Katy Evans | Curtin University, Perth (original) (raw)

Papers by Katy Evans

A thermodynamic model for phase equilibria in the H2O-CO2-NaCl system

Ore Geology Reviews, 2015

ABSTRACT Raman spectroscopic and petrographic analyses were performed on samples collected from z... more ABSTRACT Raman spectroscopic and petrographic analyses were performed on samples collected from zones distal and proximal to the Macraes gold deposit in the Otago Schist of New Zealand to characterize the features and possible origins of Carbonaceous Material (CM) and to assess the potential role of CM in the formation of gold deposits. CM is a common component in meta-sedimentary orogenic gold deposits, and it has been proposed that CM contributes to gold mineralization processes, but the details of the mechanisms responsible are not fully understood. Documentation of the origins of the Otago schist CM will improve our understanding of the role of CM in gold deposits.

A zircon grain in an orthopyroxene-garnet-phlogopite-zircon-rutile-bearing xenolith from Udachnay... more A zircon grain in an orthopyroxene-garnet-phlogopite-zircon-rutile-bearing xenolith from Udachnaya, Siberia, preserves a pattern of crystallographic misorientation and subgrain microstructure associated with crystal-plastic deformation. The zircon grain records significant variations in titanium (Ti) from 2.6 to 30 ppm that corresponds to a difference in calculated Ti-in-zircon temperatures of over several hundred degrees Celsius. The highest Ti concentration is measured at subgrain centres (30 ppm), and Ti is variably depleted at low-angle boundaries (down to 2.6 ppm). Variations in cathodoluminescence coincide with the deformation microstructure and indicate localised, differential enrichment of rare earth elements (REE) at low-angle boundaries. Variable enrichment of U and Th and systematic increase of Th/U from 1.61 to 3.52 occurs at low-angle boundaries. Individual SHRIMP-derived U-Pb ages from more deformed zones (mean age of 1799 ± 40, n = 22) are systematically younger than subgrain cores (mean age of 1851 ± 65 Ma, n = 7), and indicate that open system behaviour of Ti-Th-U occurred shortly after zircon growth, prior to the accumulation of significant radiogenic Pb. Modelling of trace-element diffusion distances for geologically reasonable thermal histories indicates that the observed variations are ~5 orders of magnitude greater than can be accounted for by volume diffusion. The data are best explained by enhanced diffusion of U, Th and Ti along deformation-related fast-diffusion pathways, such as dislocations and low-angle (<5°) boundaries. These results indicate chemical exchange between zircon and the surrounding matrix and show that Ti-in-zircon thermometry and U-Pb geochronology from deformed zircon may not yield information relating to the conditions and timing of primary crystallisation.

Journal of Metamorphic Geology, 2010

The Holland and Powell internally consistent data set version 5.5 has been augmented to include p... more The Holland and Powell internally consistent data set version 5.5 has been augmented to include pyrite, troilite, trov (Fe 0.875 S), anhydrite, H 2 S, elemental S and S 2 gas. Phase changes in troilite and pyrrhotite are modelled with a combination of multiple end-members and a Landau tricritical model. Pyrrhotite is modelled as a solid solution between hypothetical end-member troilite (trot) and Fe 0.875 S (trov); observed activity-composition relationships fit well to a symmetric formalism model with a value for w trot)trov of )3.19 kJ mol )1 . The hypothetical end-member approach is required to compensate for iron distribution irregularities in compositions close to troilite. Mixing in fluids is described with the van Laar asymmetric formalism model with a ij values for H 2 O-H 2 S, H 2 S-CH 4 and H 2 S-CO 2 of 6.5, 4.15 and 0.045 kJ mol )1 respectively. The derived data set is statistically acceptable and replicates the input data and data from experiments that were not included in the initial regression. The new data set is applied to the construction of pseudosections for the bulk composition of mafic greenschist facies rocks from the Golden Mile, Kalgoorlie, Western Australia. The sequence of mineral assemblages is replicated successfully, with observed assemblages predicted to be stable at X(CO 2 ) increasing with increasing degree of hydrothermal alteration. Results are compatible with those of previous work. Assemblages are insensitive to the S bulk content at S contents of less than 1 wt%, which means that volatilization of S-bearing fluids and sulphidation are unlikely to have had major effects on the stable mineral assemblage in less metasomatized rocks. The sequence of sulphide and oxide phases is predicted successfully and there is potential to use these phases qualitatively for geobarometry. Increases in X(CO 2 ) stabilized, in turn, pyrite-magnetite, pyrite-hematite and anhydrite-pyrite. Magnetite-pyrrhotite is predicted at temperatures greater than 410°C. The prediction of a variety of sulphide and oxide phases in a rock of fixed bulk composition as a function of changes in fluid composition and temperature is of particular interest because it has been proposed that such a variation in phase assemblage is produced by the infiltration of multiple fluids with contrasting redox state. The work presented here shows that this need not be the case.

Geochimica et Cosmochimica Acta, 2009

1 ABSTRACT 5 S K edge XANES and Mn, W and Ni XANES and EXAFS spectra of silicate glasses syn-6 th... more 1 ABSTRACT 5 S K edge XANES and Mn, W and Ni XANES and EXAFS spectra of silicate glasses syn-6 thesised at 1400 • C and 1 bar with compositions in the CaO-MgO-Al 2 O 3 -SiO 2 -S plus MnO, 7 NiO, or WO 3 systems were used to investigate sulphur speciation in silicate glasses. 8 S K-edge spectra comprised a composite peak with an edge between 2470 and 2471.4 eV, 9 which was attributed to S 2− , and a peak of variable height with an edge at 2480.2 to 2480.8 10 eV, which is consistent with the presence of S 6+ . The latter peak was attributed to sample 11 oxidation during sample storage. W-rich samples produced an additional lower energy peak 12 at 2469.8 eV that is tentatively attributed to the existence of S 3p orbitals hybridised with 13 the W 5d states. 14 Deconvolution of the composite peak reveals that the composite peak for Mn-bearing 15 samples fits well to a model that combines three Lorentzians at 2473.1, 2474.9 and 2476.2 16 eV with an arctan edge step. The composite peak for W-bearing samples fits well to the 17 same combination plus an additional Lorentzian at 2469.8 eV. The ratio of the proportions 18 of the signal accounted for by peaks at 2473.1eV and 2476.2eV correlates with Mn:Ca molar 19

Geochemistry, Geophysics, Geosystems, 2008

1] Chlorine speciation in silicate melts affects volatile exsolution, rheology, and thermodynamic... more 1] Chlorine speciation in silicate melts affects volatile exsolution, rheology, and thermodynamic properties of the melt but is poorly known. X-Ray Absorption Near Edge Structure (XANES) spectra have been used to investigate Cl speciation in 26 silicate glasses and to test the hypothesis that Cl in silicate melts is hosted by species that combine Cl and network-modifying cations such as Ca and Mg. Results indicate that Cl in CMAS (CaO-MgO-Al 2 O 3 -SiO 2 ) plus Na 2 O, K 2 O, or MnO silicate glasses exists as a combination of Ca-Cl x and Mg-Cl x species or, possibly, as mixed Ca-Mg y -Cl x species. The geometry and stoichiometry of the proposed species is unknown, but there are similarities between spectra from Ca-bearing melts and the spectra of hydrated CaCl 2 .2H 2 O, suggesting that the Ca-Cl x species could have a salt-like atomic arrangement and ionic bonding. Further investigations using XANES, alternative spectroscopic techniques, and forward modeling approaches are required to distinguish between these possibilities.