Basin provenance and its control on mineralisation within the Early Devonian Cobar Basin, western Lachlan Orogen, eastern Australia

The Cobar Basin, in central New South Wales, is an Early Devonian extensional basin that formed in the western Lachlan Orogen. The basin was filled with shallow- to deep-water sequences of the Cobar Supergroup and hosts small to large polymetallic deposits. Detrital zircon geochronology and whole-rock geochemical data collected from the Amphitheatre Group of the Cobar Supergroup provide constraints on the history of basin fill and illustrate the dynamic interplay between basin provenance and mineralisation, corresponding to the evolving tectonic regime. Data reveal provenance dissimilarity between the southern and northern parts of the basin. In the south, units of the Amphitheatre Group received abundant detritus from ca 430–410 Ma magmatic rocks situated to the southwest and southeast of the basin. By contrast, the northern successions were predominately sourced from recycled Ordovician basement found to the northwest, north, northeast of the basin, along with contributions associated with the Macquarie Arc. This spatial provenance variation, however, is less significant in the younger formations: the northern and southern sequences both exhibit an increase in older recycled detritus upwards with time. This reflects a progressive modification of basin paleogeography, during the transition from rift phase to sag phase. The rift-phase basin geography is characterised by fault-restricted deposition with predominant sediments derived from local proximal sources. The subsequent sag-phase subsidence exhibits a uniform depositional system with more homogenised basin input. This provenance variation is coeval with the stratigraphically controlled mineralisation features within the Amphitheatre Group successions, implying a provenance influence on mineralisation. Data suggest the different sediment source regions have produced distinct detrital mineral compositions between the major mineral-hosting and mineral-barren formations. The enrichment of some detrital minerals in the mineral-hosting units, such as feldspar, muscovite, Ti-minerals (and carbonate), is suggested to be an important factor for mineralisation in the basin. Detrital zircon geochronology and whole-rock geochemistry illustrate the history of basin fill for the Cobar Basin. Spatial and temporal variation of basin provenance reflects a modification of basin geography, corresponding to the evolving tectonic regimes. The change in basin source regions is one of key controls on the mineralisation within the Cobar Basin.</p

Hydroxylation of Metal-Supported Sheet-Like Silica Films

Adsorption of water on a metal-supported sheet-like silica film was studied by infrared reflection absorption spectroscopy (IRAS) and temperature-programmed desorption (TPD). As expected, the silica surface is essentially hydrophobic. Hydroxo species, primarily in the form of isolated silanols (Si–OH), were observed only upon water condensation at low temperatures and subsequent heating above 200 K. The amounts of silanol species account for less than a few percent of the surface Si atoms, and they are found to be thermally stable up to 900 K. Isotopic experiments showed that hydroxyls form almost exclusively from the adsorbed water molecules and do not undergo scrambling with the lattice oxygen atoms upon heating. Steps within the silica sheet, due to a terraced topography and/or the presence of “holes”, are proposed as the active sites for hydroxylation. The acidic properties of silanol species were studied with CO and NH₃ as probe molecules. In the case of ammonia, an H–D exchange reaction was observed between OD species and NH₃, and the same reaction was found to occur for OD­(OH) and H₂O­(D₂O), respectively. The results are compared with those reported in the literature for amorphous silica

Infrared observations of the 2006 outburst of the recurrent nova RS Ophiuchi: The early phase

We present infrared spectroscopy of the recurrent nova RS Ophiuchi, obtained 11.81, 20.75 and 55.71 d following its 2006 eruption. The spectra are dominated by hydrogen recombination lines, together with He i, O i and O ii lines; the electron temperature of ∼104 K implied by the recombination spectrum suggests that we are seeing primarily the wind of the red giant, ionized by the ultraviolet flash when RS Oph erupted. However, strong coronal emission lines (i.e. emission from fine structure transitions in ions having high ionization potential) are present in the last spectrum. These imply a temperature of 930 000 K for the coronal gas; this is in line with X-ray observations of the 2006 eruption. The emission linewidths decrease with time in a way that is consistent with the shock model for the X-ray emission