Chemistry of the Mantle and Core

Transcription

1 Chemistry of the Mantle and Core Geochemistry, University of Bristol Geochemical Reservoirs Bulk Silicate Earth Crust Depleted Mantle Bulk Earth Primitive Mantle Core Core 1

2 Mantle Seismic Profile of the Earth Outer Core must be liquid! Inner Core Birch s Velocity-Density Systematics 2

3 Earth s Core Consists of Fe % of some light element. The main reservior of siderophile elements. The chemistry of the core is an important part of the geochemical puzzle. Temperature Profile in the Earth (Geotherm) Source of the Earth s Internal Heat: Radioactive decay (e.g., 40 K). Crystallization of Inner core Adiabatic compression 3

4 Origin of the Core Homogenous Accretion (COLD) of the Earth: Accretion began after nebula was cool enough to have both metallic and silicate particles. Accumulation of planetesimals formed a well-mixed earth. Gravitational collapse released heat causing partial melting. Fe sank into the core (the iron catastrophe ; LIL s rose to the crust Heterogenous Accretion (HOT) of the Earth: Proto-Earth accreted from dense Fe-rich particles that first condensed from solar nebula. After cooling of the nebula, Si-O rich particles formed which then accreted later to form the mantle and crust. Chalcophiles, Lithophiles, Siderophiles.. 4

5 Core-Mantle Equilibrium? (From Drake and Righter, 2002) My view.. There is no longer any compelling reason to assume core and mantle are not in chemical equilibrium. This would preclude Si in the core as it would imply that the core is much more reducing than the mantle. Arguments for oxygen in the core are are based on incorrect views of bonding and crystal chemistry. This favors a cold, homogenous accretion process for the formation of the Earth. 5

6 Compositional Models for Bulk Silicate Earth (Mantle+Crust) In spite of the major problems with carbonaceous chondrites as a source for the Earth (previous lecture).. Assume that the refractory elements in the Earth are in chondritic proportions (e.g., Ca/Al = 1.1). Assume core is mostly iron and that lithophile elements are excluded from the core. Use stable isotopes to work out volatile elements. Al/Si and Mg/Si of the Silicate Earth Based on the intersection of fractionation trends, we can estimate the bulk Mg/Si to be ~1 and bulk Al/Si to be ~

7 Models for Bulk Silicate Earth (molar proportions) Pyrolite model Na 2 O CaO Chondritic model CaO MgO SiO 2 MgO SiO 2 FeO Al 2 O 3 FeO Al 2 O 3 Are Chondrites the Source of the Earth? Oxygen isotopes say no.. 7

8 Are Chondrites the Source of the Earth? Os isotopic ratios in the Primitive Upper mantle are quite different from those of C1 chondrites and suggest a late veneer of ordinary chondrites. (Drake and Righter,2003) Pressure-Induced Phase Transitions Very dense structures are favored at high pressure. 8

9 Major Minerals of the Upper Mantle Olivine (Mg,Fe)SiO 4 Si in 4-fold coordination Mg, Fe +2 in 6-fold coordination Ortho- and Clinopyroxene (Ca,Mg)(Mg,Fe)SiO 3 Mg,Fe in 6-fold coord. Ca,Fe in 8-fold coord. Major Minerals of the Transition Zone Ringwoodite (Mg,Fe)SiO 4 Si in 4-fold coordination Mg, Fe +2 in 6-fold coordination Majorite (Ca,Mg)(Mg,Fe)SiO 3 Si in 4 and 6-fold coord. Ca,Mg in 8-fold coord. B C A 9

10 Major Minerals of the Lower Mantle Perovskites (Mg,Fe)SiO 3 and CaSiO 3 Si in 6-fold coordination Mg, Ca, Fe in 12-fold coordination Ferropericlase (Mg,Fe)O Mg,Fe in 6-fold coordination Volatiles in the Mantle (?) Wadsleyite (β-mg 2 SiO 4 ) Replace Mg with 2H + Octahedral (yellow) sites for Mg. Si tetrahedra in corner-shared pairs One oxygen has a PBS of only 4/3. 10

11 Composition and Formation of the Moon Moon depleted in moderately volatile elements (Na, K) and enriched in refractory elements (Ca, Al). CaO CaO MgO SiO 2 MgO SiO 2 Al 2 O 3 FeO FeO Al 2 O 3 Bulk Silicate Earth (Chondritic Model) Bulk Silicate Moon Origin of the Moon: Giant Impactor Impact 30 minutes later 5 hours later Years later 900 million years later Today 11

12 L2 Geochemistry Magma Ocean on the Moon The impact hypothesis implies that the moon was initially molten. The anorthositic highlands (analogous to continental crust?) formed from the magma ocean. The magma ocean also explains the origin of KREEP (rocks with high concentrations of K, REE and P). Summary Crust: enriched in O, Si, Al, K, Na, Sr, Rb, P, U Mantle: enriched in O, Si, Mg, Fe, Ca Core: enriched in Fe, S, Ni, Co, Os, Ir, Re, Pt, Au Seawater: enriched in H, O, Na, Mg, Cl, B, S, Br Atmosphere: enriched in N, Ne, Ar, Xe, Kr 12

13 Summary Cosmochemical differentiation reflects volatility. Planetary differentiation reflects chalcophile, lithophile and siderophile behavior. High pressure minerals cannot accommodate incompatible elements (LIL s and HFS). Crust is enriched in LIL s and HFS elements. 13