SPRING 2014GEOLOGY 285:INTRO. PETROLOGYDr. Helen LangDept. of Geology & GeographyWest Virginia UniversityCrystallization of MagmasIgneous Rocks form by the Crystallization of Magmas at or beneath the Earth’s surfaceEffects of cooling rate• Magma cooled very suddenly freezes to glass (basalt must cool more quickly than rhyolite to form glass)• Magma cooled pretty quickly has many small crystals (microscopic or sub-microscopic) - called aphanitic• Magma cooled very slowly has big crystals (visible to the naked eye) - called phaneriticHow might a porphyritic texture develop?Phenocrysts are nepheline (a feldspathoid)What happens to a magma as it cools?• Pure liquids crystallize (freeze) at a single temperature (melting T = freezing T)– Water to ice at 0oC (32oF)–SiO2liquid to cristobalite at 1740oC• Impure liquids (like magmas) crystallize over a range of T, crystals are different composition than liquid– Different minerals form at different temperatures as the magma coolsSequence of mineral crystallization and T at which different minerals begin to crystallize depends on:• Magma composition• Pressure• Water pressure• Cooling rate• and many other factorsBowen’s Reaction Series is a generalization about “typical” Crystallization orderolivineorthopyroxeneclinopyroxenebiotiteamphibole (Hb)Ca plagioclaseNa plagioclaseNaCa plagioclasealkali feldsparmuscovitequartzlow Thigh Tcoolingresidual phasestemperatureTo really understand igneous crystallization, we need to go beyond Bowen’s Reaction Series• In the last 50+ years, geochemists have done laboratory experiments on magma-like liquids• One Method: Melt a rock, cool liquid to known T, “freeze” it, view it under the microscope, identify minerals - called “cook-and-look” method• Better Method: Start with simple systems, e.g., liquid with various proportions of pure Anorthite (Ca-plag, CaAl2Si2O8) and pure Diopside, (pyroxene, CaMgSi2O6), examine results at different TsResults are displayed on phase diagrams• What is a phase? - a homogeneous portion of a system that is physically distinct and mechanically separable from the other parts of the system(p = #phases)• We also need to know about chemical composition components = the minimum number of chemical constituents necessary to describe the compositions of all solids, liquids and gases (phases) in a system(c = #components)If the System of interest is:• Quartz, Cristobalite, Tridymite, Stishovite, Coesite and liquid SiO2• All phases can be described with the single chemical formula SiO2• 1 chemical component, SiO2• c=1For more complex systems, finding the components is a trial-and-error process• for Olivines and corresponding liquids: Mg2SiO4(Ol or liq.), Fe2SiO4(Ol or liq.), MgFeSiO4(Ol or liq.), Mg1.5Fe0.5SiO4(Ol or liq.), etc.– try elements: Mg, Fe, Si, O 4– try oxides: MgO, FeO, SiO23– try mineral end-members: Mg2SiO4, Fe2SiO42**smallest number, therefore, correct # of componentsMg2SiO4Fe2SiO4For Olivines (Mg2SiO4-Fe2SiO4) and Orthopyroxenes (MgSiO3- FeSiO3) and corresponding liquids • try mineral end-members: Mg2SiO4, Fe2SiO4, MgSiO3, FeSiO34• try oxides: MgO, FeO, SiO23*• try elements: Mg, Fe, Si, O 4*smallest number, therefore, correct # of componentsThe Phase Rule• A generalization about phase diagrams• by J. Willard Gibbs in the 1870s• f = degrees of freedom, the number of parameters that can be varied independently without changing the phases present• f = 2 + c - p (f = #degrees of freedom, c = #components, p = #phases)• Examples will show what this means• f=0 invariant (point)• f=1 univariant (line)• f=2 divariant (field or area)• f=3 trivariantDegrees of freedom or varianceOne component system: SiO2p=3c=1f=?xxxp=2c=1f=?p=1c=1f=?Real Magmas have many components (8 or 9 major ones)• We can’t draw diagrams that represent all those components• because that requires too many dimensions• But we can learn a lot about crystallization of real magmas (like basalts) by examining several different 2-component systems!2-component (Binary) Systems• We’d need 3 dimensions to represent one-phase fields (c=2, p=1, f=2+2-1=3; trivariant)• But, we can only draw diagrams in 2 dimensions easily!• Hold Pressure (P) constant, lose one degree of freedom, only need 2 dimensions• Phase rule (at constant P) becomesfP= 1 + c - pMost Common Minerals in Basalt are Plagioclase and Clinopyroxene Diabase (medium-grained “basalt”)Simple 2-component system: Diopside (CaMgSi2O6) - Anorthite (CaAl2Si2O8)Simplest representation of a basalt-like liquid (Cpx+Plag)C=2xp=1f=?p=3f=?p=2f=?xEutecticDi AnHow do we use this diagram to tell about crystallization of basalt?Diopside-rich liquid (X)80% CaMgSi2O6(Di) 20% CaAl2Si2O8(An)Draw isopleth= line of constant compositionWhat happens as liquid cools?XHow do we use this diagram to tell about crystallization of basalt?Anorthite-rich liquid (Y)65% CaAl2Si2O8(An) 35% CaMgSi2O6(Di)Draw isopleth= line of constant compositionWhat happens as liquid cools?YTie-lines and the Lever RuleabcedfXAnimated Di-An DiagramCourtesy of Professor Kenneth E. WindomDepartment of Geological and Atmospheric SciencesIowa State University, Ames, IowaPowerPoint® presentation by Kenneth E. Windom1. A bulk composition of X cools to the liquidus, at which point An crystallizes.2. Continued crystallization of An causes liquid composition to move toward Di.3. When T=1274°C, liquid has moved to eutectic; Di begins to crystallize. No change in temperature or composition of any of the 3 phases is permitted until the liquid has completely crystallized.4. At temperatures below 1274°C, only crystals of An and Di are present.XTextures of rocks in Di-An SystemPlag and Pyroxene Phenocrysts in fine-grained groundmassPerfect Equilibrium Crystallization• Crystals that have formed remain in contact with the liquid and continually equilibrate with it• Constant bulk composition• Makes little difference in Di-An system• Contrasted with fractional crystallization, where crystals are physically separated from the liquid in which they were formed, unable to react or equilibrate with liquidMore Complex 2-component systemAt a Forsterite reacts with Liquid to form Enstatitep=3f=?Y ZEn melts incongruentlyOlivine in Basalt reacted with Liquid to form OpxOl OlOpxOlOlPlagioclase, a 2-component system with Complete Solid SolutionCrystallization of Plagioclase in Basalts (real igneous plagioclase is