Phase Diagrams PDF

Igneous Petrology is the study of melts (magma) and the rocks that crystallize from such melts, encompassing an understanding of the processes involved in melting and subsequent rise, evolution, crystallization, and eruption or emplacement of the eventual rocks. Phase Diagrams  Rock-Min...

Igneous Petrology is the study of melts (magma) and the rocks that crystallize from such melts, encompassing an understanding of the processes involved in melting and subsequent rise, evolution, crystallization, and eruption or emplacement of the eventual rocks. Phase Diagrams  Rock-Mineral Associations? Nesse, Fig. 5.1 mineral stability pressure (P) is a function of: temperature (T) composition (X) Principles of igneous and metamorphic petrology: Philpott & Ague  Making Magma! Three mechanisms to produce partial melting: 1. Temperature increase 2. Pressure decrease 3. Volatile influx Peter Wyllie Dean Presnall Orthosilicate 2O – 8 Al i3 O 8 Ca AlS 2 Si Single Na chain Double Frame chain Sheet Norman N. Bowen Frame Crystallization of Makaopuhi Lava: 3 1 4 Photo by Tom Wright, 1965 (U.S. Geological Survey). From Smithsonian Institute, National Museum of Natural History 2 From Wright and Okamura, (1977) USGS Prof. Paper, 1004. Principles of igneous and metamorphic petrology: JD Winter  Crystallization Behavior of Melts 1. Magma crystallize over a range of T & P Makaopuhi Lava: 2. Many minerals crystallize throughout and more minerals with cooling 3. Overlap of crystallization T 4. Mineral composition changes 5. Melt composition changes 6. Crystallization of mineral and sequence depend on T and X of the melt 7. P can affect the types of minerals that form and the sequence 8. The nature and P of the volatiles can From Wright and Okamura, (1977) USGS Prof. Paper, 1004. also affect the minerals and their Principles of igneous and metamorphic petrology: JD Winter sequence  Zoned Crystals Diopside examples of zoned minerals in thin section growth zoning reflects variation in major and/or trace element composition acquired during growth of crystal from core to rim what controls these variations?  Phase Diagrams stability fields of mineral assemblages as a function of P, T, X can be illustrated using phase diagrams these are determined by experiment or calculated from thermodynamic principles understanding phase diagrams is the key to understanding rock-mineral associations why is there a predictable crystallisation sequence of minerals in igneous rocks (Bowen’s Reaction Series)? why are certain minerals generally zoned? why are certain minerals stable together while others are not?  Phase Rule Simple theoretical treatment to analyze simple and complex natural systems System – some part of the material universe chosen for study. Closed, open or isolated Phase – a physically separable part of a system. Crystal, magma, fluid, gas. Component – Minimum number of components necessary to describe (H2O, O2, SiO2, or NaAlSi3O8) Ice and water: 2 phases but 1 component Albite: NaAlSiO3-Single component Plagioclase: Single phase, 2 components- NaAlSiO3 and CaAl2Si2O8 Variables: Completely define the state of a system 2-types: Extensive (volume, mass, and total energy), intensive (temp., pressure, density) Reversals or fluctuations in zoning during crystal growth from a melt can be explained by several dynamic processes occurring in the magma chamber. These fluctuations indicate non-linear or variable conditions during crystallization. Here are the main explanations: Magma Mixing: Injection of new, compositionally different magma into the chamber can change the melt's chemistry. If more primitive (higher temperature, mafic) magma mixes with more evolved (lower temperature, felsic) magma, the crystallizing plagioclase can reflect this in its zoning pattern, showing reversals in the An/Ab ratio. Fractional Crystallization: As crystallization proceeds, certain elements are depleted from the melt, altering its composition. If convection or replenishment occurs, fresh melt with a different composition can come into contact with the growing crystal, leading to compositional fluctuations. Changes in Temperature and Pressure: Rapid fluctuations in temperature can cause reversals. For example, an increase in temperature can result in partial resorption of more evolved, Na-rich plagioclase (Ab), followed by the crystallization of more primitive, Ca-rich plagioclase (An) as cooling resumes. Volatile Saturation and Degassing: The presence and loss of volatiles (like water) in the melt can significantly alter crystallization. As volatiles degas, the melt composition changes, which may cause oscillatory or reversal zoning as the crystal adjusts to the evolving melt. Diffusion-Limited Growth: In cases where growth occurs faster than diffusion can equilibrate, the crystal may record transient melt compositions, resulting in fluctuations in the zoning patterns. https://serc.carleton.edu/  Phase Rule of Gibbs (1928) We can measure the temperature (T), pressure (P), volume (V), mass (m), density (ρ), composition (X), or any of a number of other possible parameters of the water. However, once a critical number of these variables is known, the others become fixed because many of the properties are interdependent. F=C–P+2 Phase diagram is a graphical representation of the F= Number of degrees of freedom physical states of a C= minimum number of chemical species substance under different required to define the system completely conditions of temperature P= Number of phases in the system and pressure. More phases = lesser the variance More components = more the variance Applied only to systems in equilibrium  Phase Diagrams: can show what phases coexist for a given composition, as a function of P or T This is an example of a 1-component system with 3 phases: phase diagram for Al2SiO5 component? axes in this case are P,T; don’t need to consider X Univariant phases? because bulk composition is curves the same everywhere on the diagram Divariant Invariant point fields Phase diagrams: Limited to three dimensions Full range of compositional and environmental variables can be calculated using computer program (e.g., MELTS) Nesse, Fig. 5.3b however, in order to be relevant to real rocks, most systems require at least 2 components  Under what conditions are low quartz, high quartz, and coesite stable together? What about high quartz, cristobalite, and liquid? Principles of igneous and metamorphic petrology: Philpott & Ague  What will happen if: Quartz is heated above 1800 °C room condition? A meteorite impact generated a pressure of 5 GPa. What phase would be stable SiO2-phase enclosed by a diamond derived from the transition zone (410 km)? Assignment: 1 Describe the following in a minimum of 250 words 1. Types of systems, variables 2. Phase rule 3. Phase diagrams 4. Single component phase diagrams with example Give 5 examples of single-component phase diagrams What are the polymorphs? Give five examples