Some notes on radiation and long‐term climatic variations

Canadian definitions of break‐up and freeze‐up are shown to have changed substantially on several occasions since 1957. This paper demonstrates that because of these changes, great care must be exercised in the tabulation and interpretation of dates of break‐up and freeze‐up. In part, these changes have been occasioned by historical concerns, so that scrupulous attention to definition is necessary if the Canadian record is to be used for scientific purposes.     

Heterogeneity and thermal modeling of ground water

Heat transport in aquifers is becoming an increasingly important topic due to recent growth in the use of ground water in thermal applications. However, the effect of heterogeneity on heat transport in aquifers has yet to be examined in the same detail as it has been for solute transport, and it is unclear what effect this may have on our ability to create accurate models. This study examines this issue through stochastic modeling using the geostatistics for two aquifers with low and high degrees of heterogeneity. The results indicate that there is considerable uncertainty in the distribution of heat associated with injection of warm water into an aquifer. Heterogeneity in the permeability field was also found to slightly reduce the ability to recover this introduced heat at a later time. These simulations also reveal that hydrodynamic macrodispersion is an important consideration in some heat flow problems.     

Associations between zircon and Fe–Ti oxides in Hiltaba event magmatic rocks,South Australia: atomic- or pluton-scale processes?

Abstract

The Hiltaba Suite intrusive rocks and penecontemporaneous Gawler Range Volcanics (GRV) comprise the 1590?Ma Gawler silicic large igneous province in the Gawler Craton, South Australia. Zircon is principally associated with Fe–Ti oxides and clusters of touching crystals in these rocks, including in the Roxby Downs Granite (RDG), host of the Olympic Dam iron oxide–copper–gold deposit, and in other intrusive rocks that comprise the Olympic Province. There has been no explicit evaluation and explanation of potential origins published for concentrations of zircon with Fe–Ti oxides (herein zircon-rich clusters) found in these and similar rocks of western North America and elsewhere. Here we use U–Pb geochronology, mineral morphologies and compositions, and insights from surface chemistry and liquid-bound particle interaction studies to investigate zircon-rich clusters and provide a model for their formation. U–Pb geochronology does not reveal any concordant zircon populations older than ca 1590?Ma, so it is unlikely that there are significant xenocrystic zircon grains or that the zircons include significant inherited cores. The lack of pre-magmatic zircon, consistent intra-grain and inter-grain zircon compositional trends, the predominance of oscillatory zoned zircon with morphologies indicating growth from hot, evolved silicate melts, and the lack of evidence for zircon recrystallisation, indicates that zircon crystallised in the host GRV and RDG magmas. Variable zircon compositions within individual clusters does not support epitaxial nucleation of zircon on Fe–Ti oxides, but it is likely that some zircon grains grew from seed crystals formed by exsolution of Zr from Fe–Ti oxides. Aggregation of isolated, liquid-bound crystals is energetically favourable, and the grainsize discrepancy between larger crystals (Fe–Ti oxides, pyroxenes) and smaller accessory minerals (zircon, apatite) maximises the disparity in particle velocities and hence enhances the opportunities for collisions and adhesion between these crystals. We propose that zircon adheres to Fe–Ti oxides with greater ease and/or with greater bond strengths, than to other phases present in the parental magmas. It is possible that this association is related to interactions between zircon and Fe–Ti oxide surface sites with opposing charges, presuming the distance between phase surfaces is sufficiently small. The occurrence of small zircon grains within Fe–Ti oxides and both euhedral zircon and zircon with asymmetric growth zonation in contact with Fe–Ti oxides indicates that several processes are responsible for the high concentrations of zircon crystals in some Fe–Ti oxide clusters.

1. Zircon is principally associated with Fe--Ti oxides in 1.59 Ga Gawler Range Volcanics (GRV) and Roxby Downs Granite (RDG)

2. U–Pb geochronology does not reveal any concordant zircon populations older than ca 1590?Ma

3. Zircon compositions and morphologies indicate that zircon crystallised in the host RDG and GRV magmas and suggest recharge, reheating and mixing occurred in these magmatic systems

4. Seed crystals, aggregation and surface chemical affinities contributed to the strong association of zircon and Fe–Ti oxides

     

Aeromagnetic surveying using a simulated unmanned aircraft system

Carleton University and Sander Geophysics are developing an unmanned aircraft system (UAS) for aeromagnetic surveying. As an early indication of the expected performance of the unmanned aircraft system, a simulated unmanned aircraft system (sUAS) was built. The simulated unmanned aircraft system is a T‐shaped structure configured as a horizontal gradiometer with two cesium magnetometers spaced 4.67 m apart, which is the same sensor geometry as planned for the unmanned aircraft system. The simulated unmanned aircraft system is flown suspended beneath a helicopter. An 8.5 km² area in the Central Metasedimentary Belt of the Grenville Province, near Plevna, Ontario, Canada, was surveyed with the simulated unmanned aircraft system suspended 50 m above ground. The survey site was chosen on the basis of its complex geological structure. The total magnetic intensity (TMI) data recorded were compared to that obtained during a conventional fixed‐wing survey and a ground survey. Transverse magneto‐gradiometric data were also recorded by the simulated unmanned aircraft system. The simulated unmanned aircraft system total magnetic intensity data have a higher resolution than the conventional fixed‐wing data and were found to have a similar resolution to that of the ground survey data. The advantages of surveying with the simulated unmanned aircraft system were: (1) the acquisition of a detailed data set free of gaps in coverage at a low altitude above the terrain and (2) substantial saving of time and effort. In the survey site, the 4.67 m simulated unmanned aircraft system gradiometer measured the transverse magnetic gradient reliably up to an altitude of 150 m above ground.     

Central meridian passage dates of coronal holes,inferred from east-west solar scans at 692 and 1415 MHz,for the period January 1968–January 1974

The presence of solar coronal holes can be inferred from one-dimensional east-west scans at 692 and 1415 MHz. The scans indicate that coronal holes are stable structures with low-emissive characteristics and with lifetimes which can span several solar rotations, in agreement with observations using other techniques. This work focuses on the first half of 1973. The 1415 MHz data presented for this period show the radio analogues of two coronal holes, commonly referred to as CH1 and CH3. These holes were observed at soft X-ray and XUV wavelengths with the Skylab satellite and at EUV with the OSO-7 satellite. The analysis is then extended to cover the period from 1968 to 1974 with a central meridian passage date and a subjective classification being assigned to each coronal hole observation. This information is tabulated and provides a consistent set of coronal hole observations during the maximum and declining phases of solar cycle 20.     

On the stereology of a spherical segment of one base,with applications to the petrography of “cleavage domes” in a metagreywacke

Mathematical Geosciences - Dome-shaped concentrations of mica adjacent to garnet porphyroblasts in a hornfelsed metagreywacke are seen in thin-section as “cleavage arcs.” This paper...     

Interplay of eustatic,tectonic and autogenic controls on a Late Devonian carbonate platform,northern Canning Basin,Australia

Frasnian reef complexes along the northern margin of the Canning Basin in northwestern Australia evolved during rifting of the Fitzroy Trough. Geological investigations of the Frasnian Hull platform, which developed on an active tilted fault‐block, reveal significant lateral and vertical facies variations superimposed on prominent metre‐scale cyclicity. This study uses numerical analyses of facies and magnetic susceptibility data from three measured sections along the Hull platform to test whether a tectonic signal can be distinguished from eustatic and other signals. Geostatistical analysis of facies variations reveals an exponential distribution of thin (<3 m) facies, characteristic of stochastic depositional processes. Thick subtidal facies predominate in the Guppy Hills (GH) and southeastern Hull Range (SHR) sections near the hangingwall margin, and thick shallow‐subtidal to intertidal facies dominate the Horse Springs drillcore (HD 14) section near the footwall margin. Power and wavelet spectral analyses indicate a strong periodic component; Average Spectral Misfit and spectral optimisation methods confirm the presence of Milankovitch eccentricity signals and suggest the presence of obliquity and precession signals. However, the results also expose strong temporal and spatial variation providing evidence for tectonic control. Spectral analyses show strongest periodicity is recorded in short intervals that are not correlated across the platform and provide evidence of variations in sedimentation rate and hiatuses. Time series for the neighbouring GH and SHR sections show no overall statistical correlation, and Markov analysis indicates weakly ordered vertical facies transitions that do not correlate across the platform. Subtidal to intertidal facies data from HD 14 core suggest that at least 35% of the section is absent, almost obscuring the Milankovitch signal. The results indicate a complex set of controls on deposition on the Hull platform with local tectonic effects having produced spatio‐temporal moderation of the underlying eustatic signals and autogenic processes adding a localised stochastic response.