Slide 1 The Earth is differentiated How and When did this occur? Two Sets of Constraints: Physical Mechanisms and Chemical Signatures.

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Presentation transcript:

Slide 1 The Earth is differentiated How and When did this occur? Two Sets of Constraints: Physical Mechanisms and Chemical Signatures

Slide 2 Useful Isotope Systems Parent nuclide 182Hf 146Sm 147Sm 176Lu 187Re 232Th 235U 238U Daughter nuclide 182W 142Nd 143Nd 176Hf 187Os 208Pb 207Pb 206Pb Half-life 9 Ma 103 Ma 106 Ga 35.9 Ga 42.2 Ga Ga Ga Ga Tracer ratio (daughter/stable) 182 W/ 184 W 142 Nd/ 144 Nd 143 Nd/ 144 Nd 176 Hf/ 177 Hf 187 Os/ 188 Os 208 Pb/ 204 Pb 207 Pb/ 204 Pb 206 Pb/ 204 Pb

Slide 3 Oxygen  -Notation 18 O/ 16 O sample - 18 O/ 16 O SMOW  18 O = 18 O/ 16 O SMOW X 1000 A scaled deviation from a standard SMOW: Standard Mean Ocean Water abundance 16 O99.76% 17 O0.037% 18 O0.200%

Slide 4 Sulfur  -Notation 33 S/ 32 S sample - 33 S/ 32 S CDT  33 S = 33 S/ 32 S CDT X 1000 A scaled deviation from a standard CDT: Canyon Diablo Troilite abundance 32 S95% 33 S0.75% 34 S4.2% 36 S0.017%

Slide 5 Mass-Dependent Fractionation Wiechert et al (2001) Science 294: 345

Slide 6 Short Lived Isotopes: Early Solar System Gilmore (2002) Science

Slide 7 LEW86010; silicate differentiation reference (4558 ± 0.5 Ma) Core segregation (4556 ± 1 Ma) Silicate differentiation (4526 ± 21 Ma) ALH84001 (4500 ± 130 Ma) Gov. Valad. (1370 ± 20 Ma) Lafayette (1320 ± 50 Ma) Y (1310 ± 30 Ma) NWA998 (1290 ± 50 Ma) Nakhla (1260 ± 70 Ma) Dhofar 019 (575 ± 7 Ma) DaG 476 (474 ± 11 Ma) Y (290 ± 40 Ma) QUE94201 (327 ± 10 Ma) NWA1195 (348 ± 19 Ma) NWA1056 (185 ± 11 Ma) LEW88516 (178 ± 9 Ma) ALH77005 (177 ± 6 Ma) EET79001B (173 ± 3 Ma) Y (173 ± 14 Ma) EET79001A (173 ± 10 Ma) NWA856 (170 ± 19 Ma) LA1 (170 ± 7 Ma) Zagami (169 ± 7 Ma) Shergotty (165 ± 11 Ma) Chassigny (1362 ± 62) 174 ± 2 Ma 1327 ± 39 Ma 332 ± 9 Ma Carbonates ALH84001 (3929 ± 37 Ma) Salts shergottites (0-175 Ma) Iddingsite nakhlites (633 ± 23 Ma) Borg & Drake Age (Ma) CAI (solar system formation reference) (4567 ± 0.6 Ma) Ages of Dated Martian Events

Slide 8 Observations/Inferences: Rocky inner, icy outer solar system Asteroid differentiation temperatures heliocentrically distributed Gross zonal structure within asteroid belt preserved The Moon had a magma ocean The solar photosphere has a composition very similar to CI carbonaceous chondrites Heat source concentrated near Sun? or Longer times to accrete object farther from the sun (less Al heating)? 26

Slide 9 Heat Sources: Solar/Magnetic Induction heating (but T-Tauri: Polar Flows) Short-lived radioisotopes ( 26 Al 0.73 Ma half life: must accrete fast) Long-lived radioisotopes (U, Th, K) (slow, only for larger bodies) Large impacts (only for larger bodies: between Moon and Mars-sized) Potential energy of core formation (larger bodies: 6300 km radius: 2300°C rise, Resonant tidal heating (Only moons: Moon?, Titan, Io, Europa) 3000 km radius: 600°C rise)

Slide 10 Timing of Core formation

Slide 11 Two Possible Mechanisms to Separate Metal from Silicate Porous FlowImmiscible Liquids and Deformation

Slide 12 Dihedral (wetting) Angle Theory The Dihedral Angle Theta is a force balance between interfacial energies

Slide 13 Sulfide Melt in an Olivine Matrix Most Fe-Ni-S melts do not form interconnected melt channels

Slide 14 0 Depth km Pressure GPa Pressure GPa after Carlson, 1994 No Crystal Settling Perovskite Settling Low Mg/Si Dunite High Mg/Si Liquid Crystal Cummulates t Quench Crust Quench Crust Magma Ocean Crystallization Cummulates should give a chemical signature

Slide 15 Lower Mantle Solidus d u s ( u p p e r b o u n d ) CoreT Multianvil Peridotite Solidus Olivine shock melting M a g n e s i o w ü s t i t e m e l t i n g Zerr et al (98), Holland & Ahrens (97) Diamond Anvil Peridotite Solidus

Slide 16 Samples Recording Planetary Differentiation

Slide 17 Pallasites: Asteroid Core-Mantle Boundary Brenham

Slide 18 Old Lunar Highland Crust

Slide 19 An Oblique Collision between the proto- Earth and a Mars-sized impactor 4.2 minutes 8.4 minutes12.5 minutes Kipp and Melosh (86), Tonks and Melosh (93)