Cygnus A: stars, dust and cones

We present new Hubble Space Telescope ( HST  ) continuum and spectral line images of the radio galaxy Cygnus A. The images show much complex structure in the central kpc². Continuum images show the central dust lane in detail, allowing detailed maps of E ( B  −  V ) to be constructed; the dust appears to follow a roughly Galactic extinction law. The emission-line components are resolved in the line images and investigated in detail. A clear 'opening cone' morphology is found, especially in the lines of Hα and [O  i ]. Blue condensations are seen in the south-eastern emission component and surrounding the central region. These are almost certainly due to star formation, which began <1 Gyr ago as deduced from the colour of the regions. More extended blue continuum is also seen and corresponds to the blue polarized component detected by other recent spectropolarimetric observations.     

Magnetisation history of lower old red sandstones from the Gamrie Outlier,Scotland

Red beds in the Lower Old Red Sandstone Gamrie Outlier on the southern margin of the Orcadian Basin originated through the diagenetic breakdown of iron silicates. The magnetic evidence indicates that haematite pigment was produced over a long time period because both normal and reversed components of magnetisation are present in individual specimens. A part of the magnetisation in these rocks is carried by coarse particle specularite but there is no evidence that this is DRM or PDRM. The composite nature of the magnetisation and its acquisition over a relatively long time period is also indicated by the fact that all sample/site mean directions are of the same polarity (reversed) but originated over a period when normal and reversed polarities were common. There is evidence that the production of early diagenetic pigment was necessary for the preservation of Devonian magnetisations in these sandstones.Another group of sandstones acquired their magnetisation in Permo-Carboniferous times and these occur in intimate association with those which retain the original composite Devonian direction. The stable NRM of these rocks is carried by specularite and they are thought to have been remagnetised because they originally contained a greater proportion of magnetite than those sandstones which retain the original magnetisation and also an unstable pigment magnetisation. In-situ oxidation of magnetite in Permo-Carboniferous times is thus thought to be responsible for the remagnetisation of these rocks.     

Microtextural analysis of a glacially ‘deformed’ bedrock: implications for inheritance of preferred clast orientations in diamictons

Although analysis of clast macrofabrics has been used to differentiate between different types of glacial diamictons and to determine palaeo‐ice flow directions, no account appears to have been made of preferred clast orientations inherited from the parental source material. Clast macrofabrics in tills are typically interpreted as having developed in response to an imposed subglacial deformation and as such provide a link between the sedimentary record and glacier dynamics. They rely on the assumption that any preferred clast orientation is a result of deformation/flow. The results of the micromorphological study of the Langholm Till exposed at North Corbelly near Dumfries (southwestern Scotland) clearly demonstrate that bedrock structure can influence clast orientation (macrofabric) within diamictons. In the lower part of the till, the orientation of elongate clasts preserves the geometry of the tectonic cleavage present within the underlying bedrock. The intensity of this steeply inclined, ‘inherited’ clast fabric decreases upward through the till, to be replaced by a more complex pattern of successive generations of clast microfabrics developed in response to deformation/flow. These results indicate potential limitations of applying clast macrofabric or microfabric analysis in isolation to establish till genesis or palaeo ice‐flow directions. Consequently, due account should be made of other glacial palaeo‐environmental and ice flow indicators, as well as rockhead depth and morphology in relation to the selection of fabric measurements sites. © British Geological Survey/Natural Environment Research Council copyright 2007. Reproduced with the permission of BGS/NERC. Published by John Wiley & Sons, Ltd.     

Monitoring playa sedimentation using sequential radar images

A series of ERS-1 radar images of the Chott el Djerid playa in Tunisia obtained at intervals of 35 days demonstrates the ability to monitor geological change from space. The C-band radar backscatter is shown from field measurements to be sensitive to changes in surface roughness at millimetric scale due to evaporative growth of halite and gypsum crystals. Backscatter increases gradually during the dry summer months and decreases rapidly in early winter as the rains begin. Episodic hydrogeological and sedimentological events including lake development, channel flow and aeolian deposition can also be detected in addition to the seasonal pattern. Multitemporal, spaceborne radar offers the prospect of being able to map the dynamics of evaporite sedimentation in continental playas.     

Basait generation at the Apollo 12 site,Part 2: Source heterogeneity,multiple melts,and crustal contamination

Abstract— The petrogenesis of Apollo 12 mare basalts has been examined with emphasis on trace-element ratios and abundances. Vitrophyric basalts were used as parental compositions for the modelling, and proportions of fractionating phases were determined using the MAGFOX program of Longhi (1991). Crystal fractionation processes within crustal and sub-crustal magma chambers are evaluated as a function of pressure. Knowledge of the fractionating phases allows trace-element variations to be considered as either source related or as a product of post-magma-generation processes. For the ilmenite and olivine basalts, trace-element variations are inherited from the source, but the pigeonite basalt data have been interpreted with open-system evolution processes through crustal assimilation. Three groups of basalts have been examined: (1) Pigeonite basalts — produced by the assimilation of lunar crustal material by a parental melt (up to 3% assimilation and 10% crystal fractionation, with an “r” value of 0.3). (2) Ilmenite basalts — produced by variable degrees of partial melting (4–8%) of a source of olivine, pigeonite, augite, and plagioclase, brought together by overturn of the Lunar Magma Ocean (LMO) cumulate pile. After generation, which did not exhaust any of the minerals in the source, these melts experienced closed-system crystal fractionation/accumulation. (3) Olivine basalts — produced by variable degrees of partial melting (5–10%) of a source of olivine, pigeonite, and augite. After generation, again without exhausting any of the minerals in the source, these melts evolved through crystal accumulation. The evolved liquid counterparts of these cumulates have not been sampled. The source compositions for the ilmenite and olivine basalts were calculated by assuming that the vitrophyric compositions were primary and the magmas were produced by non-modal batch melting. Although the magnitude is unclear, evaluation of these source regions indicates that both be composed of early- and late-stage Lunar Magma Ocean (LMO) cumulates, requiring an overturn of the cumulate pile.