1. Intro to Petrology

1. Intro to Petrology - Igneous & Metamorphic...

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Unformatted text preview: Igneous & Metamorphic Petrology Damian Nance and Keith Millam http://www.whitman.edu/geology/winter/ The Earth's Interior Major Subdivisions: Crust Oceanic crust Continental crust Mantle Upper mantle Transition zone Lower mantle Core Inner core Outer core Major subdivisions of the Earth The Earth's Interior Crust: Oceanic crust Thin: 10 km Relatively uniform stratigraphy = ophiolite suite: 3 3 3 3 3 sediments pillow basalt sheeted dikes more massive gabbro ultramafic (mantle) Continental crust Thicker: 20-90 km average ~35 km Highly variable composition 3 average ~ granodiorite The Earth's Interior Mantle: Peridotite (ultramafic) Upper to 410 km (olivine spinel) x Low Velocity Layer 60-220 km Transition Zone as velocity increases ~ rapidly x 660 km spinel perovskite-type Si Si Lower Mantle has more gradual velocity increase The Earth's Interior Core: Fe-Ni metallic alloy Outer Core is liquid x No S-waves Inner Core is solid Compositional subdivisions Variation in P and S wave velocities with depth Rheological subdivisions Relative atomic abundances of the seven most common elements that comprise 97% of the Earth's mass The Pressure Gradient q q P increases = gh Nearly linear through mantle x x ~ 30 Mpa/km (1 Kbar 3 Km) 1 GPa (10 Kbar) at base of average crust q Core: increases more rapidly since alloy more dense Pressure variation with depth Heat Sources in the Earth 1. Heat from the early accretion and differentiation of the Earth x still slowly reaching surface Heat Sources in the Earth 1. Heat from the early accretion and differentiation of the Earth x still slowly reaching surface 2. Heat released by the radioactive breakdown of unstable nuclides Heat Transfer 1. Radiation 2. Conduction 3. Convection The Geothermal Gradient q q q q Increase of temperature with depth More rapid in oceanic lithosphere (blue curves) Less rapid in continental shield (purple curves) Most rapid near surface Thickness of mature (> 100Ma) oceanic lithosphere is hatched. Thickness of continental shield lithosphere is cream colored Estimates of oceanic (blue curves) and continental shield (red curves) geotherms to a depth of 300 km The Geothermal Gradient Pattern of global heat flux variations at Earth's surface Cross-section of the mantle based on a seismic tomography model Arrows represent plate motions and large-scale mantle flow and subduction zones represented by dipping line segments. EPR =- East pacific Rise, MAR = MidAtlantic Ridge, CBR = Carlsberg Ridge. Plates: EA = Eurasian, IN = Indian, PA = Pacific, NA = North American, SA = South American, AF = African, CO = Cocos Mantle Convection Earth is likely a single layer convection system with plates forming the upper thermal boundary layer and core-mantle boundary forming the lower one. In the modern world plate convection is responsible of 90% of heat loss from mantle, while plume mode covers the remaining 10%. But proportions could have changed through Earth history. Cartoon linking seafloor spreading to circulation above 670 km boundary and hotspot formation to thermal instabilities in D layer, with subduction perhaps linking the two regions (possibly in the form of intermittent avalanching) Plate Tectonics Igneous and Metamorphic Petrogenesis 1. Mid-ocean ridges 2. Intracontinental rifts 3. Island arcs 4. Active continental margins 5. Back-arc basins 6. Oceanic hotspots 7. Continental hotspots ...
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