time are explained by Canonical Correspondance Analysis (CCA) according to biophysical environment variables.

Axis 1 of the CFA opposes ground level variations according to their amplitude, high or moderate, regardless of the direction of variation and of measurement date. Axis 2 presents a temporal variation, opposing, on one side, sedimentation sequences at post-winter terms and ablation sequences at postsummer terms, to their converse on the other side.

The CCA indicates that talwegs in their medium and lower parts are more likely to undergo hard gullying or strong accumulation, especially on steep, or conversely flat, vegetationfree slopes of oxfordian marls; on the contrary, interfluve ridges and Gonson moderate gentle to moderate slopes on cenomanian marls, possibly with some vegetation are less dynamic (canonical axis 1). This result allows to extend former observations on rythmic, contrasted variations of gullies bottoms, well different of interfluves and gonson, progressive, regular, ones.

Owing to their varied reactions to seasonal erosive agents (cryoclasty, concentrated run-off), upper parts of talwegs and lower parts of gully slopes and interfluves (canonical axis 2) originate seasonal contrasts revealed by CFA’s axis 2. However, to insure this point of view, more sampling is needed.