Large-scale Mechanical Redistribution of Orthopyroxene and Plagioclase in the Basement Sill, Ferrar Dolerites, McMurdo Dry Valleys, Antarctica: Petrological, Mineral-chemical and Field Evidence for Channelized Movement of Crystals and Melt

The Jurassic Ferrar dolerite sills of the McMurdo Dry Valleys,Antarctica represent the plumbing system for flood basalt eruptionsassociated with the breakup of Gondwana. Among the Ferrar sills,the 350–450 m thick cumulate-textured Basement Sill isdifferentiated into a Lower Marginal Zone (LMZ) gabbronorite,a thick Lower Zone (LZ) orthopyroxene–plagioclase orthocumulatepyroxenite, a strongly layered mela- to leuco-gabbronorite MiddleZone (MZ), a thick Upper Zone (UZ) gabbronorite with ferrogabbroicpods, and an Upper Marginal Zone (UMZ) gabbronorite. Texturesand mineral compositions in the LZ pyroxenite and MZ–UZgabbronorites are nearly identical, the main distinction beingthe greater relative proportion of plagioclase in the MZ–UZgabbronorites, and of pigeonite in the UZ. Most orthopyroxenein the LZ, MZ and UZ occurs as sub-euhedral, normally zonedprimocrysts, commonly with rounded plagioclase inclusions. Plagioclaseis usually sub-euhedral and normally zoned, but can containsodic cores interpreted to be xenocrystic. Orthopyroxene andfeldspar compositions thoughout the sill are generally fairlyuniform, and resemble the compositions of micro-phenocrystsin the chilled margins. We infer that the sill was filled bya c. 1250°C slurry of orthopyroxene + plagioclase phenocrystsor primocrysts that subsequently unmixed in response to buoyancyforces. The LZ websterite contains numerous anorthosite to gabbronoriteschlieren, veins and pipes (< 2 m diameter), which we interpretas fossil segregation channels. Textures and mineral compositionsin these felsic channels are very similar both to UZ and MZgabbronorites, and to the groundmass separating accumulatedorthopyroxene primocrysts in the LZ and MZ. We infer that plagioclase-charged,hydrous pore melt from the pyroxenite may have segregated, pooledand ascended through these conduits to feed growth of the UZgabbronorite. Detailed mapping shows that the pipes are separatedby about 15 m on average. Calculations suggest that this numberdensity of conduits could have drained the LZ cumulates of theirinterstitial melt + plagioclase in about 8 days. Sequences (eachc. 5–10 m thick) of layered leuco-gabbronorite in theMZ could represent intra-cumulate sills that formed from plagioclase-richslurries ascending in segregation channels. Fe–Ti-richpyroxenitic veins and pods (some pegmatitic) and an unusualcoarse-grained plagioclase facies occur at the contacts betweenmassive leuco-gabbronorite layers in the MZ. Discordant ferro-pegmatitepods and dykes occur throughout the UZ. We interpret these Fe-richpegmatoidal rocks as evolved residual melts expelled from thecompacting gabbronoritic cumulates of the MZ and UZ. KEY WORDS: Ferrar; cumulates; differentiation; Antarctica; layering     

Hydrogen and carbon isotopic ratios of the cellulose nitrate and saponifiable lipid fractions prepared from annual growth rings of a California redwood

The $\text{D}\text{H}$ and ${}^{13}\text{C}{}^{12}$C ratios of the cellulose nitrate and saponifiable lipid fractions prepared from eleven annual growth rings of a California redwood were determined. The $\text{D}\text{H}$ ratios of the two fractions are related to one another for the annual rings in the sapwood portion of the tree, but not for those in the heartwood or in the wood undergoing the transition from sapwood to heartwood. No relationship was observed between the ${}^{13}\text{C}{}^{12}$ ratios of the two fractions. These results suggest that analysis of the hydrogen isotopic composition of the saponifiable lipid fractions in plants will provide information useful for climatic reconstruction provided the initial isotopic record has not been changed by subsequent physiological or diagenetic processes.     

Atmospheric noble gases in volcanic glasses from the southern Lau Basin: origin from the subducting slab?

Noble gas concentrations and isotopic compositions have been determined for four submarine volcanic glasses from the Valu Fa Ridge (VFR) in the southern Lau Basin. The samples are the least differentiated ones from this area, and they display enrichments in fluid-mobile elements similar to the nearby island arc. ³He/⁴He ratios are slightly below average MORB (6.8–7.8 times atmospheric), whereas Ne, Ar, Kr, and Xe have isotopic compositions very similar to air. Together with previously published data from the Valu Fa Ridge and other spreading segments in the Lau Basin, our data show a systematic latitudinal variation of increasing Ne, Ar, Kr, and Xe abundances from north to south as well as Ne and Ar isotopic compositions changing from MORB-like to atmosphere-like in the same direction. Moreover, isotopic compositions and noble gas abundances of the lavas correlate strongly with Ba/Nb ratios and H₂O concentrations. Based on these observations and mass balance arguments, we propose that the atmospheric noble gases come from the subducting oceanic crust and are not due to shallow contamination with air dissolved in seawater or assimilation of old crust. Our data suggest that the noble gases released from the subducting slab are atmospheric and thus contain little or no solar He and Ne. In addition to the fact that ratios of He to heavy noble gases are small in aged ocean crust, He has possibly fractionated from the other noble gases due to its higher diffusivity, and thus He transport from the subducting slab into the mantle wedge is probably insignificant. We propose that the ³He/⁴He ratios lower than MORB observed in the VFR lavas result from radiogenic ingrowth of He in a highly depleted, and hence degassed, mantle wedge after the enrichment of U and Th released from the downgoing slab.     

Anomalously nucleogenic neon in North Chile Ridge basalt glasses suggesting a previously degassed mantle source

Fresh basalt glasses from the North Chile Ridge (NCR) in the southeastern Pacific have Ne isotopic compositions distinctly different from typical mid-ocean ridge basalts (MORB). In a three-isotope plot of ²⁰Ne/²²Ne vs. ²¹Ne/²²Ne, the NCR data define a correlation line with a slope smaller than that of the MORB correlation line, i.e. their Ne composition is more nucleogenic than that of MORB. ³He/⁴He ratios are slightly lower than the MORB average, whereas in a few stepwise heating fractions very high ⁴⁰Ar/³⁶Ar ratios up to 28,000 are found. One model to explain the data assumes contamination of the NCR mantle source by material from the continental or oceanic crust, but in addition to difficulties with quantitatively reconciling the noble gas patterns with such a model it seems unable to account for some geochemical characteristics of NCR basalts reported earlier [Bach et al., Earth Planet. Sci. Lett. 142 (1996) 223–240], such as depletions in highly incompatible elements and unradiogenic Sr isotope compositions. Therefore we favor the scenario of a mantle source which was depleted and degassed previously, possibly as a residue from mantle melting beneath the southern East Pacific Rise that was transported to the NCR and melted again. The time during which such a depleted reservoir would have to be separated from the MORB mantle is estimated at 10–100 Ma based on U/Th–Ne systematics, in reasonable agreement with the time scale deduced from the formation history of the NCR and the temporal evolution of the southeast Pacific.     

Hydrogen isotope fractionation between OH-bearing minerals and water

Hydrogen isotope fractionation factors between hydroxyl-bearing minerals and water were determined at temperatures ranging between 400 and 850°C. The hydrogen isotope exchange rates for the mineral-water pairs examined were very slow. In most cases it was necessary to use an interpolation method for the determination of the hydrogen isotope equilibrium fractionation factor, α_e.For the temperature range of 450–850°C the hydrogen isotope fractionation factors for the mica-water and amphibole-water systems are simply expressed as a function of temperature and the molar fractions of the six-fold coordinated cations in the crystal, regardless of mineral species, as follows: 10³ In α_{e(mineral-water)} = ? 22.4 (10⁶T^?2) + 28.2 + (2X_Al ? 4X_Mg ? 68X_Fe), where X is the molar fraction of the cations. As the equation indicates, for any specific composition of the OH-bearing minerals, the change of α_e with temperature, over the temperature range investigated, is the same for all minerals studied. Thus for any specified values of X_Al, X_Mg, and X_Fe for these minerals, the relationship between α_e and T is 10³ In α_e = αT^?2 + k. Consequently, hydrogen isotope fractionation among coexisting minerals is temperature independent and cannot be used as a hydrogen isotope geothermometer.Some exceptions to the above general observations exist for minerals such as boehmite and kaolinite. In these minerals hydrogen bonding modifies the equilibrium hydrogen isotopic fractionation between mineral and water.     

The potentials of lime stabilization of lateritic soils

The paper reviews lime stabilization of lateritic soils and shows that all Nigerian lateritic soils from A-1-a soil to A-7-6 soil used in the investigation, improved their engineering characteristics substantially by the addition of lime. The plasticity indices of the soils were reduced whereas the plastic limits increased; the liquid limits increased slightly, the maximum dry density decreased and the optimum moisture content increased. From the results of Durability and CBR tests, only the A-1 soil and A-2 soil have any potential as competent base materials and consequently only these require any further field tests. 6% lime is recommended for these field tests. The other soils that do not qualify for bases may be utilized as sub-base materials.     

Geochemistry and hydrothermal alteration of the Kern Mountains plutonic complex,White Pine County,Nevada and Juab County,Utah

The Kern Mountains plutonic complex is composed of multiple intrusions referred as the Cove, Tungstonia and Hoodoo Canyon plutons, whose emplacement was structurally controlled. The plutons are members of the calc-alcaline granite series, leucogranite type that are relatively alkali rich. The complex is of epizone (4–10 km) and was emplaced during Jurassic time into the carbonates and quartzite of Paleozoic age. Trace and major element data indicate that the complex was derived from the partial melting of rocks similar to Canadian Shield rocks. Analysis of variance of chemical data suggets that hydrothermal solutions carried K₂O, Na₂O and SiO₂ and raised the levels of original oxides in the plutons. Hydrothermal activity is evidenced by the occurrence of second generation muscovite, potassium feldspar and quartz in the complex and tungsten and sulphide mineralization in the area.

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Zusammenfassung Der plutonische Komplex des Kerngebirges besteht aus mehrfachen Instrusionen, genannt Cove-, Tungstonia- und Hoodoo Canyon-Pluton, deren Platznahme tektonisch kontrolliert wurde. Die plutonischen Gesteine gehören zu den Kalkalkaligraniten; sie sind leucogranitisch und damit ziemlich alkalireich. Der Komplex ist epizonal (4–10 km), er drang während des Jura in die Karbonate und Quarzite des Paläozoikums ein. Spuren- und Hauptelementmeßwerte zeigen an, daß der Komplex von aufgeschmolzenen Gesteinen abgeleitet werden kann, die den Gesteinen des Kanadischen Schildes ähneln. Die Analyse der Variation von chemischen Meßwerten deutet an, daß hydrothermale Lösungen K₂O, Na₂O und SiO₂ enthielten, die das Niveau der ursprünglichen Oxide in den plutonischen Gesteinen erhöhten. Die hydrothermale Tätigkeit wird durch das Vorhandensein von Muskowit, Kaliunifeldspat und Quartz der zweiten Generation innerhalb des Komplexes und durch Wolfram- und Sulfidmineralisation in der Umgebung bewiesen.

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Résumé L'ensemble plutonique des montagnes de Kern, consiste en de nombreuses intrusions dénommées plutons du »Cove«, de »Tungstonia«, et du »Hoodoo Canyon« dont la mise en place a été tectoniquement controlée. Ces plutons font partie de la série granitique calco-alkaline; ce sont des leucogranites relativement riches en alcali. L'ensemble est épizonal (4–10 km), et s'est mis en place au cours du Jurassique dans des Calcaires et quartzites d'âge paléozoique. Les données analytiques relatives aux élements majeurs et en trace indiquent que cet ensemble peut provenir de la fusion partielle de roches similaires à celles du Bouclier Canadien.L'analyse de la variation des données chimiques suggère aussi que l'enrichissement de ces plutons en K₂O, Na₂O, et SiO₂ s'est fait à partir de solutions minéralisantes hydrothermales, comme en témoignent non seulement la présence d'une deuxième génération de muscovite, de feldspath potassique et de quartz, mais aussi par la mineralisation en tungstène et en minerais sulfurés à leur voisinage.

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, Cove, Tungstonia Hoodoo Canyon, . - ; , , . (4–10 ); . - , , . , K₂O, Na₂O SiO₂, . , .

     

Natural entropy production in an inflationary model for a polarized vacuum

Though entropy production is forbidden in standard FRW Cosmology, Berman and Som presented a simple inflationary model where entropy production by bulk viscosity, during standard inflation without ad hoc pressure terms can be accommodated with Robertson–Walker’s metric, so the requirement that the early Universe be anisotropic is not essential in order to have entropy growth during inflationary phase, as we show. Entropy also grows due to shear viscosity, for the anisotropic case. The intrinsically inflationary metric that we propose can be thought of as defining a polarized vacuum, and leads directly to the desired effects without the need of introducing extra pressure terms.     

Sedimentological,geomorphological and dynamic context of debris-mantled glaciers,Mount Everest (Sagarmatha) region,Nepal

This paper presents the sediment, landform and dynamic context of four avalanche-fed valley glaciers (Khumbu, Imja, Lhotse and Chukhung) in the Mount Everest (Sagarmatha) region of Nepal. All four glaciers have a mantle of debris dominated by sandy boulder-gravel that suppresses melting to an increasing degree towards the snout, leading to a progressive reduction in the overall slope of their longitudinal profile. Prominent lateral–terminal moraine complexes, also comprising sandy bouldergravel, enclose the glaciers. These terminal moraines originally grew by accretion of multiple sedimentary facies of basal glacial and supraglacial origin, probably by folding and thrusting when the glaciers were more dynamic during the Little Ice Age. The four glaciers are in various stages of recession, and demonstrate a range of scenarios from down-wasting of the glacier tongue, through morainedammed lake development, to post-moraine-dam breaching. Khumbu Glacier is at the earliest stage of supraglacial pond formation and shows no sign yet of developing a major lake, although one is likely to develop behind its >250 m high composite terminal moraine. Imja Glacier terminates in a substantial body of water behind a partially ice-cored moraine dam (as determined from geophysical surveys), but morphologically appears unlikely to be an immediate threat. Chukhung Glacier already has a breached moraine and a connected debris fan, and therefore no longer poses a threat. Lhotse Glacier has an inclined, free-draining tongue that precludes hazardous lake development. From the data assembled, a conceptual model, applicable to other Himalayan glaciers, is proposed to explain the development of large, lateral-terminal moraine complexes and associated potentially hazardous moraine dams. – 2008 Elsevier Ltd. All rights reserved.