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Fundamental Concepts in Igneous Petrology (Chapter 1) Ash-Rich Strombolian Activity, Stromboli Volcano, Italy Image source: www.photovolcanica.com 1. How do we know we are dealing with i h iigneous rocks? k ? A Observational criteria (general) A. i. Field Criteria cross-cut the countryy rocks and truncate structures image source: Barb Dutrow, Dutrow 2005 1. How do we know we are dealing with i h iigneous rocks? k ? A. Observational criteria (general) i. Field Criteria Contact effects: chilled margins or contact metamorphic effects ( (above) ) Salsburyy Crags, g , Teschenite Sill (Edinburgh) (right) Hutton’s step – contact Image source: Barb Dutrow and Darrell Henry (2002) metamorphism of sandstone 1. How do we know we are dealing with i h iigneous rocks? k ? A. Observational criteria (general) i. Field Criteria Geological forms directly observed as igneous events: • cinder cones cones, stratovolcanoes, flows, etc. Ash-Rich Strombolian Activity, Stromboli Volcano, Italy Image source: www.photovolcanica.com 1. How do we know we are dealing with i h iigneous rocks? k ? A. Observational criteria (general) ii. Textural Criteria Gabbro - Rustenberg layered suite,Bushveld Complex: Image source: http://web.uct.ac.za/depts/geolsci/dlr/bv_thin.html Macroscopic/ microscopic development of interlocking texture. • first-crystallizing minerals are most euhedral and later minerals are less euhedral (paragenetic sequence) 1. How do we know we are dealing with i h iigneous rocks? k ? A. Observational criteria (general) ii. Textural Criteria Osidian from a flow Image source: Hamblin and Christiansen (2001) glassy textures Ash grain from pyroclastic p eruption: e.g. Mt. St. Helens 1. How do we know we are dealing with i h iigneous rocks? k ? A. Observational criteria (general) ii. Textural Criteria random orientation of crystals • except where there is crystal settling g or magmatic g flow Granite (above) and Gabbro (right) Image source: Darrell Henry, 2007 1. How do we know we are dealing with i h iigneous rocks? k ? A. Observational criteria (general) ii. Textural Criteria development of pyroclastic p ((explosive p eruptive p deposits materials) • rapidly cooled/partly sedimentary Rapidly cooled fragments Rapidly-cooled of rock and ash extruded as a hot volcanic ash deposit welded tuff Image source: Hamblin and Christiansen (2001) 2. What does a igneous petrologist try to assess: • • • • generation i off melts l – [how [h many?] ?] source of melting – [where?] material that is melted – [what?] processes that modifyy melts during p g crystallization, etc.? [how changed?] 3. How does a petrologist assess this? A. Experience at looking at rocks and interpreting p g textures Tokachi Volcano, Hokkaido, Japan (2006) Walter Maresch (Ruhr Univ.), Barb Dutrow (LSU) and Dan Dunkley (Univ. Tokyo). Image source: Darrell Henry 3. How does a petrologist assess this? B. Experimental data to simulate the conditions at depth p Piston cylinder apparatus and experimental assembly from Ruhr Univ.. Image source: Barb Dutrow, 1987 3. How does a petrologist assess this? C. Theoretical models to extend experimental data to other conditions i.e. thermodynamics y Theoretical melting models to relate decompression melting and the types of melts to rocks. Image source: Ed Stolper (CalTech). 3. How does a petrologist assess this? D. Knowledge of the interior of the Earth – direct samples p of the mantle Mantle xenolith in alkali basalt. Image source: Barb Dutrow, 2006 Seiad Ultramafic Complex, northern California Image source: Darrell Henry California. Henry, 1973 3. How does a petrologist assess this? D. Knowledge of interior of the Earth – indirect samples p - meteorites Meteorites - arrested early stages of development of solar nebula with no subsequent alteration or differentiation • clues to the development of the planets Meteor M t r Crater, Cr t r Arizona. Ariz n The Th explosive pl i impact of a meteorite about 25,000 ybp. Image source: Press and Siever, 2001 3. How does a petrologist assess this? D. Knowledge of interior of the Earth – indirect samples p - meteorites Irons (Fe-Ni alloy) - planetary cores? • 5% of falls [Iron meteorite MALTAHOHE, Namibia Image source: http://www.meteoriteman.com/] Stony-irons (Metal and silicate minerals) - partial differentiation • 1% of falls [Stony-iron meteorite – palllasite, Dora, New Mexico. Image source: Smithsonian Institution] 3. How does a petrologist assess this? D. Knowledge of interior of the Earth – indirect samples p - meteorites Stones (Achondrites) – differentiated – also SNC g group p • 8% of falls [Achondrite – Martian meteorite, Nakhla, Egypt, Image g source: http://www.meteoriteman.com/] p // /] Stones (Carbonaceous chondrite) – primitive/undifferentiated / • 86% of falls [Chondrite – Allende meteorite: Chihuahua, Mexico. Image source: http://www.meteoriteman.com/] 4. What do we think we know about Earth’s Earth s interior? A. The Earth is layered y Earth has layered structure: • crust (oceanic and continental) • mantle • core (inner and outer) image source: Press and Siever, 2001 4. What do we think we know about Earth’s Earth s interior? A. The Earth is layered Internal structure is largely established by variations in P- and Sseismic waves. • The Mohorovicic (Moho) discontinuity at interface of crust and upper mantle is compositional discontinuity. image source: Winter (2001) 4. What do we think we know about Earth’s Earth s interior? A. The Earth is layered Low velocity zones in upper mantle is zone of 1-10% melting • forms the asthenosphere • serves as interface of lithospheric plates and mesophere. image source: Winter (2001) 4. What do we think we know about Earth’s Earth s interior? A. The Earth is layered Damping of S waves in the outer core signals presence of liquid outer core. Sharp increase in SS and P Pwave velocities indicate solid metallic inner core. image source: Winter (2001) 4. What do we think we know about Earth’s Earth s interior? A. The Earth is layered Continental crust vs. ocean crust Thicker: 25-60 km vs. 0-10 km Older: some >4 Ga vs. <160 Ma. Si-richer: 52-75% SiO2 vs. 50-52% SiO2. Crust/upper mantle are coupled as lithospheric p p plate to depths p of initial low velocity zones. image source: Press and Siever (2001) 4. What do we think we know about Earth’s Earth s interior? A. The Earth is layered Within mantle there are additional discontinuities that mark polymorphic transitions of olivine: • 410 km olivine converts to spinel-type spinel type structures • 660 km there is a transition to a perovskite-type structure. image source: Press and Siever (2001) 4. What do we think we know about Earth’s Earth s interior? A. The Earth is layered New player: Post-perovskite high-pressure g p p phase of MgSiO g 3 • P and T stability at lowermost Earth'ss mantle. Earth mantle p derives from • “Post-perovskite" name of stable phase of MgSiO3 throughout most of Earth's mantle, i.e. perovskite. image source: Press and Siever (2001) 5. Hypothesis for origin of Earth? A Early differentiation (first few 10s of A. millions of years?) image source: Press and Siever (2001) Differentiation process likelyy results from heating due to combinations of • gravitational i i l collapse ll • accretion of planetismals • Sinking of iron to form core • radioactive decay Renewed interest in the Early Earth Possible early evolution of solar system (4.57 Ga) Alternate Early Earth scenarios P ibl events Possible nt • Moon-forming i impactor ( 4 54 Ga) (~4.54 G ) • Magma vs. watery oceans (4.4-4.0 Ga) Artist: Ron Miller The Most Ancient Earth Materials Known Detrital Zircons from a conglomerate at Jack Hills, Australia False F l color l CL image i off ancient i t zircon. (Wilde et al. 2001) Alternate Early Earth timeline Places of Other Ancient Earth Materials 5. Hypothesis for origin of Earth? B Resulting heterogeneous Earth composition B. Differentiation produced chemical zonation in the Earth Defines nature of rock types that will develop. image source: Press and Siever (2001) 6. P P--T-depth relation in the Earth? Relationship between depth and pressure a function of weight of the overlying column of material. image source: Winter (2001) 5. P P--T-depth relation in the Earth? In systems that flow (ductile), P is equal in all directions i.e. lithostatic P = ρgh • ρ = density • g = acceleration of gravity • h = height of rock column image source: Winter (2001) 5. P P--T-depth relation in the Earth? Near surface, rocks behave brittly • can accommodate small amount of differential P (a few kbars) before fracturing. density changes primarily with composition e.g. crust ~ 2.8 g/cm3 and mantle ~ 3.3 g/cm3. image source: Winter (2001) 5. P P--depthdepth-T relation in the Earth? Variation of T with depth is geothermal gradient. gradient Related to factors including cooling initiated in the early Earth and radioactive decay. decay image source: Winter (2001) Estimated ranges of oceanic and continental steady-state geotherms to a depth of 100 km using upper and lower limits based on heat flows measured near the surface. 5. P P--depthdepth-T relation in the Earth? Heat is transferred by: p • radiation to space (minor) • conduction (thermal ( vibration) ( y • convection (density differences associated with T)) • advection (transfer of heat with rocks). ) image source: Winter (2001) 7. Where are magmas from? – where it is hot enough – plate tectonics 5 3 1 6 7 4 2 200 km Continental Crust 400 Oceanic Crust Lithospheric Mantle ? 600 km Source of Melts ? Sub-lithospheric Mantle ? ? image source: Winter (2001) 1. Mid Mid--ocean Ridges 5. BackBack-arc Basins 2 Intracontinental 2. I i l Rifts Rif 6 Ocean 6. O Island I l d Basalts B l 3. Island Arcs 7. Miscellaneous Intra Intra-Continental C i lA Activity i i 4 Active 4. A i C Continental i lM Margins i kimberlites, carbonatites, kimberlites, carbonatites, anorthosites... anorthosites ...