Progressive Evaporation Research Articles

Crystallization of carbon tetrachloride in confined geometries

The thermal behaviour of carbon tetrachloride confined in silica gels of different porosities was studied by differential scanning calorimetry. Both the melting point and the low temperature phase transition were measured and found to be inextricably dependant on the degree of confinement. The amount of solvent was varied through two sets of experiments, sequential addition and original progressive evaporation allowing the measurement of DSC signals for the various transitions as a function of the amount of CCl4. These experiments allowed the determination of the transition enthalpies in the confined state, which in turn allowed the determination of the exact quantities of solvent undergoing these transitions. A clear correlation was found between the amounts of solvent (both free and confined) undergoing the two transitions, demonstrating that the formation of the adsorbed layer t does not interfere with the second transition. The thickness of this layer and the porous volumes of the two silica samples were measured and found to be in very close agreement with the values determined by gas sorption.

Influence of thermal aging on the electrical properties of poly(vinyl chloride)

AbstractThis article reports on the study of the thermal aging of poly(vinyl chloride) (PVC) used in medium‐ and high‐voltage cables. It is shown that the thermal aging leads to the degradation of the material and to the modification of its electrical properties. The degradation is all the more important and faster as the temperature is high. This degradation is attributed to a progressive evaporation of the plasticizer at the beginning of aging and to a weight loss of stabilizer followed by a change in the color of polymer and a release of hydrochloric acid at more advanced stages of aging. It also results in a crosslinking of the material and a shrinking of samples. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 4728–4733, 2006

HISTORY OF MOLLUSC COMMUNITY TYPES AND FAUNAL DYNAMICS IN CONTINENTAL SALINE ECOSYSTEMS OF THE SOUTH MEDITERRANEAN QUATERNARY

Quaternary continental aquatic ecosystems characterised by poly/eusaline waters, and without a connection to the Mediterranean Sea, are dominated by the potamidid gastropod Potamides conicus (Blainville) and the cardiid bivalve Cerastoderma glaucum (Poiret). Metasaline environments are characterised by monotypic occurrences of C. glaucum . The primarily freshwater-dwelling thiarid gastropod Melanoides tuberculata (Muller), in contrast, colonised oligo/mesosaline habitats, similarly to its precursors from the Upper Messinian Lago-Mare ecosystems. In case of the establishment of saline conditions in former freshwater environments, characteristic taphocoenoses document the dynamic development in the context of progressive evaporation: the Melanoides palaeocommunity from a Holocene ecosystem at In Ecker (S Algeria) is accompanied by freshwater inhabitants, which did not reach their adult size of complete development due to the temporal character of their habitats. Oligo/mesosaline conditions are reflected in oxygen isotope signatures of the Melanoides shells. High variations in carbon isotope values indicate changes in the composition of the diet during ontogeny, and may support the interpretation that the ecosystem developed progressively. In comparison to Neogene and fossil Quaternary assemblages, the composition of extant eusaline communities is considerably more diversified: larger North African lakes in Libya and Egypt are characterised by neozoan elements originating from the Mediterranean fauna. The dynamic development of these faunas was predominantly supported by anthropochory in the context of fish cultures. The mode of early ontogenetic development played an important role with regard to the introduction of gastropods to continental environments, since the entire spectrum of euryhaline coastal gastropods with planktotrophic larval development is absent from these ecosystems. The preadaptational loss of planktotrophy enabled Potamides to establish persisting populations. Apart from its brooding mode, Melanoides is characterised by parthenogenesis, and thus a single individual, introduced to a continental ecosystem, can establish a population.

Montmorillonite based artificial nacre prepared via a drying process

Taking advantage of the specific behaviour of Na/Ca montmorillonite in aqueous dispersion, textured films were prepared by progressive evaporation of dispersions with low concentrations of delaminated platelets. The increase of the concentration of ions during evaporation changes the nature of the clay mineral platelet face interactions from repulsive to attractive. After complete drying, a dense brick-like structure is obtained when a sodium salt is used as deflocculant. The bending strength of the textured film is strongly affected by cracks formation during drying, specially when the sample thickness increases. After optimisation, crack-free textured samples with a 140 μm thickness, 20 cm × 25 cm area and 120 MPa bending strength were obtained.

Sedimentary facies and the context of dolocrete in the Lower Triassic Sherwood Sandstone group: Corrib Field west of Ireland

The Lower Triassic Sherwood Sandstone Group (SSG) reservoir in the Corrib Field is predominantly fine- to medium-grained fluvial sandstone with minor aeolian and playa sediments that formed under semi-arid to arid conditions. There is no evidence of roots, burrows or organic-rich soils in the SSG suggesting an environment with sparse vegetation in the catchment and sedimentation from a predominantly braided sand-dominated river. Dolomite is the main porosity-occluding cement (< 25 vol.%) in the Sherwood Sandstone Group reservoir in the Corrib Gas Field, offshore west of Ireland. Dolomite is demonstrably early diagenetic since it is non-ferroan, brightly luminescent, occurs in rocks with high (uncompacted) intergranular volumes and grew before all burial diagenetic cements. Conversely, calcrete is the dominant early diagenetic cement in well 27/5-1 in the SSG, ∼50 km south of Corrib. Dolocrete and/or calcrete are represented throughout the entire SSG interval and typically form under conditions of very low sediment accumulation rates. Dolocrete occurs equally in all facies showing that it is not a result of depositional processes. Core goniometry data in the Corrib Field revealed that the general palaeoflow direction was from south to north. During progressive evaporation of river- and ground-water calcite formed first, followed by dolomite further downstream with gypsum formed last if evaporation continues. This well-established pattern indicates that well 27/5-1 has calcrete and Corrib has dolocrete since they both result from evaporative concentration of ground- and river-water during flow down the very shallow Triassic palaeogradient.

Neoarchaean ( c . 2.58 Ga) halite casts: implications for palaeoceanic chemistry

Possibly the most extensive and best-preserved Archaean halite casts yet discovered occur in the c . 2.58 Ga upper Black Reef and basal Oaktree formations, Transvaal Supergroup, in Mpumalanga Province, South Africa. Halite casts are isolated on bedding planes, range in size from c . 1 mm to 20 mm, and have cubic, dumbbell and triangular shapes, as well as hopper-like pyramidal hollows on cube faces. Some of the casts display distinct hopper shapes characteristic of halite crystals. The halite cast-bearing pavements are developed within silicified mudstone interbedded with siltstone or stromatolitic dolomite. Associated sedimentary structures pointing to subaerial exposure include adhesion ripples and warts, desiccation and prism cracks, rill marks and tepee structures. Halite cast-bearing beds are interpreted as supratidal flat or sabkha deposits. The presence of isolated casts and hopper-shaped crystals suggests that halite resulted from displacive growth within the sediment from supersaturated residual brines after mudstone deposition. Absence of any indication of the former presence of gypsum or anhydrite supports previous contentions that the Neoarchaean ocean was deficient in sulphate or contained a high bicarbonate to calcium ratio such that with progressive evaporation, most calcium was consumed before the gypsum stability field was reached. The association of halite and carbonate in the upper Black Reef and basal Oaktree formations constrains the palaeolatitude of the Transvaal Basin at 2.58 Ga to subequatorial (10–30°).

Burning of weathering and emulsified oil spills

In previous works, we described small-scale experiments that dealt with the combustion of fuel slicks on water and provided some features of the events that can lead to boilover. The experiments showed that the violent-eruptive vaporization may be due to boiling nucleation near the heating fuel/water interface that has been superheated, and the mechanisms causing this phenomenon to occur have been discussed. If an attempt is made to burn crude oils, which are multicomponent fuels, the problem becomes complicated with the “weathering” of slicks (progressive evaporation of lighter volatiles) and the possibility to have water-in-oil emulsions. The objective of the present work is to study how the combustion of crude oil-layers floating on water is influenced by the aging of oil (degree of oil evaporation) and the formation of emulsions. The experiments are dealing with burning rate, time to the start of boilover, burned mass ratio and boilover intensity. The results are discussed in relation to the main factors influencing the phenomenon: initial oil-layer thickness and pool diameter. The behavior of weathered crude oil agrees closely with the general behavior of single oils with increasing boiling point (the surface temperature will increase with weathering as the residual crude oil becomes less volatile). In contrast, the case of emulsions is more complex and somewhat difficult to model (thermal breakage of emulsified layers, boilover difficult to discern). Finally, the work is complemented by a study of smoke production during the pre-boilover phase. The procedure permits measurements of exhaust gas composition by means of a flow-through system. Results show that weathering increases both yield of soot and CO but that emulsification decreases these yields. The former effect is due to a residual liquid progressively less volatile, the later to vaporised water interacting chemically in the flame.

Mode of gypsum deposition in southeastern Iranian soils as revealed by isotopic composition of crystallization water

Variations in δD and δ 18O values of water of crystallization and meteoric water were used to elucidate the origin of mother brine and the gypsum mode of formation in selected southeastern Iranian soils. Thirteen representative pedons were sampled at various geomorphic positions around the city of Rafsanjan. Crystallization water in different parts of three gypsum pendants was also analyzed isotopically. The slope of the δD and δ 18O plot for crystallization water was found to be 2.8, showing that evaporation was the major process for gypsum accumulation in the studied area. Comparison of data for eight rainwater samples and the crystallization water shows that gypsum formation occurred in a climate that was probably lower in temperature than that of today. The mean δD and δ 18O values of gypsum crystallization water vary from −41.9‰ and +6.5‰ for rock pediment to −36.9‰ and +10.0‰ for mantled pediment and −28.8‰ and +13.8‰ for transitional surfaces and finally, −8.8‰ and +19.2‰ for playa margins. The existence of such a close relationship between the isotopic composition of gypsum crystallization water and geomorphic positions could be attributed to the deposition of gypsum in different landforms at various stages of evaporation and/or progressive evaporation of moving surface and subsurface waters. An increase in δD and δ 18O values was also observed with increasing depth in almost all soil profiles studied. Different climates at the time of gypsum formation in different depths seem to be the possible reason. With increasing depth from the pebble surface in gypsum pendants, δD and δ 18O values decrease, which is also attributed to climatic changes during pendant development.

Melting Glaciers: A Major Source of Persistent Organochlorines to Subalpine Bow Lake in Banff National Park, Canada

Organochlorine pesticides and polychlorinated biphenyls (PCBs) are ubiquitous and persistent in the environment. They are known to concentrate in cold environments as a result of progressive evaporation from warm regions, and condensation in colder regions. In this study we show that melting glaciers supply 50 to 97% of the organochlorine inputs to a subalpine lake in Alberta, Canada, while contributing 73% of input water. Tritium analyses indicated that during the mid- to late summer warm period, at least 10% of the glacial melt originated from ice that was deposited in 1950-1970, when it was more contaminated with organochlorines. This finding suggests that climate warming may cause melting glaciers to become increasing sources of contaminants to freshwaters. Organochlorines from glacial streams were largely in dissolved form because the organic-poor glacial clays had a limited sorption capacity for the more hydrophobic chemicals.

Fluid mixing and recycling during Pyrenean thrusting: evidence from fluid inclusion halogen ratios

Syntectonic fluids have been sampled through fluid inclusion microthermometry and crush-leach analyses (cations and halogens) from a 50 km N-S transect through the central-southern Pyrenees. The fluid inclusions are contained in syntectonic quartz veins in Triassic redbeds, Cretaceous carbonates and Hercynian basement rocks, with some calcite and dolomite data from limestones and evaporites in more external parts of the belt. The main datasets come from (1) Alpine shear zones cutting the Néouvielle granodiorite in the Hercynian Axial Zone at the north end of the transect; (2) An imbricate zone beneath the Alpine Gavarnie Thrust at the Pic de Port Vieux; (3) Several localities in the footwall and hangingwall of the Gavarnie Thrust on the southern margin of the Axial Zone. The inclusion fluids generally decrease in salinity from 27–35% at the northern end of the transect to 7–22% on the southern margin of the Axial Zone. The majority of the inclusions have Cl/Br ratios lower than seawater and are interpreted as relict fluids after seawater evaporation and halite precipitation in the upper Trias. This interpretation is supported by Cl-Br-Na systematics, which are consistent with a change from halite to halite + sylvite precipitation with progressive evaporation. Fluids in the basement shear zones are interpreted to have essentially the same evaporitic origin as those still contained in sedimentary formations, although it is possible that final concentration of brines in the Néouvielle Massif involved retrograde hydration reactions with removal of water by precipitation of hydrous minerals. The fluids are also very similar in salinity and halogen chemistry to those found in veins associated with Mesozoic Pb-Zn-F deposits which predate the thrusting. The lower salinities seen at the southern margin of the Axial Zone are interpreted to reflect mixing of the brines with a higher level fluid (connate or meteoric water) circulating within the Mesozoic carbonates of the higher thrust sheets. At one locality where Triassic evaporites are still present, high Cl/Br ratios at relatively low salinities are present in inclusions within the underlying Triassic redbeds, but low Cl/Br ratios at higher salinities are seen lower in the sequence. This is consistent with dissolution of halite by a dilute fluid, but with limited penetration downwards. We suggest that the fluid history of the Pyrenees evolved through a series of stages: (1) Upper Triassic evaporite formation with sinking of brines into underlying redbeds and fractured basement rocks; (2) Circulation of brines with formation of Pb-Zn deposits along faults at some time between the Triassic and the Upper Cretaceous; (3) Renewed extension with erosion of Triassic rocks in many areas and further drawing down of Triassic brines into the basement; (4) Deposition of U. Cretaceous and Palaeocene carbonates containing connate waters of marine origin; (5) Formation of the Pyrenean thrust belt with overpressuring and expulsion of the brines along shear zones and faults; (6) Creation of topography with a high-level circulation system in the Mesozoic thrust sheets driven largely by topography. At the southern margin of the Axial Zone there was limited mixing of the deeper, overpressured brines with these more dilute, hydrostatically pressured fluids. An important point is that because of their density, hypersaline brines are difficult to expel from the upper crust, and may be involved in a succession of alteration and mineralisation events in the same general area over hundreds of millions of years.

Authigenesis of Trioctahedral Smectite in Magnesium-Rich Carbonate Speleothems in Carlsbad Cavern and other Caves of the Guadalupe Mountains, New Mexico

AbstractTrioctahedral smectite is a constituent of Mg-rich carbonate crusts and moonmilks (pasty deposits) in caves of the Guadalupe Mountains of southeastern New Mexico. Energy dispersive X-ray microanalysis of individual crystallites and their aggregates along with the X-ray diffraction analysis indicates that the smectite is probably stevensite. Saponite is likely present in some samples also. The smectite is intimately associated with dolomite crusts and huntite moonmilks in Carlsbad Cavern, Lechu-guilla Cave, and other dolostone caves. Clay particles appear as fibers and films, with aggregates comprising decimicron-sized filamentous masses that envelop crystals of dolomite, huntite, and magnesite. The occurrence of smectite is related to the genesis of the Mg-rich carbonate minerals. In water films, progressive evaporation and carbon dioxide loss results in the sequential precipitation of Mg-rich calcite, aragonite, dolomite, huntite, and magnesite. This sequence of carbonate precipitation removes Ca and greatly increases the Mg/Ca ratio in the solutions. Silica is commonly available probably because of high pH conditions, and consequently, smectite forms in the Mg-rich alkaline environment. Along with the Mg-rich carbonate minerals, opal, quartz, and uranyl vanadates may precipitate with the smectite.

Evaporite mineral assemblages in the nakhlite (martian) meteorites

A mineralogical study of the three nakhlite (martian) meteorites has revealed that they contain evaporite mineral assemblages. Lafayette has Ca-siderite and clay minerals (smectite/illite) along fractures within olivine; Governador Valadares contains clay mineral veins in olivine, with siderite, gypsum and anhydrite in interstitial areas; Nakhla has clay and gypsum veins in olivine, with Mg-, Mn-rich siderite, anhydrite and halite in interstitial sites. Minor goethite is also present in the three meteorites. Lafayette siderite has the range of compositions (mol%) CaCO 3 21.6–36.8, MnCO 3 4.2–35.3, MgCO 3 0.1–1.6, FeCO 3 27.4–67.0; Governador Valadares has CaCO 3 3.6–11.1, MnCO 3 1.1–2.1, MgCO 3 9.0–29.2, FeCO 3 64.3–77.8; Nakhla has CaCO 3 0.1–5.7, MnCO 3 1.0–39.9, MgCO 3 2.0–40.9, FeCO 3 23.2–87.0. Trace element abundances for clay, siderite and gypsum are all similar with LREE, Y>HREE, Zr, Nb and La 0.9–95×CI; Y 0.2–2.4×CI. This pattern of abundances reflects the trace element contents of the parent fluid, which in turn were derived through dissolution of LREE-enriched feldspathic mesostasis. The close similarities in silicate petrography and radiometric ages determined by other workers for these olivine clinopyroxenites suggests that the parent rocks were close to one another on Mars and therefore the same fluid may have been responsible for the precipitation of the evaporite mineral assemblages. Lafayette contains the mineral assemblage and siderite composition which are least soluble in water and Nakhla contains the most soluble minerals and carbonate composition. On the basis of our new data we consider a new model of progressive evaporation from a Na–Mg–Fe–Ca–SO 4–Cl–H 2O–HCO 3 − acidic brine in an area of enclosed drainage (e.g. crater or low-lying flood plain) on Mars. Partial dissolution of near-surface rocks by the acidic brine released Fe, Mg and trace elements from mesostasis and olivine into the fluid. The Lafayette assemblage was formed where >25% volume of the water remained following evaporation, Governador Valadares 20% and Nakhla <10%. This model is consistent with martian ground temperatures at >0°C and a thicker atmosphere (pCO 2=30–100 mbar) at least over brief periods of time ≤1.3 Ga.

Stable Isotope (C, Cl, and H) Fractionation during Vaporization of Trichloroethylene

Stable isotope fractionation during vaporization of trichloroethylene has been measured, with possible application as a technique to investigate subsurface behavior. The equilibrium value of Δ13Cvapor-liquid has been measured between 5 and 35 °C, and Δ13Cvapor-liquid, Δ37Clvapor-liquid, and ΔDvapor-liquid have been measured during progressive evaporation of liquid trichloroethylene at 22 ± 2 °C. Equilibrium values of Δ13Cvapor-liquid show a total range of 0.07−0.82‰, with a trend of decreasing Δ13Cvapor-liquid with increasing temperature, from approximately +0.7‰ at 5−15 °C to approximately +0.1‰ at 35 °C. Progressive evaporation experiments yield values of Δ13Cvapor-liquid = +0.35‰ and +0.24‰, Δ37Clvapor-liquid = −1.64‰, and ΔDvapor-liquid = +8.9‰. The positive values for carbon and hydrogen isotope fractionation, while unexpected, are consistent with available quantitative and qualititative data for trichloroethylene and other contaminant hydrocarbons, but a satisfactory explanation for these observations, particularly in combination with the negative value for chlorine, remains elusive. Vapor−liquid fractionation factors have application to the investigation of the behavior of trichloroethylene at contaminated sites, particularly sites undergoing remediation by techniques such as soil vapor extraction and soil bioventing.

Dedolomitization and other early diagenetic processes in Miocene lacustrine deposits, Ebro Basin (Spain)

Abstract A variety of meteoric diagenetic features reveal the development of a syngenetic karst on lacustrine deposits of the Ebro Basin. Diagenetic processes that operated on lacustrine laminated and stromatolitic carbonates include the following. (1) A first syndepositional stage with processes such as dolomitization, desiccation and related breccia formation and sulphate precipitation, either as lenticular gypsum crystals or nodules. This stage took place under progressive evaporation due to lake level fall, when the previous carbonate deposits became exposed as a supra-littoral fringe surrounding saline mud flats of adjacent sulphate depositional environments. (2) A second early diagenetic stage in which processes such as sulphate dissolution and collapse brecciation, dedolomitization, calcite spar cementation and silicification occurred as a result of meteoric water input that caused a progressive rise in lake level. Light isotopic compositions (δ 13 C and δ 18 O) of diagenetic calcites, versus heavier compositions in primary laminated and stromatolitic limestones, confirm a meteoric influence. The syngenetic karst is best developed at the boundary between two allostratigraphic units and coincided with one of the extensive stages of sulphate deposition at the end of the Early Miocene. The karst facies occurred in an area that was a low-relief barrier that separated two sites of sulphate deposition during low lake levels. This indicates that the karst development was controlled by topographic changes within the basin and record a shift from arid to wetter climatic conditions, as suggested by the overlying freshwater carbonate deposits. The presence of diagenetic features such as those described in the central Ebro Basin affecting saline lacustrine carbonates is relevant because they can be used as indicators of subaerial exposure periods in terrestrial environments and they also reveal important palaeogeographic and palaeoclimatic events of basinal extent.

Sedimentology and diagenesis of Miocene colemanite-ulexite deposits (western Anatolia, Turkey)

ABSTRACT The Neogene borate deposits of Anatolia have thin sedimentary covers and have never been deeply buried. The major rock-forming, calcium-bearing borates are colemanite and ulexite. Under progressive evaporation, borate precipitation occurred in a number of settings, from stable playas to perennial lakes, the latter evolving to shallow lakes and playa lakes. In all these lakes, shallowing-upward cyclicity is common. Colemanite facies occupy the margins, and ulexite facies the centers of the basins; this mineral zonation is depositional and cannot be ascribed to diagenetic processes. Colemanite and ulexite formed mainly as interstitial growths under synsedimentary conditions, and as mineralogically primary, displacive and/or cementing lithofacies, which were affected by progressive compaction. Heliothermal conditions might have favored the precipitation of colemanite in some lakes; in others, subaerial (playa) conditions seem to have been appropriate. Locally, some colemanite is secondary and proceeds from the (macroscopic) replacement of precursor ulexite; this replacement occurred during either early diagenesis or (more probably) late diagenesis. Minor evidence exists for colemanite formation from replacement of precursor inyoite (?).

Chemical and boron isotope compositions of non-marine brines from the Qaidam Basin, Qinghai, China

The general chemistry and boron isotope composition were investigated in fresh waters and hypersaline brines from the Qaidam Basin, northern Qinghai-Xizang (Tibet) Plateau, China. This basin is a large, tectonically active system, isolated from the ocean and composed of thick clastic and evaporite sediments. The modern playas are subject to intense evaporation and are characterized by hypersaline brines as well as potash and borate evaporites. The chemical composition of the dissolved solutes in the modern brines and waters reveals three main sources: (1) inflow of hot springs enriched in sodium, sulphate and boron. Evaporation of these waters leads to a high Na/Cl ratio (>1), a NaClSO 4 brine and an evaporite mineral assemblage of halite-mirabilite-borate (Lakes Daqaidam and Xiaoqaidam); (2) inflow surface river waters which are modified by preferential dissolution of halite and potassium and magnesium salts characterized by a Na(Mg)Cl solute type with low Na/Cl (<1), Br Cl, Li Cl and B Cl ratios; and (3) Ca-chloridic subsurface brines which are controlled by both salt dissolution and dolomitization processes. Evaporation and salt crystallization of mixtures of the latter two types leads to a “marine-like” brine (e.g., NaMgCl type, Na Cl ratio ⪡ 1) and mineral assemblages similar to that predicted for progressive evaporation of seawater (e.g., Qarhan playa: halite-sylvite-carnallite-bischofite). The δ 11 B-values of the input waters to the Qaidam Basin (range of −0.7 to + 10.9%. vs. NBS- 951 and brines from salt lakes (+0.5 to + 15.0%.) are similar to those of associated granitic rocks ( δ 11B= −2.3 to +3.7%.; n=3) and hence indicate the non-marine origin of these fluids. The highest δ 11B-values are associated with fluids with low B Li ratios, indicating selective removal of elemental boron and 10B by adsorption onto clay minerals. The magnitude of 11B enrichment due to adsorption is ∼15–20%., and thus non-marine brines are well distinguished from marine-derived brines ( δ 11B of +39 to +59%.) preserving the large isotopic difference (∼40%.) of their source waters. It is proposed to use this distinctive isotopic signature for tracing the origin of ancient evaporite environments.

Geochemical processes affecting meltwater chemistry and the formation of saline ponds in the Victoria Valley and Bull Pass region, Antarctica

Major ion, trace element and nutrient concentrations have been determined in meltwater streams, frozen lakes and isolated saline ponds of the Victoria Valley and Bull Pass regions in Victoria Land, Antarctica. Geochemical processes affecting glacial meltwater composition with time and distance from the source glaciers include solute acquisition by soil salt leaching and solute concentrations by evaporation. Evaporation in the marginal lake melt and in isolated saline ponds appears to increase the Mg/Ca ratio of these meltwaters relative to that of meltwater streams. With progressive evaporation gypsum and calcite may precipitate, leading to the development of Na-Cl and Na-HCO3 brine ponds. These ponds may be vertically stratified with respect to temperature and salinity if they experience partial or complete freezing over the winter season. The chemical characteristics and nutrient concentrations of meltwaters in the Victoria Valley are similar to those of other drainage systems in the region, although the Ca-Cl brines reported from the Wright Valley immediately to the south were not observed. Trace element (Cu, Pb, Zn, Cd, Mn and Fe) concentrations measured in the lakes and large ponds do not show any evidence of unusual enrichment in the drainage.

New constraints on Precambrian ocean composition.

The Precambrian record of carbonate and evaporite sedimentation is equivocal. In contrast to most previous interpretations, it is possible that Archean, Paleoproterozoic, and to a lesser extent, Meso to Neoproterozoic seawater favored surplus abiotic carbonate precipitation, as aragonite and (hi-Mg?) calcite, in comparison to younger times. Furthermore, gypsum/anhydrite may have been only rarely precipitated prior to halite precipitation during evaporation prior to about 1.8 Ga. Two effects may have contributed to these relationships. First, sulfate concentration of seawater may have been critically low prior to about 1.9 Ga so the product mCa++ x mSO4-- would not have produced gypsum before halite, as in the Mesoproterozoic to modern ocean. Second, the bicarbonate to calcium ratio was sufficiently high so that during progressive evaporation of seawater, calcium would have been exhausted before the gypsum field was reached. The pH of the Archean and Paleoproterozoic ocean need not have been significantly different from the modern value of 8.1, even at CO2 partial pressures of a tenth of an atmosphere. Higher CO2 partial pressures require somewhat lower pH values.

Characterisation of Ultrafine Microstructures Using a Position-Sensitive Atom Probe (POSAP)

ABSTRACTA new development in the experimental techniques of atom probe microanalysis is described, which involves the use of a position sensitive detector system. This detector subtends a large solid angle (∼20°) at the specimen, and therefore permits the collection of ions from a substantial fraction of the whole surface area of the emitter. Progressive pulsed field evaporation leads to the construction of a three-dimensional map of the atomic chemistry of the specimen. The new instrument is ideally suited to the investigation of complex, ultrafine microstructures. Applications to the study of age-hardened aluminium alloys and Alnico permanent magnet materials are described.

THE TUNGUSKA EVENT AND THE METEORS WITH TERMINAL FLARES

The comparison of the Tunguska body explosion with the effect of terminal flares of meteors and fireballs leads us to the conclusion that these events are of a similar nature but differ only by their scale. We consider that the dynamics of progressive breaking and evaporation of meteoric bodies during their entry into the terrestrial atmosphere could explain the terminal burst. An extremely porous body model for the Tunguska meteorite was analysed and rejected as unsatisfactory. The realistic values of the initial velocity (∼30 km/sec) and of the inclination angle for the Tunguska's trajectory (5–15°) give orbital elements not in contradiction with the cometary origin of the Tunguska body.