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GEOL 4700 ‘Field’ Mineralogy Earth’s Interior http://ruby.colorado.edu /~smyth/G4700.html GEOL 4700 ‘Field’ Mineralogy Earth’s Interior Multi-anvil Press 5000 ton (Bayerisches Geoinstitut) Earth’s Interior http://ruby.colorado.edu /~smyth/G4700.html Earth’s Interior The Core is Metal The Mantle is oxygen minerals 1 There could be significant hydration in the Transition Zone Water Storage Capacity in Ocean Units • Nominally Anhydrous Minerals (NAMs) can incorporate up to 10 times the total Ocean mass if saturated. How do we know what is inside the Earth? • Seismology: Earthquake waves • Cosmochemistry: Meteorites • Experiments: Laboratory synthesis • Xenoliths Major Parts • Ocean water 0.025% of mass • Crust (Above the Moho) 0.5% of mass – Oceanic (7 to 10 km of basalt & gabbro) – Continental (30 to 60 km of granite) • Mantle - Moho to 2900 km - Solid Rock – 65% of total mass – Upper (Moho to 410km) (Olivine + pyroxene) – Transition Zone (410 -670 km) (Silicate Spinels) – Lower 670 to 2900 km (Perovskite + periclase) • Core (2900 to 6367km) 35% of mass – Outer Core (Liquid Metal) – Inner Core (Solid Metal) Mantle Mineralogy: Modal Fraction Review Mantle Mineralogy Crust • Oceanic Crust (Basalt and Gabbro) – Thin (7 - 10 km) – Dense (2.9 - 3.1 g/cm3) – Young (< 250 my) • Continental Crust (Granite + Diorite + Seds) – Thick (30 - 60 km) – Light (2.7 - 2.8 g/cm3) – Old (250 - 4000 my) 2 Mantle • Upper Mantle (Moho to 410 km) Seismology • S-wave shadow – No S-waves opposite-side – Peridotite (Olivine + pyroxene) – Eclogite (Pyroxene + Garnet) • Transition Zone (410 - 670 km) – Spinels and Spinelloids earthquakes – Core (outer) must be molten • P and S reflections – Discontinuities at 410 and 670 km – Phase Changes • Lower Mantle (670 - 2900 km) – MgSiO3 - Perovskite – MgO - Periclase • P and S travel time anomalies – Tomography – Hot and cold regions Ringwoodite Core • Outer Core (2900 to 5150km) – Molten iron metal (+ ~10% lighter element) – No S-wave transmission • Inner Core (5150 - 6378 km) – Solid Iron Metal – May slowly rotate relative to mantle • • • • Formula (Mg,Fe)2SiO4 Up to 2.4 %H2O by weight Stable at depths of 525 to 660 km. Third most abundant mineral in Earth Wadsleyite How do we know what’s there? • Seismology – S-wave shadow – P and S reflections and conversions • Meteorites: – Earth formed from primitive meteorites • Xenoliths from kimberlites & basalts • Experiments • • • • Formula (Mg,Fe)2SiO4 Up to 3.2 %H2O by weight Stable at depths of 410 to 525 km. Fourth most abundant mineral in Earth 3 Seismology P-waves S-waves Xenoliths Velocity Structure and Earthquakes at N. Tonga Xenoliths (VanderHilst, Nature (1995) Red = slow = hot green = fast = cold Xenoliths Xenolith-Peridotite • Xenoliths are ‘strange rocks’ • Xenoliths are inclusions in kimberlites and basalts • Xenoliths provide natural high pressure rocks. – Kimberlites bring diamonds and other rock samples from as deep as 670 km. – Basalts are derived from the lithospheric mantle 4 Breaking Xenoliths at a Diamond Mine in South Africa Meteorites • Meteorites tell us of Earth’s bulk composition – Chondrites are primitive meteorites – Earth is “chondritic” in composition. • Composition is similar to sun minus H, He and other volatiles. Meteorites Meteorites Experiments • Multi-Anvil Press – Synthesis experiments to 1000 km depth – Samples 1-5 mm3 • Diamond Anvil Cell – Spectroscopy – Ultrasound – X-ray diffraction Multi-anvil Press 5000 ton (Bayerisches Geoinstitut) 5 Multi-anvil Press Sample Assembly Experiments • Multi-Anvil Press – Synthesis experiments to 1000 km depth – Samples 1-5 mm3 • Diamond Anvil Cell – Spectroscopy – Ultrasound – X-ray diffraction Multi-anvil Press Sample Assembly Experiments • Multi-Anvil Press – Synthesis experiments to 1000 km depth – Samples 1-5 mm3 • Diamond Anvil Cell – Spectroscopy – Ultrasound – X-ray diffraction Carbide cubes retain a lot of strain Multi-anvil Press Octahedral Assembly: After and Before 6 Multi-Anvil Press Octahedron Interior Terms Diamond-anvil Cell Interior Terms Interior Terms 7 Interior Terms Interior Terms DAC on Diffractometer Mica Crystal with Quartz @ 5GPa Interior Terms 8 525 km Wadsleyite 410km: Olivine Density 3.2 Olivine Ringwoodite Ringwoodite Crystal Wadsleyite 3.5 g/cm3 410km:Olivine Ringwoodite Crystal with Quartz Wadsleyite Wadsleyite 9 660 km Ringwoodite Density 3.6 Perovskite + MgO 4.2 g/cm3 Convection in solid mantle • Driven by U, Th, K decay • • • • • 660 km Ringwoodite Tetrahedral Si Perovskite + MgO Octahedral Si (internal engine) Moves about 2 cm/yr Causes earthquakes Moves plates Splits and rejoins continents One Cell or two? Convection in solid mantle Mantle Density vs Depth 4800 Magnetic Field • Due to electrical currents from 4600 Earth’s dynamo. • Convection in liquid metal outer core. • Convection is coupled to Earth’s rotation. • Field is subject to sporadic reversals. 4400 4200 Lower Mantle 4000 3800 Transition Zone 3600 – Last reversal was 30,000 years ago. 3400 • Field holds ozone layer (UV shield) in Upper Mantle 3200 place. 3000 0 200 400 600 800 1000 1200 10 Magnetic Field Reversals 11