Thermal annealing-induced electric dipole relaxation in natural alexandrite

Electrical properties of natural alexandrite (BeAl₂O₄:Cr³⁺) are investigated by the thermally stimulated depolarization current (TSDC) technique. Samples are submitted to consecutive annealing processes and TSDC is carried out after each annealing, yielding bands with different parameters. These bands are fitted by a continuous distribution of relaxation parameters: activation energy and pre-exponential factor of the Arrhenius equation. It has been observed that annealing influences the dipole relaxation behavior, since it promotes a modification of Fe³⁺ and Cr³⁺ impurity distributions on sites of distinct symmetry: Al₁ and Al₂. In order to have a reference for comparison, TSDC is also carried out on a synthetic alexandrite sample, where the only impurity present is Cr³⁺ ion.     

Angular velocities of Nubia and Somalia from continuous GPS data: implications on present-day relative kinematics

This study focuses on the break-up of the African tectonic plate into separate Nubian and Somalian blocks, based on recent Global Positioning System (GPS) data. A new, unique velocity field has been obtained by processing all available observations of permanent GPS stations on Africa since 1996. The quantity and distribution of the stations and the length of the time-series of observations exceed that of previous studies by a considerable margin, allowing one to derive a reliable estimate of the differential motion between the Nubia and Somalian plates, which are considered as a single (African) block in the prevailing global tectonic plate models. The estimated relative pole of rotation of Somalia with respect to Nubia is located at 54.8°S; 37.0°E with magnitude −0.069°/Ma, implying distinct opening in the Ethiopian Rift of magnitude ≈7 mm/year and azimuth ≈N94°E, whereas in southeastern South Africa this value is reduced to ≈2 mm/year in almost the same direction. This is in accordance with some of the independent geological and geophysical tectonic models of the Nubia-Somalia plate boundary region. However, the spatial density of the current tracking network is still not optimal to establish the exact location of the entire Somalia-Nubia plate boundary; in particular, the possible branch east of Lake Victoria and heading towards the Mozambique Channel is impossible to confirm or reject at this moment.     

Thermobarometry, Sm/Nd Ages and Geophysical Evidence for the Location of the Suture Zone Between Cuyania and the Western Proto-Andean Margin of Gondwana

Basement rocks comprising ortho- and paragneisses and schists whose tectono-metamorphic evolution is poorly known, are exposed in the Sierras de Umango, Maz-Espinal and Las Ramaditas, in the northwest of the La Rioja Province, Argentina. These units were included in the Maz, El Taco, El Zaino complexes, as well as the Tambillos Metamorphics that would be part of the northern end of the Cuyania terrane, a microcontinent derived from Laurentia that collided with Western Gondwana during the early Paleozoic, or belong to the active margin of the continent. To recognize rocks belonging to each of these tectonic units and to understand the history and physical conditions of accretion were some of the main goals of the multidisciplinary investigation whose preliminary results are presented here. Geochemical studies, trace and REE elements and Sm-Nd model ages allowed the recognition of several episodes of crustal accretion in these rocks. The oldest one occurred at ca. 2.2 Ga in an arc/back-arc environment along the eastern segment of the Sierra de Maz, and was possibly coeval with development of a early Proterozoic continental crust that acted as source to sediments of Maz Complex. The following episode of crustal accretion that formed rocks in this region was at ca. 1.4 Ga and is registered by tonalites emplaced in an extensional environment cropping out in the western flank of the Sierra del Espinal.

In the Sierra de Umango, an arc/back-arc sequence registered an episode of crustal accretion during the Grenvillian Cycle (ca. 1.3 Ga). The last episode of crustal accretion detected in this area (800 Ma) is represented by an old alkaline volcanism in the Sierra de Umango. This episode could be representing the first stage of break-up of the Rodinia supercontinent during the Neoproterozoic.

The metamorphic grade reached by these rocks is mostly represented by fabrics with mineral assemblages of intermediate to high pressures and high temperatures, typical of collisional environments. The oldest rock-forming fabrics tectono-metamorphic episode recognized is of middle Proterozoic age (ca. 1.04 to 0.969 Ga, garnet-whole-rock Sm/Nd age) being registered by metapelites from Maz Complex that attained temperatures of 650°C-6.3 kbar. A younger metamorphic event (463 Ma, garnet - whole-rock Sm/Nd age) is verified in metatonalites intrusive in these metapelites. Another metamorphic event at ca. 301 Ma (garnet-WR Sm/Nd age) was recognized in metasediments from El Taco Complex. Peak metamorphic conditions of this event, probably registering the last major tectonic episode that affected rocks of this area was established in 868°C-9.8 kbar. It is impossible to distinguish fabrics belonging to totally different tectonic episodes based on structural or metamorphic data. Therefore, distinction between major tectono-thermal events of totally different ages such as the high-T middle Proterozoic deformation and with N-NWestwards tectonic transport direction registered in the Sierras de Maz-Espinal and Umango from the youngest one (ca. 301 Ma) that attained the highest-P/T conditions, recognized in the Sierra de Las Ramaditas, had to be done on the basis of Sm/Nd ages.

Geophysical evidence indicates the presence of extensive WNW-oriented lineaments that separate basements blocks of different magnetic and gravimetric signatures that are thought to represent ancient Grenvillian age suture zones. On the other hand, the northern segment of the Valle Fértil lineament that runs between Sierras de Umango and Maz-Espinal is at present interpreted as marking the eastern boundary of the Cuyania terrane. This is supported by isotopic data as well as the contrasting history of tectono-metamorphic events as determined for both of these segments of the NW Sierras Pampeanas.     

An Experimental Performance Evaluation of Spatio-Temporal Join Strategies

Many applications capture, or make use of, spatial data that changes over time. This requirement for effective and efficient spatio‐temporal data management has given rise to a range of research activities relating to spatio‐temporal data management. Such work has sought to understand, for example, the requirements of different categories of application, and the modelling facilities that are most effective for these applications. However, at present, there are few systems with fully integrated support for spatio‐temporal data, and thus developers must often construct custom solutions for their applications. Developers of both bespoke solutions and of generic spatio‐temporal platforms will often need to support the fusion of large spatio‐temporal data sets. Supporting such requests in a database setting involves the use of join operations with both spatial and temporal conditions – spatio‐temporal joins. However, there has been little work to date on spatio‐temporal join algorithms or their evaluation. This paper presents an evaluation of several approaches to the implementation of spatio‐temporal joins that build upon widely available indexing techniques. The evaluation explores how several algorithms perform for databases with different spatial and temporal characteristics, with a view to helping developers of generic infrastructures or custom solutions in the selection and development of appropriate spatio‐temporal join strategies.     

Input of geophysics to understand hydrogeology towards the assessment of geoenvironmental conditions in Beira city,Mozambique

As the fourth largest city in Mozambique, and located right beside the Indian Ocean, Beira faces sanitary and environmental problems that are a strong concern to the local and national authorities. One of the major problems that Beira city has to deal with is the existence of undrinkable groundwater in many dug-wells that supply a large amount of the city population. The cooperation project among Direcção Nacional de Geologia of Mozambique, the Laboratório Nacional de Energia e Geologia of Portugal and the Instituto de Apoio ao Desenvolvimento Português that took place from 2007 to 2011 has as one of its main goals the production of the Geological and Geoenvironmental Maps of Beira at scale 1:50,000, intending to create a tool to support the implementation of governmental policies regarding sustainable groundwater and natural resources exploitations, land management and environmental protection. These goals were achieved through the identification, characterization and mapping of the different geological units and the evaluation of the city environmental problems regarding water and soil contamination in urban areas, involving the use of several methodologies that were subsequently integrated and jointly interpreted. These comprised remote sensing techniques, hydrogeology analysis and evaluation and the use of geochemistry and electrical geophysical methods. This work emphasizes the input of geophysical and hydrogeological methods to assess the geoenvironmental conditions of Beira, namely those related with the prediction of occurrence of saline and brackish water in the shallow dug-wells that supply a large amount of Beira populations, especially in the crowded suburbs. Several geophysical surveys using electrical and electromagnetic methods were conducted and interpreted regarding the evaluation of the several hypotheses that were stood for this feature. Results are related with hydrogeological conditions and organic and inorganic hydrochemical analysis. Considering these facts, this study was an important input for mapping areas with saline and brackish water imprisoned in the subsurface geological formations, using a wider set of methodologies that were integrated in these results.     

An early-time infrared and optical study of the Type Ia Supernova 1998bu in M96

We present first-season infrared (IR) and optical photometry and spectroscopy of the Type Ia Supernova 1998bu in M96. We also report optical polarimetry of this event. SN 1998bu is one of the closest type Ia supernovae of modern times, and the distance of its host galaxy is well determined. We find that SN 1998bu is both photometrically and spectroscopically normal. However, the extinction to this event is unusually high, with     We find that SN 1998bu peaked at an intrinsic     Adopting a distance modulus of 30.25 (Tanvir et al.) and using Phillips et al.'s relations for the Hubble constant, we obtain     Combination of our IR photometry with those of Jha et al. provides one of the most complete early-phase IR light curves for a SN Ia published so far. In particular, SN 1998bu is the first normal SN Ia for which good pre- t _{B max} IR coverage has been obtained. It reveals that the J , H and K light curves peak about 5 days earlier than the flux in the B -band curve.     

Morphology of pockmarks along the western continental margin of India: Employing multibeam bathymetry and backscatter data

This study addresses the morphology of pockmarks along the western continental margin of India using multibeam bathymetry and backscatter data. Here, for the first time we have utilized the application of ArcGIS (Geographical Information System) for understanding the morphology of pockmarks from the western continental margin of India. The pockmarks observed in water depths of 145–330 m are circular, elliptical or elongated in plan-view, with an average length and width of 157 (±72) m and 83 (±19) m respectively. The average pockmark relief and perimeter are 1.9 (±0.9) m and 412 (±181) m, respectively. The pockmarks have average areas and volumes of 10 759 m² and 15 315 m³ respectively. Spatial separation that coincides with 210 m isobath divides the pockmarks into two groups with differing distributions and morphologies. These pockmarks originated from seepages of biogenic or thermogenic gas or from pore fluids from deeper sources, migrated vertically along the faults. Besides a possible structural control, the pockmark morphologies are also affected by bottom currents and/or by submarine slumping. The average acoustic backscatter strength from pockmark centre is higher (−35 dB) than the average backscatter of the total area (−40 dB), which suggests their possible linkage to the precipitation of diagenitic minerals from biodegradation of seepage material.     

Circulation and fine-sediment dynamics in the Amazon Macrotidal Mangrove Coast

The Amazon Macrotidal Mangrove Coast (AMMC) is a large (~7500 km²) contiguous mangrove fringe eastwards from the Amazon River mouth. It encompasses dozens of interconnected bays intercalated with mangrove peninsulas. Mud accumulates on the mangrove flats, whereas the bed of the bays and channels is generally sandy. In this study we investigated the circulation, sediment transport and deposition in a central site at one of these mangrove peninsulas. The study was undertaken during the dry period, when there is no influence of the Amazon River plume and minimum local freshwater inflow. Current and suspended-sediment concentration were monitored in a feeder channel on the mangrove flat along a ~1000 m section oriented along the peninsula axis. Sediment deposition was also measured on the flat. Our results show there was a strong exchange between the neighboring bays. Channel currents were flood dominant, reaching up to >1 m s⁻¹, with residual water and sediment transport westwards. Suspended sediment concentration (SSC) in the channel was directly related to velocity magnitude, ranging between 50 and 350 mg L⁻¹. The flat was flooded in a way that indicated the tidal wave evolves westwards, nearly parallel to the AMMC shoreline. Currents on the flats were much slower than those in the channel and showed slight ebb dominance. However, SSC was higher during the flood than ebb, clearly indicating settling during the current deceleration and limited erosion during the following ebb–flow acceleration. The net sediment transport across the section was 60 tons westwards for the period of the experiment (~4 days). The mean deposition rate was 0.006 kg m⁻² s⁻¹ (or 1.4 kg m⁻² per tide), which was higher than rates from other reported assessments in mangroves. The set of results indicate very large internal sediment reworking in the AMMC. © 2019 John Wiley & Sons, Ltd.     

Fine-grained volatile components ubiquitous in solar nebula: Corroboration from scoriaceous cosmic spherules

The scoriaceous cosmic spherules (CSs) that make up to a few percent (for sizes >150 μm size) of total micrometeorite flux are ubiquitous and have remained enigmatic. The present work provides in-depth study of 81 scoriaceous CSs, from observed ~4000 CSs, collected from Antarctica (South Pole water well) and deep-sea sediments (Indian Ocean) that will allow us to analyze the nature of these particles. The fine-grained texture and the chemical composition of scoriaceous particles suggest that they are formed from matrix materials that are enriched in volatiles. The volatile components such as water, sulfide, Na, K, etc. have vanished due to partial evaporation and degassing during Earth's atmospheric entry leaving behind the vesicular features, yet largely preserving the elemental composition. The elemental ratios (Ca/Si, Mg/Si, Al/Si, Fe/Si, and Ni/Si) of interplanetary dust particles (IDPs) are compatible with the scoriaceous CSs, which in turn are indistinguishable from the matrices of CI and CM chondrites signifying similarities in the nature of the sources. Furthermore, the texture of cometary particles bears resemblance to the texture of the scoriaceous particles. The compilation of petrographic texture, chemical, and trace element composition of scoriaceous CSs presents a strong case for matrix components from hydrated and volatile-rich bodies, such as CI and CM chondrites, rather than chondrules. We conclude that the fine-grained scoriaceous CSs, the matrix materials of hydrated chondrites, IDPs, and cometary particles that overlap compositionally were widespread, indicating a dominant component in the early solar nebula.     

Oxygen isotopic and chemical composition of chromites in micrometeorites: Evidence of ordinary chondrite precursors

We identified 66 chromite grains from 42 of ~5000 micrometeorites collected from Indian Ocean deep‐sea sediments and the South Pole water well. To determine the chromite grains precursors and their contribution to the micrometeorite flux, we combined quantitative electron microprobe analyses and oxygen isotopic analyses by high‐resolution secondary ion mass spectrometry. Micrometeorite chromite grains show variable O isotopic compositions with δ¹⁸O values ranging from ?0.8 to 6.0‰, δ¹⁷O values from 0.3 to 3.6‰, and Δ¹⁷O values from ?0.9 to 1.6‰, most of them being similar to those of chromites from ordinary chondrites. The oxygen isotopic compositions of olivine, considered as a proxy of chromite in chromite‐bearing micrometeorites where chromite is too small to be measured in ion microprobe have Δ¹⁷O values suggesting a principal relationship to ordinary chondrites with some having carbonaceous chondrite precursors. Furthermore, the chemical compositions of chromites in micrometeorites are close to those reported for ordinary chondrite chromites, but some contribution from carbonaceous chondrites cannot be ruled out. Consequently, carbonaceous chondrites cannot be a major contributor of chromite‐bearing micrometeorites. Based on their oxygen isotopic and elemental compositions, we thus conclude with no ambiguity that chromite‐bearing micrometeorites are largely related to fragments of ordinary chondrites with a small fraction from carbonaceous chondrites, unlike other micrometeorites deriving largely from carbonaceous chondrites.