Woolshed Creek fossil site: a key part of Canberra’s scientific and cultural heritage

The Woolshed Creek fossil site near the Royal Military College, Duntroon, Canberra, contains brachiopods Atrypa duntroonensis (early Homerian, early Silurian, ca 430.5?Ma) within a mudstone of the Canberra Formation. Their discovery in 1844 by the Reverend William B. Clarke (“the Father of Australian Geology”), and subsequent comparison with other fossil collections from around the world, contributed significantly to the nineteenth century debate about the oldest rocks in Australia. The fossil site is now on the ACT Government Heritage List and recent site improvements make it readily accessible via a pathway from the sports grounds of the Royal Military College.     

Use of in situ-produced Be in carbonate-rich environments: A first attempt

We have determined the production rate of in situ-produced ¹⁰Be in calcite through measurements of ¹⁰Be and ³⁶Cl concentrations in the same calcite samples and of ¹⁰Be concentrations in depth profiles of flint from the same erosional surface. This yields a ¹⁰Be production rate in calcite of 37.9 ± 6.0 at/g/yr at sea level and high latitude, approximately sixfold higher than production in the coexisting flint. We propose that this higher rate of production may be due to high production cross sections for C spallation by cosmic rays with energies below 50 MeV. These results also open the possibility of dating burial events in carbonate-rich environments by differential radioactive decay of ¹⁰Be and ³⁶Cl.     

A methodology for making initial estimates of groundwater recharge from groundwater vulnerability mapping

Recharge to an aquifer can be estimated by first calculating the effective rainfall using a soil moisture budgeting technique, and then by applying a recharge coefficient to indicate the proportion of this effective rainfall that contributes to groundwater recharge. In the Republic of Ireland, the recharge coefficient is determined mainly by the permeability and thickness of the superficial deposits (subsoils) that overlie the country’s aquifers. The properties of these subsoils also influence groundwater vulnerability, and a methodology has been developed for determining the recharge coefficient using the groundwater vulnerability classification. The results of four case studies have been used to develop a quantified link between subsoil permeability, aquifer vulnerability, recharge and runoff. Recharge and runoff coefficients are each classed into three groupings: high, intermediate and low. A high recharge coefficient equates to a low runoff coefficient, and vice versa. A GIS-based tool enables preliminary estimates of recharge to be made using these recharge coefficient groupings. Potential recharge is calculated as the product of effective rainfall and recharge coefficient. The actual recharge is then calculated taking account of the ability of the aquifer to accept the available recharge. The methodology could be applied to other temperate climate zones where the main aquifers have a substantial covering of superficial deposits.     

Estimation of groundwater recharge in a major sand and gravel aquifer in Ireland using multiple approaches

Groundwater recharge was investigated in the most extensive sand and gravel aquifer (area of approximately 200 km²) in the Republic of Ireland as part of a wider study seeking to derive recharge estimates using aquifer vulnerability mapping. The proportion of effective rainfall (total rainfall minus actual evapotranspiration) that leads to recharge is known as the recharge coefficient. The recharge investigation involved a variety of approaches, including soil moisture budgeting, well hydrograph analysis, numerical modelling and a catchment water balance. The adoption of multiple techniques provided insights on recharge and also on aquifer properties. Comparison of two soil moisture budgeting approaches (FAO Penman-Monteith with Penman-Grindley) showed how variations in the effective rainfall values from these methods influence groundwater levels simulated in a numerical groundwater model. The catchment water balance estimated the recharge coefficient to be between 81 and 85%, which is considered a reasonable range for this aquifer, where overland flow is rarely observed. The well hydrograph analysis, using a previous estimate of specific yield (0.13), gave recharge coefficients in the range of 40–80%, considered low for this aquifer: a revised specific yield of 0.19 resulted in a more reasonable range of recharge coefficients of between 70 and 100%.     

Polarized X-ray absorption spectroscopy of metal ions in minerals

We have made use of the nearly complete linear polarization of synchrotron radiation to study the polarization dependence of X-ray absorption near-edge structure (XANES) and extended fine structure (EXAFS) in oriented single crystals of gillespite (BaFe²⁺ Si₄O₁₀; Fe^{2 +} in square-planar coordination, point symmetry C ₄), anatase (TiO₂; Ti⁴⁺ in octahedral coordination, point symmetry D _2d), and epidote (Ca₂(Al, Fe³⁺)₃SiO₄)₃(OH); Fe³⁺ in distorted octahedral coordination, point symmetry (C _s). For gillespite, the Fe K-XANES spectrum varies strongly with E-vector orientation of the incident X-ray beam. When the E-vector lies in the plane of the FeO₄ group (i.e., perpendicular to the c-axis), multiple-scattering features at 7127 and 7131 eV intensify, whereas when the E-vector is perpendicular to the plane of the FeO₄ group (i.e., parallel to the c-axis), a strongly-polarized 1s to 4p bound state transition occurs at 7116 eV and a localized continuum resonance occurs at 7122 eV. The Fe-K-EXAFS spectrum of gillespite is also highly polarization dependent. When the E-vector is perpendicular to c, all four nearest-neighbor oxygens around Fe²⁺ contribute to the EXAFS signal; when E is parallel to c, the EXAFS signal from nearest-neighbors is reduced by at least 86%. The unpolarized Ti K-XANES spectrum of anatase has three relatively strong pre-edge features at 4967.1, 4969.9, and 4972.7 eV which have resisted definitive interpretation in past studies. The lowest energy feature has a strong xy polarization dependence, suggesting a large amount of 4p _x,y character, and it is also very sharp, indicating a well-defined transition energy. Both of these observations are consistent with an excitonic state with a binding energy of 2.8 eV. The two higher energy features, which are characteristic of octahedrally-coordinated Ti⁴⁺, show little polarization dependence and are probably due to 1s to 3d bound-state transitions, with a small degree of np character in the final state wavefunction. Interpretation of the polarization dependence of Fe K-XANES spectra for epidote is not as straightforward due to the lower space group symmetry (P2₁/m) relative to gillespite (P4/ncc) and anatase (I4₁/amd) and the lower point group symmetry (C _s) of the M(3) site which contains most of the Fe³⁺ in the epidote structure. However, the presence of a shoulder at 7121 eV in the E parallel to b spectrum and its absence in the E normal to bc spectrum are consistent with it being a 1s to 4p _z bound-state transition. Strong, weakly x, y polarized features near 7126 eV in both spectra are most likely due to localized continuum transitions. Also, the 1s to 3d pre-edge intensity varies in intensity with E-vector orientation which is consistent with displacement of Fe³⁺ from the center of the M(3) octahedral site. Analysis of EXAFS spectra of epidote in these two polarizations yields bond distances which are within 0.04 Å of previous single-crystal X-ray diffraction analysis. This study demonstrates the utility of polarized X-ray absorption spectroscopy in quantifying the energies and orbital compositions of final state wavefunctions associated with various X-ray induced transitions in transition-metal containing minerals. It also shows that reasonably accurate M-O distances can be obtained for individual bonds oriented in crystallographically non-equivalent directions.     

Stratiform copper deposits and interactions with co-existing atmospheres, hydrospheres, biospheres and lithospheres

Fundamental evidence from world-class stratiform copper deposits repeatedly indicates that they originated by a two-stage mineralization process: a syn-diagenetic enrichment of the host sediment in iron sulfide, followed by a sub-surface addition of copper during early diagenesis of the sediment. In contrast to the previously favored syngenetic hypothesis, which suggested an open equilibration between atmosphere, hydrosphere, biosphere and upper lithosphere during mineralization, the two-stage model implies that only the initial deposition of iron sulfides should take place with open exchanges between the upper “spheres”. The sub-surface emplacement of copper should occur in relative isolation from surface environments and, therefore, the occurrence and distribution of stratiform copper deposits through time and space should not be used to make direct interpretations of contemporaneous conditions in the upper “spheres”.     

Lithologic and stratigraphic setting of zinc mineralization in Grenville metasediments,Quebec

For many years, high-grade metamorphism and multiphase deformation have discouraged mineral exploration in the Grenville terrain of the Canadian Shield. However, detailed studies of known zinc occurrences in the Grenville Supergroup metasediments of Quebec now reveal that mineralization is stratigraphically controlled and consistently limited to mapable lithologic units. Within the vast regions composed predominantly of calcitic marbles, zinc mineralization is found to be confined to the much more limited areas of dolomitic marble, and more precisely, to the lithologic transition from metaclastic beds to meta-dolomites. Massive sphalerite mineralization of the Balmat-type is very closely restricted to this contact zone, while disseminated sphalerite with sparse amounts of galena are commonly observed in the adjacent dolomite. With these guidelines, and with evidence that mineralization is fundamentally stratiform in nature, it has now been demonstrated that mineralized stratigraphic horizons can be located beyond the areas of previously known sphalerite showings, and consequently the potential for important discoveries of zinc mineralization in the Grenville Supergroup has become very attractive to the present-day mining industry.     

Phase relations of pyroxene and amphibole in greenstone,blueschist and eclogite of the Franciscan Complex,California

The phase relations of pyroxenes, amphiboles and associated minerals in metamorphic rocks of the Franciscan Complex can be graphically depicted on a ternary diagram which has at its apices the metamorphic clinopyroxene end members, viz NaAl-NaFe³⁺-Ca(Fe²⁺, Mg). Phases are plotted by projection from a constant subassemblage of minerals. This analysis allows interpretation of the effects of pressure, temperature, bulk rock composition and fluid composition on stability of minerals within the Franciscan.Pyroxenes in meta-igneous rocks and metagraywackes have a limited compositional range and fall into two groups: the omphacites, with 50±5% diopside +hedenbergite component; and the jadeitic pyroxenes with 10±5% diopside+hedenbergite. Pyroxenes intermediate between these two groups are unstable relative to assemblages containing Na-amphibole+other minerals.Coexisting pyroxenes and amphiboles in eclogites and associated coarse blueschists comprise equilibrium assemblages, and the proportion of pyroxene to amphibole is a function of rock composition. Eclogites are stable at higher temperature than regionally developed fine-grained greenstones and blueschists in the Franciscan, and at higher pressure than amphibolites. X _H2O ^fluid is not an important factor in the stability of Franciscan eclogite relative to amphibolite.     

Structure energies of the alkali feldspars

Structural energetics of the alkali feldspars have been studied using a “lattice” or structure energy model. Electrostatic energies, U _e,for 20 well-refined, non-intergrown alkali feldspars were calculated using Bertaut's (1952) summation procedure and average about ?13,400 kcal/mol; the repulsive energies of the alkali site in each structure (～15 kcal/mol) were calculated using repulsive parameters for K-O and Na-O interactions estimated from bulk modulus data for NaF and KF and the exponential form of the repulsive potential. Using a procedure in which the position of the alkali cation was varied while the oxygen cage was kept fixed, structure energy gradients for the alkali sites of high albite and a hypersolvus Ab₄₂Or₅₈ structure were computed. In both cases, a broad structure energy well, elongated approximately parallel to c and subparallel to the observed split Na positions, was found. In both structures there is a single energy minimum corresponding closely with the observed single alkali positions. Comparison of U _e values for the alkali feldspars with different K/Na ratios shows that intermediate compositions are predicted to be less “stable” than either endmember and that the potassic end-member is predicted to be less “stable” than the sodic one, assuming that all other factors contributiong to the free energies of each phase are approximately the same. Comparison of U _e values for the high albite and low sanidine structures with different Al/Si distributions and a fixed tetrahedral framework indicates that the ordered charge distributions are 63.0 and 54.8 kcal/mol, respectively, more “stable” than the disordered distributions. Smaller, more realistic energy differences were obtained by using U _evalues averaged from four separate calculations with a +3 charge on a different T site in each and with +4 charges on the other T sites. If, in addition, the charges on cations and oxygen are reduced to half their nominal formal charges, in agreement with Pauling's electroneutrality principle and the results of recent molecular orbital calculations on silicates, the predicted electrostatic energy differences are reduced to 3.6 and 1.6 kcal/mol, respectively. These calculations also indicate that the T₁O site in the high albite structure energetically favors Al and that the Al/Si distribution determines the Na position within the alkali site.