Invariant manifolds, phase correlations of chaotic orbits and the spiral structure of galaxies

In the presence of a strong   m = 2  component in a rotating galaxy, the phase-space structure near corotation is shaped to a large extent by the invariant manifolds of the short-period family of unstable periodic orbits terminating at L ₁ or L ₂. The main effect of these manifolds is to create robust phase correlations among a number of chaotic orbits large enough to support a spiral density wave outside corotation. The phenomenon is described theoretically by soliton-like solutions of a Sine–Gordon equation. Numerical examples are given in an N -body simulation of a barred spiral galaxy. In these examples, we demonstrate how the projection of unstable manifolds in configuration space reproduces essentially the entire observed bar–spiral pattern.     

Seismic hazard monitoring in Kamchatka and its applications to the M = 6.6 earthquake of 6 October 1987

The earthquake of 6 October 1987 (M = 6.6), which occurred near the Shipunsky Cape, Kamchatka, was the largest crustal event in the vicinity of the main city of Kamchatka — Petropavlovsk-Kamchatsky — during the last three decades. It was followed by numerous aftershocks. This earthquake allowed us to test the effectiveness of the seismic hazard monitoring in Kamchatka, including the seismological, geodetic and hydrogeochemical surveys. The seismic survey provided the location and source nature of the main shock and aftershocks and the seismic environment of the main shock. The geodetic and hydrogeochemical surveys have yielded data on the response to earthquakes of the Earth's surface deformations, water level, and chemical elements concentration in the underground water. As a result, the following data were obtained:

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The earthquake of 6 October had a seismic moment 4–10 E18 Nm, thrust type of faulting and the source volume of 20 × 20 × 10 km³. The maximum intensity was VI–VII (MSK-64 scale) and maximum acceleration 88 cm/s².

Before this event, a relative increase in the number of the upper mantle (depth more than 100 km) moderate magnitude earthquakes during 5 years and a one-year period of seismic quiescence for small shallow earthquakes, were recognized. Significant anomalies in HCO₃ and H₃BO₃ concentrations in the underground waters were observed in the wells a week before the main shock.

     

Grain-scale variations in trace element composition of fluid-altered zircon,Acasta Gneiss Complex,northwestern Canada

SEM back-scattered electron images of zircon separates from a 2.9 Ga granitic dyke from the Acasta Gneiss Complex, Slave Province, reveal primary igneous oscillatory zoning which is transgressively overprinted by irregular alteration domains. Electron microprobe analyses show that altered zircon domains are depleted in Zr and Si relative to unaltered zircon while retaining a constant Zr/Si ratio. SIMS trace element analyses indicate that LREE are preferentially enriched in the altered domains. Altered zircon contains elevated concentrations of Ba (up to 580 ppm), Ca (up to 2% weight of the element) and common Pb (>50 ppb). The presence of LREE, Ca, Ba and common Pb in the zircons is interpreted to be the result of secondary alteration by an aqueous fluid, and not by primary incorporation during crystallization. We propose that the alteration-related elements are located in amorphous zircon domains.     

Gamma-Hadron Separation using Čerenkov Photon Density Fluctuations

In the atmospheric Čerenkov technique γ-rays are detected against the abundant background produced by hadronic showers. In order to improve the signal to noise ratio of theexperiment, it is necessary to reject a significant fraction of hadronic showers. Traditional background rejection methods based on image shape parameters have been extensively used for the data from imaging telescopes. However, non-imaging Čerenkov telescopes have to develop very different means of statistically identifying and removing cosmic ray events. Some of the parameters, which could be potentially important for non-imaging arrays, are the temporal and spectral differences, the lateral distributions and density fluctuations of Čerenkov photons generated by γ-ray and hadron primaries. Here we study the differences in fluctuations of Čerenkov photon density in the light pool at the observation level from showers initiated by photons and those initiated by protons or heavier nuclei. The database of simulated events for the PACT array has been used to evaluate the efficiency of the new technique. Various types of density fluctuations like the short range and medium range fluctuations as well as flatness parameter are studied. The estimated quality factors reflect the efficiencies with which the hadrons can be rejected from the data. Since some of these parameters are independent, the cuts may be applied in tandem and we demonstrate that the proton rejection efficiency of ∼90% can be achieved. Use of density fluctuations is particularly suited for wavefront sampling observations and it seems to be a good technique to improve the signal to noise ratio. This revised version was published online in July 2006 with corrections to the Cover Date.     

Compositional Zoning of the Bishop Tuff

Compositional data for >400 pumice clasts, organized accordingto eruptive sequence, crystal content, and texture, providenew perspectives on eruption and pre-eruptive evolution of the>600 km³ of zoned rhyolitic magma ejected as the Bishop Tuffduring formation of Long Valley caldera. Proportions and compositionsof different pumice types are given for each ignimbrite packageand for the intercalated plinian pumice-fall layers that eruptedsynchronously. Although withdrawal of the zoned magma was lesssystematic than previously realized, the overall sequence displaystrends toward greater proportions of less evolved pumice, morecrystals (0·5–24 wt %), and higher FeTi-oxide temperatures(714–818°C). No significant hiatus took place duringthe 6 day eruption of the Bishop Tuff, nearly all of which issuedfrom an integrated, zoned, unitary reservoir. Shortly beforeeruption, however, the zoned melt-dominant portion of the chamberwas invaded by batches of disparate lower-silica rhyolite magma,poorer in crystals than most of the resident magma but slightlyhotter and richer in Ba, Sr, and Ti. Interaction with residentmagma at the deepest levels tapped promoted growth of Ti-richrims on quartz, Ba-rich rims on sanidine, and entrapment ofnear-rim melt inclusions relatively enriched in Ba and CO₂.Varied amounts of mingling, even in higher parts of the chamber,led to the dark gray and swirly crystal-poor pumices sparselypresent in all ash-flow packages. As shown by FeTi-oxide geothermometry,the zoned rhyolitic chamber was hottest where crystal-richest,rendering any model of solidification fronts at the walls orroof unlikely. The main compositional gradient (75–195ppm Rb; 0·8–2·2 ppm Ta; 71–154 ppmZr; 0·40–1·73% FeO*) existed in the melt,prior to crystallization of the phenocryst suite observed, whichincluded zircon as much as 100 kyr older than the eruption.The compositions of crystals, though themselves largely unzoned,generally reflect magma temperature and the bulk compositionalgradient, implying both that few crystals settled or were transportedfar and that the observed crystals contributed little to establishingthat gradient. Upward increases in aqueous gas and dissolvedwater, combined with the adiabatic gradient (for the 5 km depthrange tapped) and the roofward decline in liquidus temperatureof the zoned melt, prevented significant crystallization againstthe roof, consistent with dominance of crystal-poor magma earlyin the eruption and lack of any roof-rind fragments among theBishop ejecta, before or after onset of caldera collapse. Amodel of secular incremental zoning is advanced wherein numerousbatches of crystal-poor melt were released from a mush zone(many kilometers thick) that floored the accumulating rhyoliticmelt-rich body. Each batch rose to its own appropriate levelin the melt-buoyancy gradient, which was self-sustaining againstwholesale convective re-homogenization, while the thick mushzone below buffered it against disruption by the deeper (non-rhyolitic)recharge that augmented the mush zone and thermally sustainedthe whole magma chamber. Crystal–melt fractionation wasthe dominant zoning process, but it took place not principallyin the shallow melt-rich body but mostly in the pluton-scalemush zone before and during batchwise melt extraction. KEY WORDS: Bishop Tuff; ignimbrite; magma zonation; mush model; rhyolite     

Anomalous Behavior of the Linear Polarization of the Herbig Ae Star WW Vul during the 1997 Minimum

In the course of photopolarimetric monitoring of the Herbig Ae star WW Vul, an unusual variation of linear polarization upon energence from a deep minimum was recorded, which cannot be explained by the standard mechanism for such objects (UX Ori stars): a variable contribution of radiation scattered by circumstellar dust. An analysis of possible causes of the anomalous behavior of polarization shows that it was most likely due to the presence of aligned, aspherical dust grains in the dust cloud crossing the line of sight at that time.     

Bizarre geochemistry of komatiites from the Crixás greenstone belt,Brazil

Komatiite lava flows in the Crixás greenstone belt, Goiás, Brazil, have textures and volcanic structures typical of Archean komatiites, but are geochemically most unusual. The flows are porphyritic and massive, or layered with spinifex upper parts and olivine cumulate lower parts. MgO contents range from 18 to 40%. In such lavas, only olivine (and minor chromite) can have crystallized, but neither major nor trace elements fall on olivine control lines. In MgO variation diagrams, CaO and Sr fall on lines with slopes steeper than olivine control lines; SiO₂, FeO, Na₂O, K₂O and Y show little systematic variation; Zr shows a large variation that does not correlate with MgO; and Al₂O₃ decreases markedly with decreasing MgO. The aberrant behaviour is highlighted by the REE (rare earth elements) in spinifex and olivine cumulate layers from three flows: in the spinifex layers, chondrite-normalized REE patterns are hump-shaped with maxima at Nd or Sm ((La/Sm)_N=0.6, (Gd/Yb)_N=1.6–2.3), whereas cumulate zones in the same flows have steadily sloping patterns, with LREE enriched relative to HREE ((La/Sm)_N=1.3, (Gd/Yb)_N=1.4). Neither normal magmatic processes acting during emplacement of the komatiites, nor thermal erosion and wall-rock assimilation can explain these effects, and we speculate that elements commonly thought of as “immobile” (e.g. Al, Zr, REE) migrated during hydrothermal alteration or metamorphism. A Pb-Pb whole rock isochron gave an age of 2,728±140 Ma and selected Sm-Nd analyses an apparent isochron age of 2,825±98 Ma (ɛ_Nd≈0). The Pb-Pb age is believed to be the approximate time of emplacement. Interpretation of the Sm-Nd data is complicated by the evidence of mobility of REE.