Apophyllite (001) surface alteration in aqueous solutions studied by HAFM

Depending on pH and temperature, two different types of surface reactions occur on the apophyllite (001) surface in aqueous HCl-solutions at temperatures from 20 to 130 °C. At low pH, laterally spreading hillocks cover the surface. The hillocks are softer than the pristine surface, chemical analysis shows a depletion in Ca + K, and the spreading velocity of hillocks depends on pH. This indicates a change in chemical bond strength, non-stoichiometric dissolution and a mechanism involving protons. External disturbances such as the AFM scanning tip cause the upper surface layers to peel off revealing that the active sites of hillock formation are between the silicate layers of apophyllite. The observed process can therefore be described by a penetrative ion-replacement reaction which proceeds well below the surface monolayer. By this ion-replacement, the silicate layers eventually become destabilised. The observed reaction, therefore, is equivalent to an incongruent dissolution process. Despite structural similarities, this process is only superficially similar to the ion-exchange occurring in clay minerals or zeolites. In these minerals, the structural backbone is not destabilized. At a more neutral pH and high temperatures, step retreat and etch pit formation can be observed on the apophyllite (001) surface thus indicating a more congruent dissolution mechanism.     

History and origin of aubrites

The cosmic ray exposure (CRE) ages of aubrites are among the longest of stone meteorites. New aubrites have been recovered in Antarctica, and these meteorites permit a substantial extension of the database on CRE ages, compositional characteristics, and regolith histories. We report He, Ne, and Ar isotopic abundances of nine aubrites and discuss the compositional data, the CRE ages, and regolith histories of this class of achondrites. A Ne three-isotope correlation reveals a solar-type ratio of ²⁰Ne/²²Ne = 12.1, which is distinct from the present solar wind composition and lower than most ratios observed on the lunar surface. For some aubrites, the cosmic ray-produced noble gas abundances include components produced on the surface of the parent object. The Kr isotopic systematics reveal significant neutron-capture-produced excesses in four aubrites, which is consistent with Sm and Gd isotopic anomalies previously documented in some aubrites. The nominal CRE ages confirm a non-uniform distribution of exposure times, but the evidence for a CRE age cluster appears doubtful. Six meteorites are regolith breccias with solar-type noble gases, and the observed neutron effects indicate a regolith history. ALH aubrites, which were recovered from the same location and are considered to represent a multiple fall, yield differing nominal CRE ages and, if paired, document distinct precompaction histories.     

Die Grödener Schichten (Perm,Südalpen). Sedimentologische und geochemische Untersuchungen zur Unterscheidung mariner und kontinentaler Sedimente

Within the sedimentation area of the Gröden formation two facies units have been observed: The attributes of one facies correspond with a deposition in continental environment. Kaolinitic weathering and fresh-water algae support this interpretation. The other facies is deposed in marine environment (supply of Mg for dolomitization and formation of chlorites, scarce forams). The transition facies shows characteristics of a costal region with marginal lagoons. Paleontological data (tetrapod traces, drifted cephalopods and forams) are in accordance with this model. The paleogeographical development during the Middle Permian is as follows: Continental sedimentation at the basis, marine transgression in the Carnic Alps and the western Karawanken during lower Gröden stage, progressiv transgression during the middle Gröden stage to the west with a coast line near Gröden valley and a coast line near River Etsch at the end of the Middle Permian.     

Zur Stabilität des Ferrocordierits

Ferrocordierite with Fe⁺²/Fe⁺²+Mg=0,92 was synthesized as a single phase at 600° C and 1000 bars total pressure using cristobalite and natural monoclinic chloritoid as starting materials. Oxygen fugacities during synthesis were those given by the buffering power of the hydrothermal bombs. Pure ferrocordierite without Mg synthesized from various artificial starting materials was never obtained as a single phase, but coexisted with variable amounts of metastable hercynite+quartz. The ferrocordierites made in this study are orthorhombic with distortion indices Δ up to 0,26° as a function of pressure, temperature, and duration of the runs. The lattice constantsa ₀ = 17,234 and b₀ = 9,824 are considerably larger, c₀ = 9,298 smaller than those of pure Mg-cordierite; mean indices of refraction vary within the range 1,569–1,573±0,002. In successful breakdown experiments natural Fe-rich cordierite intergrown with quartz from Mujinazawa, Japan, as well as the most Fe-rich natural cordierite so far discovered, from Dolni Bory, Moravia, were used. Ferrocordierite is stable only above relatively high temperatures (450°–600° C depending on pressure), which are within the limits of determination identical to the lower stability limits of Mg-cordierite. In contrast to the latter phase, however, ferrocordierite becomes unstable at a yet undefined pressure between 6000 and 10000 bars. The stable breakdown products at low temperatures are chloritoid+quartz or - possibly only at very low pressures - assemblages with 7 Å-chamosite, which have been synthesized (metastably ?) over a more extended PT-range. Runs seeded with chloritoid + cristobalite did not yield critical evidence as to the more stable ,assemblage. Except for occasional small amounts of pyrophyllite there is considerable uncertainty concerning the aluminous phase coexisting with chamosite; kaolinite with all critical peaks covered by chamosite orγ-Al₂0₃ amorphous to X-rays may be present. At high temperatures and at a pressure of 10000 bars ferrocordierite broke down to assemblages containing an orthoamphibole, probably ferrogedrite. It is uncertain, however, whether these assemblages represent stable equilibrium, or whether they are metastable substitutes for other parageneses, e.g. almandite silimanite + quartz. Given the requisite bulk compositions and geothermal gradients ferrocordierite remains stable under static conditions to crustal depths of at least 20 km. ThusChinner's mechanism of differential breakdown of Fe-bearing cordierites with increasing pressures may operate in deep zones of regional metamorphism within orogenic belts, but appears unlikely in shallow posttectonic contact metamorphism, unless this metamorphism is caused by hot basic magmas producing temperatures above the incongruent melting of ferrocordierite. In contrast to the relations found for Mg-cordierite the formation of feroocordierite is very sluggish, the more so the higher the temperature and pressure. Intermediate metastable assemblages of other phases appear in its place. It seems possible that these unfavorable reaction kinetics are - opposed to thermodynamic equilibrium - in part responsible for the rare occurrence of Fe-rich cordierites in nature. The overlap of the stability fields of ferrocordierite and chloritoid as demonstrated in this study allows the stable coexistence of these two phases within a limited temperature range.     

Die tektonische Geschichte der Küstenkordillere Chiles

Zusammenfassung Die schmale Küstenkordillere Chiles wird von einem Sockel präkambrischer und paläozoischer Gesteinsserien aufgebaut. Dieser Unterbau wurde mehrfach und scharf gefaltet sowie durch eine frühe Regionalmetamorphose und jüngere Kontaktmetamorphosen umgeprägt. Seit dem Beginn der Trias wurden in der Küstenkordillere keine nennenswerten Biegungsverformungen mehr angelegt. Die viele Kilometer dicken Vulkanite und Sedimente des mesozoischen Deckgebirges erfuhren vielmehr eine schollenartige Zerstückelung. Die andine Orogenese schuf im Oberbau somit vorwiegend Bruchverformungen. Im Bereich der Pazifik-Küste gibt es Bruchstrukturen, die noch im Holozän angelegt sind.Im Mesozoikum erfolgte in drei Phasen eine Mobilisierung ausgedehnter granitischer und granodioritischer Magmenkörper. Diese Intrusionen stehen in keinem greifbaren Zusammenhang mit tektonischen Paroxysmen.

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The narrow cordillera on the coast of Chile is constructed by a fundament of precambrian and paleozoic rocks. This substratum was strongly folded several times and altered by an early regional-metamorphism and a later contactmetamorphism. Since the beginning of the trias no remarkable bending-deformations in the coastal-cordillera were produced. Those volkanites and sediments of the mesozoic superstructure, many kilometers thick, were rather broken into thrusted blocks. The andine orogenesis produced mainly fracture-deformations in the superstructure. On the Pacific coast there are fracture-structures, dating from the holocene.During the mesozoic time a mobilisation of large granitic and grano-dioritic magmas took place in three phases. These intrusions do not show any visible connection with tektonic paroxysmes.

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Résumé La cordillère étroite sur la côte du Chili est formée d'un socle de séries de roches précambriques et paléozoiques. Cette infrastructure fut fortement pliée à différentes reprises et transformée par un métamorphisme primitif régional et des métamorphismes de contact plus récentes. Depuis le trias il n'y avait plus de nouvelles déformations de tournant d'importance dans la cordillère sur la côte du Chili. Les roches volcaniques et les sédiments de la superstructure mesozoique, ayant une épaisseur de beaucoup de kilomètres, furent morcelés. Dans la superstructure l'orogénèse andine a créé principalement des déformations par rupture. Sur la côte du Pacifique il y a des textures de rupture résultant du holocène.Pendant le mesozoique une mobilisation de vastes magmas granitoides et granodioritiques ent lieu en trois phases. Ces intrusions n'ont aucun rapport saisissable aux paroxysmes tectoniques.

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Herrn Prof. Dr. Dr. h. c.Erich Bederke zum 70. Geburtstag gewidmet.     

Oil shales,evaporites and ore deposits

The relationships between oil shales, evaporites and sedimentary ore deposits can be classified in terms of stratigraphic and geochemical coherence. Oil shale and black shale deposition commonly follows continental red beds and is in turn followed by evaporite deposition. This transgressive-regressive sequence represents an orderly succession of depositional environments in space and time and results in stratigraphic coherence. The amount of organic carbon of a sediment depends on productivity and preservation, both of which are enhanced by saline environments. Work on Great Salt Lake. Utah, allows us to estimate that only 5% of TOC originally deposited is preserved. Inorganic carbonate production is similar to TOC production, but preservation is much higher.Oil shales and black shales commonly are enriched in heavy metals through scavenging by biogenic particles and complexation by organic matter. Ore deposits are formed from such rocks through secondary enrichment processes, establishing a geochemical coherence between oil shales and ore deposits. The Permian Kupferschiefer of N. Europe is used as an example to define a Kupferschiefer type (KST) deposit. Here oxygenated brines in contact with red beds become acidified through mineral precipitation and acquire metals by dissolving oxide coatings. Oxidation of the black shale leads to further acid production and metal acquisition and eventually to sulfide deposition along a reducing front. In order to form ore bodies, the stratigraphic coherence of the red bed-black shale-evaporite succession must be joined by the geochemical coherence of the ore body-evaporite-black shale association. The Cretaceous Cu-Zn deposits of Angola, the Zambian Copperbelt as well as the Creta, Oklahoma, deposits are other KST examples. In the Zambian Copperbelt, evaporites are indicated by the carbonate lenticles thought to be pseudomorphs after gypsum-anhydrite nodules. MVT deposits are also deposited by acid brines, but at more elevated temperatures and with carbonates as principal host rocks. The Pine Point deposits are cited for their close association with evaporites.Alkaline, metal-rich brines are postulated for the HYC deposit of McArthur River, Australia. Such brines are known from the Green River Formation and deposits formed from such brines constitute the GRT class. They can be recognized by the presence of Magadi-type cherts and zeolite-analcime-K-spar tuffs. The Cu-Co ore bodies of Outokumpu, Finland, might also belong to this type. A new classification of sedimentary ore deposits is proposed, based on their geochemical environment. KST and MVT are formed from acid ore fluids, while GRT and CT (Creede type) are derived from basic ore fluids. pH of the fluids is best evaluated not from the ores themselves, but from their effect on the host-rocks.     

Geochemistry of Great Salt Lake,Utah I: Hydrochemistry since 1850

The hydrochemistry of Great Salt Lake, Utah, has been defined for the historic period, 1850 through 1982, from published data combined with new observations. The water balance depends largely on river inflow, atmospheric precipitation onto the lake surface and evaporation. Input of the major solutes can best be accounted for by mixing dilute calcium-bicarbonate type river waters with NaCl-dominated hydrothermal springs.Prior to 1930, lake concentrations fluctuated inversely with lake volume in response to small climatic variations. Since then, salt precipitation and dissolution have significantly modified lake brine compositions and have led to density stratification and the formation of brine pockets of differing composition. Brine mixing has become an important component of brine evolution. We have used calculated evaporation curves with mineral precipitation and dissolution to clarify these processes.Pore fluids represent important storage for solutes. Solute profiles can be modeled by simple one-dimensional diffusion calculations. Short-term historic variations in lake composition affect shallow pore fluids in the upper 2 metres of sediment.