Secondary Alteration Products Research Articles

Determination of volatiles in volcanic rocks and minerals with a Directly Coupled Evolved Gas Analyzing System (DEGAS) -Part I: Interpretation of degassing profiles (DEGAS-profiles) of minerals and rocks on the basis of melting experiments

Volatile components in magma strongly influence many physical properties of melts and minerals. The temperature resolved degassing analysis of volcanic crystalline and vitreous rocks gives detailed information about volatile compounds in the melt. Aspecial high-temperature mass-spectrometry device in combination with a thermo-balance allows a quantitative determination of different volatile species. It enables a differentiation between the primary gas content in the magma and the gas released from decomposition of secondary alteration products. The gas release profiles give the following indications: i) during the littoral explosions of Pahoehoe lava the content of volatiles is not changed by interaction with air or sea water; ii) the degassing profiles of vitreous black sand verify the primary content of volatiles in the erupted melt, only CO2 was detected; iii) the oxygen release profile gives significant indications for oxygen undersaturation of the erupted magma; iv) remelting of black sand in air at 1450°C for 0.45 h causes an oxygen saturation of the basaltic melt; v) remelting of black sand in argon atmosphere confirms the oxygen undersaturation of the melt; vi) remelting of black sand-black shale mixtures affects a significant change in the degassing profiles, especially in CO2-release. With the first investigations we can demonstrate that gas release curves of volcanic rocks are qualified for a) detection of the primary gas content of erupted magma; b) detection of alteration processes of the igneous glass; c) detection of contamination of the magma with adjacent rocks.

Low-Temperature Hydrothermal Alteration of Trachybasalt at Conical Seamount, Papua New Guinea: Formation of Smectite and Metastable Precursor Phases

AbstractThe conversion of volcanic glass to secondary alteration products is one of the most common mineralogical transformations during low-temperature hydrothermal alteration of submarine basalts. To better understand the mechanism and kinetics of this transformation, porphyritic and formerly glassy trachybasalt, recovered from Conical Seamount, Papua New Guinea, was studied in detail. Low-temperature interaction of trachybasalt with hydrothermal fluids at this submerged volcano occurred in response to the formation of submarine epithermal-style gold mineralization. Alteration of the coherent volcanic rocks is heterogeneous with pronounced differences in alteration intensity occurring between igneous minerals and the surrounding glassy groundmass. In comparison to the volcanic glass, the crystalline phases were less prone to hydrothermal alteration with the alteration susceptibility decreasing from clinopyroxene through biotite to feldspar. Low-temperature alteration of clinopyroxene resulted in the formation of abundant saponite-like smectite with no topotactic relationship being observed between the two phases. In contrast, the conversion of biotite to smectite involved structural inheritance as the orientation of common structural blocks was maintained during alteration. Transmission and analytical electron microscopy revealed that pervasive alteration of interstitial glass in the groundmass of the trachybasalt resulted in the formation of montmorillonite- and saponite-like smectite whereby smectite composition is strongly influenced by the glass chemistry. The occurrence of poorly crystalline domains with a 0.3 to 0.4 nm layer spacing in the altered interstitial glass suggests that the transformation of glass to smectite involved the formation of a transitional alteration product. Comparison with the results of previous studies highlights the fact that the glass-to-smectite transformation can proceed through more than one reaction pathway. Reaction style and reaction progress are controlled by kinetic factors such as the mode of fluid transport triggering alteration in the low-temperature hydrothermal environment. Alteration of the trachybasalt at Conical Seamount is inferred to have taken place at a comparably low fluid-rock ratio as the low permeability and the absence of primary fractures and joints restricted fluid circulation through the coherent volcanic rocks.

Phosphorus-bearing sulfides and their associations in CM chondrites

Phosphorus-bearing Fe and Ni sulfides represent a new type of phosphorus compounds and are characteristic accessory phases of CM chondrites. The proportions of atoms in the sulfides can be approximated by the equation (Fe + Ni)/P = 0.965 ± 0.003 (1σ) · S/P + 1.255 ± 0.036 (1σ). Sulfides with high S/P ratios are systematically richer in Fe and poorer in Ni compared with low-S/P sulfides. Their characteristic minor elements are Cr, Ca, Co, K, and Na. The contents of Cr and Ca may reach several weight percent, but their incorporation does not affect the relation between (Fe + Ni)/P and S/P. This is also true of light elements (O and H), which probably occur in the P-bearing sulfides in certain amounts. The sulfides are usually associated with schreibersite, barringerite, eskolaite, and daubreelite. A negative correlation was observed between the Fe/Ni ratios of coexisting P-bearing sulfides and phosphides. Metallic iron was never found in association with the sulfides. It can be suggested that P-bearing sulfide is a primary phase rather than a secondary alteration product formed under the conditions of the CM chondrite parent body. This phase had to be stable in the solar nebula after the formation of Ca-Al inclusions and before the condensation of Fe-Ni metal. At high temperatures, P-bearing sulfide with low Fe/Ni and S/P ratios coexists with schreibersite in the solar gas. During condensation schreibersite is replaced by barringerite, which is accompanied by a decrease in the Fe/Ni ratio of phosphides and an increase in the S/P and Fe/Ni ratios of P-bearing sulfides. Trace element data suggest that the P-bearing sulfides could be formed in the solar nebula by the sulfidization of a precursor phase of extrasolar origin.

Mineralogy and environmental stability of slags from the Tsumeb smelter, Namibia

Three types of smelting slags originating from historically different smelting technologies in the Tsumeb area (Namibia) were studied: (i) slags from processing of carbonate/oxide ore in a Cu–Pb smelter (1907–1948), (ii) slags from Cu and Pb smelting of sulphide ores (1963–1970) and (iii) granulated Cu smelting slags (1980–2000). Bulk chemical analyses of slags were combined with detailed mineralogical investigation using X-ray diffraction analysis (XRD), scanning electron microscopy (SEM/EDS) and electron microprobe (EPMA). The slags are significantly enriched in metals and metalloids: Pb (0.97–18.4 wt.%), Cu (0.49–12.2 wt.%), Zn (2.82–12.09 wt.%), Cd (12–6940 mg/kg), As (930–75,870 mg/kg) and Sb (67–2175 mg/kg). Slags from the oldest technology are composed of primary Ca- and Pb-bearing feldspars, spinels, complex Cu–Fe and Cu–Cr oxides, delafossite–mcconnellite phases and Ca–Pb arsenates. The presence of arsenates indicates that these slags underwent long-term alteration. More recent slags are composed of high-temperature phases: Ca–Fe alumosilicates (olivine, melilite), Pb- and Zn-rich glass, spinel oxides and small sulphide/metallic inclusions embedded in glass. XRD and SEM/EDS were used to study secondary alteration products developed on the surface of slags exposed for decades to weathering on the dumps. Highly soluble complex Cu–Pb–(Ca) arsenates (bayldonite, lammerite, olivenite, lavendulan) associated with litharge and hydrocerussite were detected. To determine the mineralogical and geochemical parameters governing the release of inorganic contaminants from slags, two standardized short-term batch leaching tests (European norm EN 12457 and USEPA TCLP), coupled with speciation-solubility modelling using PHREEQC-2 were performed. Arsenic in the leachate exceeded the EU regulatory limit for hazardous waste materials (2.5 mg/L). The toxicity limits defined by USEPA for the TCLP test were exceeded for Cd, Pb and As. The PHREEQC-2 calculation predicted that complex arsenates are the most important solubility controls for metals and metalloids. Furthermore, these phases can readily dissolve during the rainy season (October to March) and flush significant amounts of As, Pb and Cu into the environment in the vicinity of slag dumps.

Brochantite-2M2from Pierre Plate Mine, Vizille

Evidence for the recently described 2M2polytype of brochantite from X-ray powder diffraction investigation of secondary alteration products of ore material from the Pierre Plate Mine, Vizille, Isère, France is presented. This report is the first to describe the 2M2polytype from locations outside of Italy and through the use of powder XRD methods. In the procedure used herein, developed in a study by Merlinoet al.[Eur. J. Mineral15, 267–275 (2003)], we have used family reflections, common to both main types of brochantite, as the source of approximate cell parameters from which we obtain positions of characteristic reflections to demonstrate the unique choice of polytype, before final refinement stage. This method demonstrates that the determination of polytype is possible from powder data, for samples typical of both geological and urban environments. Least-squares refined cell parameters for the 2M2polytype from Pierre Plate area=12.7409(8) Å,b=9.8371(6) Å,c=6.0109(3) Å, anda=90.135(9)°, constrained in space groupP21/n11.

Ceramic Waste Forms for Actinides

The concept of nuclear waste forms based on minerals that contain actinides has led to the development of polyphase and special-purpose crystalline ceramics. These ceramics are considered by many to be attractive media for the long-term storage of actinides in geological repositories. The available data show that monazite, pyrochlore, zircon, and zirconolite are all highly durable in both natural and synthetic aqueous systems at low temperatures. In comparison, perovskite is prone to dissolution and conversion to anatase and other secondary alteration products. The titanate and silicate phases of interest become metamict (amorphous) as a result of irradiation. Several compounds, including monazite, cubic zirconia, and the defect fluorite structure types with Zr on the B site, exhibit the attractive property of radiation resistance. These results, together with other materials properties, are discussed briefly with respect to criteria for waste form performance.

Cl-amphibole in the nakhlite MIL 03346: Evidence for sediment contamination in a Martian meteorite

The Pathfinder, Spirit and Opportunity missions revealed that Martian soils and rocks are enriched in chlorine (up to 0.6 wt.% Cl). Significant piles of evaporitic sedimentary rocks have also been observed at Meridiani Planum. The SNC meteorites, inferred to come from Mars, hardly ever contain Cl-rich minerals except in secondary alteration products such as clay-rich veinlets and massive halite trapped within olivine from Nakhla. Here we describe the first extra-terrestrial Cl-rich amphibole, in the newly found nakhlite MIL 03346. It is a chloro-potassichastingsite (up to 8% Cl wt.%) that crystallized exclusively as a daughter mineral in some of the melt patches sequestered inside augite phenocryst clusters. It is never found as fracture-filling material nor as discrete grains in the mesostasis. This Cl-rich amphibole crystallized from the evolved residual liquid at some intermediate magmatic stage. This heterogeneous contamination at a magmatic stage provides the first evidence for contamination of a nakhlite parent magma by Cl-rich particles of evaporitic sediments. Such a contamination appears to be very limited to melt patches present in a small number of augite crystals. Because of dilution and devolatilization effects, this contamination remains cryptic in the mesostasis.

Raman spectroscopy of Japanese artists' materials: The Tale of Genji by Tosa Mitsunobu

AbstractMicro‐samples from the earliest complete copy of the Japanese masterpiece The Tale of Genji were analyzed by Raman and FTIR spectroscopy to determine the materials used. The album was commissioned in 1509 and consists of 54 illustrations by Tosa Mitsunobu with matching calligraphic pages painted by six different court calligraphers. The pages were originally pasted onto a folding screen but were remounted in an album in the 17th century and a frontispiece and finispiece illustration painted by Tosa Mitsuoki. The illustration for chapter 52 is a replacement of unknown date.The following materials were identified in the illustrations: azurite, indigo, malachite, atacamite, botallackite, chrysocolla, gamboge, vermilion, lead(II,IV) oxide (red lead), red ochre/haematite, calcite and basic lead carbonate (lead white). A smaller range was used on the calligraphic pages: vermilion, lead(II,IV) oxide (red lead), indigo, gamboge, yellow ochre and calcite.Most of the materials identified are part of the traditional Japanese palette, and a wide range of tones was created by mixing pigments from this relatively restricted set. The hydrated copper chlorides, atacamite and botallackite, may be alteration products of original malachite. The chrysocolla, a natural amorphous copper silicate, was probably incorporated accidentally with the malachite, as the two can occur together in nature. Chrysocolla occurs with malachite only on the replacement illustration for chapter 52, which lacks copper chlorides.The lead carbonate and red lead are both discolored in many areas. Lead sulfate was identified in some altered areas of red lead and probably represents a secondary alteration product. The primary alteration product of both lead pigments is assumed to be lead(II) sulfide, as silver in the manuscript has altered to silver sulfide. The alternative alteration product, lead(IV) oxide, is considered less likely. Copyright © 2006 John Wiley & Sons, Ltd.

Archaeological glass alteration products in marine and land-based environments: morphological, chemical and microtextural characterization

A study was performed on the natural degradation of glass from Italian archaeological sites, both marine and land-based, with the aim of understanding the role that the chemico-physical conditions of the environment had on the morphology and composition of the alteration products. Marine samples shows two different textural morphologies of alteration: multiple layers of iridescent lamellae in colored glass, or an opaque white crust on the surface of colorless glass, characterized by a spongy, distorted, ramified structure extending more and more deeply into the bulk of the glass. Buried glass presents only iridescent lamellae as alteration products. These are mainly composed of a hydrated ‘silica gel’, differing essentially in the extension of alteration lamellae in both submerged and buried glass. The principal events in the process may be summarized as ion exchange reaction, which causes selective leaching of alkalis and protons entering the glass to produce a hydrated, alkali-deficient layer, and congruent dissolution of the glass matrix, with destruction of the silica network and subsequent precipitation of hydrous silica-gel layers as secondary alteration products.

Seafloor bioalteration of sulfide minerals: results from in situ incubation studies

We present results of incubation studies conducted at low temperatures (∼4°C) in the vicinity of a seafloor hydrothermal vent system. We reacted Fe-, S-, Cu-, and Zn-bearing minerals including pyrite, marcasite, chalcopyrite, sphalerite, elemental sulfur, and a portion of a natural chimney sulfide structure for 2 months at the Main Endeavour Segment of the Juan de Fuca Ridge in the Pacific Ocean. Our study utilizes Fluorescent In Situ Hybridizations (FISH), Scanning and Transmission Electron Microscopy (SEM, TEM), and light microscopic analysis. The surfaces of these minerals are solely colonized by Bacteria and not by Archaea. Colonization densities vary over an order of magnitude with the following sequence: elemental sulfur > chimney sulfide > marcasite > pyrite > sphalerite > chalcopyrite, and correspond well with the abiotic oxidation kinetics of these materials, excepting elemental sulfur, which is both the least reactive to oxidizing species and the most heavily colonized. Colonization densities also correspond with apparent degree of reaction (dissolution pitting + accumulation of secondary alteration products). Heavy accumulations of secondary Fe oxides on Fe-bearing minerals, most notably on the chimney sulfide, form in situ as the result of mineral dissolution and the activity of neutrophilic Fe-oxidizing bacteria. Results suggest that mineral-oxidizing bacteria play a prominent role in weathering of seafloor sulfide deposits, and that microbial utilization of mineral substrates contributes to biomass production in seafloor hydrothermal environments.

Sorption behaviour of Am on precorroded HLW glass in water and brines

Summary In the alteration layer of HLW glasses corroded in aqueous solutions, less soluble elements like REE, actinides and other elements are retained by sorption or by formation of secondary alteration products controlling the solution concentrations. In order to quantify the sorption effect on the retention of metal ions, first sorption tests with U, Th and Eu have been performed using precorroded, simulated HLW glass as substrate. To demonstrate that the sorption behaviour of Eu is also representative for homologue trivalent actinides additional sorption tests were performed with Am241 in deionized water, NaCl and MgCl2 brine. Distribution ratios R s (R s=Amsorb/ Amsol× V/m) for Am were found independent of the starting concentration in the pH range 2 to 6. At higher pH, a high amount of Am-containing colloidal particles were detected. The R s values of Am show a strong increase with pH, quite similar to the R s values of Eu. In general, the sorption behaviour of Eu and Am is very similar in deionized water and in brines. Clay minerals formed in the gel layer as secondary phases during long-term glass corrosion are assumed to act primilary as strong sorbents. Therefore, the sorption of REE and Am onto synthetic poor crystalline smectite precursors and a "well crystallized" natural smectite is compared with the findings obtained from precorroded glass. The sorption of Am on the precorroded glass is found to be much stronger than the retention on the investigated pure clay phases, indicating a different number of active sites or occurance of a different sorption mechanism.

Constraints on the petrogenesis of Martian meteorites from the Rb-Sr and Sm-Nd isotopic systematics of the lherzolitic shergottites ALH77005 and LEW88516

Detailed Rb-Sr and Sm-Nd isotopic analyses have been completed on the lherzolitic shergottites ALH77005 and LEW88516. ALH77005 yields a Rb-Sr age of 185 ± 11 Ma and a Sm-Nd age of 173 ± 6 Ma, whereas the Rb-Sr and Sm-Nd ages of LEW88516 are 183 ± 10 and 166 ± 16 Ma, respectively. The initial Sr isotopic composition of ALH77005 is 0.71026 ± 4, and the initial εNd value is +11.1 ± 0.2. These values are distinct from those of LEW88516, which has an initial Sr isotopic composition of 0.71052 ± 4 and an initial εNd value of +8.2 ± 0.6. Several of the mineral and whole rock leachates lie off the Rb-Sr and Sm-Nd isochrons, indicating that the isotopic systematics of the meteorites have been disturbed. The Sm-Nd isotopic compositions of the leachates appear to be mixtures of primary igneous phosphates and an alteration component with a low 143Nd/144Nd ratio that was probably added to the meteorites on Mars. Tie lines between leachate-residue pairs from LEW88516 mineral fractions and whole rocks have nearly identical slopes that correspond to Rb-Sr ages of 90 ± 1 Ma. This age may record a major shock event that fractionated Rb/Sr from lattice sites located on mineral grain boundaries. On the other hand, the leachates could contain secondary alteration products, and the parallel slopes of the tie lines could be coincidental.Nearly identical mineral modes, compositions, and ages suggest that these meteorites are very closely related. Nevertheless, their initial Sr and Nd isotopic compositions differ outside analytical uncertainty, requiring derivation from unique sources. Assimilation-fractional-crystallization models indicate that these two lherzolitic meteorites can only be related to a common parental magma, if the assimilant has a Sr/Nd ratio near 1 and a radiogenic Sr isotopic composition. Further constraints placed on the evolved component by the geochemical and isotopic systematics of the shergottite meteorite suite suggest that it (a) formed at ∼4.5 Ga, (b) has a high La/Yb ratio, (c) is an oxidant, and (d) is basaltic in composition or is strongly enriched in incompatible elements. The composition and isotopic systematics of the evolved component are unlike any evolved lunar or terrestrial igneous rocks. Its unusual geochemical and isotopic characteristics could reflect hydrous alteration of an evolved Martian crustal component or hydrous metasomatism within the Martian mantle.

Iron-nickel sulfides in anhydrous interplanetary dust particles

Fe,Ni-sulfide grains in nine anhydrous chondritic porous (CP) interplanetary dust particles (IDPs) and one hydrated chondritic smooth (CS) IDP were examined using 200 and 400 keV transmission electron microscopy.Crystal structures of the grains were investigated using selected area electron diffraction, electron microdiffraction, and high-resolution lattice fringe imaging. Grain compositions were measured using quantitative energy-dispersive x-ray spectroscopy. Three types of sulfide grains were examined: 10 to 100 nm diameter nanocrystals within and on the surfaces of GEMS (glass with embedded metal and sulfides), small 100 to 500 nm diameter isolated grains, and large 0.5 to 5 μm diameter grains dispersed throughout the fine-grained matrices of IDPs.Most sulfide nanocrystals within and on the surfaces of GEMS are low-Ni pyrrhotite (0 ≤ Ni ≤ 5.5 atomic %) with a hexagonal unit cell where ao = 0.34 nm and co = 0.57 nm. Small 100 to 500 nm mafic grains dispersed throughout the matrices of the IDPs include hexagonal pyrrhotite (0 ≤ Ni ≥ 20 atomic %), ordered hexagonal pyrrhotite exhibiting prominent superlattice reflections, and a cubic sulfide that appears to have a sulfur-deficient “spinel-like” (Fd3m) structure. A large (∼2 × 5 μm) grain within one anhydrous IDP is a polycrystalline mixture of hexagonal pyrrhotite and cubic sulfide. Electron diffraction and lattice fringe imaging show that hexagonal and cubic sulfide are coherently intergrown on a unit cell scale.When heated in the electron beam the cubic sulfide transforms into hexagonal pyrrhotite. Therefore, it is possible that most of the pyrrhotite, the dominant sulfide in anhydrous chondritic IDPs, is a secondary thermal alteration product of frictional heating and partial sulfur loss during atmospheric entry. Neither troilite (FeS) nor pentlandite was identified in any of the nine anhydrous IDPs. Pentlandite was identified in the single hydrated IDP, in accordance with a previous study of sulfides in chondritic IDPs.

Remagnetization of igneous rocks in Gupis area of Kohistan arc, northern Pakistan

The Kohistan arc was formed due to subduction of neo-Tethyan oceanic crust beneath Asia. The arc is comprised of volcanic, plutonic and sedimentary rocks of Mesozoic to Tertiary age, formed prior and after the suturing of the Indian and Asian continents. Paleomagnetic investigations have been carried out on Paleocene volcanic and plutonic rocks exposed in the northern part of the arc. A total of 110 samples from 16 sites were drilled. According to rock-magnetic studies the main magnetic carrier is magnetite. Optical microscopy study reveals that low-grade metamorphism have effected all rocks. Magnetite is found as both a primary magmatic mineral and secondary alteration product in all samples. Samples of volcanics yield post tilting characteristic remanent magnetizations (ChRM). The in situ mean direction of the ChRMs of the intrusives is similar to the in situ mean direction of volcanics. The presence of secondary magnetite in plutons, the similarity of in situ mean ChRM of plutons with that of post-tilting ChRMs of similar age volcanics and dissimilarity of the mean ChRM of plutons from expected directions at the time of formation of plutons support a secondary origin for the ChRM of plutons. As the ChRM directions of the volcanics and intrusives are the same, it implies that both ChRMs were acquired during the same remagnetization event. Comparing the mean paleolatitude (25 ± 6°N) from Gupis area with those from Indian APWP and considering the fact that there was prevailing heating event in Lower Tertiary in the area, the acquisition age of this secondary remanent magnetization can be bracketed between 50 and 35 Ma.

Alpha-decay damage and aqueous durability of actinide host phases in natural systems

This paper provides an overview of the geochemical alteration and alpha-decay damage effects observed in the titanate mineral phases brannerite, perovskite, pyrochlore, zirconolite, and the silicate mineral zircon. The available data show that brannerite, pyrochlore, zircon, and zirconolite are all highly durable in natural systems, often surviving the complete destruction of the host rock during weathering. In comparison, perovskite is prone to dissolution and conversion to anatase and other secondary alteration products at moderate to low temperatures. The mineralogical studies provide quantitative data on the crystalline–amorphous transformation and the structure of the metamict state. These studies indicate that the titanate and silicate phases of interest become metamict (amorphous) as a result of the gradual accumulation of alpha-recoil collision cascades. When compared to the data for synthetic pyrochlore and zirconolite doped with 238Pu and 244Cm, dose-age data indicate that the critical dose values for geologically young samples of natural pyrochlore and zirconolite are higher by a factor of approximately 2–3. The available geological data indicate that most of the natural samples occur in rock types that cooled very rapidly to 300°C or lower, followed by slow cooling over geological time periods at lower temperatures without subsequent reheating (metamorphism). Average storage temperatures are on the order of 100–200°C, indicating that the higher critical dose values of the natural samples are due to thermal annealing over geological time periods. The dose-age data also provide evidence for the long-term annealing of isolated alpha-recoil collision cascades in perovskite, pyrochlore, zirconolite, and zircon. Rate constants obtained from the current data sets suggest similar long-term annealing rates of 1–2×10 −9 yr −1 for all four minerals. Additional recovery of damage from long-term annealing at ambient conditions will not be significant for time periods up to approximately 10 7 yr.

40Ar/39Ar dating of Ordovician K-bentonites in Laurentia and Baltoscandia

Several Ordovician K-bentonites occurring widely in eastern North America and western Europe were dated using the 40 Ar/ 39 Ar technique to test previously suggested inter-continental correlations. The three thickest and most widespread bentonites – Deicke, Millbrig (North America) and Kinnekulle (Sweden and Denmark) – were examined. Single-grain analyses of phenocrystic biotites yield 100% concordant plateau ages of 449.8±2.3 Ma (2σ internal error) (Deicke), 448.0±2.0 Ma (Millbrig) and 454.8±2.0 Ma (Kinnekulle in Sweden). The altered biotites from the Denmark Kinnekulle sample show discordant age spectra with highly variable plateau ages being most likely caused by recoil redistribution of Ar isotopes between biotite and interlayer secondary alteration products during neutron irradiation . Such recoil artifacts are likely evident in previously published 40 Ar/ 39 Ar data for these units, causing bias towards spuriously old ages. In such cases where intragrain alteration is evident and discordant age spectra are common, geologically meaningless plateau ages may be obtained which are less accurate than integrated ages, hence a 100% concordance criterion for acceptance is useful. The inconsistent 40 Ar/ 39 Ar ages and geochemical contrasts between the North American and Swedish bentonites imply distinct volcanic episodes, and preclude a specific correlation between the Millbrig and Kinnekulle K-bentonites. The precise 40 Ar/ 39 Ar ages indicate a minimum duration of 7±3 Myr for the Diplograptus multidens graptolite biozone. In comparison with previous U/Pb data, the results are useful for quantifying the bias (ca. 1%) between current calibrations of the 40 Ar/ 39 Ar and U/Pb geochronometric systems. This bias is most likely due to errors related to 40 K decay constants and neutron fluence monitors used in 40 Ar/ 39 Ar dating.

The Transformation of Uranyl Oxide Hydrates: The Effect of Dehydration on Synthetic Metaschoepite and Its Alteration to Becquerelite

The U(VI) solid phases schoepite, metaschoepite, and dehydrated schoepite are important reservoirs of mobile uranium in the environment. These simple uranyl oxide hydrates result from weathering of uranium minerals and the corrosion of anthropogenic uranium solids. We have studied the role of hydrational water among these phases and in subsequent tranformation to other secondary metal-U(VI) oxide hydrates. Synthetic metaschoepite (MS, UO3·2.0H2O), its dehydrated phases, and its secondary alteration products were characterized using X-ray powder diffraction (XRD), scanning electron microscopy (SEM), X-ray energy dispersive spectroscopy (EDS), and high-resolution thermogravimetric analysis (HRTGA). Drying MS at 105 °C resulted in the formation of a dehydrated phase (UO3·0.9H2O) that was structurally distinct from natural dehydrated schoepite (DS, UO3·0.75H2O) reported by others. Unlike natural DS, our dehydrated material was easily rehydrated, although crystallinity of the rehydrated phase was reduced. The ...

Long-Term Stability of Spent Nuclear Fuel Waste Packages in Gorleben Salt Repository Environments

The results are summarized of 15 yr of German research on spent fuel with respect to its suitability as a waste form disposed of in a repository located in the Gorleben salt dome. Within the multibarrier system for long-term isolation of high-level waste (HLW), the innermost engineered barrier “canistered spent fuel” contributes essentially to isolating radionuclides from the biosphere if a salt brine were to come into contact with the waste form. A large fraction of the radionuclide contents of the reacted fuel mass would become reimmobilized within secondary alteration products and on container corrosion products, but inevitably a certain nuclide-specific fraction would be released into the aqueous geochemical environment. The corrosion resistance of the fuel and the radionuclide mobility are not inherent materials properties but also depend on geological disposal conditions, packing concepts, and radioactive decay. In particular, the availability of oxidants is critical, controlling spent-fuel alteration rates and alteration products as well as radionuclide solubilities. Spent fuel is at least as suitable for final disposal as is HLW glass.

Geology and ore genesis of the manganese ore deposits of the Postmasburg manganese-field, South Africa

The Postmasburg Mn/Fe-ores occur exclusively in dolomitic Precambrian sinkhole structures with siliceous breccias and shales as hostrocks. The main manganese minerals are braunite and bixbyite, apart from secondary alteration products of the psilomelane-manganomelane family. Various generations of ore minerals could be identified. The ore mineralization is subdivided into three different genetic types. They are classified either as pure karst deposits or as combined formations of karst origin and shallow marine sedimentation due to the transgression of the Banded Iron Formation (BIF) sea. Post-sedimentary metamorphism is identified as very low grade. The development of the different ore types is illustrated schematically.

Ion microprobe studies of Efremovka CAIs: I. Magnesium isotope composition

Five coarse-grained Ca-Al-rich refractory inclusions (CAIs) from the Efremovka CV3 chondrite are analysed for their magnesium isotopic composition using an ion microprobe. The analysed inclusions represent the three common CAI types (A, B1, and B2). Data for magnesium isotopic composition of the major mineral phases (melilite, spinel, fassaite, and anorthite) in these inclusions are used to determine intrinsic magnesium isotopic fractionations and to identify nonlinear effect in 26Mg due to the decay of extinct 26Al. The magnesium isotopic mass fractionation, F( Mg), generally favours the heavier isotopes, as is expected in the case of coarse-grained CAIs. The F( Mg) data for two of the inclusions, however, show contrasting trends. Sympathetic behaviour found in both isotopic and petrographic data in one type B1 inclusion suggests that the parent melt of this inclusion experienced a short-duration volatilization event prior to crystallization. The isotopic data also suggest possible presence of relict spinel in this inclusion that constrains the cooling rate of the parent melt to >70°C/h during crystallization. Hot and dense nebular settings that allow for such fast cooling can only be localized in nature. The pristine nature of the Efremovka CAIs, as evident from the lack of secondary alteration products in them, is well supported by the isotopic data. Three of the five inclusions yield well-behaved Mg-Al isochrons with ( 26Al 27Al ) 0 close to the canonical value of 5 × 10 −5. The anorthite data for another inclusion suggest minor disturbance in the isotopic systematics and yield a nearly well-behaved isochron with a slightly lower value of ( 26Al 27Al ) 0. Data for melilite and anorthite in the fifth inclusion define apparent isochrons that suggest partial re-equilibration and exchange of magnesium isotopes between these two phases. The Efremovka data are suggestive of a relatively homogeneous distribution of 26Al in the solar nebula.