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191.
Svend Stouge David A. T. Harper George D. Sevastopulo Darren O'Mahony John Murray 《Geological Journal》2016,51(4):584-599
The Middle Ordovician Rosroe Formation consists of some 1350 m of coarse, mainly siliciclastic to volcaniclastic sedimentary rocks, deposited in a submarine fan environment, and is restricted to the southern limb of the South Mayo Trough, western Ireland. Discrete allochthonous blocks, reaching 5 m in size, are present in the formation at several localities. Conodonts recovered from these blocks, collected from two separate locations, are of late Early and mid Mid Ordovician age. The conodonts have high conodont‐alteration indices (CAI 5) indicative of temperatures as high as 300o to max. 480 °C; some found in the Lough Nafooey area have abnormally high indices (CAI 6), which correspond to temperatures of about 360o to max. 550 °C. The oldest fauna is dominated by Periodon aff. aculeatus and characterized by Oepikodus evae typical of the Oepikodus evae Zone (Floian Stage; Stage Slices Fl2–3, Lower Ordovician). The younger conodont assemblage, characterized by Periodon macrodentatus associated with Oistodella pulchra, is referred to the P. macrodentatus conodont Biozone (lower Darriwilian; Stage Slices Dw1–2). The Rosroe conodont assemblages are of Laurentian affinity; comparable faunas are well known from several locations along the east to south‐eastern platform margin of Laurentia and the Notre Dame subzone of central Newfoundland, Canada. The faunal composition from the limestone blocks suggests a shelf edge to slope (or fringing carbonate) setting. The faunal assemblages are coeval with, respectively, the Tourmakeady Formation (Floian–Dapingian) and Srah Formation (Darriwilian) in the Tourmakeady Volcanic Group in the eastern part of the South Mayo Trough and probably are derived from the same or similar laterally equivalent short‐lived carbonate successions that accumulated at offshore ‘peri‐Laurentian’ islands, close to and along the Laurentian margin. During collapse of the carbonate system in the late Mid Ordovician, the blocks were transported down a steep slope and into deep‐water by debris flows, mixing with other rock types now found in the coarse polymict clastics of the Rosroe Formation. The faunas fill the stratigraphical ‘gap’ between the Lower Ordovician Lough Nafooey Volcanic Group and the upper Middle Ordovician Rosroe Formation in the South Mayo Trough and represent a brief interval conducive to carbonate accumulation in an otherwise siliciclastic‐ and volcaniclastic‐dominated sedimentary environment. Copyright © 2015 John Wiley & Sons, Ltd. 相似文献
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Abstract– The Stardust mission captured particles from the comet 81P/Wild 2 in gradient density silica aerogel and returned the collected samples to earth in 2006. The analyses of these particles have revealed several new insights into the formation of our solar system. However, since the aerogel used as the capture material was silica, the elemental analyses of the silica‐rich particles were made more complicated in certain ways due to the mixing of the silicon of the particles and that of the aerogel. By using a nonsilica aerogel, future elemental analyses of silica–rich particles captured in aerogel could be made more straightforward. Resorcinol/formaldehyde (RF), alumina, and zirconia aerogels were impact tested with meteoritic fragments and the captured fragments were mapped with synchrotron‐based X‐ray microprobe (XRM) and the particles were analyzed with X‐ray fluorescence (XRF). The resorcinol/formaldehyde aerogel proved to be the best capture material, in that it could be keystoned and XRF could be used to locate and analyze particles that were less than 10 μm. 相似文献
194.
The use of invariant manifolds for transfers between unstable periodic orbits of different energies 总被引:1,自引:0,他引:1
Kathryn E. Davis Rodney L. Anderson Daniel J. Scheeres George H. Born 《Celestial Mechanics and Dynamical Astronomy》2010,107(4):471-485
Techniques from dynamical systems theory have been applied to the construction of transfers between unstable periodic orbits
that have different energies. Invariant manifolds, trajectories that asymptotically depart or approach unstable periodic orbits,
are used to connect the initial and final orbits. The transfer asymptotically departs the initial orbit on a trajectory contained
within the initial orbit’s unstable manifold and later asymptotically approaches the final orbit on a trajectory contained
within the stable manifold of the final orbit. The manifold trajectories are connected by the execution of impulsive maneuvers.
Two-body parameters dictate the selection of the individual manifold trajectories used to construct efficient transfers. A
bounding sphere centered on the secondary, with a radius less than the sphere of influence of the secondary, is used to study
the manifold trajectories. A two-body parameter, κ, is computed within the bounding sphere, where the gravitational effects of the secondary dominate. The parameter κ is defined as the sum of two quantities: the difference in the normalized angular momentum vectors and eccentricity vectors
between a point on the unstable manifold and a point on the stable manifold. It is numerically demonstrated that as the κ parameter decreases, the total cost to complete the transfer decreases. Preliminary results indicate that this method of
constructing transfers produces a significant cost savings over methods that do not employ the use of invariant manifolds. 相似文献
195.
An Alfven Wave Reflection (AWR) model is proposed that provides closure for strong field-aligned currents (FACs) driven by the magnetopause reconnection in the magnetospheres of planets having no significant ionospheric and surface electrical conductance. The model is based on properties of the Alfven waves, generated at high altitudes and reflected from the low-conductivity surface of the planet. When magnetospheric convection is very slow, the incident and reflected Alfven waves propagate along approximately the same path. In this case, the net field-aligned currents will be small. However, as the convection speed increases, the reflected wave is displaced relatively to the incident wave so that the incident and reflected waves no longer compensate each other. In this case, the net field-aligned current may be large despite the lack of significant ionospheric and surface conductivity. Our estimate shows that for typical solar wind conditions at Mercury, the magnitude of Region 1-type FACs in Mercury’s magnetosphere may reach hundreds of kilo-Amperes. This AWR model of field-aligned currents may provide a solution to the long-standing problem of the closure of FACs in the Mercury’s magnetosphere. 相似文献
196.
George Pavlovich Zaraisky Valentina Korzhinskaya Nataliya Kotova 《Mineralogy and Petrology》2010,99(3-4):287-300
It is well established that the fractionation of Li–F granitic magmas at depth leads to the accumulation of flux elements such as F and Li, and metal cations such as Ta and Nb in residual melts. However, it remains to be determined whether magmatic fractionation is sufficient to concentrate Nb and Ta into economically significant quantities, and what role hydrothermal–metasomatic processes play in the formation of such ore deposits. In the literature, reliable data about the solubility of Ta and Nb in hydrothermal solutions is missing or incomplete. This study provides a quantitative experimental estimation of the possible contribution from hydrothermal processes in Ta enrichment in cupolas of albitized and greisenized Li–F granite. Experimental studies of Ta2O5 and columbite–tantalite (Mn,Fe)(Nb,Ta)2O6 solubility were carried out in fluoride solutions consisting of HF, NaF, KF, and LiF. At low fluoride concentrations (0.01 and 0.1 m), Ta2O5 solubility at 550°C and 100 MPa under Co–CoO oxidizing conditions is low (near 10?5–10?4 m) in all fluoride solutions (HF, NaF, KF, LiF). At high fluoride concentrations (1 and 2 m) the highest Ta2O5 concentrations (10?1 m) were detected in HF solutions. In KF, NaF, and LiF solutions, the Ta2O5 solubility is also high (10?3–10?2 m). The dependence of columbite–tantalite (Nb2O5-59 wt. %, Ta2O5-18 wt. %) solubility as a function of solution composition, T, and P has also been investigated. Tantalum and Nb concentrations have the highest values in HF solutions at reduced conditions (up to 10?3 to 10?2 m Ta in 1 m HF). In 1 m NaF solutions, the concentrations of Nb and Ta are, respectively, 2.5 and 3 orders of magnitude less than those in the 1 m HF solutions. Solubility of Ta and Nb in KF solutions has intermediate values. It is established that in NaF and KF solutions the dependence of solubility on pressure is distinctly negative. The Nb and Ta contents increase with increasing concentrations of HF and KF in solution, however, they do not change with increasing NaF concentration. In NaHCO3, Na2CO3, and HCl solutions columbite–tantalite solubility is low. Even in 1 m chloride solutions the content is within the limits of 10?5 m for Nb and 10?6 to 10?8 m for Ta. We conclude that hydrothermal transport of Ta and Nb is possible only in concentrated fluoride solutions. 相似文献
197.
George W. Luther III 《Aquatic Geochemistry》2010,16(3):395-420
In the aquatic geochemical literature, a redox half-reaction is normally written for a multi-electron process (n > 2); e.g., sulfide oxidation to sulfate. When coupling two multi-electron half-reactions, thermodynamic calculations indicate possible reactivity, and the coupled half-reactions are considered favorable even when there is a known barrier to reactivity. Thermodynamic calculations should be done for one or two-electron transfer steps and then compared with known reactivity to determine the rate controlling step in a reaction pathway. Here, thermodynamic calculations are presented for selected reactions for compounds of C, O, N, S, Fe, Mn and Cu. Calculations predict reactivity barriers and agree with one previous analysis showing the first step in reducing O2 to O2 ? with Fe2+ and Mn2+ is rate limiting. Similar problems occur for the first electron transfer step in these metals reducing NO3 ?, but if reactive oxygen species form or if two-electron transfer steps with O atom transfer occur, reactivity becomes favorable. H2S and NH4 + oxidation in a one-electron transfer step by O2 is also not favorable unless activation of oxygen can occur. H2S oxidation by Cu2+, Fe(III) and Mn(III, IV) phases in two-electron transfer steps is favorable but not in one-electron steps indicating that (nano)particles with bands of orbitals are needed to accept two electrons from H2S. NH4 + oxidation by Fe(III) and Mn(III, IV) phases is generally not favorable for both one- and two-electron transfer steps, but their reaction with hydroxylamine and hydrazine to form N2O and N2, respectively, is favorable. The anammox reaction using hydroxylamine via nitrite reduction is the most favorable for NH4 + oxidation. Other chemical processes including photosynthesis and chemosynthesis are considered for these element–element transformations. 相似文献
198.
199.
William R. Dickinson George E. Gehrels 《International Journal of Earth Sciences》2010,99(6):1247-1265
Individual U–Pb ages for 5,655 detrital zircons (DZ) in 61 sandstone samples from Mesozoic strata of the Colorado Plateau
and nearby areas provide insights into paleogeographic relations across the interior of North America and the paleotectonic
evolution of North American continental margins. Pre-Mesozoic DZ grains derived either directly, or ultimately through sediment
recycling, from distant sources in eastern North America are more abundant than DZ grains derived from the nearby Cordilleran
magmatic arc of western North America. Sediment dispersal patterns included Triassic fluvial transport of detritus westward
from the Ouachita orogen uplifted along the northern flank of rift highlands precursor to the oceanic Gulf of Mexico, Jurassic
eolian transport southward into widespread ergs from deflation of floodplains of transcontinental paleorivers with headwaters
in pre-Atlantic Appalachian highlands, and Jurassic-Cretaceous recycling of eolianite DZ from retroarc Sevier thrust sheets
and from sedimentary cover of the Mogollon paleohighlands flanking the Border rift system. 相似文献