Journal ArticleDOI
Assessing the extent of disequilibrium and overstepping of prograde metamorphic reactions in metapelites from the Bushveld Complex aureole, South Africa
David J. Waters,D. P. Lovegrove +1 more
TLDR
In this article, a detailed record of the relative timing of porphyroblast growth and metamorphic reactions in the Bushveld Complex, South Africa has been inferred from microstructures.Abstract:
Andalusite–staurolite–biotite hornfels metamorphosed beneath the mafic layered rocks of the Bushveld Complex, South Africa, preserves a detailed record of the relative timing of porphyroblast growth and metamorphic reactions. The sequence inferred from microstructures shows considerable overlap of the period of growth of porphyroblasts of staurolite, cordierite, biotite and andalusite, and the persistence over a similar interval of the reactant porphyroblastic phase chloritoid. This is inconsistent with calculations of equilibrium phase relations, and implies that disequilibrium processes controlled the prograde reaction sequence, despite the slow heating rates involved (1 °C per 10 000 yr). The early appearance of cordierite by a metastable reaction and its subsequent disappearance indicates that delayed nucleation of porphyroblastic phases, rather than simply sluggish reaction, is required to account for the sequence of growth. The predicted reactions for the first appearance of andalusite and staurolite have low entropy of reaction, and do not occur until they have been overtaken in terms of reaction affinity by high-entropy devolatilisation reactions involving the breakdown of chlorite. Once the porphyroblasts have nucleated, metastable chloritoid-breakdown reactions also contribute to their growth. The implied magnitude of the critical overstepping for andalusite nucleation is around 5 kJ mole−1 (equivalent to 40 °C for the chlorite-breakdown reaction), and that for other phases is expected to decrease in the order andalusite>staurolite>cordierite. Coupling between nucleation rate, crystal growth rates and the resulting grain size distribution suggests that the rate constants of natural reactions are at least an order of magnitude lower than those measured in the laboratory. Pseudomorphs after chloritoid and cordierite conserve volume but not Al or other species of low mobility, suggesting a breakdown mechanism controlled by an interface process such as the slow dissolution of the refractory porphyroblast phase, rather than by a transport step.read more
Citations
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Book
A Practical Guide to Rock Microstructure
TL;DR: In this article, a glossary of microstructural terms for sedimentary and metamorphic rocks is presented, along with a discussion of the relationship between sedimentary, igneous and deformed rocks.
Journal ArticleDOI
Quantifying geological uncertainty in metamorphic phase equilibria modelling; a Monte Carlo assessment and implications for tectonic interpretations
TL;DR: In this paper, the authors compare the relative importance of geology uncertainty related to natural petrographic variation at the hand sample-and/or thin section-scale on the sample's bulk composition, which is the primary control on its equilibrium phase relationships.
Journal ArticleDOI
Direct observations of pseudomorphism: compositional and textural evolution at a fluid-solid interface
TL;DR: In this paper, the pseudomorphism is initiated by epitaxial growth at the fluid-mineral interface, when the dissolution of the parent phase results in an interfacial fluid layer that is supersaturated with respect to a different solid composition.
Journal ArticleDOI
Contact metamorphic devolatilization of shales in the Karoo Basin, South Africa, and the effects of multiple sill intrusions
TL;DR: In this article, the authors provide geochemical and numerical analyses from the organic-rich Ecca Group in the Karoo Basin, South Africa, which was affected by contact metamorphism from multiple sill intrusions in the Early Jurassic.
Journal ArticleDOI
The role of water in the evolution of the continental crust
TL;DR: In this paper, the composition and role of fluids through the crustal cycle of burial, metamorphism and uplift are investigated. But the authors do not make a clear distinction between diagenetic and metamorphic fluids, and extensive equilibration with host rocks means that few chemical or isotopic characteristics survive to provide tracers for deep fluid origins.
References
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Journal ArticleDOI
Relating formulations of the thermodynamics of mineral solid solutions; activity modeling of pyroxenes, amphiboles, and micas
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