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

Useful Isotope Systems Parent nuclide   182Hf 146Sm 147Sm 176Lu 187Re 232Th 235U 238U Daughter nuclide   182W 142Nd 143Nd 176Hf 187Os 208Pb 207Pb 206Pb Tracer ratio (daughter/stable)   182W/184W 142Nd/144Nd 143Nd/144Nd 176Hf/177Hf 187Os/188Os 208Pb/204Pb 207Pb/204Pb 206Pb/204Pb Half-life   9 Ma 103 Ma 106 Ga 35.9 Ga 42.2 Ga 14.01 Ga 0.7038 Ga 4.468 Ga

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

Pallasites: Asteroid Core-Mantle Boundary Brenham

Samples Recording Planetary Differentiation

Ages of Dated Martian Events Salts shergottites (0-175 Ma) Iddingsite nakhlites (633 ± 23 Ma) Carbonates ALH84001 (3929 ± 37 Ma) Shergotty (165 ± 11 Ma) Zagami (169 ± 7 Ma) LA1 (170 ± 7 Ma) NWA856 (170 ± 19 Ma) 174 ± 2 Ma EET79001A (173 ± 10 Ma) Y793605 (173 ± 14 Ma) EET79001B (173 ± 3 Ma) ALH77005 (177 ± 6 Ma) LEW88516 (178 ± 9 Ma) NWA1056 (185 ± 11 Ma) Y980459 (290 ± 40 Ma) 332 ± 9 Ma QUE94201 (327 ± 10 Ma) NWA1195 (348 ± 19 Ma) DaG 476 (474 ± 11 Ma) Dhofar 019 (575 ± 7 Ma) Nakhla (1260 ± 70 Ma) NWA998 (1290 ± 50 Ma) Y000593 (1310 ± 30 Ma) 1327 ± 39 Ma Lafayette (1320 ± 50 Ma) Chassigny (1362 ± 62) Gov. Valad. (1370 ± 20 Ma) ALH84001 (4500 ± 130 Ma) Silicate differentiation (4526 ± 21 Ma) Core segregation (4556 ± 1 Ma) LEW86010; silicate differentiation reference (4558 ± 0.5 Ma) CAI (solar system formation reference) (4567 ± 0.6 Ma) 1000 2000 3000 4000 4657 Age (Ma) Borg & Drake

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

Solar/Magnetic Induction heating (but T-Tauri: Polar Flows) 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, 3000 km radius: 600°C rise) Resonant tidal heating (Only moons: Moon?, Titan, Io, Europa)

Timing of Core formation

Two Possible Mechanisms to Separate Metal from Silicate Porous Flow Immiscible Liquids and Deformation

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

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

Magma Ocean Crystallization No Crystal Settling Crystal Cummulates 15 22.5 7.5 15 22.5 7.5 t Quench Crust Quench Crust Liquid Pressure Depth Pressure GPa Liquid km GPa 250 Dunite High Mg/Si Liquid 500 Perovskite Settling Low Mg/Si 750 Cummulates should give a chemical signature after Carlson, 1994

Lower Mantle Solidus Pressure (GPa) 2000 T e m p r a t u ( K ) 3000 Diamond Anvil Peridotite Solidus Pressure (GPa) 2000 T e m p r a t u ( K ) 3000 4000 5000 20 40 80 120 CMB M n l A d i b s o Olivine shock meltin g n g e l t i m e ü s t i t w e s i o g n n d ) Core T M a o u r b p p e s ( u d u Multianvil Peridotite Solidus Zerr et al (98), Holland & Ahrens (97)

Old Lunar Highland Crust

Impact Rate over time

Giant Impact during Accretion Don Davis artwork

An Oblique Collision between the proto-Earth and a Mars-sized impactor 4.2 minutes 8.4 minutes 12.5 minutes Kipp and Melosh (86), Tonks and Melosh (93)

Pre Type II Nucleosynthesis (s-process) 21 solar mass star ratio to solar abundance Rauscher et al. (2002)

Type II SN Nucleosynthesis (r-process) 25 solar mass star Rauscher et al. (2002)

Galactic Composition evolution Chiappini (2004)

Nearby Supernova Knie et al. (2004)

Interstellar shocks Clayton (1979)

Silicate Condensation Clayton (1979)

Oxygen d-Notation A scaled deviation from a standard 18O/16Osample - 18O/16OSMOW d18O = X 1000 18O/16OSMOW SMOW: Standard Mean Ocean Water abundance 16O 99.76% 17O 0.037% 18O 0.200%

Sulfur d-Notation A scaled deviation from a standard 33S/32Ssample - 33S/32SCDT d33S = X 1000 33S/32SCDT CDT: Canyon Diablo Troilite abundance 32S 95% 33S 0.75% 34S 4.2% 36S 0.017%

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

Earth Structure

Ray Paths

D’’ Heterogeneity

Great Earthquakes

Temp