Rb/Sr isotopic and geochemical evolution of a recrystallized shear (mylonite) zone at Broken Hill

Relatively narrow (1 to 1,000 metres) planar zones of intense shear deformation and retrograde metamorphism (retrograde shist zones, RSZ) are common in the Willyama Complex around Broken Hill. A Rb/Sr isotopic study of one of these zones has revealed an unexpected isotopic pattern. All of the analysed points lie to the left of and above the 1665 Ma isochron of the Potosi Gneiss host. This result indicates a different isotopic composition of the zone as a whole rather than a chemical redistribution of Potosi Gneiss within a closed system. The isochron plot scatter is similar to that of ubiquitous metamorphic pegmatites in the basement of the Broken Hill region. The shear zone observation, however, is shown to be unrelated to the pegmatite segregation process and requires the introduction, by elemental exchange, of a more radiogenic strontium, whilst maintaining relatively fixed Rb/Sr ratios. Significant proportions of SiO₂, K₂O and Ba were removed at the same time. Na₂O, Al₂O₃, TiO₂, MgO, P₂O₅, Zr, Ni, Sc, Y, Nb, Rb remain essentially constant after allowing for volume loss of the removed elements. CaO and Sr increase slightly overall.This pattern of elemental exchange is consistent with the breakdown of K-feldspar to muscovite and the essentially isochemical recrystallization of the other major phases (quartz, plagioclase and biotite) that are observed petrographically. Silica produced by the K-feldspar breakdown and a significant amount of the free quartz must also have been removed. Large scale fluid transport is required to achieve these results.The total rock isochron gives no indication of the age of the retrograde events. However two biotite-total rock joins indicate that the last internal isotopic redistribution occurred 458 Ma ago, shortly after the time of a regional low grade metamorphism. ⁸⁷Sr/⁸⁶Sr v 100/⁸⁶Sr plots confirm that a mixing process took place at about this time and not at 1,490 Ma or the present day.The introduced fluid had an ⁸⁷Sr/⁸⁶Sr ratio of about 0.794. If this figure still represents the original source value then it has come from what was long established mature crust at 458 Ma and not the mantle. The large fluid volumes required for the silica dissolution preclude a source in the local rock porosity, and suggest a meteoric fluid which has reacted extensively with the Willyama Complex metamorphics.     

Pressure dependence of the thermal Grüneisen parameter,with application to the earth's lower mantle and outer core

By considering high-temperature (classical) thermal oscillations of atoms in certain simple crystal structures with purely central interatomic forces, the treatment of anharmonic oscillations is generalised to random three-dimensional motion, yielding the Vashchenko and Zubarev relationship for the Grüneisen ratio γ at any pressure. If one-dimensional atomic oscillations only are considered the equation reduces to the Dugdale-MacDonald expression. To account for non-central forces additional terms must be introduced, giving:

$\text{γ=}\text{1}\text{2}\text{d}\text{K}\text{d}\text{P}\text{?}\text{5}\text{6}\text{+}\text{2}\text{9}\text{P}\text{K}\text{?}\text{f}\text{18K}\text{+}\text{1}\text{6}\text{d}\text{f}\text{d}\text{P}\text{1?}\text{4}\text{3}\text{P}\text{K}\text{+}\text{f}\text{3K}$

where f = 0 for purely central forces. Calculations of f in terms of the Poisson ratio for different crystal structures have not been made, but for many materials the central-force approximation suffices. This is believed to be true both for the outer core $\text{(}\text{γ}\text{≈1.4)}$ and for the close-packed structures of the lower mantle $\text{(}\text{γ}\text{≈1.0)}$. For the upper mantle non-central atomic forces are important and we have no estimate of ($\text{γ}$ independently of laboratory values for plausible minerals which suggest γ ≈ 0.8.     

Anhysteretic remanent magnetization (ARM) in magnetite grains

Summary ARM has been measured in a range of inducing, steady fields up to 50 oersteds and for 6 sizes of magnetite grains with average diameters 5 m to 174 m. For all sizes a slight non-linearity of ARM with inducing field was found, apparently comprising a non-linear contribution independent of grain size plus a linear contribution which increased with decreasing grain size. In the largest grains induced ARM agreed well with multidomain grain theory. Relative enhancement of ARM in smaller grains is comparable to the enhancement of thermoremanence and therefore appears to indicate a pseudosingle domain contribution to ARM in small grains. However the observations allow an alternative explanation in terms of more extreme dimension ratios in the smaller grains. Presentation of the equations for multidomain ARM and TRM using observed instead of intrinsic susceptibilities makes it appear that the inadequacy of multidomain theory (and consequent necessity for pseudo-single domain theory) are less serious than has been supposed.     

The origin of thermoremanent magnetization: Contribution of pseudo-single-domain magnetic moments

The pseudo-single-domain (PSD) intensity and stability of thermoremanent magnetization (TRM) in multidomain magnetite grains 0.05–15 μm in size are attributed to residual magnetic moments not removed by demagnetization. Barkhausen discreteness in domain wall positions is a possible source of such moments, but the observed grain size and applied field dependences of TRM in the lower PSD range are more convincingly explained by a new theory (F.D. Stacey and S.K. Banerjee, 1974) in which the moments of domain walls and the surface terminations of domain walls play a central role. The magnitudes of PSD moments and the average number of moments per grain required by this theory are deduced from low-field (0–25 Oe) TRM measurements on magnetite grains of controlled sizes between 0.04 and 0.22 μm. The predicted maximum PSD moment is about equal to the saturation moment in grains ≤ 0.1 μm in size but is only 10% of the saturation moment in 0.22-μm grains. Since blocking temperature and hysteresis data independently suggest two-domain structure in 0.22-μm grains and wall-like domain structure in smaller grains, the predicted PSD moments are quantitatively reasonable.     

Halite karst geohazards (natural and man-made) in the United Kingdom

In the United Kingdom, Permian and Triassic halite (rock salt) deposits have been affected by natural and artificial dissolution producing karstic landforms and subsidence. Brine springs from the Triassic salt have been exploited since Roman times or possibly earlier, indicating prolonged natural dissolution. Medieval salt extraction in England is indicated by the names of places ending in "wich", indicating brine spring exploitation at Northwich, Middlewich, Nantwich and Droitwich. Later, Victorian brine extraction in these areas accentuated salt karst development, causing severe subsidence problems which remain a legacy. The salt was also mined, but the mines flooded and consequent brine extraction caused the workings to collapse, resulting in catastrophic surface subsidence. Legislation was enacted to pay for the damage and a levy is still charged for salt extraction. Some salt mines are still collapsing, and the re-establishment of the post-brine extraction hydrogeological regimes means that salt springs may again flow, causing further dissolution and potential collapse.     

Wave propagation along freely propagating surface gravity current fronts

This work deals with the propagation and evolution of disturbances which move along freely propagating two-dimensional gravity current fronts. Examples of evolving perturbations on fronts are displayed in real-aperture radar images of gravity currents in the coastal zone. The theory of Cooper et al. (2001), which is based upon the ray tube formulation of Whitham (1974), is employed to simulate disturbances of the sort seen in this imagery and in the larger body of literature. Initial anomalies in both shape and velocity are introduced and allowed to evolve, and several types of new and interesting behaviors emerge. Shape perturbations of the form x=a sech δy evolve into two anomalies, which separate in time as they propagate in opposite directions along the front. When the value of a is increased, the disturbances, which propagate along the gravity current, can break, forming breaking frontal waves (BFWs). These manifest themselves as sharp angular features to either side of the main bulge. Two types of velocity disturbances are employed. The first has the form U=U₀(1+â sech δy), and evolves to preserve a single frontal bulge that increases in amplitude and width as it propagates. Here again, large values of â result in BFWs. In this case, they replicate the general behavior present in the imagery. The second type of velocity perturbation used is U=U₀(1+â cos δy). The smallest values of a generate no BFWs, but yield fronts which oscillate in space and time. Larger values produce a string of BFWs which are qualitatively similar to the cusp-and-trough morphology observed so frequently in nature. The largest values of a allow the gravity current to form a string of large, bulbous structures which intersect one another as they propagate forward and spread laterally. And finally, we make an effort to correlate the results of the simulations with the shapes seen in radar and visible imagery in the literature.     

Effects of sediment iron mineral composition on microbially mediated changes in divalent metal speciation: Importance of ferrihydrite

Dissimilatory metal reducing bacteria (DMRB) can influence geochemical processes that affect the speciation and mobility of metallic contaminants within natural environments. Most investigations into the effect of DMRB on sediment geochemistry utilize various synthetic oxides as the Fe^III source (e.g., ferrihydrite, goethite, hematite). These synthetic materials do not represent the mineralogical composition of natural systems, and do not account for the effect of sediment mineral composition on microbially mediated processes. Our experiments with a DMRB (Shewanella putrefaciens 200) and a divalent metal (Zn^II) indicate that, while complexity in sediment mineral composition may not strongly impact the degree of “microbial iron reducibility,” it does alter the geochemical consequences of such microbial activity. The ferrihydrite and clay mineral content are key factors. Microbial reduction of a synthetic blend of goethite and ferrihydrite (VHSA-G) carrying previously adsorbed Zn^II increased both [Zn^II-aq] and the proportion of adsorbed Zn^II that is insoluble in 0.5 M HCl. Microbial reduction of Fe^III in similarly treated iron-bearing clayey sediment (Fe-K-Q) and hematite sand, which contained minimal amounts of ferrihydrite, had no similar effect. Addition of ferrihydrite increased the effect of microbial Fe^III reduction on Zn^II association with a 0.5 M HCl insoluble phase in all sediment treatments, but the effect was inconsequential in the Fe-K-Q. Zinc k-edge X-ray absorption spectroscopy (XAS) data indicate that microbial Fe^III reduction altered Zn^II bonding in fundamentally different ways for VHSA-G and Fe-K-Q. In VHSA-G, ZnO₆ octahedra were present in both sterile and reduced samples; with a slightly increased average Zn-O coordination number and a slightly higher degree of long-range order in the reduced sample. This result may be consistent with enhanced Zn^II substitution within goethite in the microbially reduced sample, though these data do not show the large increase in the degree of Zn-O-metal interactions expected to accompany this change. In Fe-K-Q, microbial Fe^III reduction transforms Zn-O polyhedra from octahedral to tetrahedral coordination and leads to the formation of a ZnCl₂ moiety and an increased degree of multiple scattering. This study indicates that, while many sedimentary iron minerals are easily reduced by DMRB, the effects of microbial Fe^III reduction on trace metal geochemistry are dependent on sediment mineral composition.     

The changing dietary habits of nineteenth‐century Australian explorers

In the first decade of white settlement in New South Wales, ground exploration was limited by the critical shortage of horses and the inability to live off the land. Bush skills were lacking and muskets were extremely inaccurate. Subsequently, the introduction of kangaroo dogs, the willingness to consume even the least appetising animals, and the identification of edible native plants made explorers less dependent on what they could carry. Such skills were to become largely irrelevant after 1840, as fully mounted expeditions began to penetrate the interior, where game was scarce and there was no time for hunting.