Lecture 5 Phase Equilibria I

Lecture 5 Phase Equilibria I - x Liquid Phase Equilibria I...

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Unformatted text preview: x Liquid Phase Equilibria I Silica mineral + liquid Forsterite + liquid 1557 GEOL 320 -- Petrology Enstatite+ liquid 1543 Enstatite + silica mineral Forsterite + Enstatite Case Study: Makaopuhi Lava Lake Makaopuhi pit crater (1 km x 1.6 km x 110 m deep) along the East Rift Zone of Kilauea, Hawai'i (N. Nesvadba, 08.05.01) Makaopuhi Lava Lake: 1965 magma samples recovered from various depths beneath solid crust from April 1965 Feb. 1969 thermocouple attached to sampler to measure T melt = liquid + crystals + gas crust melt Makaopuhi Lava Lake 1020C Magma samples recovered from various depths beneath solid crust crust melt From Wright and Okamura, (1977) USGS Prof. Paper, 1004. Makaopuhi Lava Lake Magma samples recovered from various depths beneath solid crust 1030C crust melt From Wright and Okamura, (1977) USGS Prof. Paper, 1004. Makaopuhi Lava Lake Magma samples recovered from various depths beneath solid crust 1070C crust melt From Wright and Okamura, (1977) USGS Prof. Paper, 1004. Makaopuhi Lava Lake 1250 1200 1150 1100 1050 1000 950 900 Temperature oc 0 10 20 30 40 50 60 70 80 90 100 Percent Glass Fig. 6-1. From Wright and Okamura, (1977) USGS Prof. Paper, 1004. Makaopuhi Lava Lake Minerals that form during crystallization Olivine 1250 Clinopyroxene Plagioclase Opaque Liquidus 1200 Temperature oC olivine decreases below 1175oC 1150 1100 1050 Melt Crust 1000 Solidus 950 0 10 0 10 20 30 40 50 0 10 20 30 40 50 0 10 Fig. 6-2. From Wright and Okamura, (1977) USGS Prof. Paper, 1004. Makaopuhi Lava Lake Mineral composition during crystallization 100 Olivine 90 Augite Plagioclase Weight % Glass 80 70 60 50 .9 .8 .7 .9 .8 .7 .6 80 70 60 Mg / (Mg + Fe) Mg / (Mg + Fe) An Fig. 6-3. From Wright and Okamura, (1977) USGS Prof. Paper, 1004. Crystallization Behavior of Melts 1. Cooling melts crystallize from a liquid to a solid over a range of temperatures (and pressures) Crystallization Behavior of Melts 1. Cooling melts crystallize from a liquid to a solid over a range of temperatures (and pressures) 2. Several minerals crystallize over this T range, and the number of minerals increases as T decreases Crystallization Behavior of Melts 1. Cooling melts crystallize from a liquid to a solid over a range of temperatures (and pressures) 2. Several minerals crystallize over this T range, and the number of minerals increases as T decreases 1. The minerals that form do so sequentially, with considerable overlap Crystallization Behavior of Melts 1. Cooling melts crystallize from a liquid to a solid over a range of temperatures (and pressures) 2. Several minerals crystallize over this T range, and the number of minerals increases as T decreases 1. The minerals that form do so sequentially, with considerable overlap 2. Minerals that involve solid solution change composition as cooling progresses Crystallization Behavior of Melts 1. Cooling melts crystallize from a liquid to a solid over a range of temperatures (and pressures) 2. Several minerals crystallize over this T range, and the number of minerals increases as T decreases 1. The minerals that form do so sequentially, with considerable overlap 2. Minerals that involve solid solution change composition as cooling progresses 3. The melt composition also changes during crystallization Crystallization Behavior of Melts 1. Cooling melts crystallize from a liquid to a solid over a range of temperatures (and pressures) 2. Several minerals crystallize over this T range, and the number of minerals increases as T decreases 1. The minerals that form do so sequentially, with considerable overlap 2. Minerals that involve solid solution change composition as cooling progresses 3. The melt composition also changes during crystallization 4. The minerals that crystallize (as well as the sequence) depend upon the T and X of the melt Crystallization Behavior of Melts 1. Cooling melts crystallize from a liquid to a solid over a range of temperatures (and pressures) 2. Several minerals crystallize over this T range, and the number of minerals increases as T decreases 1. The minerals that form do so sequentially, with considerable overlap 2. Minerals that involve solid solution change composition as cooling progresses 3. The melt composition also changes during crystallization 4. The minerals that crystallize (as well as the sequence) depend upon the T and X of the melt 1. Pressure can affect the types of minerals that form and the sequence Crystallization Behavior of Melts 1. Cooling melts crystallize from a liquid to a solid over a range of temperatures (and pressures) 2. Several minerals crystallize over this T range, and the number of minerals increases as T decreases 1. The minerals that form do so sequentially, with considerable overlap 2. Minerals that involve solid solution change composition as cooling progresses 3. The melt composition also changes during crystallization 4. The minerals that crystallize (as well as the sequence) depend upon the T and X of the melt 5. Pressure can affect the types of minerals that form and the sequence 6. The nature and pressure of volatiles can affect the minerals and their sequence. How Do We Determine How Melts Crystallize? Cross section: sample in red q q q Samples of actively crystallizing lava Experimental petrology 3 melt existing rocks 3 melt "synthetic rocks" Rocks and magmas behave according to laws of physics and chemistry the sample 800 Ton Ram Carbide Pressure Vessle Talc Talc SAMPLE Graphite Furnace 1 cm Furnace Assembly Fig. 6-5. After Boyd and England (1960), J. Geophys. Res., 65, 741-748. AGU Equilibrium q q a geologic system (rock or magma) is in a state where there is no driving force for change conditions of the system change (typically P or T), chemical rxns proceed to restore the system to equilibrium system some portion of the universe you want to study Magma chamber Ex: System Cools q q additional minerals will crystallize proportion of minerals to melt increases Magma chamber Types of Systems: Closed q q q q Thermal and mechanical energy of system can be exchanged with surroundings Mass cannot be exchanged P-T can change Chemical constituents cannot change mass balance preservation of a constant bulk composition as chemical and physical processes operate on rocks Magma chamber Types of Systems: Open q q Energy and mass can be exchanged Ex: assimilation of country rock = changes the composition of the magma Magma chamber Phase Diagrams Primary tool for petrologists, material Forsterite + scientists, ceramists, and liquid metallurgists to depict crystallization and melting Silica mineral + liquid 1557 q Graphical portrayal of stability ranges of minerals and melts as a function of Enstatite+ liquid bulk composition(s), temperature, Forsterite + and Enstatite 1543 pressure q Enstatite + silica mineral x Liquid Developing the Phase Rule The phase rule is a theoretical Forsterite + treatment that allows us to understand liquid how a system can vary or react to changes in the system, such as Silica mineral + liquid 1557 increases or decreases in temperature and pressure, the introduction of Enstatite+ liquid additional constituents, etc. Forsterite + q 1543 Enstatite + silica mineral Enstatite x Liquid So What is a Phase? x Liquid A physically distinct part of a system Forsterite that is mechanically separable fromliquid + other phases in the system (at least theoretically) Silica mineral + liquid q May be a solid (mineral), liquid, or gas q Enstatite+ liquid 1543 Enstatite + silica mineral Forsterite + Enstatite 1557 So What is a Phase? x Liquid q Composition: Fixed ex: quartz, SiO2 3 Variable ex: solid solution phasesForsterite + such liquid as olivine 3 forsterite Silica mineral + liquid fayalite Fe2SiO4 1557 Mg2SiO4 1543 More than one phase can have the same composition as long Enstatite+ liquidphase has as each Forsterite + distinct physical properties Enstatite 3 Ex: polymorphs of SiO2: 3 cristobalite, stishovite, tridymite Enstatite + silica mineral How many phases are present? ice Two: (1)Liquid H2O (2)Ice H2O liquid So What is a Component? x Liquid A chemical constituent, such as Si, Forsterite + H2O, O2, SiO2, or NaAlSi3O8 liquid q When it comes to the phase rule, the number of components refers to the Silica mineral + liquid 1557 minimum number of chemical species required to completely define the system and all of its Enstatite+ liquid phases Forsterite + q 1543 Enstatite + silica mineral Enstatite What are the number of components of this system? One: H2O ice liquid System: Olivine Consider only the zoned olivine grains in the image as our system. What are the number of components of this system? Two: forsterite and fayalite Backscattered electron photomicrogaph of martian meteorite NWA2046 (Scott Kuehner, NASA) So What Are System Variables? x Liquid q q Aspects of a system that can change Forsterite + Two types: liquid 3 Silica mineral + liquid v Extensive depends on the quantity of material in the system Ex: mass, volume, number of moles 1557 3 1543 Intensive don't depend on the size of the system and are properties of the Forsterite + Enstatite+ liquid substances that compose a system Enstatite v Ex: P, T, density () Enstatite + silica mineral The Phase Rule F=c+2 F = # degrees of freedom The number of intensive variables that must be specified in order to completely define the system The Phase Rule F=c-+2 F = # degrees of freedom The number of intensive variables that must be specified in order to completely determine the system = # of phases phases are mechanically separable constituents The Phase Rule F=c-+2 F = # degrees of freedom The number of intensive variables that must be specified in order to completely determine the system = # of phases phases are mechanically separable constituents c = minimum # of components (chemical constituents that must be specified in order to define all phases) The Phase Rule F=C-+2 F = # degrees of freedom The number of intensive variables that must be specified in order to completely determine the system = # of phases phases are mechanically separable constituents c = minimum # of components (chemical constituents that must be specified in order to define all phases) 2 = 2 intensive parameters Usually = temperature and pressure for us geologists One Component Systems SiO2 F=C-+2 Fig. 6-6. After Swamy and Saxena (1994), J. Geophys. Res., 99, 11,787-11,794. AGU One Component Systems 2. The system H2O Fig. 6-7. After Bridgman (1911) Proc. Amer. Acad. Arts and Sci., 5, 441-513; (1936) J. Chem. Phys., 3, 597-605; (1937) J. Chem. Phys., 5, 964-966. ...
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This note was uploaded on 04/07/2008 for the course GEOL 320 taught by Professor Milam during the Winter '08 term at Ohio University- Athens.

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