Petrology,chemistry, and isotopic compositions of the lunar highland regolith breccia Dar al Gani 262

Abstract— Lunar meteorite Dar al Gani 262 (DG 262)—found in the Libyan part of the Sahara—is a mature, anorthositic regolith breccia with highland affinities. The origin from the Moon is undoubtedly indicated by its bulk chemical composition; radionuclide concentrations; noble gas, N, and O isotopic compositions; and petrographic features. Dar al Gani 262 is a typical anorthositic highland breccia similar in mineralogy and chemical composition to Queen Alexandra Range (QUE) 93069. About 52 vol% of the studied thin sections of Dar al Gani 262 consist of fine-grained(100 μm) constituents, and 48 vol% is mineral and lithic clasts and impact-melt veins. The most abundant clast types are feldspathic fine-grained to microporphyritic crystalline melt breccias (50.2 vol%; includes recrystallized melt breccias), whereas mafic crystalline melt breccias are extremely rare (1.4 vol%). Granulitic lithologies are 12.8 vol%, intragranularly recrystallized anorthosites and cataclastic anorthosites are 8.8 and 8.2 vol%, respectively, and (devitrified) glasses are 2.7 vol%. Impact-melt veins (5.5 vol% of the whole thin sections) cutting across the entire thin section were probably formed subsequent to the lithification process of the bulk rock at pressures below 20 GPa, because the bulk rock never experienced a higher peak shock pressure. Mafic crystalline melt breccias are very rare in Dar al Gani 262 and are similar in abundance to those in QUE 93069. The extremely low abundance of mafic components and the bulk composition may constrain possible areas of the Moon from which the breccia was derived. The source area of Dar al Gani 262 must be a highland terrain lacking significant mafic impact melts or mare components. On the basis of radionuclide activities, an irradiation position of DG 262 on the Moon at a depth of 55–85 g/cm³and a maximum transit time to Earth <0.15 Ma is suggested. Dar al Gani 262 contains high concentrations of solar-wind-implanted noble gases. The isotopic abundance ratio ⁴⁰Ar/³⁶Ar < 3 is characteristic of lunar soils. The terrestrial weathering of DG 262 is reflected by the occurrence of fractures filled with calcite and by high concentrations of Ca, Ba, Cs, Br, and As. There is also a large amount of terrestrial C and some N in the sample, which was released at low temperatures during stepped heating. High concentrations of Ni, Co, and Ir indicate a significant meteoritic component in the lunar surface regolith from which DG 262 was derived.     

Airborne gravimetry used in precise geoid computations by ring integration

Two detailed geoids have been computed in the region of North Jutland. The first computation used marine data in the offshore areas. For the second computation the marine data set was replaced by the sparser airborne gravity data resulting from the AGMASCO campaign of September 1996. The results of comparisons of the geoid heights at on-shore geometric control showed that the geoid heights computed from the airborne gravity data matched in precision those computed using the marine data, supporting the view that airborne techniques have enormous potential for mapping those unsurveyed areas between the land-based data and the off-shore marine or altimetrically derived data. Received: 7 July 1997 / Accepted: 22 April 1998     

Environmental risk factors for diseases transmitted by vectors: a case study in North-Argentina

The study quantifies the environmental risk factors for two diseases with different vectors and cycles of transmission: malaria and Chagas' disease in N.W. Argentina near the Bolivian border. This is the area within Argentina where malaria is still a serious health problem. Chagas' disease is to some extent present in many parts of the country. The field work for the study concerned 5903 people in 1466 dwellings. The study resulted in detailed maps of risk factors: particularly water quality and contacts with migrants from Bolivia in the case of malaria; and thatched roofs and dogs in the case of Chagas' disease. This revised version was published online in August 2006 with corrections to the Cover Date.     

The metamorphosed mafic-ultramafic complex of Mokuro, Ilesha Schist Belt, southwestern Nigeria

In the Proterozoic Schist Belt of Nigeria, lenticular bodies of metabasites and meta-ultramafics are frequently intercalated within staurolite bearing metapelitic schists. Such a metamorphosed mafic-ultramafic complex is particularly well exposed in the Mokuro riverbed between the towns of Ife and Ilesha. These outcrops display contact relationships with the surrounding metasediments, as well as between the individual mafic and ultramafic rock types. The most common mafic rocks are indistinctly layered amphibolites, accompanied by apatite rich amphibolites and massive amphibolites, in part rich in ilmenite and pyrrhotite. Among the generally massive ultramafic rocks, nearly monomineralic amphibole rocks predominate, while chlorite-amphibole, talc-chlorite-amphibole and talc bearing olivine-chlorite-amphibole rocks occur in subordinate amounts.Field, textural and geochemical evidence suggest that the mafic-ultramafic complex derived from a thick, structurally differentiated basaltic sill that contained doleritic portions in its interior. Slow cooling rates in these inner parts enabled crystal settling with the formation of ultramafic cumulates. Due to the enrichment of volatiles during the crystallisation process, higher amounts of apatite and sulphides, as well as late magmatic amphibole, were formed in parts of the mafic-ultramafic body.Mineral assemblages in the mafic-ultramafic complex testify to a metamorphic overprint under amphibolite-facies conditions. Thermodynamic modelling in the system CMFASH leads to an estimated P–T range of 1.5–3 kbar and 550–620°C for the metamorphic peak assemblage talc-olivine-chlorite-Ca amphibole-orthoamphibole.     

Flow of groundwater with variable density and viscosity, Atikokan Research Area, Canada

 Flow of groundwater with variable density and viscosity was simulated at the Atikokan Research Area (ARA) in northwestern Ontario, Canada. An empirical viscosity–concentration equation was modified to include total-dissolved-solids (TDS) data from the ARA. The resulting equation was used successfully to estimate reasonably accurate viscosity values over the expected range of temperature and concentration, in comparison with experimental values derived for sodium chloride solutions. A three-dimensional finite-element code, MOTIF, developed by Atomic Energy of Canada Limited, was used in the simulations. The inclusion of the effects of depth-increasing temperature and TDS-dependent fluid-density distribution, while maintaining only a temperature-dependent viscosity relationship in a simulation, resulted in a more penetrative flow against expected buoyancy effects (i.e., the physics of the system was not honored). Accounting for concentration in the viscosity equation caused water to be less penetrative and more in accordance with the expected physics of the system. A conclusion is that fluid concentration should be considered simultaneously in calculating the density and viscosity of a fluid during modeling of variable-density flow in areas underlain by fluids with high TDS. Results of simulations suggest that both flow directions and magnitudes should be employed simultaneously during the calibration of a model. Large-scale groundwater movement in the ARA may be analyzed with carefully selected vertical no-flow boundaries. By incorporating the geothermal temperature gradient, groundwater recharge increases by 12%; thus, this gradient plays a significant role in groundwater flow at the ARA. Variability in the fluid concentration at the ARA neither decreases nor increases recharge into the groundwater system. The hypothesis that an isolated continuous regional flow system may exist at depth in the ARA is not supported by these simulations. Received, September 1996 Revised, September 1997, February 1998 Accepted, February 1998     

Water vapour content over two Indian sites tested for a millimeter-wave radio observatory

Measurements of the water vapour content over two possible sites for a millimeter-wave radio-observatory in South India are described. The results of the survey, made with an infrared spectral hygrometer built at the Meudon Observatory, France are compared with those of similar surveys made at some other sites in Europe and America.     

Sources and sinks of atmospheric methane

In 1972 average mixing ratio of methane in the troposphere was 1.41 ppm and 1.3 ppmv for the northern and southern hemisphere, respectively, which corresponds to a total amount of 4×10¹⁵ g of CH₄ present in the atmosphere. Most is of recent biologic origin.¹⁴C analyses show that no more than 20 percent is released by fossil sources. The various ecosystems producing CH₄ are discussed and the total annual production is estimated to lie between 5.5×10¹⁴ g/yr and 11×10¹⁴ g/yr. The corresponding turnover times for atmospheric CH₄ range from 4 to 7 yrs. The destruction of CH₄ takes place mainly in the troposphere, most probably through the reaction of CH₄ + OH CH₃ + H₂O. About 10 percent of the CH₄ is destroyed in the stratosphere. The CH₄ cycle contributes on the order of 1 percent to the atmospheric carbon cycle.     

Snow sedimentation — Diagenesis and avalanches: A correlation with sedimentary rocks

The paper describes some processes and phenomena relating to snow, such as sedimentation and resedimentation, diagenesis and gravity-induced mass transports (avalanches). It is shown that the type of avalanche is largely governed by the bedding sequence within the snow cover. From this it is concluded that snow could act as a valuable correlation model to gain a better understanding of some gravity-induced sediment transport modes, e.g. slumping—sliding and turbidity flow.     

Sedimentäre Eisenerze und ihr geochemisches Bildungsmilieu in den tertiären Deckschichten der Rheinischen Braunkohle,Tagebau Frechen

Zusammenfassung Die hangenden tertiären Schichteinheiten der Rheinischen Braunkohle sind im Tagebau Frechen der Rheinische Braunkohlenwerke AG teils sideritisch, teils markasitisch-pyritisch vererzt. Beide Vererzungsarten sind postsedimentär und können daher nicht direkt als Milieuindikatoren für das Sedimentationsmilieu der Ablagerungen interpretiert werden. Die Existenz der Sideritgeoden, auf die tonigen Sedimente der Indener Schichten beschränkt, legt jedoch eine Ablagerung der Geodenmatrix in einem von Moorwässern bestimmten, limnischen Milieu nahe, da nur hier die Voraussetzungen für eine geologisch relevante Bereitstellung von Eisen gegeben erscheinen. Es wird angenommen, daß dieses zur Sideritbildung notwendige Eisen zunächst bevorzugt als Humat in die Tone eingelagert wurde. Im Rahmen der postsedimentären Oxidation organischer Substanzen bildeten sich dann CO₂-reiche Porenlösungen, in denen das Eisen, einem pH-Gefälle folgend, erneut transportiert und unter hohen pH-Werten schließlich als Siderit ausgefällt wurde. Markasit und Pyrit deuten eine ähnliche pH-Entwicklung der sie bildenden Porenlösungen an. Auch hier beobachtet man mit der Zeit eine Zunahme der Basizität der Lösungen, ausgedrückt in der Ausscheidungsfolge Markasit-Pyrit. In tonigen Sedimenten, mit geschlossenen Mikrosystemen der Porenräume, wird diese Folge nur einmal durchlaufen, in Sanden und Kiesen deuten wiederholte Ausscheidungen von Markasit und Pyrit ein offenes Porensystem an, in das im Verlauf der Diagenese auch jüngere Grundwässer Ehund pH-bestimmend eingreifen konnten.

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In the Frechen open cast mine (Rheinische Braunkohlenwerke AG) the hanging wall of the Rhenish lignites is partly mineralized by siderite and marcasite-pyrite. The mineralizations are postsedimentary and therefore, the environment existing during the deposition of the host rocks cannot be interpreted directly from the presence of siderite and marcasite-pyrite. However, since the occurrence of sideritic concretions is restricted to clayey sediments of the Inden beds (Miocene), the depositional environment may be deduced for geochemical reasons: Large-scale transport of iron is only possible in a limnetic environment influenced by bog water. It is suggested that the iron, necessary for the formation of siderite, was deposited together with the clay minerals, mainly as iron humates. Postsedimentary oxidation of organic materials formed CO₂-rich pore fluids which again facilitated mobilization of iron. In a basic environment siderite was precipitated. For the pyrite and marcasite bearing sedimentary beds a similar development of the interstitial waters is suggested. The precipitation sequence of marcasite, followed by pyrite, is indicative for an increasingly basic character of the pore fluids. In the clay sediments with closed microsystems of pore volumes, one marcasite-pyrite cycle was precipitated. The sand and gravel beds contain repeated marcasite-pyrite cycles, indicating a system open to repeated changes of its Eh-pH states by interaction with more recent groundwaters.

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Resumé Les couches tertiaires susjacentes aux lignites rhénaniennes sont minéralisées partiellement sidéritiquement et partiellement markasitiquement-pyritiquement dans les mines à ciel ouvert du Rheinische Braunkohlenwerke AG à Frechen. Ces deux types de minéralisation sont postsédimentaires et c'est ce qui explique qu'ils ne peuvent être interprétés comme des indicateurs du milieu existant à l'endroit de sédimentations. L'apparition des géodes de sidérite qui est limitée aux sédiments argileux de la formation Indènes (Miocene) rend vraisemblable que la déposition des sédiments argileux surviendra dans un milieu limnique influencé par des eaux marécageuses, étant donné que les conditions pour un transport des ions ferreux dans des quantités géologiquement intéressantes, n'existent que dans un tel milieu. Il est supposé que le fer est réparti dans les argiles sous la forme de sels humiques. Pendant l'oxidation des matériaux organiques, des solutions intersticielles riches en CO₂ se forment dans desquelles le fer est transporté de nouveau suivant l'augmentation du pH et où il est précipité sous forme de sidérite dans le milieu avec les valeurs pH plus élevées. Les formations des précipitations de markasite et de pyrite indiquent un développement semblable de leur solution intersticielle. On peut aussi observer dans ce cas une augmentation avec le temps de la basicité des solutions, marquée par la séquence de la précipitation markasite-pyrite. Cette séquence n'est suivie qu'une fois dans des sédiments argileux ayant un système intersticiel fermé; plusieurs précipitations observées dans des sables et graviers indiquent un système ouvert dans lequel les eaux souterraines peuvent intervenir déterminant le Eh-pH des processus des sédimentations plus jeunes.

     

Carbonaceous chondrites—I. Characterization and significance of carbonaceous chondrite (CM) xenoliths in the Jodzie howardite

Mineralogical, chemical, textural, and isotopic studies of the abundant carbonaceous inclusions in the Jodzie howardite are consistent with CM characteristics. These CM xenoliths show regolith alteration on a level comparable to the Murray and Murchison meteorites but less than Nogoya, flow-oriented development of phyllosilicates and ‘poorly characterized phases’, and partial oxidation of sulfides. Temperature-programmed pyrolysis mass spectrometry (25°–1400°C) indicates that gas release patterns of volatiles and hydrocarbon components and percent contents of N(0.15), C(2.3) and S(2.4) are typical of CM meteorites. Release of significant amounts of SO₂ is attributed to the thermal breakdown of ‘poorly characterized phases’ (Fe-Ni-C-S-O) that formed during low temperature aqueous alteration in the CM parent body.Noble gas abundances are well within the reported range of CM meteorites. The fact that the Ne composition is typical for ‘solar’ values and the isotopic structure of Xe is ‘planetary’ argues that these gases were entrapped by different mechanisms. Cosmic ray exposure ages for the xenoliths (³He, 5 × 10⁶; ²¹Ne, 6.7 × 10⁶; ³⁸Ar, 6.9 × 10⁶ yr) agree with the reported exposure age for the eucritic host. Volatile abundances, presence of intact organic molecules, and phyllosilicates in the CM xenoliths preclude regolith temperatures in excess of 200°C after CM incorporation. Mixing of the host and xenoliths probably occurred during a low-velocity collision of main belt asteroids.