Timing of advance and basal condition of the Laurentide Ice Sheet during the last glacial maximum in the Richardson Mountains,NWT

This study presents new ages for the northwest section of the Laurentide Ice Sheet (LIS) glacial chronology from material recovered from two retrogressive thaw slumps exposed in the Richardson Mountains, Northwest Territories, Canada. One study site, located at the maximum glacial limit of the LIS in the Richardson Mountains, had calcite concretions recovered from aufeis buried by glacial till that were dated by U/Th disequilibrium to 18,500 cal yr BP. The second site, located on the Peel Plateau to the east yielded a fossil horse (Equus) mandible that was radiocarbon dated to ca. 19,700 cal yr BP. These ages indicate that the Peel Plateau on the eastern flanks of the Richardson Mountains was glaciated only after 18,500 cal yr BP, which is later than previous models for the global last glacial maximum (LGM). As the LIS retreated the Peel Plateau around 15,000 cal yr BP, following the age of the Tutsieta phase, we conclude that the presence of the northwestern margin of the LIS at its maximum limit was a very short event in the western Canadian Arctic.     

Mineralogical and chemical evolution of the Ernest Henry Fe oxide–Cu–Au ore system, Cloncurry district, northwest Queensland, Australia

The Ernest Henry Cu–Au deposit was formed within a zoned, post-peak metamorphic hydrothermal system that overprinted metamorphosed dacite, andesite and diorite (ca 1740–1660 Ma). The Ernest Henry hydrothermal system was formed by two cycles of sodic and potassic alteration where biotite–magnetite alteration produced in the first cycle formed ca 1514±24 Ma, whereas paragenetically later Na–Ca veining formed ca 1529 +11/−8 Ma. These new U–Pb_titanite age dates support textural evidence for incursion of hydrothermal fluids after the metamorphic peak, and overlap with earlier estimates for the timing of Cu–Au mineralization (ca 1540–1500 Ma). A distal to proximal potassic alteration zone correlates with a large (up to 1.5 km) K–Fe–Mn–Ba enriched alteration zone that overprints earlier sodic alteration. Mass balance analysis indicates that K–Fe–Mn–Ba alteration—largely produced during pre-ore biotite- and magnetite-rich alteration—is associated with K–Rb–Cl–Ba–Fe–Mn and As enrichment and Na, Ca and Sr depletion. The aforementioned chemical exchange almost precisely counterbalances the mass changes associated with regional Na–Ca alteration. This initial transition from sodic to potassic alteration may have been formed during the evolution of a single fluid that evolved via alkali exchange during progressive fluid-rock interaction. Cu–Au ore, dominated by co-precipitated magnetite, minor specular hematite, and chalcopyrite as breccia matrix, forms a pipe-like body at the core of a proximal alteration zone dominated by K-feldspar alteration. Both the core and K-feldspar alteration overprint Na–Ca alteration and biotite–magnetite (K–Fe) alteration. Ore was associated with the concentration of a diverse range of elements (e.g. Cu, Au, Fe, Mo, U, Sb, W, Sn, Bi, Ag, F, REE, K, S, As, Co, Ba and Ca). Mineralization also involved the deposition of significant barite, K(–Ba)–feldspar, calcite, fluorite and complexly zoned pyrite. The complexly zoned pyrite and variable K–(Ba)–feldspar versus barite associations are interpreted to indicate fluctuating sulphur and/or barium supply. Together with the alteration zonation geochemistry and overprinting criteria, these data are interpreted to indicate that Cu–Au mineralization occurred as a result of fluid mixing during dilation and brecciation, in the location of the most intense initial potassic alteration. A link between early alteration (Na–Ca and K–Fe) and the later K-feldspathization and the Cu–Au ore is possible. However, the ore-related enrichments in particular elements (especially Ba, Mn, As, Mo, Ag, U, Sb and Bi) are so extreme compared with earlier alteration that another fluid, possibly magmatic in origin, contributed the diverse element suite geochemically independently of the earlier stages. Structural focussing of successive stages produced the distinctive alteration zoning, providing a basis both for exploration for similar deposits, and for an understanding of ore genesis.     

What is a flux tube? On the magnetic field topology of buoyant flux structures

We study the topology of field lines threading buoyant magnetic flux structures. The magnetic structures, visually resembling idealized magnetic flux tubes, are generated self-consistently by numerical simulation of the interaction of magnetic buoyancy and a localized velocity shear in a stably stratified atmosphere. Depending on the parameters, the system exhibits varying degrees of symmetry. By integrating along magnetic field lines and constructing return maps, we show that, depending on the type of underlying behaviour, the stages of the evolution, and therefore the degree of symmetry, the resulting magnetic structures can have field lines with one of three distinct topologies. When the x -translational and y -reflectional symmetries remain intact, magnetic field lines lie on surfaces but individual lines do not cover the surface. When the y symmetry is broken, magnetic field lines lie on surfaces and individual lines do cover the surface. When both x and y symmetries are broken, magnetic field lines wander chaotically over a large volume of the magnetically active region. We discuss how these results impact our simple ideas of a magnetic flux tube as an object with an inside and an outside, and introduce the concept of 'leaky' tubes.     

Luminosity and surface brightness distribution of K-band galaxies from the UKIDSS Large Area Survey

We present luminosity and surface-brightness distributions of 40 111 galaxies with K -band photometry from the United Kingdom Infrared Telescope (UKIRT) Infrared Deep Sky Survey (UKIDSS) Large Area Survey (LAS), Data Release 3 and optical photometry from Data Release 5 of the Sloan Digital Sky Survey (SDSS). Various features and limitations of the new UKIDSS data are examined, such as a problem affecting Petrosian magnitudes of extended sources. Selection limits in K - and r -band magnitude, K -band surface brightness and K -band radius are included explicitly in the  1/ V _max  estimate of the space density and luminosity function. The bivariate brightness distribution in K -band absolute magnitude and surface brightness is presented and found to display a clear luminosity–surface brightness correlation that flattens at high luminosity and broadens at low luminosity, consistent with similar analyses at optical wavelengths. Best-fitting Schechter function parameters for the K -band luminosity function are found to be   M *− 5 log  h =−23.19 ± 0.04, α=−0.81 ± 0.04  and  φ*= (0.0166 ± 0.0008)  h ³ Mpc⁻³  , although the Schechter function provides a poor fit to the data at high and low luminosity, while the luminosity density in the K band is found to be   j = (6.305 ± 0.067) × 10⁸ L_⊙  h  Mpc⁻³  . However, we caution that there are various known sources of incompleteness and uncertainty in our results. Using mass-to-light ratios determined from the optical colours, we estimate the stellar mass function, finding good agreement with previous results. Possible improvements are discussed that could be implemented when extending this analysis to the full LAS.     

On the iron interpretation of the 6.4 keV emission line from SGR 1900+14

A 6.4 keV emission line was discovered in an unusual burst from the soft gamma repeater SGR 1900+14 with the Rossi X-ray Timing Explorer (RXTE). The line was detected in part of a complex multipeak precursor that preceded the unusual burst of 1998 August 29, i.e. two days after the giant flare of August 27 from the source. The origin of the line was not firmly identified and two possible interpretations were equally plausible including (a) Kα fluorescence from a small iron rich material that was ejected to the magnetosphere during the August 27 flare, and (b) proton or α-particle cyclotron resonance. If the iron scenario was correct, we expect to find evidence for the line during the intervening interval between the flare and the August 29 burst, i.e. on August 28. Here we present the results of the August 28 burst observation, taken with RXTE. We detect a total of seven bursts whose individual and joint spectra do not show evidence for spectral lines. We also investigated a sample of nine bursts before and after the August 29 burst (from 1998 June to December) that do not reveal evidence for a spectral line near 6.4 keV or elsewhere. These results disfavor the iron scenario and make the proton/α-particle cyclotron resonance interpretation more plausible. The appearance of the emission line in part of a complex burst and its absence from the studied sample indicate that the line is likely due to a transient phenomenon that may depend on the burst morphology, energetics and the properties of the emission region.      

The NASA Lunar Impact Monitoring Program

NASA’s Meteoroid Environment Office has implemented a program to monitor the Moon for meteoroid impacts from the Marshall Space Flight Center. Using off-the-shelf telescopes and video equipment, the Moon is monitored for as many as 10 nights per month, depending on weather. Custom software automatically detects flashes which are confirmed by a second telescope, photometrically calibrated using background stars, and published on a website for correlation with other observations. Hypervelocity impact tests at the Ames Vertical Gun Range facility have begun to determine the luminous efficiency and ejecta characteristics. The purpose of this research is to define the impact ejecta environment for use by lunar spacecraft designers of the Constellation manned lunar program. The observational techniques and preliminary results will be discussed. The U.S. Government's right to retain a non-exclusive, royalty-free license in and to any copyright is acknowledged.