Pre-existing Layer Research Articles

A Geochemical Process Model for the Mount Isa Copper Orebodies

In this paper we present new data and review existing data on the distribution, lithochemistry and paragenesis of copper ore at Mount Isa and provide the first description of a body of anhydrite, dolomite, barite, and hematite recently intersected at the northern end of the copper system. It has been previously proposed that the extraordinary spatial coincidence of giant copper and lead-zinc deposits at Mount Isa is due to the emplacement of copper into host rocks made chemically favorable for copper mineralization by enrichment in dolomite and/or pyrite and mechanically favorable by the presence of preexisting layers of ductile lead and zinc sulfide ore. Several recent studies, however, suggest that copper and much if not all of the lead and zinc were deposited contemporaneously. We provide evidence that high-grade (>2.5%) copper is associated with fractured and silicified rocks. Early breccia-cementing quartz is commonly euhedral and contains numerous growth zones defined by disordered carbon and brine inclusions containing CH4. This quartz is inferred to have been deposited from a cooling brine (“basement brine”) that equilibrated with rocks beneath the level of the deposit. Consistent with earlier studies, we have noted that there is often late dolomite (dolomite II) infilling of vuggy porosity defined by this euhedral quartz. Chalcopyrite, however, postdates both phases and there is textural evidence of both dolomite and quartz dissolution during chalcopyrite formation. The nature of the copper-bearing fluid (“basinal brine”) was deduced from the intimate association of chalcopyrite and anhydrite and from previous fluid inclusion studies. This brine was oxidized and sulfur rich but undersaturated in quartz, consistent with this fluid being introduced from above as proposed in several previous studies. New reaction path modeling suggests that reduction of the oxidized brine by the carbonaceous and Urquhart Shale was the most likely depositional process. Many other rock types in the region also had the capacity to reduce oxidized ore-forming brines, thus ore can be anticipated in these other units, provided that sufficient porosity and permeability existed during ore formation. The modeling further suggests that the presence of dolomite is not a prerequisite to economic copper grades, although carbonate dissolution possibly contributed (with fracturing) to enhanced porosity and permeability. Furthermore, sulfide-rich rocks were no more effective at precipitating copper than sulfide-absent rocks, consistent with several lines of evidence demonstrating that pyrite or other sulfide minerals were not consumed during ore formation. Mixing of the basinal and basement brines could also have caused precipitation of copper, but the predicted gangue assemblages differ from those observed in nature. Various radiometric ages are consistent with formation of the Mount Isa copper deposits after initiation of the unconformable South Nicholson basin, although we recognize that more precise and accurate dating of the ore and fluid-flow events are required. Basal red-bed sediments of the South Nicholson basin would have provided suitable aquifers for the oxidized, copper-rich basinal brines. We speculate that the driving force for brine flow was the development of topography to the east of Mount Isa.

AC-anodising of aluminium: Contribution to electrical and efficiency study

AC-anodising of AA1050 aluminium alloy in sulphuric and phosphoric acid solutions has been examined, with the contributions of the anodic and cathodic processes evaluated from the electrical signal analysis. The porous anodic film morphology and the potential-time behaviour confirm the similarity with DC-anodising, although lower anodic charge and film formation efficiencies are evident from gravimetric measurements. The anodic charge loss is mainly related to the capacitive current required to re-establish (between successive charge cycles) the electric field strength for ionic migration across the pre-existing and healed barrier layer oxide. Cathodic hydrogen gas evolution is considered to proceed at flaw sites in the barrier layer oxide, leading to high localized current densities that result in high cathodic potentials. Hydrogen gas retention at flaws and pores of the anodic film also contributes to the high cathodic potentials.

Modeling studies of the upper ocean response to a tropical cyclone

A coupled ocean and boundary layer flux numerical modeling system is used to study the upper ocean response to surface heat and momentum fluxes associated with a major hurricane, namely, Hurricane Dennis (July 2005) in the Gulf of Mexico. A suite of experiments is run using this modeling system, constructed by coupling a Navy Coastal Ocean Model simulation of the Gulf of Mexico to an atmospheric flux model. The modeling system is forced by wind fields produced from satellite scatterometer and atmospheric model wind data, and by numerical weather prediction air temperature data. The experiments are initialized from a data assimilative hindcast model run and then forced by surface fluxes with no assimilation for the time during which Hurricane Dennis impacted the region. Four experiments are run to aid in the analysis: one is forced by heat and momentum fluxes, one by only momentum fluxes, one by only heat fluxes, and one with no surface forcing. An equation describing the change in the upper ocean hurricane heat potential due to the storm is developed. Analysis of the model results show that surface heat fluxes are primarily responsible for widespread reduction (0.5°–1.5°C) of sea surface temperature over the inner West Florida Shelf 100–300 km away from the storm center. Momentum fluxes are responsible for stronger surface cooling (2°C) near the center of the storm. The upper ocean heat loss near the storm center of more than 200 MJ/m2 is primarily due to the vertical flux of thermal energy between the surface layer and deep ocean. Heat loss to the atmosphere during the storm’s passage is approximately 100–150 MJ/m2. The upper ocean cooling is enhanced where the preexisting mixed layer is shallow, e.g., within a cyclonic circulation feature, although the heat flux to the atmosphere in these locations is markedly reduced.

Rifting through a heterogeneous crust: insights from analogue models and application to the Gulf of Corinth

Abstract We used analogue models to study the fault evolution produced by extension through a heterogeneous crust. In the experiments, the heterogeneous crust consisted of a gently dipping silicone layer surrounded by brittle material. The viscous silicone level simulates a weak, upper crustal nappe stack that formed during a previous phase of shortening. X-ray scanner facilities allowed us to acquire 3D images of the experimental models at regular time invervals and hence to study the fault pattern development and the location of the main depocenters during rifting. The experimental results show that the inherited weak nappe stack acts as a décollement and localizes deformation. In the early stages of extension a system of conjugate high-angle normal faults initiates close to the upper tip of the gently dipping silicone layer near the free surface and propagates upwards, resulting in an initial symmetrical graben configuration. Further extension results in (1) a progressive asymmetry of the rifted zone, due to migration of its right margin down the nappe, (2) a shift of the main depocentre downward along the décollement, and (3) the simultaneous activity of several normal faults within the rifted zone. When the pre-existing silicone layer is oblique to the extension, the normal faults develop in an en echelon array, with a strike intermediate between the azimuth of the gently dipping silicone layer and the extension direction. The experiments also show how rheological differences between areas with potential intracrustal weak layers and adjacent domains without décollement level can lead to significant differences in fault pattern, dimension and orientation of the rifted zone. Complete asymmetry of a rift and switches in fault dip direction between adjacent domains can be explained by the presence of pre-existing upper crustal heterogeneities.

NOTES AND CORRESPONDENCE: Polarimetric Radar Observation of Multiple Freezing Levels

Abstract An unusual multiple freezing-level event observed with polarimetric radar during the second phase of the Improvement of Microphysical Parameterization through Observational Verification Experiments (IMPROVE-2) field program is described. The event occurred on 28 November 2001 when a warm front moved over the Oregon Cascade Mountains. As the front approached, an elevated melting layer formed above a preexisting melting layer near ground. Continued warming of the lower atmosphere eventually dissipated the lower melting layer. The polarimetric measurements are used to estimate the height of the freezing levels, document their evolution, and deduce hydrometeor habits. The measurements indicate that when the two freezing levels were first observed melting was incomplete in the upper melting layer and characteristics of particles that passed through the two melting layers were similar. As warming progressed, the character of particles entering the lower melting layer changed, possibly becoming ice pellets or frozen drops. Eventually, the refreezing of particles ended and only rain occurred below the elevated melting layer. The Doppler radial winds showed a well-defined wind maximum apparently associated with a “warm conveyor belt.” The jet intensified and descended through the elevated melting layer with time. However, the increase in wind speed did not appear connected with melting or result in precipitation enhancement.

Bottom-type scattering layers and equatorial spread <i>F</i>

Abstract. Jicamarca radar observations of bottom-type coherent scattering layers in the post-sunset bottomside F-region ionosphere are presented and analyzed. The morphology of the primary waves seen in radar images of the layers supports the hypothesis of kudeki+bhattacharyya-1999 that wind-driven gradient drift instabilities are operating. In one layer event when topside spread F did not occur, irregularities were distributed uniformly in space throughout the layers. In another event when topside spread F did eventually occur, the irregularities within the pre-existing bottom-type layers were horizontally clustered, with clusters separated by about 30km. The same horizontal periodicity was evident in the radar plumes and large-scale irregularities that emerged later in the event. We surmise that horizontal periodicity in bottom-type layer irregularity distribution is indicative of large-scale horizontal waves in the bottomside F-region that may serve as seed waves for large-scale Rayleigh Taylor instabilities. Key words. Ionosphere (equatorial ionosphere; ionospheric irregularties; plasma waves and instabilities)

Geometric and kinematic analysis of a transpression terrane boundary: Minas fault system, Nova Scotia, Canada

Abstract The Minas fault system is an ENE-WSW trending transpressional boundary between the Appalachian Meguma and Avalon tectono-stratigraphic terranes of Nova Scotia, Canada. Along this boundary there is large-scale partitioning of deformation into distinct external (contractional) and internal (shear) zones. With the increase in strain from external to internal zones there is progressive localization of deformation, culminating in the discrete shear band domain. Deformation has produced materially, temporally and spatially distinct folds and faults throughout the fault system history. Ductile structures are generally composite features derived from multiple transposition of pre-existing layers. The partitioning of deformation found amongst fault rock units can in turn be associated with contrasting deformation micromechanisms. The distinctive variation in mechanical response and microstructures provides an insight into the role of localization, partitioning and distribution of deformation. Kinematic analysis has demonstrated that the Minas fault system segment examined here is a thinning deformation zone, in which strain is accommodated within progressively narrower volumes of rock. Deformation can be summarized as a broad, initially diffuse zone of triclinic transpression that has evolved, with the accumulation of finite strain, into zones of distinct structural style and variation in finite strain. It is not possible to demarcate ‘deformed shear zone’ and ‘undeformed host rocks’. Instead, the Minas fault system is described in terms of discontinuous transitions in finite strain and deformation style within a large scale movement picture.

Spatial distribution of melt-bearing structures in anatectic rocks from Southern Brittany, France: implications for melt transfer at grain- to orogen-scale

In this study, we present quantitative spatial information on the one- and two-dimensional distribution of inferred melt-bearing structures in anatectic supracrustal rocks of the Southern Brittany Migmatite Belt, south of the transcurrent South Armorican Shear Zone (SASZ); based on these data, we infer the mechanism of melt extraction from partially molten crust. Former melt-bearing structures include foliation-parallel leucosomes and cross-cutting granitic leucosomes that infill inter-boudin partitions and extensional shear surfaces, as well as discordant dykes of granite. Petrographic (i.e., mineralogical and microstructural) continuity of granite from structure to structure suggests that they once formed a continuous melt-bearing network. Measurements along one-dimensional line traverses perpendicular to layering of stromatic migmatite exposed in clean, sub-horizontal outcrop surfaces provide information about thickness and spacing distributions of foliation-parallel leucosomes. Most leucosome thicknesses fall in the range of 1–10 mm, with upper limits around 20–30 mm. The number of thicker layers decreases abruptly with increasing thickness, which is inconsistent with scale-invariance. This suggests that leucosome formation was controlled by short-range melt movement along grain boundaries to form melt-rich layers constrained by pre-existing compositional layering. Spacing distributions also are not scale-invariant; however, the large percentage of leucosomes (40–60%) in these line traverses suggests that spacing distributions may be controlled in part by impingement of leucosomes, making it difficult to derive genetic information from these data. Qualitative observation of inferred melt-bearing structures in mutually perpendicular two-dimensional exposures from the same outcrop reveals anisotropy of the leucosome network related to a well-developed sub-horizontal quartz–feldspar lineation reflecting stretching associated with transcurrent movement along the SASZ. Analysis of these two-dimensional distributions using the box-counting method corroborates the observed anisotropy, but indicates that leucosome morphology (and perhaps distribution) is not scale-invariant. The applicability of the box-counting method, or of fractal analysis, to understanding melt movement in migmatites is discussed in light of these results. Based on the anisotropy of melt-bearing structures, we infer that melt-movement in structures now represented by layer-parallel leucosomes was primarily sub-horizontal. These layers fed steeply dipping structures now represented by cross-cutting leucosomes, in particular those developed at inter-boudin partitions, and granite dykes. The formation and orientation of these steeply dipping structures was in part controlled by far-field stresses related to dextral displacement along the SASZ. Melt extraction is inferred to have occurred along these steeply dipping structures; extracted melt accumulated in plutons at higher crustal levels, such as the Quiberon, Sarzeau, and Guérande granites.

Bias-induced spatially resolved growth and removal of Si-oxide by atomic force microscopy

Three mechanisms for spatially resolved growth and removal of oxide on silicon substrates have been investigated. Thermally grown oxide layers with thicknesses in the range 2–6 nm were the distinctive feature of the system. The layers were characterized and manipulated by methodologies based on atomic force microscopy (AFM) with conducting probes in a vacuum environment of 10-2–10-3 Pa. The probe is then effectively a travelling electrode that generates an electrostatic field between the tip and the substrate. Oxide growth was induced for a positive sample bias greater than 5 V, but below the level corresponding to dielectric breakdown. Application of a short pulse of amplitude marginally above that corresponding to dielectric breakdown, on the other hand, had the effect of producing pits of inner diameter of about 10 nm in the pre-existing oxide layer at the point of tip-to-oxide contact. Application of a low positive sample bias (less than that required for measurable oxide growth) in combination with high linear scan speed had the effect of removing a pre-existing oxide layer from the scanned field of view. The most plausible mechanisms are based on transverse ionic diffusion (for oxide growth), controlled dielectric breakdown (for formation of pits) and lateral transport of silicaceous species (for oxide removal).

Deformation partitioning during folding and transposition of quartz layers

Banded iron formation (BIF) from the Quadrilátero Ferrı́fero (southeastern Brazil) shows a compositional layering with alternating iron-rich and quartz-rich layers. This layering was intensively folded and transposed at a centimeter/millimeter scale through a component of bedding-parallel shear related to flexural slip at middle to high greenschist facies conditions (400–450 °C). The microstructure and c-axis fabrics of normal limbs, inverted limb and hinge zones of a selected isoclinal fold were analyzed combining optical and scanning electron microscopy (SEM) and digital image analysis. In the normal limbs, recrystallized quartz grains show undulose extinction, relatively dry grain boundaries, c-axes at high angle to foliation and a pervasive grain shape fabric (GSF) indicating operation of crystal-plastic processes. In the inverted limb, quartz grains show more serrated and porous (“wet”) grain boundaries; the GSF is similar to that of the normal limb, but c-axes are oriented at 90° to those of the normal limb. We interpreted these characteristics as reflecting operation of solution-precipitation deformation in inverted limbs, as a consequence of grains having been rotated to an orientation that was hard to basal 〈 a〉 glide, but easy to dissolution-precipitation creep. This deformation partitioning between crystal-plasticity and solution-transfer during folding/transposition of quartz may explain the common occurrence of layered quartz rocks, where individual layers show alternating c-axis fabrics with opposite asymmetries but a consistent GSF orientation. Such characteristics may reflect an earlier event of pervasive folding/transposition of a preexisting layering.

Influence of Metal/Metal Ion Concentration on the Photocatalytic Activity of TiO2−Au Composite Nanoparticles

Gold-capped TiO2 nanocomposite particles have been synthesized by reducing [AuCl4]- on the surface of preformed TiO2 colloids in acetonitrile. Such semiconductor−metal nanocomposites are considered to be beneficial toward improving the efficiency of the photocatalytic oxidation process. Whereas photogenerated electrons of TiO2 colloids readily reduce Au(III) ions in deaerated solution, the photogenerated holes are capable of oxidizing the preexisting gold layer on TiO2 particles in aerated solutions. The experimental conditions under which photocatalysis experiments are carried out dictate the net concentration of metal and metal ion species at the semiconductor/metal interface and hence the overall catalytic activity. Using photocatalytic oxidation of SCN- as a probe, we have investigated the influence of Au0 and Au(III) on interfacial hole transfer. The ratio of metal/metal ion concentration (viz., [Au0]/[Au(III)]) at the TiO2 interface was found to dictate the overall photocatalytic process in the semi...

Unusual Fe-Ti-Cr spinels from discordant bodies of iron-rich ultramafic pegmatite at the Amandelbult Platinum mine, northwestern Bushveld Complex

AbstractSpinels associated with discordant bodies of iron-rich ultramafic pegmatite are described from the Amandelbult Platinum mine in the northwestern part of the Bushveld Complex. The spinels are divided into three groups, disseminated Ti-magnetite, disseminated Fe-Ti-Cr spinel and massive Fe-Ti-Cr spinel. The Fe-Ti-Cr spinels show a range of unusual compositions intermediate between chromite and Ti-magnetite. A relationship was found between stratigraphic height and spinel-type, with the Fe-Ti-Cr spinels restricted to pegmatites from the Upper Critical zone and Ti-magnetite to pegmatites from the Lower Main zone. Ilmenite is a ubiquitous component of all of the pegmatites examined here. The massive Fe-Ti-Cr oxide pegmatites are found only where earlier-formed chromitite layers are juxtaposed with sheet-like bodies of olivine-clinopyroxene pegmatite. A distinct thickening of the original chromitite layers in this situation, and compositional gradients within them, points to accretion of Fe-Ti-Cr spinels onto them prior to partial sub-solidus re-equilibration. Analytical data are presented for these spinels and for the Ti-magnetite.The composition of the Fe-Ti-Cr spinels is not duplicated by cumulus spinels in the Bushveld Complex, but the compositions and microtextures of the disseminated Ti-magnetite are very similar to cumulus Ti-magnetite from the Upper zone. Accordingly, it is deduced that the Ti-magnetite in the pegmatites from the Lower Main zone, together with the ilmenite, crystallized at magmatic temperatures from a suitable Fe-Ti-rich silicate-oxide melt. No evidence has been found to link the pegmatites to hydrothermal fluids. The Cr-rich nature of the disseminated spinels in pegmatites from the Upper Critical zone suggests that the pegmatite melt was richer in chromium at this stratigraphic height, although re-equilibration with earlier-formed cumulus chromite also occurred. Formation of the Fe-Ti-Cr oxide pegmatites reflects a complex process that is incompletely understood and why new oxides plate onto pre-existing chromitite layers that are juxtaposed with Fe-rich ultramafic pegmatites is a matter of conjecture.

Modification of wetting properties of SiO x surfaces by Ar implantation

The aim of this experiment is to better understand the mechanism leading to the modification of the wetting properties of insulating (quartz) and non-insulating (Si) surfaces by ion and atom implantation. Coupons of quartz and silicon (with its native oxide layer) have been irradiated by means of 3 keV Ar ions and atoms with a fluence of 1.8×10 16 Ar/cm 2. Some exposures to Ar ions have been performed under oxygen partial pressure (∼5×10 −5 Torr). The samples have been characterized before and after implantation by means of contact angle hysteresis measurements, Rutherford backscattering spectroscopy (RBS) and angle resolved X-ray photoelectron spectroscopy (ARXPS). Irradiation with argon ions or atoms has produced a more hydrophilic surface immediately after implantation. Ar depth profiling by means of RBS has shown that 4% of implanted (atoms and ions) Ar has been retained in quartz, while 13% and 21% of Ar has been retained in silicon after Ar implantation with and without oxygen partial pressure, respectively. However, no difference in the depth distribution has been measured. Characterization by means of ARXPS has shown a noticeable change in the composition of the quartz and silicon oxide surfaces (implantation of Si under an O 2 partial pressure producing a thicker oxide layer) which can explain the differences in the reported Ar retention. In general, the irradiation have removed a large portion of oxygen present in the pre-existing carbonaceous layer on the surface of the samples. The ion beam irradiation has been more efficient than atom beam to both, increase the wettability of the quartz surfaces and enhance the concentration of the carbonaceous layer in “dispersed islands” on the surfaces.

An hypothesis on the early evolution of the development of the isocortex

We propose an hypothesis on the evolutionary origin of the unique inside-out developmental gradient of the isocortex, in which deep layers originate before superficial layers. This contrasts with the development of the reptilian cortex, which originates in an outside-in gradient. In mice, a mutated protein, reelin, produces the reeler phenotype, whose cortex has an outside-in neurogenetic gradient like in reptiles. Reelin is normally located in the marginal layer of the developing cerebral cortex, and its normal function has been proposed to be a stop signal that prevents radially migrating cells from moving into the marginal zone. Additionally, mutations on the kinase Cdk5, or in its neuronal-specific activator p35, produce a deficit similar to reeler in that the neurogenetic gradient is outside-in. However, contrary to reeler, in which no cell-sparse layer I is observed, in these mice, a well-defined layer I exists, which suggests that migrating cells respond normally to reelin. Apparently, Cdk5/p35 participate in permitting cortical cells to move across pre-existing (earlier produced) cortical layers, in order to be able to contact reelin once they reach the marginal zone. We suggest that the evolutionary advent of the mammalian cortical inside-out gradient became partly possible through the activation of the Cdk5/p35 pathway, which permitted migrating cells to move across layers of older cells. At about the same time, reelin became an important element in cortical development as it prevented neuronal migration into the marginal zone (cortical layer I) and facilitated the migration of neurons past postmigratory elements.

Pressure effects on high temperature steam oxidation of zircaloy-4

The pressure effects on the oxidation of Zircaloy-4 (Zry-4) cladding in high temperature steam have been analyzed. A double layer autoclave was made for the high pressure/high temperature oxidation tests. The experimental test temperature range was 700–900°C, and pressures were 0.1–1.5 MPa. Steam partial pressure was found to be a more important factor than total pressure. Steam pressure enhances the oxidation rate of Zry-4 exponentially and depends on the temperature, the maximum being between 750–800°C. A preexisting oxide layer decreases the enhancement by about 40–60%. The acceleration of the oxidation rate caused by high pressure steam seems to originate from the formation of cracks due to an abrupt transformation of the tetragonal phase in the oxide, where the instability of the tetragonal phase comes from the reduction of surface energy due to steam.

Embryonic central nervous system angiogenesis does not involve blood‐borne endothelial progenitors

We asked, whether, in the blood of avian embryos, endothelial precursor cells circulate that actually contribute to the growing vascular system in and around the central nervous system (CNS). We compared the morphology and distribution of QH1-positive cells after transplantation of quail paraxial mesoderm, after blood transfusion, in quail-chick parabiosis, or after quail bone-marrow transplantation. After head mesoderm transplantation from quail to chick, we observed sprouting endothelial cells (ECs), capillary tube formation, and chimeric endothelial lining of large arteries in the host brain. These QH1-positive quail cells showed EC morphologies that demonstrated three different aspects of CNS angiogenesis: invasion by means of filopodia, clonal proliferation and tube formation, and integration into preexisting EC layers. After blood transfusion or in chick-quail parabiosis, blood-borne QH1+ cells were found in the lumen of but not integrated into the wall of the host vascular system. Neither were QH1+ cells observed in the capillary walls of parabiotic chick chorioallantoic membranes. In both cases, the quail cells showed typical macrophage morphology. In chicks that had received quail bone marrow transplants onto their chorioallantoic membranes, QH1+ cells with macrophage, but not EC shape were occasionally seen near the inoculation site. We conclude that (1) blood-borne cells do not become ECs or directly contribute to angiogenesis inside, or in vascular plexuses around the CNS during embryonic development; (2) blood-borne cells do not contribute to the intraneural macrophage population of the embryonic CNS.

Method for nanostructuring of Ag atoms on Ni(111) surfaces

The nanostructuring process of Ag atoms on the Ni(111) surface was investigated in two different ways: (1) the use of a morphological change caused by Pb deposition onto the Ag-covered Ni(111) surface and (2) the use of Volmer-Weber growth mode by Ag depositions onto the Pb-covered Ni(111) surface. In case (1), the preexisting Ag layer was transformed into three-dimensional islands (nanocrystals) during Pb deposition at room temperature: The nanocrystals were directly on the Ni(111) surface and surrounded by a monoatomic Pb layer. In case (2), the Ag nanoparticles were formed not on the bare Ni(111) surface, but on the preexisting Pb layer: The particles were found to be mobile under the interaction with scanning tunneling microscopy tip. The results were discussed in terms of simple thermodynamic considerations and growth kinetics.

Interdependence of On-Load Corrosion, Creep-Rupture, and Copper Deposit in Augmenting Failure Processes of Boiler Tubes

Long-term overheating of boiler tubes in fossil fuel-fired thermal power plants intrinsically is linked with the formation and growth of internal oxide scale at high-heat flux zones, known as hot spots. Preferential and selective deposition of highly aggressive corrodents such as alkalis, chlorides, and some inorganic oxides like cupric oxide/cuprous oxide (CuO/Cu2O) take place at these hot spots. Caustic corrosion develops as the localized concentration of sodium hydroxide (NaOH) at the metal-oxide interface increases either through a wick-boiling process or simply by seepage between cracked oxide layers. At high-pH levels, the magnetite (Fe3O4) layer starts dissolving and results in rapid corrosion. In addition, the rate of corrosion is found to accelerate under sustained loads (hoop stress) and with fluid temperature. Copper corrosion products (typically coming from brass condenser tubes) concentrate at the site of attack and plate out onto the metal surface, aggravating the corrosion process further by galvanic corrosion. The end result is a thickening of preexisting oxide layers and an eventual loss of localized heat-transfer rate. As the metal temperature escalates, the tube creeps much faster and fails in an embrittled manner. In the present paper, the sequence of mechanisms involved in the failure process is discussed, and their interdependence, causing overheating and failure, is described. In addition, the role of copper deposits in expediting the failure process is analyzed critically. A case study is presented to substantiate the proposed mechanisms involved in long-term overheating failures.

A Mathematical Model for Crater Defect Formation in a Drying Paint Layer

Certain deep indentations observed in dry coatings are referred to as “craters”. They are believed to arise from gradients in the coating surface tension. A mathematical model of surface-tension-gradient-driven flow, using the lubrication approximation for thin layers, is developed to study the formation of craters. The paint is modeled as consisting of an evaporating “solvent” part and a nonvolatile “resin” part. Surface tension gradients on the coating surface arise due to a nonuniform distribution of surfactant. Axisymmetric numerical simulations using the model are performed to explore two candidate crater production mechanisms: an initial release of concentrated surfactant and a steady surfactant source. The effects of changes in various properties, such as the paint drying rate, the surfactant diffusivity, and the viscosity increase during drying, are examined. The model produces craters with large diameters, pronounced rims, and central peaks, similar to those seen in practice. Drying rate has a large influence on crater diameter and depth, by limiting flow due to surface tension gradients within a given time. Reduction of the paint viscosity increase during drying causes increased flow rates, leading to larger craters. A preexisting layer of surfactant on the paint surface sharply reduces the extent of cratering. Surfactant diffusion also tends to reduce the severity of cratering by alleviating surface tension gradients. In some cases, a simplified form of the drying model may be used to quickly approximate the results of the full model. The model provides useful insights into the craters seen in industrial coating applications.

Geochemistry of Late Cenozoic basalts from the Crary Mountains: characterization of mantle sources in Marie Byrd Land, Antarctica

Late Cenozoic (9.34 to 0.04 Ma) alkaline basalts from the Crary Mountains, eastern Marie Byrd Land, Antarctica have low 87 Sr/ 86 Sr (0.70267–0.70283), moderately high 143 Nd/ 144 Nd (0.51286–0.51291) and high 206 Pb/ 204 Pb (19.9–20.9) ratios. The Crary Mountains basalts together with Hobbs Coast basalts from western Marie Byrd Land have the strongest HIMU signature ( 206 Pb/ 204 Pb>20.6 ) in the West Antarctic Rift System (WARS). There is no evidence of crustal contamination and their trace-element patterns are similar to HIMU basalts from oceanic islands. Higher concentrations of some large-ion lithophile (LIL) elements, particularly K, relative to oceanic HIMU, suggest melting of an enriched mantle. Models for small degrees of partial melting between 2% and 3% of a mantle peridotite containing 2.5% amphibole are consistent with trace-element variations in the basalts. The Crary data confirm the presence of a moderately depleted, lower-μ ( μ= 238 U/ 204 Pb ) mantle component with a 206 Pb/ 204 Pb signature between 19.3 and 19.8 in the source regions of West Antarctic basalts. The lower-μ component, defined by the focus of West Antarctic data arrays on 2-D and 3-D isotope plots, is prevalent as a mixing end-member throughout the WARS, whereas the HIMU component, defined by 206 Pb/ 204 Pb ratios greater than 20.5, is present at only a few localities. The positive correlation of many highly incompatible trace-elements with 206 Pb/ 204 Pb ratios indicate that smaller-degree melts preferentially sample the HIMU component, whereas larger degree melts sample the lower-μ component. We also suggest, based on variations in elements moderately incompatible to highly compatible in the mantle phases phlogopite and amphibole, that the lower-μ component is more hydrous than the HIMU source. The lower-μ and HIMU components are also found in alkaline basalts from Tasmania and New Zealand which were adjacent to the Antarctica coast prior to the mid-Cretaceous fragmentation of the proto-Pacific margin of Gondwanaland. We propose that before continental breakup, a HIMU-type mantle plume was trapped and stored beneath a much more extensive pre-existing metasomatised layer within the Gondwanaland lithosphere. Source regions for all known extreme HIMU basalts in the continental borderlands of the southwest Pacific would be encompassed by a modest plume head, 600–800 km in diameter, emplaced around 100 Ma. We suggest that an earlier metasomatic event, possibly related to the Jurassic Bouvet-plume, enriched the originally depleted upper mantle in highly incompatible elements. A fertile lithosphere formed by plume-driven melts and fluids during the Jurassic would allow for the widespread development of the present-day isotopic signature of the lower-μ source.