Evaporation Sequence Research Articles

Understanding field evaporation sequences by in-situ correlative microscopy and simulation

Understanding field evaporation sequences by in-situ correlative microscopy and simulation

A tale of three fluids: Fluid-inclusion and carbonate clumped-isotope paleothermometry reveals complex dolomitization and dedolomitization history of the Latemar platform

Abstract This work focuses on an exceptionally complex natural laboratory, the Triassic Latemar isolated platform in the Dolomite Mountains of northern Italy. It explores spatial and temporal gradients in processes and products related to contact metamorphism, dolomitization, and the dedolomitization of marine limestones. Rock samples were studied using dual fluid-inclusion thermometry and clumped-isotope thermometry. Independent of the spatial position at Latemar, Δ47 clumped-isotope and fluid-inclusion data provide contrasting paleotemperature estimates. An apparent lack of systematic patterns in fluid-inclusion data (homogenization temperature, salinity, density) results from analyses of micrometer-sized growth zones within a single crystal. The composition of the individual fluid inclusions represents a “snapshot” of fluid mixing with variable endmember elemental ratios. The bulk crush-leach data and slopes in Caexcessversus Nadeficit diagrams indicate different water–rock interactions and fluid signatures with evaporation sequences and crystalline rocks. The presence of three fluid types (crystalline basement brine, halite-dissolution brine, seawater) in all carbonates suggests that all fluids coexisted during contact metamorphism and dolomitization of Latemar carbonates. Non-equilibrium processes overruled thermodynamic controls on the precipitation of diagenetic phases. Fluid mixing resulted in the precipitation of two complex carbonate successions. The Δ47 data represent bulk temperatures, averaging the mixing ratio of fluids with different temperatures and their respective volume. Fluid-inclusions record patterns of remarkable complexity and shed light on the complexity of a multi-fluid system. Data shown here provide answers to the controversial interpretation of dolomitizing fluid temperature in the Latemar and exemplify the strengths of multi-proxy paleotemperature studies.

A Bottom-Up Volume Reconstruction Method for Atom Probe Tomography.

This paper describes a reconstruction method for atom probe tomography based on a bottom-up approach accounting for (i) the final tip morphology (which is frequently induced by inhomogeneous evaporation probabilities across the tip surface due to laser absorption, heat diffusion effects, and inhomogeneous material properties), (ii) the limited (and changing) field of view, and (iii) the detector efficiency. The reconstruction starts from the final tip morphology and reverses the evaporation sequence through the pseudo-deposition of defined small reconstruction volumes, which are then stacked together to create the full three-dimensional (3D) tip. The subdivision in small reconstruction volumes allows the scheme to account for the changing tip shape and field of view as evaporation proceeds. Atoms within the same small reconstruction volume are reconstructed at once by placing atoms back onto their possible lattice sites through a trajectory-matching process involving simulated and experimental hit maps. As the ejected ion trajectories are simulated using detailed electrostatic modeling inside the chamber, no simplifications have been imposed on the shape of the trajectories, projection laws, or tip surface. We demonstrate the superior performance of our approach over the conventional reconstruction method (Bas) for an asymmetrical tip shape.

A radio-frequency Bose–Einstein condensate magnetometer

We report on a radio frequency magnetometer employing a Bose–Einstein condensate of 87Rb atoms held in a dipole trap. An AC sensitivity of 296 pT/Hz is achieved at a probing volume of 3.2×10−8 cm3, leading to a volume-normalized sensitivity of 53 fT/Hz cm−3. At larger probing volumes with the atoms released from the magnetic trap used in the initial phase of the evaporation sequence, the AC sensitivity is improved to 1.4 pT/Hz, allowing a two-mode approach for applications requiring improved sensitivity. Immediate application in high-resolution electromagnetic induction imaging is expected when compared to performance of other induction-based imaging platforms.

Utica Shale Play Oil and Gas Brines: Geochemistry and Factors Influencing Wastewater Management.

The Utica and Marcellus Shale Plays in the Appalachian Basin are the fourth and first largest natural gas producing plays in the United States, respectively. Hydrocarbon production generates large volumes of brine ("produced water") that must be disposed of, treated, or reused. Though Marcellus brines have been studied extensively, there are few studies from the Utica Shale Play. This study presents new brine chemical analyses from 16 Utica Shale Play wells in Ohio and Pennsylvania. Results from Na-Cl-Br systematics and stable and radiogenic isotopes suggest that the Utica Shale Play brines are likely residual pore water concentrated beyond halite saturation during the formation of the Ordovician Beekmantown evaporative sequence. The narrow range of chemistry for the Utica Shale Play produced waters (e.g., total dissolved solids = 214-283 g/L) over both time and space implies a consistent composition for disposal and reuse planning. The amount of salt produced annually from the Utica Shale Play is equivalent to 3.4% of the annual U.S. halite production. Utica Shale Play brines have radium activities 580 times the EPA maximum contaminant level and are supersaturated with respect to barite, indicating the potential for surface and aqueous radium hazards if not properly disposed of.

Evaporation and Electrowetting of Sessile Droplets on Slippery Liquid-Like Surfaces and Slippery Liquid-Infused Porous Surfaces (SLIPS).

Sessile droplet evaporation underpins a wide range of applications from inkjet printing to coating. However, drying times can be variable and contact-line pinning often leads to undesirable effects, such as ring stain formation. Here, we show voltage programmable control of contact angles during evaporation on two pinning-free surfaces. We use an electrowetting-on-dielectric approach and Slippery Liquid-Infused Porous (SLIP) and Slippery Omniphobic Covalently Attached Liquid-Like (SOCAL) surfaces to achieve a constant contact angle mode of evaporation. We report evaporation sequences and droplet lifetimes across a broad range of contact angles from 105°–67°. The values of the contact angles during evaporation are consistent with expectations from electrowetting and the Young-Lippman equation. The droplet contact areas reduce linearly in time, and this provides estimates of diffusion coefficients close to the expected literature value. We further find that the total time of evaporation over the broad contact angle range studied is only weakly dependent on the value of the contact angle. We conclude that on these types of slippery surfaces, droplet lifetimes can be predicted and controlled by the droplet’s volume and physical properties (density, diffusion coefficient, and vapor concentration difference to the vapor phase) largely independent of the precise value of contact angle. These results are relevant to applications, such as printing, spraying, coating, and other processes, where controlling droplet evaporation and drying is important.

Isothermal drying of plant-based food material: An approach using 2D polydimethylsiloxane (PDMS) micromodels

The aims of this study were to build PDMS micromodels that simulated plant-based food material, and use them to investigate the drying mechanisms at pore-scale, the drying front morphologies and the drying curves during the isothermal drying process. Micromodels were built using a modified photolithographic technique. Three normal pore size distribution functions were developed to simulate the plasmodesma, intercellular space, and cell wall. Micromodels were impregnated with a pyranine aqueous solution and placed on a balance illuminated with ultraviolet light. Isothermal drying was developed by natural convection (20 °C). Drying front morphologies displayed a hierarchical behavior in which pores with the largest diameter dried first. Drainage and surface evaporation sequences and internal evaporation controlled the drying of PDMS micromodels. The micromodels were useful to simulate plant-based food material and study isothermal drying of such porous media. The results led to the proposal of some drying mechanisms that were involved.

Compositions of Planetary Debris around Dusty White Dwarfs

Abstract The photospheres of some white dwarfs (WDs) are “polluted” by accretion of material from their surrounding planetary debris. WDs with dust disks are often heavily polluted and high-resolution spectroscopic observations of these systems can be used to infer the chemical compositions of extrasolar planetary material. Here, we report spectroscopic observation and analysis of 19 WDs with dust disks or candidate disks. The overall abundance pattern very much resembles that of bulk Earth and we are starting to build a large enough sample to probe a wide range of planetary compositions. We found evidence for accretion of Fe-rich material onto two WDs as well as O-rich but H-poor planetary debris onto one WD. In addition, there is a spread in Mg/Ca and Si/Ca ratios and it cannot be explained by differential settling or igneous differentiation. The ratios appear to follow an evaporation sequence. In this scenario, we can constrain the mass and number of evaporating bodies surrounding polluted WDs.

Mineralogically-induced metal partitioning during the evaporative precipitation of efflorescent sulfate salts from acid mine drainage

Efflorescent sulfate salts constitute a transient storage of acidity and metals during the dry season in mining areas affected by acid mine drainage, especially under semiarid climates. The main goal of this work was to study the metal partitioning among the dissolved and solid phases through the evaporative precipitation sequence of extremely metal-rich mine waters. The evaporative sequence was induced in the laboratory under controlled conditions for 24 days. The loss of water caused a progressive decrease of pH values (from 2.71 to 1.33) and increase of metal concentrations (e.g., from 1826 to 17800mg/L of Al, 836mg/L to 9783mg/L of Fe and 301 to 2879mg/L of Zn), which caused the precipitation of efflorescent sulfate salts. The precipitated salts were mainly composed of a mixture of minerals of Ca (gypsum, CaSO4·2H2O), Al (alunogen, Al2(SO4)3·17H2O), Fe (copiapite, Fe2+Fe3+4(SO4)6(OH)2·20H2O and melanterite Fe2+SO4·7H2O) and Mg (hexahydrite, MgSO4·6H2O), although the proportion of each mineral phase varied throughout the experiment. A preferential precipitation of Al and Mg sulfate salts, together with melanterite, was observed until the second week of the experiment, with evaporation rates lower than 70%. The precipitation of gypsum predominated with higher evaporation rates, reaching values higher than 80% of the mineral assemblage. The evaporative precipitation sequence was modeled using PHREEQC, obtaining good agreement with experimental data for gypsum, but failing in turn to accurately reproduce the evaporative sequence of other minerals such as hexahydrite, alunogen and melanterite. A metal partitioning pattern was observed during the evaporative precipitation sequence. Fe-sulfate minerals (e.g., copiapite and melanterite) present a higher affinity for Cu, Y and Th (and other trace metals such as Mn, Cd or Sc) while Al and Mg sulfate salts would retain Zn, Co, Ni and to a lesser extent Cr. In the case of gypsum, found prominently in the mineral assemblage during the experiment, it seems to have affinity for REE. This metal partitioning pattern is not observed in field data reported in literature. Such discrepancies, as well as those observed between modeled and experimental results, must be investigated in further studies.

An atom probe analysis of self-assembled monolayers: A novel approach to investigate mixed and unmixed self-assembled monolayers (SAMs) on gold

Atom probe tomography was used to analyze self-assembled monolayer of octanethiol and (1H,1H,2H,2H) perfluoro decanethiol (PFDT) in a mixed and unmixed state on a gold surface. The first part of this report, we have analyzed the two SAMs with a direct comparison, whereas in the second part, a binary mixture of these two SAMs (with a 1:1 ratio) are analyzed. Au was taken as a metal substrate in both cases. It has been observed that, although having similar structure and belongs from the same SAMs group, the sequence of evaporation of both chains appeared significantly different molecular fractions in the first case, while in the mixed phase condition the two SAMs chains were arranged in a patchy domain by forming a small nanoscale cluster on the gold surface. Primarily, this study gives an overall understanding of evaporation sequence and surface morphology of the SAMs in an unmixed and mixed condition on the gold substrate, which may lead to the further design process in various applications.

Assessing the Spatial Accuracy of the Reconstruction in Atom Probe Tomography and a New Calibratable Adaptive Reconstruction.

We define a measure for the accuracy of tomographic reconstruction in atom probe tomography, named here the spatial error index. We demonstrate that this index can be used to compare rigorously the spatial accuracy of various different approaches to the calculation of tomographic reconstruction. This is useful, for example, to evaluate the performance of alternate tomographic reconstruction approaches, and ensures that the comparisons are independent of individual data quality or other instrumental parameters. We then introduce a new "adaptive reconstruction" formalism that uses a progression of reconstruction parameters based on a per-atom correction from the cube root of the inverse of the voltage, along with linear correction factors linked to the evaporation sequence. We apply the measure for spatial accuracy to this new reconstruction protocol.

Correlative transmission electron microscopy and atom probe tomography on field evaporation mechanism of a bulk LaAlO3 oxide

Laser-assisted atom probe tomography (APT) has commonly been performed to determine the composition of non-conductive oxide materials at the atomic scale. However, its field evaporation mechanism is still insufficiently understood due to the complexity of laser-matter interaction on the surface of oxide tips. To understand the physical mechanism underlying laser-assisted field evaporation in a bulk oxide LaAlO3 (LAO), we conducted an interrupted experiment that combines transmission electron microscopy with APT. This correlated technique visualized the shape evolution of LAO tips during laser-assisted APT. We demonstrated that the evaporation field strength of the tips depends on the APT experimental parameters including applied laser energy and base temperature. An increase in the laser energy led to a reduction in the field evaporation strength of the tips and the formation of apex asymmetries on the tips, thereby considerably influencing the mass resolution of the technique. Moreover, an increase in the base temperature caused a slight decrease in the evaporation strength but did not significantly affect the mass-resolving ability of APT. The results discussed in this work suggest that it is the employed laser energy that primarily affects APT's resolving capability and the field evaporation strength of oxide materials. Our interrupted technique provides fundamental insight into the field evaporation sequences of LAO oxide tips under laser illumination, which can be feasibly applied to the study of various metallic and oxide materials.

Introducing a Crystallography-Mediated Reconstruction (CMR) Approach to Atom Probe Tomography.

Current approaches to reconstruction in atom probe tomography produce results that exhibit substantial distortions throughout the analysis depth. This is largely because of the need to apply a multitude of assumptions when estimating the evolution of the tip shape, and other pseudo-empirical reconstruction factors, which vary both across the face of the tip and throughout the analysis depth. We introduce a new crystallography-mediated reconstruction to improve the spatial accuracy and dramatically reduce these in-depth variations. To achieve this, we developed a barycentric transform to directly relate atomic positions in detector space to real space. This is mediated by novel crystallographic analysis techniques, including: (1) calculating the orientation of a crystal directly from the field evaporation map, (2) tracking pole locations throughout the evaporation sequence, and (3) accounting for the evolving tip radius in a manner that removes the dependence on the geometric field factor. By improving the in-depth spatial accuracy of the atom probe reconstruction, a greater accuracy of the atomic neighborhood relationships is available. This is critical in modern materials science and engineering, where an understanding of the solid solution architecture, precipitate dispersions, and descriptions of the interfaces between phases or grains are key inputs to microstructure-property relationships.

Pinning-Free Evaporation of Sessile Droplets of Water from Solid Surfaces.

Contact-line pinning is a fundamental limitation to the motion of contact lines of liquids on solid surfaces. When a sessile droplet evaporates, contact-line pinning typically results in either a stick-slip evaporation mode, where the contact line pins and depins from the surface in an uncontrolled manner, or a constant contact-area mode with a pinned contact line. Pinning prevents the observation of the quasi-equilibrium constant contact-angle mode of evaporation, which has never been observed for sessile droplets of water directly resting on a smooth, nontextured, solid surface. Here, we report the evaporation of a sessile droplet from a flat glass substrate treated with a smooth, slippery, omni-phobic covalently attached liquid-like coating. Our characterization of the surfaces shows high contact line mobility with an extremely low contact-angle hysteresis of ∼1° and reveals a step change in the value of the contact angle from 101° to 105° between a relative humidity (RH) of 30 and 40%, in a manner reminiscent of the transition observed in a type V adsorption isotherm. We observe the evaporation of small sessile droplets in a chamber held at a constant temperature, T = (25.0 ± 0.1) °C and at constant RH across the range RH = 10-70%. In all cases, a constant contact-angle mode of evaporation is observed for most of the evaporation time. Furthermore, we analyze the evaporation sequences using the Picknett and Bexon ideal constant contact-angle mode for diffusion-limited evaporation. The resulting estimate for the diffusion coefficient, DE, of water vapor in air of DE = (2.44 ± 0.48) × 10-5 m2 s-1 is accurate to within 2% of the value reported in the literature, thus validating the constant contact-angle mode of the diffusion-limited evaporation model.

Optimizing the Deepwater Completion Process Offshore Israel

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 189637, “Optimizing the Deepwater Completion Process: Case History of the Tamar 8 Completion Design, Execution, and Initial Performance Offshore Israel,” by John Healy, SPE, Steven M. Waggoner, Ian Magin, SPE, Matt Beavers, SPE, Kevin Williams, and Russell Hebert, SPE, Noble Energy, prepared for the 2018 IADC/SPE Drilling Conference and Exhibition, Fort Worth, Texas, USA, 6–8 March. The paper has not been peer reviewed. This paper describes the successful delivery of one ultrahigh-rate gas well (more than 250 MMscf/D) completed in a significant gas field offshore Israel with 7-in. production tubing and an openhole gravel pack (OHGP). The Tamar 8 well was completed approximately 4 years after the start of initial production from the Tamar development. Several operational innovations and process improvements were implemented that resulted in a significant reduction in rig time. Introduction The Tamar field was discovered in 2009 in 5,505 ft of water at a total depth (TD) of 14,967 ft. Tamar is one of several recent gas discoveries made in the deep waters offshore northern Israel and Cyprus in the Levant Basin (Fig. 1). The field consists of three gas-bearing sandstone layers separated by two shaley units. The trap for the reservoir is a large four-way anticline cross-cut by northwest bearing faults. There is an approximately 4,900-ft-thick evaporate sequence in the shallow overburden above the field consisting of mostly halite, with interbedded anhydrite and clastics. Tamar was designed as a subsea development with five initial wells tied back 150 km to a new shallow-water production-processing platform located near the existing Mari-B platform. As the world’s longest subsea tieback, the Tamar field came online in March 2013 at gas rates ranging from 600 to 950 MMscf/D from a production platform designed for 1,200 MMscf/D. Since 2013, all wells have produced with high reliability and minimal difficulty. Tamar 8 In early 2016, a decision was made to add an additional well, Tamar 8, to the existing well stock. Accelerated well planning was conducted thereafter to be ready for a completion start date of 1 December 2016. On the basis of front-end engineering studies, several operational statements of requirements (SORs) were specified consistent with the previous Tamar completion campaign. These SORs included the following: The production (9 7/8-in.) casing was to be set approximately 10 ft inside the reservoir section. The reservoir interval must be underreamed to 12¼ in. The reservoir interval must be vertical to a low angle (less than 20°). Only 115 ft of reservoir interval would be drilled/completed. Completion Sandface. The Tamar sandface completion is an OHGP that, in simple terms, is a drilled section of the reservoir [using a qualified and fit-for-purpose reservoir drill-in fluid (RDIF)] in which a gravel pack has been placed. A key geometric attribute of the Tamar OHGP is the requirement to enlarge (underream) the drilled reservoir section; this hole enlargement (from 8½ to 12¼ in.) significantly increases the effective wellbore radius from which multiple production benefits are derived.

Rheological Properties and Ignition and Combustion Characteristics of Biochar–Algae–Water Slurry Fuels

This study examined the rheological properties, ignition, and combustion characteristics of biochar–algae–water (BAW) slurry fuels. A pine sawdust biochar with a median particle size (D50) of approximately 12 μm and algae Chlorella vulgaris in dry powder form with D50 of approximately 19 μm were used. The BAW slurries with a constant solid loading of 40 wt % and algae/biochar ratio varying from 0 to 0.2 by weight were prepared. The apparent viscosity was measured using a Haake VT550 cone-and-plate viscometer. The stability of the slurries was characterized using a “drop rod” method. Ignition and combustion characteristics of the slurries were studied using a suspended single-droplet technique. A single droplet of a slurry fuel with a diameter ranging from 0.5 mm to 1.5 mm was suspended on a silicon carbide fibre and burned in air at 1023 K in an electrically heated tube furnace. The ignition and combustion processes of the droplet were recorded using a CCD camera at 200 fps. The ignition delay time, burnout time, and burning rate were determined. The BAW slurries showed shear-thinning flow behavior. The slurries had higher viscosity and greater stability at higher algae proportion in the solid. The ignition and combustion process of BAW slurries followed the sequence of water evaporation, devolatilization, ignition, and combustion of the solid residue. The combustion of the residual solid was diffusion controlled under the experimental conditions and the burning rates of the BAW slurry droplets ranged from 0.15 to 0.25 mm2 s−1.

Methods for reconstruction of paleo-seawater pH based on boron isotopes in evaporative depositional sequences: case study using the Cambrian–Lower Ordovician evaporite sequence in the Tarim Block, NW China

Evaluation of paleo-seawater pH is an important aspect to study the paleo-ocean environment. The Paleozoic strata lack foraminiferal shells; therefore, reconstruction of paleo-seawater pH is difficult. In this paper, the Cambrian–Lower Ordovician evaporate sequence present in the Tarim Basin is used as an example to work out methods for reconstruction of paleo-seawater pH for evaporate sequences through study of boron isotopes. Analysis of diagenetically unaltered samples yields δ11B values for normal seawater in an open environment ranging from 7.5 to 12.6‰ (average = 9.4‰) and those of evaporative salt-lake facies ranging from − 4.7 to − 1.8‰ (average = − 3.3‰). With increase in the rate of evaporation of seawater, its pH decreases gradually and the δ11B values of tricoordinated and tetracoordinated compounds decrease synchronously. Using a salt-lake brine with pH 7, δ11B value in sedimentary rock of − 3.3‰ and unfractionated δ11B in tetracoordinated B(OH) 4 − in solution, the average δ11B of the Middle Cambrian–Early Ordovician paleo-seawater came to be 16.2‰. It establishes a workable relationship between δ11B in sedimentary rock and seawater pH. The Middle Cambrian–Early Ordovician normal paleo-seawater pH estimated using the δ11B value of sedimentary rock representing the weakest evaporation intensity is ~ 9.1 that is 0.9 higher than pH of modern seawater. In these calculations, it is assumed that the total dissolved inorganic carbon is unchanged between the Cambrian–Early Ordovician paleo-seawater and modern seawater and the [CO3 2−] content in the paleo-seawater is greater by a factor of 2–3 than that in modern seawater. This increase in [CO3 2−] content is inferred to be one of the main factors for the widespread development of dolomite.

Superficial alteration mineralogy in active volcanic systems: An example of Poás volcano, Costa Rica

The alteration mineralogy in the crater area of Poás volcano (Costa Rica) has been studied to constrain acid fluid-rock interaction processes and conditions relevant for the formation of sulphate-bearing mineral assemblages found on the surface of Mars. Individual sub-environments, which include the hyperacid lake (Laguna Caliente), ephemeral hot springs, fumarole vents and areas affected by acid rain and/or spray from the lake, are marked by distinct secondary mineral associations, with sulphates commonly as prevailing component. The sulphates occur in a wide mineralogical diversity comprising gypsum/anhydrite, various polyhydrated Al-sulphates, alunite-jarosite group minerals, halotrichite-, voltaite- and copiapite-group minerals, epsomite and römerite. Depending on the sub-environment, they are variably associated with clay minerals (kaolinite-group and smectite-group), zeolites, SiO2-polymorphs, Fe-(hydro)oxides, Ti-oxides, native sulphur, sulphides, chlorides, fluorides, phosphates and carbonates. Geochemical modelling was performed to identify mechanisms responsible for the formation of the secondary minerals found in the field, and to predict their possible stability under conditions not seen at the surface. The results indicate that the appearance of amorphous silica, hematite, anhydrite/gypsum, pyrite, anatase and kaolinite is relatively insensitive to the degree of acidity of the local aqueous system. On the other hand, alunite-jarosite group minerals, elemental sulphur and Al(OH)SO4 only form under acidic conditions (pH<4). The presence of polyhydrated Mg- and Fe2+-sulphates is restricted to olivine-bearing rocks exposed to acid rain or brine spray. Modelling suggests that their formation required a repetitive sequence of olivine dissolution and evaporation in an open system involving limited amounts of fluid. The mineral variety in the crater of Poás is remarkably similar to sulphate-bearing assemblages considered to be the product of acid-sulphate alteration on Mars. The analogy suggests that comparable fluid-rock interaction controls operated in Martian volcanic environments.

Understanding of Capping Effects on the Tip Shape Evolution and on the Atom Probe Data of Bulk LaAlO3 Using Transmission Electron Microscopy.

Two challenges exist in laser-assisted atom probe tomography (APT). First, a drastic decline in mass-resolving power is caused, not only by laser-induced thermal effects on the APT tips of bulk oxide materials, but also the associated asymmetric evaporation behavior; second, the field evaporation mechanisms of bulk oxide tips under laser illumination are still unclear due to the complex relations between laser pulse and oxide materials. In this study, both phenomena were investigated by depositing Ni- and Co-capping layers onto the bulk LaAlO3 tips, and using stepwise APT analysis with transmission electron microscopy (TEM) observation of the tip shapes. By employing the metallic capping, the heating at the surface of the oxide tips during APT analysis became more symmetrical, thereby enabling a high mass-resolving power in the mass spectrum. In addition, the stepwise microscopy technique visualized tip shape evolution during APT analysis, thereby accounting for evaporation sequences at the tip surface. The combination of "capping" and "stepwise APT with TEM," is applicable to any nonconductors; it provides a direct observation of tip shape evolution, allows determination of the field evaporation strength of oxides, and facilitates understanding of the effects of ultrafast laser illumination on an oxide tip.

Interpretación geológica y geofísica de la Elevación de Río Grande, margen brasileño suroriental: tectónica extensional y rifting de las cortezas continental y oceánica

This paper discusses the geological and geophysical interpretation of rift structures in the region extending from the Rio Grande Rise, in the Southeastern Brazilian margin, towards the Cabo Frio High, which separates the Campos and Santos basins. We have analysed potential field data (gravity and magnetic) from the Argentine to the Brazilian oceanic basins and extending over the Pelotas, Santos and Campos basins. The Rio Grande Rise shows a relatively negative Bouguer anomaly in an area that corresponds to a major positive bathymetric feature between the Argentine and Brazil basins. North–south propagators related to the early spreading centres of the Atlantic Ocean are observed from Argentina towards the southern Santos Basin, which is characterized by an elevated basement topography relative to the Pelotas Basin. The region adjacent to the Florianopolis Fracture Zone between the Santos and Pelotas basins is also characterized by an elevated basement region aligned in an east–west direction, and locally it is marked by rift structures aligned along a NW–SE direction, forming a lineament or shear zone (Cruzeiro do Sul lineament) that extends from the Cabo Frio High towards the Rio Grande Rise, thus involving both continental and oceanic crusts. The Rio Grande Rise is associated with the east–west-trending fracture zones, which are characterized by several aligned magnetic anomalies in the southern Santos Basin. The Rio Grande Fracture Zone continues landward as the Sao Paulo Ridge, and extends towards the platform as the Florianopolis High. Oceanic propagators are identified from Argentina towards the Pelotas and Santos basins, and locally we observe rupturing of the salt layer by igneous intrusions or possibly by mantle exhumation. The Florianopolis (or Rio Grande) Fracture Zone is marked by an abrupt topographic offset separating the Pelotas Basin from the southern Santos Basin, and the associated volcanic belts limit the southernmost occurrence of the late Aptian evaporate sequence. The evaporite sequence in this segment of the continental margin shows remarkable layering of halite, anhydrite and carnalite. Conjugate to the Rio Grande Rise, the Walvis Ridge, offshore Namibia, is similarly a topographic high, but rift structures as observed in the Brazilian side are apparently unique in the South Atlantic. Alternative interpretations for the origin of the Rio Grande Rise include: a volcanic edifice or plateau rooted in the mantle; an intraplate shear zone affecting both continental and oceanic crust; an oceanic area of igneous over-productivity caused by a hotspot or a thermal anomaly in the mantle; a palaeo-spreading centre in the Cretaceous Atlantic Ocean; an area of excessive volcanic activity resulting from mantle differentiation due to adiabatic decompression; or perhaps an isolated remnant of continental crust left outboard of the Brazilian continental margin during the drifting process.