Iron-formations and epigenetic ironstones in the Palaeoproterozoic Willyama Supergroup, Olary Domain, South Australia

Summary Iron-formations occur as massive to compositionally layered, Fe oxide-rich, concordant bodies in the Palaeoproterozoic Willyama Supergroup of the Olary Domain, South Australia. They have constitutional similarities to those occurring in the neighbouring Broken Hill Domain. The most abundant iron-formations are in the Quartzofeldspathic Suite and comprise magnetite-quartz assemblages (± hematite, barite, actinolite, apatite). Hematite, magnetite, albite, quartz, Ca(Na) amphibole(s), CaNaFe clinopyroxene and andraditic garnet are major constituents of rare calc-silicate iron-formations in the Bimba and Calcsilicate Suites, whereas magnetite, quartz, almandine-spessartine, manganoan fayalite, manganoan grunerite and apatite form manganiferous iron-formations in the Pelite Suite. The pronounced differences in mineralogy of the three iron-formation types are the result of regional metamorphism of diverse hydrothermal precipitates with variable elastic components, together with the local effects of high-temperature metasomatic alteration. Metasomatic fluids were produced as a result of devolatilisation of the evaporite-bearing volcanosedimentary sequence, during and following amphibolite grade metamorphism and deformation, which led to localised and regional-scale hydrothermal alteration. In places, there was extensive metasomatic reconstitution (veining, brecciation, replacement) of iron-formations and associated rocks, caused by high-temperature (350°–650°C), oxidising, saline fluids. The resulting epigenetic ironstones are dominated by magnetite-hematite-quartz with minor sulfides and display enrichment in Fe, Ti, Cu, Au, Sc, U, V, Y, Zn and HREE relative to parental iron-formations.

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Eisenformationen und epigenetische Eisensteine in der Paläoproterozoischen Willyama Supergroup, Olary Domäne, Südaustralien

Zusammenfassung Eisenformationen kommen als massige bis in der Zusammensetzung geschichtete, Eisenoxidreiche, konkordante Körper in der paläoproterozoischen Willyama Supergroup der Olary Domäne, Südaustralien, vor. Sie haben konstitutionelle Ähnlichkeiten mit Vorkommen in der benachbarten Broken Hill Domäne. Die häufigsten Eisenformationen befinden sich in der Quartzofeldspathic Suite and bestehen aus Magnetit und Quarz (± Hämatit, Baryt, Aktinolit, Apatit). Hämatit, Magnetit, Albit, Quarz, Ca(Na) Amphibol(e), CaNaFe Klinopyroxen und andraditischer Granat sind Hauptbestandteile von seltenen Kalksilikat-Eisenformationen in den Bimba und Calcsilicate Suites, während Magnetit, Quartz, Almandin-Spessartin, manganhaltiger Fayalit, manganhaltiger Grunerit und Apatit manganhaltige Eisenformationen in der Pelite Suite bilden. Die ausgeprägten Unterschiede in der Mineralogie der drei Typen von Eisenformationen sind durch Regionalmetamorphose von diversen hydrothermalen Ausfällungen mit variablen klastischen Komponenten verursacht worden, zusammen mit lokalen Wirkungen einer hock-Temperatur metasomatischen Alteration. Metasomatische Fluide wurden während und nach der Amphibolitmetamorphose und Deformation durch Devolatilisation der evaporithaltigen vulkanosedimentären Abfolge produziert, die sowohl zu lokaler, wie auch zu weiträumiger hydrothermaler Alteration führten. Örtlich kam es zu umfangreicher metasomatischer Rekonstitution (Gangbildung, Brekkzierung, Verdrängung) von Eisenformationen und assoziierten Gesteinen verursacht durch hoch-Temperatur (350°–650°C), oxidierte, saline Fluide. Die resultierenden epigenetischen Eisensteine bestehen hauptsächlich aus Magnetit, Hämatit, und Quarz mit Sulfiden und weisen eine Anreicherung in Fe, Ti, Cu, Au, Sc, U, V, Y, Zu und SREE relativ gegenüber den ursprünglichen Eisenformationen auf.

     

Molecular hydrogen line emission from the reflection nebula Parsamyan 18

The newly commissioned University of New South Wales Infrared Fabry–Perot (UNSWIRF) has been used to image molecular hydrogen emission at 2.12 and 2.25 μm in the reflection nebula Parsamyan 18. P 18 is known to exhibit low values of the (1–0)/(2–1) S(1) ratio, suggestive of UV-pumped fluorescence rather than thermal excitation by shocks. Our line ratio mapping reveals the full extent of this fluorescent emission from extended arc-like features, as well as a more concentrated thermal component in regions closer to the central exciting star. We show that the emission morphology, line fluxes and gas density are consistent with the predictions of photodissociation region (PDR) theory. Those regions with the highest intrinsic 1–0 S(1) intensities also tend to show the highest (1–0)/(2–1) S(1) line ratios. Furthermore, variations in the line ratio can be attributed to intrinsic fluctuations in the incident radiation field and/or the gas density, through the self-shielding action of H₂. An isolated knot of emission discovered just outside P 18, and having both an unusually high (1–0)/(2–1) S(1) ratio and relative velocity, provides additional evidence for an outflow source associated with P 18.     

Modelling the chloride signal at Plynlimon,Wales, using a modified dynamic TOPMODEL incorporating conservative chemical mixing (with uncertainty)

The application of a modified version of dynamic TOPMODEL for two subcatchments at Plynlimon, Wales is described. Conservative chemical mixing within mobile and immobile stores has been added to the hydrological model in an attempt to simulate observed stream chloride concentrations. The model was not fully able to simulate the observed behaviour, in particular the short‐ to medium‐term dynamics. One of the primary problems highlighted by the study was the representation of dry deposition and cloud‐droplet‐deposited chloride, which formed a significant part of the long‐term chloride mass budget. Equifinality of parameter sets inhibited the ability to determine the effective catchment mixing volumes and coefficients or the most likely partition between occult mass inputs and chloride mass inputs determined by catchment immobile‐store antecedent conditions. Some success was achieved, in as much as some aspects of the dynamic behaviour of the signal were satisfactorily simulated, although spectral analysis showed that the model could not fully reproduce the 1/f power spectra of observed stream chloride concentrations with its implications of a wide distribution of residence times for water in the catchment. Copyright © 2006 John Wiley & Sons, Ltd.     

Crustal structure beneath exposed accreted terranes of Southern Alaska

Summary. The crustal structure beneath the exposed terranes of southern Alaska has been explored using coincident seismic refraction and reflection profiling. A wide-angle reflector at 8–9 km depth, at the base of an inferred low-velocity zone, underlies the Peninsular and Chugach terranes, appears to truncate their boundary, and may represent a horizontal decollement beneath the terranes. The crust beneath the Chugach terrane is characterized by a series of north-dipping paired layers having low and high velocities that may represent subducted slices of oceanic crust and mantle. This layered series may continue northward under the Peninsular terrane. Earthquake locations in the Wrangell Benioff zone indicate that at least the upper two low-high velocity layer pairs are tectonically inactive and that they appear to have been accreted to the base of the continental crust. The refraction data suggest that the Contact fault between two similar terranes, the Chugach and Prince William terranes, is a deeply penetrating feature that separates lower crust (deeper than 10 km) with paired dipping reflectors, from crust without such reflectors.     

Swift observations of the X-ray and UV evolution of V2491 Cyg (Nova Cyg 2008 No. 2)

We present extensive, high-density Swift observations of V2491 Cyg (Nova Cyg 2008 No. 2). Observing the X-ray emission from only one day after the nova discovery, the source is followed through the initial brightening, the super-soft source phase and back to the pre-outburst flux level. The evolution of the spectrum throughout the outburst is demonstrated. The UV and X-ray light curves follow very different paths, although changes occur in them around the same times, indicating a link between the bands. Flickering in the late-time X-ray data indicates the resumption of accretion. We show that if the white dwarf (WD) is magnetic, it would be among the most magnetic known; the lack of a periodic signal in our later data argues against a magnetic WD, however. We also discuss the possibility that V2491 Cyg is a recurrent nova, providing recurrence time-scale estimates.     

Phytoliths infer locally dense and heterogeneous paleovegetation at FLK North and surrounding localities during upper Bed I time, Olduvai Gorge, Tanzania

The phytolith content of 10 samples collected immediately under Tuff IF (~ 1.785 Ma) at FLK N and other surrounding localities (~ 2 km²) provides a direct botanical evidence for woody vegetation in the eastern margin of Olduvai Gorge during uppermost Bed I time. Observation and counting of 143 phytolith types (5 to >150 μ) reveal dense but heterogeneous woody cover (~ 40–90%) of unidentified trees and/or shrubs and palms associated to the freshwater springs surrounding FLK N, and more open formation (presumably ~ 25–70% woody cover) in the southeast at localities VEK, HWK W and HWK E. The paleovegetation is best described as groundwater palm forest/woodland or bushland, which current analogue may be found near Lake Manyara in similar hydrogeological context (freshwater springs near saline/alkaline lake). Re-evaluating the published pollen data based on this analogy shows that 70% of the pollen signal at FLK N may be attributed to thicket-woodland, Acacia groundwater woodland, gallery and groundwater forests; while < 30% is attributed to swamp herbage and grasslands. Micro-botanical, isotopic, and taphonomical studies of faunal remains converge on the same conclusion that the area surrounding FLK N, which attracted both carnivores and hominins in the early Pleistocene, was densely wooded during uppermost Bed I time.