Corona textures in Proterozoic olivine melanorites of the Equeefa Suite,Natal Metamorphic Province,South Africa

Summary Olivine-plagioclase and phlogopite-plagioclase coronas have been identified from olivine melanorites of the Mid- to Late Proterozoic Equeefa Suite in southern Natal, South Africa. Olivine, in contact with plagioclase, is mantled by a shell of clear orthopyroxene, in turn rimmed by pale green (pargasitic) clinoamphibole. Locally a third rim, composed of a fine pargasite-spinel symplectite is developed adjacent to the plagioclase. The second corona reaction has produced greenish-brown pargasite at phlogopite-plagioclase interfaces. A third, less obvious reaction, between olivine and phlogopite is also noted. Analytical data of all the mineral phases present, along with the coronas, are given. Two-pyroxene thermometry yields magmatic core temperatures ( 1120°C), with rim compositions indicating equilibration at 850°C. Consistent with this, the modelled olivine-plagioclase reaction occurs between 830–1050°C with a_waterbetween 0.1 and 1.0 at 7 kbar. The three reactions took place during a prolonged history of cooling and partial hydration of the magmatic olivine melanorites from over 1000°C down to 600°C. The P-T conditions indicated by the reactions suggest this cooling process was essentially isobaric, indicating that the area was not subjected to rapid uplift or burial throughout this entire period.

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Korona-Texturen in proterozoischen Olivin-Melanoriten der Equeefa Suite, Natal Metamorphic Province, Südafrika

Zusammenfassung Aus Olivin-Melanoriten der mittel- bis spätproterozoischen Equeefa Suite im südlichen Natal, Südafrika, wurden Olivin-Plagioklas und Phlogopit-Plagioklas-Koronartexturen beobachtet. Olivin, der mit Plagioklas im Kontakt steht, wird von einem klaren Saum vom Orthopyroxen ummantelt, der seinerseits von blaßgrünem (pargasitischem) Klinoamphibol umsäumt wird. Stellenweise ist ein dritter Saum, bestehend aus feinkörnigem symplektitischem Pargasit-Spinell im Kontakt mit Plagioklas ausgebildet. Die zweite koronabildende Reaktion resultiert in Bildung eines grünbraunen Pargasites an Phlogopit-Plagioklas Kornkontakten. Eine dritte, weniger auffällige Reaktion zwischen Olivin und Phlogopit wurde ebenfalls beobachtet. Zwei-Pyroxen-Thermometrie ergab magmatische Temperaturen der Kernbereiche ( 1120°C) und belegt eine Gleichgewichtseinstellung in den Randzonen bei ca. 850°C. Olivin-Plagioklas-Modellreaktionen liegen ebenfalls in einem Temperaturbereich von 830–1050°C bei Wasseraktivitäten von 0.1 bis 1.0 und einem Druck von 7 kbar. Die drei Reaktionen liefen im Zuge einer länger andauernden Abkühlung unter teilweiser Hydratisierung der magmatischen Melanorite in einem Temperaturbereich von 1000°C bis ca. 600°C ab. Die aus den Reaktionen ableitbaren P-T-Bedingungen sprechen für eine im wesentlichen isobare Abkhlungsgeschichte und zeigen, daß dieses Gebiet wáhrend dieser gesamten Periode keiner raschen Hebung bzw. keiner Versenkung unterworfen worden ist.

     

Crystal chemistry of a highly disordered Mg-Al natural spinel

Summary A Mg-Al spinel from a xenolith found in the Sacrofano volcanic area (Latium, Italy) was investigated by X-ray single crystal diffraction and electron microprobe. Its main structural parameters were: a₀ = 8.0922(3) Å and the oxygen coordinate u = 0.26213(4), which gave to tetrahedral and octahedral bond distances of 1.9220(6) Å and 1.9299(3) Å respectively. Cation distribution, obtained from structural and chemical data, showed a very high Mg-Al disorder, with an Al(T) content of 0.205 and an inversion parameter, i, of about 0.23. Comparison with the cation distribution of a spinel with similar chemical composition but occurring in chlorite schists (Shishimsk Mountains, Urals, Russia) with a₀ = 8.1003(3) Å and u=0.26330(3), and with data from the literature concerning spinels from metamorphosed limestones (Kalkbro, Sweden) and websteritic dikes (Balmuccia, Italy), all characterized by much lower Mg-Al disorder (i about 0.13–0.12), suggests a completely different thermal history, with rapid cooling rates and consequent high closure temperature of intracrystalline exchange, consistent with the Sacrofano geological environment.

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Kristallchemie eines stark ungeordneten, natürlichen Mg-Al-Spinells

Zusammenfassung Ein Mg-Al-Spinell aus einem Xenolith des Vulkangebiets von Sacrifano (Latium, Italien) wurde mit Röngten-Einkristallmethoden und der Elektronenmikrosonde untersucht. Die wichtigsten Strukturparameter sind a_o = 8,0922(3) Å und die Sauerstoffkoordinate u = 0,26213(4), was zu den tetraedrischen und oktaedrischen Bindungsabständen von 1,9220(6) Å bzw. 1,9299(3) Å führt. Die aus den strukturellen und chemischen Daten erhaltene Kationenverteilung zeigt eine sehr starke Unordnung Mg-Al mit einem Al (T)-Gehalt von 0,205 und einem Inversionsparameteri von ca. 0,23. Der Vergleich mit der Kationenverteilung eines Spinells ähnlicher chemischer Zusammensetzung, aber aus Chloritschiefern (Schischimsk-Berge, Ural, Rußland) mit a₀ = 8,1003(3) Å und u=0,26330(3), sowie mit Literaturdaten über Spinelle aus metamorphen Kalken (Kalkbro, Schweden) und websteritischen Gängen (Balmuccia, Italien), die alle durch eine viel geringere Mg-Al-Unordnung charakterisiert sind (i etwa 0,13–0,12), weist auf eine völlig andere thermische Geschichte hin, und zwar mit rascher Abkühlungsgeschwindigkeit und hoher Schließungstemperatur des intrakristallinen Austausches, was mit den geologischen Verhältnissen von Sacrofino in Übereinstimmung steht.

     

Geology of the volcanic-hosted Brockman rare-metals deposit,Halls Creek Mobile Zone,northwest Australia. l. Volcanic environment,geochronology and petrography of the Brockman volcanics

Summary Rare-metals mineralization of the Brockman deposit (Halls Creek Mobile Zone, NW Australia) is hosted in a fluorite-bearing, rhyolitic volcaniclastic unit informally termed the Niobium Tuff. The Tuff, more correctly described as a tuffaceous volcaniclastic deposit, is the lowermost unit of a sequence of trachyte-to-rhyolite lavas, trachyandesite subvolcanic rocks, and volcaniclastic units of the Brockman volcanics located within the Halls Creek Group, a thick, early Proterozoic volcano-sedimentary sequence. High precision SHRIMP ion-microprobe zircon dating of the Niobium Tuff gives an eruption age of 1870 ± 4 Ma. Regional geochronological constraints indicate the Niobium Tuff was deposited about 15 Ma before major orogenic activity affected the area. Despite folding, faulting and low-grade metamorphism, the Brockman volcanics show excellent preservation of primary volcanological features, including pillow-lavas and pillow-breccias, that suggest a dominantly subaqueous, below-wave-base emplacement environment. The style of eruption products and magma volume constraints suggest the trachyte-dominated volcanics were erupted from a small shield volcanic complex probably in a rift-related basin in a shallow-marine setting. The tectonic setting was intraplate but differs from most Tertiary to recent trachyte volcanic complexes which are largely subaerial, are built on relatively thick continental crust, and show no post-eruptive orogenic history. Brockman-style rare-metal deposits are characterized by preservation of subaqueous volcanics beneath a thick sedimentary sequence, eruption of early incompatible-element enriched products followed by less differentiated magmas, and fine-grained mineralogy influenced by alteration processes. Prospects exist for discovery of analogous deposits, particularly in early Proterozoic mobile belts and Tertiary intraplate shield volcanic provinces.

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Geologie der an Vulkanite gebundenen Seltene-Metalle-Lagerstätte Brockman, Halls Creek Mobile Zone, Nordwest-Australien. I. Vulkanologie, Geochronologie und Petrographie der Brockman-Vulkanite

Zusammenfassung Selten-Metall-Vererzung der Brockman-Lagerstätten (Halls Creek Mobile Zone, NW Australien) sitzt in einer Fluorit-führenden, rhyolitischen vulkanoklastischen Einheit auf, die informell als der Niob-Tuff bezeichnet wird. Es handelt sich hier um ein vulkanoklastisches Tuff-Sediment; dieses ist die unterste Einheit einer Abfolge vor Trachyt bis Rhyolit-Laven, trachyandesitischen Subvulkaniten and vulkanoklastischen Einheiten der Brockman Vulkanite innerhalb der Halls Creek Gruppe, einer mächtigen frühproterozoischen vulkanosedimentdren Abfolge. Prüzisionsdatierungen des Niob Tuffs mit der SHRIMP Ionen-Mikrosonde ergeben ein Eruptionsalter von 1870 +-4 Millionen Jahren. Regionale geochronologische Zusammenhänge zeigen, daß der Niob Tuff etwa 15 Millionen Jahre vor einer größeren Orogenese, die das Gebiet betroffen hat, abgelagert wurde. Trotz Faltung, Bruchtektonik and niedriggradiger Metamorphose zeigen die Brockman-Vulkanite einen hervorragenden Erhaltungszustand primärer vulkanologischer Erscheinungen. Diese urnfassen auch Kissenlaven und KissenBreckzien, die eine vorwiegend subaquatische Ablagerung in ruhigem Wasser erkennen lassen. Die Art der Eruptionsprodukte and das Magmavolumen zeigen, daß die Trachytdominierten Vulkanite von einem kleinen Schildvulkan stammer, wahrscheinlich in einem Becken in einer Rift-Situation im seichten marinen Milieu. Die tektonische Situation war intraplate, aber unterscheidet sich von den meisten tertiären bis rezenten trachytischen Vulkan-Komplexen, die hauptsächlich subaerisch sind, auf einer relativ mächtigen kontinentalen Kruste aufsitzen, and keine post-eruptive orogene Entwicklung zeigen. Seltene-Metalle-Lagerstatten des Brockman-Typs rind durch die Erhaltung subaquatischer-Vulkanite unterhalb einer machtigen sedimentdren Abfolge gekennzeichnet; welters durch frühe Eruptionsprodukte, die an inkompatiblen Elementen angereichert sind, auf die dann weniger differenzierte Magmen folgten, und schließlich durch einen feinkörnigen Mineralbestand, der vor Umwandlungsprozessen betroffen war. Es besteht die Möglichkeit der Entdeckung analoger Lagerstätten, besonders in frühproterozoischen mobilen Gürteln und in tertidren intraplate Schildvulkan Provinzen.

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Optical anisotropy of zoned magnetites due to form birefringence

Summary Five polished samples of zoned magnetites from skarn deposits showed optical anisotropy effects under the ore microscope, though X-ray powder diffraction investigations indicated cubic magnetite. Using high magnification ore microscopy (oil immersion lenses, monochromatic light), extremely fine zones with slightly different reflectivity were observed. Scanning electron microscopy (SEM) of etched samples confirmed zones with a thickness between a few m and 100 nm. EDX analyses showed differences in the composition of these zones, ranging from almost pure magnetite to contents of CaO, MgO, A1₂O₃, SiO₂ up to 4 wt.% in total. Thus, the extremely thin, parallel zones of magnetite, with slightly different compositions and different optical constants (reflectivity, absorption, refraction), obviously lead to optical anisotropy due to form birefringence.

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Optische anisotropie zonarer Magnetite aufgrund von formdoppelbrechung

Zusammenfassung Fünf Anschliffe von zonar gebauten Magnetiten aus Skarn-Lagerstätten zeigten unter dem Erzmikroskop optische Anisotropie-Erscheinungen, obwohl Röntgen-Pulverauf-nahmen kubischen Magnetit ergaben. Mittels hochauflösender Erzmikroskopie (Öl-Immersion, monochromatisches Licht) wurden extrem feine Zonierungen mit etwas unterschiedlichem Reflexionsvermögen beobachtet. Rasterelektronenmikroskopie (SEM) von angeätzten Proben bestätigte eine Zonendicke zwischen einigen Yin und 100 nm. EDX Analysen zeigten Unterschiede in der Zusammensetzung dieser Zonen, die von fast reinem Magnetit bis zu CaO-, MgO-, A1₂O_3-, SiO₂-Gehalten von insgesamt 4 Gew.% reichen. Offensichtlich fiihren die extrem diinnen, parallelen Zonierungen mit unterschiedlicher Zusammensetzung, und folglich unterschiedlichen optischen Konstanten (Reflexion, Absorption, Lichtbrechung) zu optischer Anisotropie durch Formdoppelbrechung.

     

Some remarks on certain special BDT-FRW solutions

We wish to point out that the BDT-FRW (k=1) special dust solution obtained by Dehnen and Obregón (1971) and the BDT-FRW (k=1) radiation solution given by Obregón and Chauvet (1978) are contained as special cases in the special perfect fluid Jordan-Brans-Dicke solution first given by Brill (1962). In addition we give a simple proof that the special BDT-FRW (k=1) vacuum solution given by Dehnen and Obregón (1972) does not exist. We finally consider the special case =0 and present some new solutions.     

Comment on the LRS-BDT-Bianchi type-V vacuum solution

We wish to point out that the anisotropic LRS-BDT-Bianchi type-V vacuum solution recently given by Ram and Singh is wrong. Moreover, the correct solution is nothing but an isotropic FRW (k=–1) solution given by us previously.     

The main problem of lunar theory solved by the method of Brown

Brown's method for solving the main problem of lunar theory has been adapted for the computation by machine with the help of an algebraic processor. Brown's results are first recovered and refined. The solution is then expanded to include most terms of order nine. The terms in the series for the longitude and latitude are listed with an accuracy of 0.000 01 and of 0.000 001 for the parallax.This research was supported in parts by the National Science Foundation grant MCS 78-01425.     

Tilted electromagnetic Bianchi type III cosmological solution

An exact cosmological solution to the Einstein-Maxwell equations for the case of Bianchi type III symmetry with stiff matter and an electromagnetic field is constructed. The behaviour of the solution near the singularity and at late stages of the expansion is investigated.     

Mountain restoration: Soil and surface wildlife habitat

Much wildlife habitat is being destroyed by extractive resource industries in mountain environments. This article illustrates how mountain wildlife habitat was restored in a devastated area. A strip mine for coal on the east slopes of the Alberta Rockies, occupied during its operations by Rocky Mountain bighorn sheep (Ovis canadensis canadensis, Shaw 1803), was reclaimed as bighorn habitat. By considering the behaviour and habitat requirements of bighorns in conjunction with an operating coal mine and reclamation process, a large coal mining company has been able to reconstruct the mine site restoration to benefit mountain sheep. Although bighorn habitat requirements are well known, it was not until the animal and its requirements were studied against the backdrop of an operating coal mine that obvious recommendations regarding the maintenance and placement of high steep excavated rock faces were made and accepted by the government regulatory agencies. An open pit mine is a devastated landscape; trees, soil and overburden are removed to retrieve coal that may be up to 215 m underground. Overburden and soil are returned, but the disturbance compares to the barren landscapes left behind by glaciers. Contouring of the land, grading of overburden and soil, seeding with grass and legume mixtures and extensive fertilization are designed to speed plant colonization and soil development. By examining the bighorn's biological needs with respect to specific mining operations, a unique opportunity arose to employ McHarg's design with nature concept. This approach incorporates environmental and societal values into every aspect of development, and promotes the evaluation of the constraints and opportunities arising. The sheep numbered about 200. Their seasonal movements were similar to those found on native ranges. They used the reclaimed areas as winter range and for the mineral licks exposed during mining; in summer, the sheep moved to nearby alpine areas. Two thirds of all sightings were confined to 1.3 km² of reclaimed grassland; its average productivity (4190 kg/ha) exceeded native ranges (1700 kg/ha). The body mass of female sheep in autumn equalled that of the largest in Alberta, but the skull dimensions were not larger than those of adjacent ranges. Infestation with lungworms was moderate. Lamb production and survival were high. Design criteria should be: feeding areas should be dry and lie within 300 m of escape terrain, which should have a slope of 40% and contain at least three benches. Rock piles should be placed on grazing areas. Mineral licks, a vital welfare factor, already existed within the high walls created by strip mining.     

Mountain hazards

The review examines mountain land risks, especially those that involve sudden, more or less localized bouts of damage and disasters. Geographers' studies to date have mainly dealt with natural hazards, emphasizing those like avalanches, large landslides or natural dams that are uniquely associated with mountain environments. Such work is briefly reviewed. Earthquake risks are then singled out to help define and discuss problems faced by this work. They are shown to depend strongly upon aspects of mountain environments other than seismicity. Human vulnerability appears to be mainly dependent upon socioeconomic and habitat conditions which, although they determine seismic impacts, may and usually do develop independent of it. Moreover, these ingredients of risk are most closely associated with, or definitive of, the mountain people and areas. They are most strongly influenced by human agency, both in deciding who and what are exposed to risk, and in actually increasing or decreasing unfavorable responses to earthquakes. Recent earthquake disasters in the mountains also record how rapid changes in these social and habitat conditions, rather than seismicity, are increasing the scale and altering the forms of damage. These changes in turn, however, are largely dependent upon developments, initiatives and penetration from outside the mountains, and responses of mountain people more or less enforced by them. The importance of so-called highland-lowland interactions, is even more obvious in the most destructive of human hazards discussed, those of war and other armed violence. Mountain lands and peoples are shown to have been subject to recurrent warring, and a disproportionate share of the deployment of state violence in this century. Most of the casualties are resident civilians. They have been usually threatened by other uses of armed violence, including insurgency and counter-insurgency warfare and genocidal actions by state forces. Refugees and expellees from war zones have included great numbers of mountain folk. Another fast growing hazard of the late twentieth century, not unrelated to war and militarism, the international trafficking in and addiction plagues of hard drugs, has a unique relation to mountains. The South American growers of the coca leaf, and Southwest and Southeast Asian growers of the opium poppy are mountain farmers. Their participation is bringing a range of severe risks to their high valleys, as well as benefits. Finally the paper addresses the conceptual problems brought about by the compounding of determinist assumptions in both mountain land and hazards research. These include not only environmental determinist notions, or related Neo-Malthusian and Social Darwinist ideas, but the various historicist ones of assumed technological, economic and political stages of development. It is argued that such social constructions of the problem of hazards and disaster are dubious, and have become a major impediment to understanding and risk mitigation. Alternative frameworks require more attention to the regional, state, and global patterns of influence shaping risk in mountain lands, but also to the human ecology of mountain societies taking account of matters to which the expediences of technocratic expertise have made us blind.