An isotopic study of hydrocarbon generation processes

The sources of reservoired liquid hydrocarbons in Australia differ substantially from their Northern Hemisphere counterparts. Not only are Australian source rocks largely of terrigenous origin but the concepts accepted as defining the hydrocarbon potential of source rocks have recently been challenged. Although isotopic measurements on Australian source rocks and liquid hydrocarbon products closely parallel those published from elsewhere, new interpretations of these data have been made, particularly with regard to D/H ratios in coals. As interesting as such results may be, the most important findings relate to the origins and histories of methane, the higher gaseous hydrocarbons and carbon dioxide, and the interpretation of these data in terms of hydrocarbon genesis. Simple Northern Hemisphere concepts which characterize gases as being biogenic, oil-associated, etc., on the bases of their isotopic and chemical compositions are clearly not applicable to the Australian scene. Indeed justification of such precise classifications in the Northern Hemisphere on the available evidence is also questioned.Isotopic and compositional data from gases from the Cooper, Bowen, Sydney and Gippsland Basins and from the North West Shelf (Australia) are reviewed, and assessed in terms of generation pathways.     

A tem microstructural study of dolomite with curved faces (saddle dolomite)

Electron diffraction, analytical electron microscopy, and high voltage, high resolution electron microscopy have been used to investigate crystal defects in calcium-rich saddle dolomites having pronounced curvature of the faces. Results show that branching, ribbon-like defects in these so-called saddle dolomites are thin, coherent laths of calcitic material. The ribbons are profuse and explain the characteristic calcium excess found in most saddle dolomites. Because the lattice spacings of calcite are between 3.8% and 6.7% larger than the corresponding lattice spacings of dolomite, a calcitic ribbon causes local distortion of the host dolomite. The branching ribbons have a predominant {10¯14} orientation and are generally present in high density. They may represent the source of crystal distortion that ultimately manifests itself on the macroscopic scale. The calcitic ribbons form during growth from aqueous solution, although they have features in common with similar defects found in carbonatite carbonates. This fine-scale intergrowth microstructure may be a variant of even finer-scale modulated structures found in other sedimentary calcian dolomites.     

Lithic breccias in pyroclastic flow deposits on St. Kitts,West Indies

Lithic-rich breccias are described from within a sequence of young (2000–3000 yrs B.P.) scoria and ash flow deposits erupted from Mount Misery and an older pumice and ash flow deposit (ignimbrite) on St. Kitts. Cross sections constructed through pyroclastic flow fans in well-exposed sea cliffs 4–6 km from the vent show that the lithic breccias are lensoid deposits which seem to occur as channel-shaped accumulations (up to > 20 m thick and > 150 m wide) within flow units. The best-developed example infills a deeply incised channel cut into older flow units. The coarsest lithic breccias are clast supported and fines depleted and grade laterally and vertically through finer-grained, matrix-supported breccias into scoria and ash flow deposits. Coarse scoria-concentration zones mainly occur at the tops of scoria and ash flow units but also at the bases, and gas-segregation pipes are common. The lithic breccias are a type of body-concentration deposit as they pass laterally into normal scoria and ash flow deposits and, where best developed, clearly occur above a reversely graded basal shear zone or layer. Grain-size studies indicate the lithic breccias and parent flows are strongly fines depleted and were highly fluidized. We suggest this may be a feature of many Lesser Antillean pyroclastic flows because of increased turbulence-induced fluidization resulting from a high degree of surface roughness caused by the steep (up to 40 °) irregular slopes, densely vegetated sinuous gullies of the tropical volcanoes, and ingestion and ignition of large amounts of lush vegetation. Accumulation of batches of lithics concentrated in the highly fluidized flows began at the break in slope where flows moved from gullies across hydraulic jumps onto the outer coastal flanks. The accumulations of breccias continued to move and be channelled down the central parts of the flows. Initially, on crossing onto the lower slopes, some of these flows seem to have had very powerfully erosive, nondepositional heads, and in the extreme example a deep channel as long as 1–2 km may have cut through underlying flow units at least as far as the present coastline. Much of the overriding remainder of the flow then drained away laterally. Thin, fine-grained ash flow deposits may form a marginal overbank facies to the pyroclastic flow fans.     

Radon-222 in groundwater of the Long Valley caldera,California

In the Long Valley caldera, where seismicity has continued essentially uninterrupted since mid-1980 and uplift is documented, samples of water from hot, warm, and cold springs have been collected since September, 1982, and their²²²Rn concentrations analyzed. Concurrently, rocks encompassing the hydrologic systems feeding the springs were analyzed for their radioelement contents, because their uranium is the ultimate source of the²²²Rn in the water.The²²²Rn concentration in the springs varies inversely with their temperature and specific conductance. High concentrations (1500 to 2500 picocuries per liter) occur in dilute cold springs on the margins of the caldera, while low contents (12 to 25 pCi/l) occur in hot to boiling springs. Springwater radon concentrations also correlate slightly with the uranium content of the encompassing rocks.A continuous monitoring system was installed in August, 1983, at a spring issuing from basalt, to provide hourly records of radon concentration. A gamma detector is submerged in a natural pool, and we have observed that the radioactivity measured in this manner is due almost entirely to the²²²Rn concentration of the water. Initial operation shows diurnal and semidiurnal variations in the²²²Rn concentration of the springwater that are ascribed to earth tides, suggesting that those variations are responding to small changes in stress in the rocks encompassing the hydrologic system.     

Solutions of the integral equation of diffusion and the random walk model for continuous plumes and instantaneous puffs in the atmospheric boundary layer

The integral equation method is related to the random walk modelling that has proved so effective and popular in recent years. The I.E. method, by using simple probability techniques, avoids the inefficient determination of thousands of trajectories in order to build up concentration profiles. In fact it is so simple and efficient it can be run on a conventional programmable calculator. The method is applied to passive material being released from an elevated source within a neutrally stable surface layer over a uniform surface, and also to an instantaneous release when the effect of wind shear is examined. The latter scenario is also studied using random walk techniques and a comparison of the solutions obtained. Agreement is very good, although downwind spread is shown to be quite sensitive to gridlength size in the I.E. method.     

Beryllium-10 contents of core samples from the St. Severin meteorite

Eleven samples taken from along the 35-cm core AIII of the LL-chondrite St. Severin have ¹⁰Be contents ranging from 20 to 28 dpm/kg and averaging 24.5 ± 0.9 dpm/kg. The ¹⁰Be contents increase with depth in the outermost 8 cm while at depths greater than 8 cm the ¹⁰Be contents change little. Calculations based on cross sections for proton-induced reactions that make ¹⁰Be disagree seriously with the measured values. Relatively large cross sections were constructed for neutron-induced reactions below 1 GeV. They give calculated ¹⁰Be production rates that agree with the measured values to within 10%. Cosmogenic ¹⁰Be in stony meteorites is better described as a medium-energy product than as a high-energy one.     

Public participation in policy making: the state-of-the-art in Canada

This paper critically reviews the current status of public participation in Canada. Postulating that public participation implies an open, democratic form of planning and policy making, the paper examines factors which have influenced the attainment of meaningful public input in practice. Public participation in Canada has evolved greatly in the past 15 years but several key issues remain unresolved: provisions for participation remain, in large part, discretionary; procedures for the funding of intervenors require further development; and the emphasis in many jurisdictions remains upon the operational level involvement of the lay public, with a public hearing as the final arbiter of public opinion. More attention to the formal evaluation of participatory exercises is essential to the resolution of these concerns. In particular, more attention should be given to the context within which participation occurs. Moreover, greater consideration should be given to public involvement at the normative and strategic levels of planning. The paper concludes by briefly exploring the implications for practice that the concept of ‘normative participation’ might have in Canada.     

Paleohydrologic regimes in the southwestern Great Basin, 0–3.2 my ago,compared with other long records of “gobal” climate

The oxygen-isotope record from fossil foraminifera in deep-sea sediments is commonly used as a proxy for global ice volume. The linkage between δ¹⁸O and ice volume, however, is probably nonlinear. We have developed a simple numerical model of the isotopic response of the oceans to ice-volume change. The major features it simulates are (1) the changing mean isotopic composition of snow as a function of ice volume (colder snow temperatures forced by climate change and higher-elevation accumulation areas imply more negative mean δ¹⁸O); (2) the nonequilibrium isotopic composition of ice sheets (the past history of an ice sheet is integrated into its mean isotopic composition, which introduces a lag of isotopic “ice volume,” i.e., the measured δ¹⁸O record, scaled to ice-volume units, behind true ice volume); (3) selective preservation of isotopically more negative (colder, higher-latitude) ice (this geographic effect can selectively amplify or dampen the isotopic response to the ice-volume signal). We illustrate the response of our model to simple hypothetical ice-volume transitions of ice growth and ice decay. Sensitivity tests are illustrated for all model parameters. The results suggest that oxygen-isotope records reproduce the general patterns of ice-volume change fairly accurately. The foraminiferal isotope record, however, may misrepresent the true amplitude of the ice-volume signal and lag true ice volume by 1000 to 3000 yr.     

Zoned minerals in garnet peridotite nodules from the Colorado Plateau: implications for mantle metasomatism and kinetics

Some garnet peridotite nodules from The Thumb, a minette neck on the Colorado Plateau in the southwestern United States, contain zoned minerals. Zoning does not exceed 1.5 wt.% for any oxide, but some relative changes are large: in one garnet TiO₂ and Cr₂O₃ ranges are 0.05–0.65 and 3.5–5.0 wt.%, respectively. In two porphyroclastic nodules, garnet rims are depleted in Mg and enriched in Fe, Ti, and Na compared to cores, and one garnet is irregularly zoned in Ti and Cr. Olivine crystals in these rocks are unzoned, and pyroxene zoning is slight, yet matrix olivine and pyroxene contain more Fe and Ti and less Mg and Cr than inclusions of these phases in garnet. In three coarse nodules, garnet rims are Ti-rich compared to cores, and Ca, Fe, Mg, and Cr zoning patterns are complex. Several nodules appear to have partially equilibrated near 1200° C and 35 kb, and under these conditions cation mobility in pyroxene was greater than in garnet. The zoning partly reflects Fe and Ti metasomatism in the mantle. Calculations indicate that Fe-Mg gradients in garnet could have persisted for only a short time in the mantle, perhaps thousands of years or less, so the metasomatism occurred shortly before eruption. The minette host, a likely source of the Fe and Ti, is rich in light rare earth elements: since the nodules are much poorer in these elements, little or no infiltrated minette was trapped in them. Diffusion is a possible mechanism for nodule metasomatism. Some fertile peridotite nodules from kimberlites may have been affected by similar events. Compositional differences between inclusions in garnet and matrix phases are intriguingly similar to some of the differences between most peridotite inclusions in diamonds and common lherzolite phases.     

An electron microprobe study of some Alberta placer gold

Summary The detailed characteristics of 435 placer gold grains from the North Saskatchewan and Athabasca rivers have been investigated using a combination of optical microscopy, energy dispersive electron microprobe analysis and SEM techniques. Most grains show high-fineness rims surrounding lower-fineness cores (giving an extreme range of 4.3%–46.1% Ag within an individual grain). Characteristics of the rims lend support to the hypothesis that they are the products ofin situ leaching of silver followed by transport which largely destroys a spongy texture produced by the leaching. There is no change in the average Ag content of grains of any given size range with distance downstream, which also indicates that all significant leaching takes place before transportation. However, differences are apparent in the average Ag content with grain size.The clustering about certain values of fineness of the nuclei of grains which is seen in samples from both rivers, suggests that Au was largely derived from specific sources in the mountains (Omineca geanticline) to the west rather than redistributed from Shield localities during glaciation.

---

Mikrosonden-Untersuchungen an Seifengold aus Alberta, Kanada

Zusammenfassung 435 Körner aus Goldseifen im North Saskatchewan und im Athabasca-Fluß wurden einer detaillierten Untersuchung mit optischer Mikroskopie, energie-dispersiver Mikrosondenanalytik und Rasterelektronenmikroskopie unterzogen.In den meisten Körnern umgibt ein Rand mit hohem Feinheitsgrad einen Kern mit niedrigem Feinheitsgrad (dies führt zu beträchtlichen Schwankungen des Ag-Gehaltes innerhalb des einzelnen Kornes; im Extremfall 4,3%–46,1% Ag). Die Charakteristik der Ränder gibt Anlaß zu der Hypothese, daß das Silber dieser Ränder in situ herausgelöst worden ist, bevor die Körmer transportiert wurden; die während des Transportes auf die Körner einwirkenden Kräfte haben dann die poröse Textur, die durch das herauslösen des Silbers entstanden waren, weitgehend zerstört. Es findet sich keine Änderung des durchschnittlichen Ag-Gehaltes mit zunehmender Transportweite flußabwärts, solange nur Kömer aus einem Größenintervall betrachtet werden. Dies deutet ebenfalls darauf hin, daß der überwiegende Teil des Silbers vor dem Transport herausgelöst wurde. Man findet jedoch Unterschiede im durchschnittlichen Ag-Gehalt zwischen Körnern verschiedener Größe.Das Auftreten von mehreren bestimmten Werten-Clustern der Feinheitsgrade, die in Proben aus beiden Flüssen zu finden ist, legt nahe, daß Au zum größten Teil aus mehreren, spezifischen Ursprungsgesteinen in den westlich gelegenen Gebirgen (Omineca Geanticline) stammt. Eine glazigene Umverteilung von Gold aus verschiedenen Lokalitäten des Kanadischen Schildes erscheint weniger realistisch.

---

---

With 9 Figures