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The Formation of Igneous Rocks

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  1. The Formation of Igneous Rocks Magma Genesis, Transport, and Modification

  2. Igneous Rocks: Review With your neighbors, discuss and answer: • What is fractionation? How does this process occur? Why is it important in understanding how igneous rocks form and evolve? • What is Bowen’s Reaction Series (BRS)? Sketch as much as you can remember about BRS. • What is the geothermal gradient? Why is it important in discussing the formation of igneous rocks?

  3. Igneous Rocks: Importance and Occurrence • Occurrence: • Mantle (ultramafic, plutonic) • Oceanic crust (SIMA - mafic) • Continental crust (SIAL - intermediate to felsic) • Specific rock types closely tied to tectonic setting • Importance: • Make up >90% of earth’s crust • Occur on all terrestrial planets • Volcanic hazards • Economic deposits (diamond, Cr, Ni, Co, Mo, Sn, W, Ti, Li) Mt. Vesuvius, Naples

  4. Introduction to Igneous Rocks • Understanding igneous rock petrography requires: • Knowledge of characteristic igneous rock textures and structures • Knowledge of characteristic compositions (mineral assemblages) • Understanding igneous rock petrogenesis requires: • Understanding the origin of magma • Understanding magmatic evolution and cooling history • Understanding magma transport, storage, and eruption mechanisms • Understanding relationships between igneous rock formation and tectonic settings

  5. Igneous Rock Textures • Texture is largely determined by rate of cooling • Intrusive (plutonic) • Crystallize slowly below the earth’s surface • Holocrystalline • Phaneritic • Extrusive (volcanic) • Crystallize more quickly at the earth’s surface • Porphyritic to aphanitic • Glassy • Vesicular • Pyroclastic

  6. Igneous Rock Composition • Natural range of igneous rock compositions • Reflected in mineral associations (assemblages)

  7. Igneous Rock Compositions • Bowen’s Reaction Series • Predicts the order in which minerals crystallize from a cooling magma • Idealized model for equilibrium crystallization in a magmatic system • Discontinuous mineral crystallization series • Continuous mineral crystallization series • Composition of natural magmas determines the extent to which crystallization follows BRS • As a magma cools, all minerals have a characteristic crystallization (melting) temperature and crystallize from either: • All liquid or • Some liquid & some crystals • Crystals interact with the liquid and change composition as cooling takes place • Crystallization temps depend on magma composition

  8. Mafic Hi density Low viscosity Hi Temp Felsic Low density Hi viscosity Low Temp Composition: BRS

  9. Igneous Rock Classification Based on • Texture • Phaneritic • Aphanitic/ porphyritic • Vesicular • Glassy • Composition (essential minerals) • Ultramafic • Mafic • Intermediate • Felsic (silicic)

  10. Petrogenesis: The Formation of Igneous Rocks • Magma genesis • Source rock • Mechanism to cause melting and create magma • Mechanism to transport magma from source to crystallization site • Brittle: dikes and fractures • Ductile: diapirs • Magma cooling and crystallization to form rock • Compositional modification (fractional and equilibrium crystallization, assimilation, mixing) • Combination of these processes produces the diversity of igneous rocks on earth

  11. The Formation of Igneous Rocks • Closely tied to tectonic setting and processes • Divergent boundaries (& hot spots) = primitive magma • Convergent boundaries = recycled magma

  12. What is Magma? • Magma = • Liquid (molten rock) ± crystals ± dissolved gasses (volatiles) *Also varies with temperature and water content

  13. Separation of two “fractions” in a source material through Partial melting and/or Partial crystallization Two fractions are Different in composition Different from the original material Separate through gravity settling/upward movement of melt Magma Fractionation

  14. Bowen’s Reaction Series Idealized model for equilibrium melting or crystallization Minerals have different melting/ crystallization temps Minerals lower on BRS melt first Hydrous minerals melt first Melt will generally be LESS mafic than starting rock Solid residue will generally be MORE mafic than starting rock Origin of Magma: Partial Melting

  15. The Origin of Magma Magma genesis requires: • Source rock • Mantle (ultramafic) • Oceanic crust (mafic) • Continental crust (intermediate to felsic) • Unlikely that any preexisting rock will melt 100% to make magma • Magma sources: • Primitive magma = mantle rock • Recycled magma = crustal & mantle rock

  16. Average crustal geothermal gradient = 20ºC/km The Origin of Magma • Magma genesis: • The mantle and crust are SOLID • Seismic evidence • Xenolith evidence • Experimental evidence • Requires mechanism to cause melting (anatexis) • Increase heat • Lower rock melting temperature

  17. Decompression melting Caused by convection Hot, deep material rises faster than heat is lost to surroundings Raises local geothermal gradient Process responsible for generating primitive magma at DIVERGENT MARGINS and HOT SPOTS The Origin of Magma • Raising the geothermal gradient (increase heat) • Frictional heat • Caused by faulting • Localized heating and melting

  18. Caused by adding volatiles (H2O, CO2) Sea water in pore spaces in rock Water bound in hydrous minerals (clay, serpentine, mica, amphibole) Water is released as rocks reach higher temps & pressures, melts surrounding rocks Process responsible for generating recycled magma at CONVERGENT MARGINS The Origin of Magma • Lowering the solidus • Flux melting = addition of component(s) that lower rock melting temperature

  19. The Origin of Magma • Adding hot material (raise geothermal gradient) • Crustal anatexis = melting of continental crust • Basaltic magmas generated in mantle rise into the crust • Lower density can prevent magma from rising to surface • Hot magma heats and partially melts surrounding crustal rocks • Process responsible for generating felsic magmas at CONVERGENT MARGINS and CONTINENTAL RIFTS

  20. Partial melting of source rock (via decompression, flux melting, or crustal anatexis) produces Magma Crystal residue Magma migrates upward due to density contrast Magma less dense than surrounding rocks Two major mechanisms of movement Brittle: fractures and dikes Ductile: diapirism Magma Transport Basalt dike

  21. Fracturing opens spaces that magma can invade Fracturing caused by Tensional stress (rock pulled apart by tectonic forces) Expansion of magma and upward buoyancy Process most common in mafic magmas (basalt) Process common in brittle crust (upper crust; extension) Magma Transport • Allows rapid transport of magma (scale of days to years) Dike and volcanic neck, Ship Rock, NM

  22. Diapir = elliptical to tear-shaped mass that rises toward the surface Upward migration due to density contrast Magma less dense than surrounding rocks Process most common in intermediate to felsic magmas, particularly granite bodies Process common in mantle and ductile lower crust Magma Transport • Slower transport, scale of years to many thousands of years

  23. After origin and transport, magmas are commonly modified before/during crystallization to form rocks DIFFERENTIATION = processes that modify composition of the magma Magma mixing Assimilation Crystal Fractionation CRYSTALLIZATION = solidification of magma to form rock Equilibrium crystallization (Bowen’s Reaction Series) Fractional crystallization Magma Modification and Evolution

  24. Assimilation Magma can melt, react with, and/or dissolve surrounding rocks Difficult to fully assimilate melted rock due to density and viscosity contrasts Magma Modification and Evolution • Magma Mixing (or mingling) • Two magmas of different compositions blend together to form a single magma • New magma has a composition partway between the two original magmas • Incomplete mixing = mingling

  25. Crystallization (equilibrium or fractional) changes the composition of the liquid Elements are preferentially partitioned into certain minerals 5 MgO 5 SiO2 10 Total 5/10 * 100 = 50% MgO 5/10 * 100 = 50% SiO2 4 MgO 5 SiO2 9 Total 4/9 * 100 = 44% MgO 5/9 * 100 = 56% SiO2 Magma Modification and Evolution

  26. Equilibrium crystallization Magma Modification and Evolution • Crystals that form remain in direct contact with melt • Crystals and melt continually equilibrate • Composition of the system is constrained by the bulk composition of the original melt • Example: equilibrium crystallization of plagioclase feldspar

  27. Bowen’s Reaction Series = idealized model for equilibrium crystallization (and melting) Crystallizing minerals are in equilibrium with the melt Melt changes composition as crystals form and melt cools Earlier formed crystals will no longer be in equilibrium with the melt, and will be dissolved to form new minerals Process gives rise to many (BUT NOT ALL) diverse igneous rocks Magma Modification and Evolution

  28. Natural examples of BRS reactions: Early forming crystals are generally euhedral Early forming crystals may be surrounded by later forming crystals Early forming crystals may be resorbed, suggesting a reaction with the melt to form later crystals (olivine, left photo) Minerals that undergo solid solution may be zoned (plagioclase, right photo) Magma Modification and Evolution

  29. Fractional crystallization Magma Modification and Evolution • Crystals that form are immediately removed from the melt • Floating or sinking of crystals • Filter pressing • Flow segregation • “Armoring” of crystals • Composition of the system is NOT constrained by the original bulk composition • Example: fractional crystallization of plagioclase feldspar

  30. Cumulate textures, suggesting crystal removal by settling (layered mafic intrusions) Magma Modification and Evolution • Natural examples of fractional crystallization processes: • High-temp crystals may have a “rim” of low-temp crystal

  31. Few (perhaps no) igneous rocks are simple crystallized melts of a parent rock Igneous rocks represent a complex interplay of Source rock type and conditions of melting Fractionation of magma from its source Modification by contamination (mixing, assimilation) Fractionation as magma cools and crystallizes (likely some each of equilibrium and fractional crystallization) Conditions of cooling (volcanic vs. plutonic) Igneous Rock Formation

  32. Answer the following and turn in (1 per group): Use the plagioclase feldspar binary T-X diagram to determine the equilibrium crystallization of a melt with starting bulk composition of 40% An, and answer the questions on the worksheet. Use the plagioclase feldspar binary T-X diagram to determine the fractional crystallization of a melt with starting bulk composition of 60% An, and answer the questions on the worksheet. How might you distinguish a rock formed by the process in question 1 from a rock formed by the process in question 2 (what physical and/or chemical characteristics would you look for)? [Answer on back of worksheet] Begin Homework Part 1 (modeling of fractional crystallization in a M&M magma chamber). Collaborative Activity