Topics in Theoretical Metamorphic Petrology
AuthorWheller, Catherine Jane
AffiliationSchool of Earth Sciences
Document TypePhD thesis
Access StatusThis item is embargoed and will be available on 2022-04-02. This item is currently available to University of Melbourne staff and students only, login required.
© 2019 Catherine Jane Wheller
The aim of this thesis is to progress research on three factors influencing phase equilibrium modelling of metamorphic rocks. The first involves updating the activity-composition model for the ilmenite-hematite solid solution so that the P-T conditions for the evolution of rocks containing ilmenite can be modelled across a wider range of composition. The second investigates how to model high-grade rocks that have equilibrated domainally. The third explores whether mechanical closure can be a primary control on mineral assemblage evolution in subsolidus rocks. A new thermodynamic model suitable for the ilmenite-hematite solid solution has been calibrated in the system FMTOMn (FeO-MgO-TiO2-Fe2O3-MnO). The activity-composition relationships are parameterised in the symmetric formalism, and the model now allows inclusion of long-range cation-order of Fe2+, Mg and Mn. Calibrated using relatively new cation-ordering data from Harrison, Becker and Redfern (2000), Harrison, Redfern and Smith (2000) and Harrison and Redfern (2001), the model is internally-consistent with the thermodynamic database of Holland and Powell (2011). As disorder in mineral solid solutions is a contributing factor to mineral stability at high temperatures, this model should be an improvement for use in calculations at higher temperature. Interpretation of the metamorphic history of a rock is commonly made through the application of P-T pseudosections which use the effective bulk composition (e.g. whole-rock XRF analysis) as a basis for modelling. This becomes difficult if the rock has equilibrated domainally post the metamorphic peak. Texturally-distinct microdomains can form when there are small-scale effective-composition variations in the rock where localised mineral reaction occurs at the grain scale. A method for interpreting the P-T history of a rock with domainal textures using a P-T grid and compatibility diagrams is investigated, using a texturally complex sapphirine-quartz bearing granulite from the Anosyen tectonic domain in southern Madagascar. Future studies that involve interpreting the conditions of formation of high-grade rocks may benefit from the proposed P-T grid and compatibility diagram method if effective bulk composition is inhomogeneous on a thin-section scale. The P-T history of the oxidised Anosyen terrane is further constrained using the traditional pseudosection approach on homogeneous samples in the same area. With ferric in the sapphirine model (Wheller and Powell, 2014), it is possible to explore the P-T conditions for the evolution of rocks across oxidation state. For the first time, rocks from highly oxidised to reduced sapphirine granulites can be modelled. The peak P-T conditions experienced in the south-east of the Anosyen tectonic domain attained at least 850C and pressures greater than 7.6 kbar. This also shows that the stability of sapphirine+quartz can occur below 900C and may not be a universal ultra-high-temperature (UHT) terrane indicator as commonly suggested. Finally, rocks with reactions textures that would normally be inferred to have formed due to sluggish kinetics may instead have formed through reaction along an isochor, the rock evolving at constant volume. In order to investigate the consequences of a constant volume process, a calculated volume--temperature (V-T) phase diagram is calculated for a granulite sample that has been reworked in subsolidus fluid-absent conditions. Constant volume paths show that reaction at equilibrium can be attenuated due to the reaction `vessel' maintaining constant volume, with the external environment unable to accommodate volume change. This has implications for the assumption that depth can be constrained by the mineral assemblage.
Keywordsthermocalc; sapphirine; ilmenite; ultra-high-temperature; metamorphism; activity-composition; thermodynamic model; equilibrium; pseudosection; microdomains; Madagascar; Anosyen; kinetics; constant volume; rework
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