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Igneous and Metamorphic Petrology Convection demo “Lava Lamp” Density displacement demo: oil and water are immiscible. Marble Demo :Fractionation The Texts • An Introduction to Igneous and Metamorphic Petrology 1st ed by J. D. Winter • or Principles of Igneous and Metamorphic Petrology J.D. Winter 2nd ed. The Earth’s Interior Crust: Oceanic crust Usually < 10 km ophiolite suite: list Continental Crust Thicker: 20-90 km average ~35 km Variable composition but average a granodiorite O2 - The Silicate Tetrahedron 2_25 Si4+ O2 - O2 The basis of most rock-forming minerals, charge - 4 O2 - The Mantle is mostly Silicates The Earth’s Interior Mantle: Peridotite (ultramafic) Upper Mantle to 410 km olivine, pyroxenes, spinel - structure minerals, and garnet Low Velocity Layer 60-220 km Aesthenosphere Transition Zone as velocity increases 410 660 km , olivine not stable, replaced by high P polymorphs with ~ same composition: wadsleyite (beta-spinel structure), and ringwoodite (gamma-spinel structure) Lower Mantle 660 Upper minerals unstable, perovskite-type structure SiIV SiVI Seismic Tomography The Earth’s Interior Core: Fe-Ni metallic alloy Sulfur Outer Core is liquid No S-waves Inner Core is solid Discussions: Differentiation Iron Meteorites, Impactor Density and Buoyancy LVZ Note how S-wave velocities drop to zero in the Liquid outer core Source: Recommended Text Kearey and Vine (1990), Global Tectonics. Upper Mantle Samples • Samples of the upper mantle occasionally appear where faulting has exposed it in oceanic fracture zones, thrust it up in collision zones, or where brought up in diatreme and basalt eruptions. The rock revealed is usually Peridotite, which is three-quarters Dunite (pure olivine) and one-quarter basalt. The Basalt forms by the partial melting of this peridotite, which drives off the basaltic melt, leaving behind the solid “depleted “ dunite (basaltic components removed). • The original (fertile) mantle has more Al, Ca, Ti, Na, and K and lower Mg# = Mg/(Mg +Fe) than Dunite So some of the above go into the basalt. Fo Mg++ 1900C Fa Fe++ 1500C Molten- VERY Hot No solids 1900 oC First mineral to crystallize out Independent Tetrahedra 1553 oC 3-D Single chains Double chains “Basaltic” sheets “Andesitic” 3-D 3-D Molten- Not so hot sheets 3-D 100% Solid “Granitic” Dark Green Gray Gray Pink to Salmon Fine crystals Need a microscope Low silica, HOT, fluid Course crystals Easily seen Intermediate High silica, warm, viscous If crystals are left in contact with melt … http://www4.nau.edu/meteori te/Meteorite/Eucrite.html • Ultramafic to Basaltic • Gray needles are Plagioclase (Plag) Feldspar, Yellow-brown crystals are Pyroxene (Py), brightly colored crystals are Olivine (Ol). At lower Temps, the Olivine xtals have been partially resorbed by the melt, their atoms reused to make Py & Plag. Plagioclase Feldspar Stable composition varies with Temperature If the first formed crystals of Calcium-rich (Ca) Plagioclase Feldspar are left in contact with the melt , as the melt cools more stable sodium-rich layers will be deposited on their outer rims Zoned feldspar (plagioclase) showing change in composition with time in magma chamber (calcium-rich in core to sodium-rich at rim) Isolated Olivine crystals • Early formed Olivine crystals can sink to the bottom of a magma chamber, so they are isolated from the very reactive ions in the melt. If early crystals are removed (isolated), the melt becomes richer in Silica Remove Fe, Mg, Ca Some Si Left with K and Al Most of Si You can start with a Mafic (silica-poor) magma and end up with some Felsic (silica-rich) Granites. Marble Demo A melt will crystallize its mafic components first, and the remaining melt may be granitic We need to be able to estimate pressures Pressure Gradient P increases = rgDh 1 GPa at base of crust • Linear increase mantle • ~ 30 MPa/km • Core: r increases more rapidly since Fe-Ni alloy more dense Pressure Calcs • To calculate pressures at the base of a stack of layers with different densities, start from the top layer, calculate the pressure at the base as • P0-1 = r0-1gDh0-1 • For the second layer, • P2 = P0-1 + r1-2gDh1-2 Etc. Multi-layer Pressure Calc Example • Upper crust 25 km thick, density 2.75 Mg/m3 r0-1 = 2.75 Mg/m3 x 1000 kg/1Mg = 2.75 x 103 kg/m3 • P1 = r0-1gDh0-1 • = 2.75 x 103kg/m3 2 x 9.81 m/s2 x 25 x 103 m • = 6.744 x 108 kg . m/s2 x 1/m2 (aka “Pascals”) • Next layer down, 10 km basalt r1-2 = 3 x 103 kg/m3 • P2 = P1 + r1-2gDh1-2 Etc. See the handout, after the lecture Mg3Al2(SiO4)3 Olivine Example • At high TP, the a olivine structure is no longer stable. • Below depths of about 410 km olivine undergoes an exothermic phase transition to the sorosilicate, wadsleyite , the b Olivine • At about 520 km depth, wadsleyite transforms exothermically into ringwoodite, the g Olivine, which has the spinel structure. • At a depth of about 670 – 700 km ringwoodite decomposes into silicate perovskite ((Mg,Fe)SiO3) and ferropericlase ((Mg,Fe)O) in an endothermic reaction. • These phase transitions lead to a discontinuous increase in the density of the Earth's mantle that can be observed by seismic methods. They are also thought to influence the dynamics of mantle convection in that the exothermic transitions reinforce flow across the phase boundary, whereas the endothermic reaction hampers it. • This leads some workers to believe that the 700 km boundary blocks convection from the core mantle boundary, and upper mantle convection cells are distinct. Exothermic materials heat, expand, more buoyant Phase diagram for aluminous Notice the mantle will 4-phase Lherzolite: not melt under normal Al-phase = ocean geotherm! Ca++ Plagioclase shallow (< 50 km) Spinel Lherzolite Spinel is MgAl2O4 50-80 km Garnet Lherzolite 80-400 Si [4] => Si [6] km Si[4] Si[6] coord. > 400 km Figure 10-2 Phase diagram of aluminous Lherzolite with melting interval (gray), sub-solidus reactions, and geothermal gradient. After Wyllie, P. J. (1981). Geol. Rundsch. 70, 128-153. Heat Sources in the Earth • Impact heat from the early accretion and differentiation of the Earth – Convection cells redistribute heat to cold surface Heat Sources in the Earth 1. Heat from the early accretion and differentiation of the Earth still slowly reaching surface 2. Heat released by the radioactive breakdown of unstable nuclides Heat Transfer 1. Radiation Requires transparent medium Rocks aren’t (except perhaps at great depth) 2. Conduction Rocks are poor conductors Very slow 3. Convection Material movement (requires ductility) Heat-induced expansion and buoyancy Much more efficient than conduction Geothermal Gradient Cool Silica-rich rocks (with Quartz, K-feldspar) melt at cooler temperatures. Melts are viscous Silica-poor rocks (with Olivine, Pyroxene, Ca-feldspar) melt at higher temperatures Melts are very fluid Hot Lithosphere Buoyancy Ocean and Continental Lithosphere Thermal Gradients Melting depths vary w\ volcanic province Most within upper few hundred kilometers Heat highest at MOR, suggests rising convection cells there Highest at MORs Origin of Basaltic Magma - MOR Harry Hess’ Seafloor Spreading • Role of Pressure in divergent margin – Reducing the pressure lowers the melting temperature – the mantle partially melts – Mid-ocean ridge and rift valley: called decompression melting http://volcanoes.usgs.g ov/about/edu/dynamicpl anet/nutshell.php Mantle loses heat at surface, becomes denser. Pulls lithosphere down into “Subduction Zone” Origin of Basaltic Magma 2 Subduction Zone • Role of volatiles - WATER INITIALLY BASALTIC Origin of Basaltic Magma 3 Plumes, also basaltic Assimilation and magmatic differentiation Why are the continents so silica rich? Weathering dissolves high-temp. minerals, but also: Fractionation: if early crystals settle out, remaining melt is relatively richer in silica Show Samples Origin of Andesite & Diorite: intermediate silica content Basaltic here Good diagram for the Andes Mountains Small blobs, not much heat in them Assimilate some crust, fractionate Origin of Granitic Rocks Magma rises further distance, more fractionation. Passes through thicker crust, more assimilation. Huge blobs w/ low temps but lots of magma, fractionation & assimilation => Granite Batholiths Can also get small amounts of granites from deep felsic rock passed by ascending magma Plate Tectonic - Igneous Genesis 1. Mid-ocean Ridges 2. Intracontinental Rifts 3. Island Arcs 4. Active Continental Margins 5. Back-arc Basins 6. Ocean Island Basalts 7. Miscellaneous IntraContinental Activity kimberlites, carbonatites, anorthosites... Or, for Kimberlites (7) Many workers think plumes from the core-mantle boundary can punch through the endothermic 670-700 km transition. Diamonds formed from subducted organic carbon are lifted by rising plumes that happen to hit a subducted slab of ocean lithosphere. Isotope Signatures • Plate tectonic provinces have a characteristic stable isotope signature