Planetary accretion and core segregation:Processes and chromnology
Asteroid belt
Earth-Moon
VenusMercury
Mars
Solar system: general zoning from metal and silicates to gas and ice
Terrestrial planets: metal and rock
Gas giants: H, He
Ice giants: ice + gas
Kuiper belt: ice
Compression effect
Composition of stellar and planetary materialsSolar abundance of the elements
Earth and planetary redox conditions
Terrestrial planetsMercury, Venus, Earth, Moon, Mars, Vesta
Mantle / core -ratio ~ O/Fe-ratio of accretion zone
Cosmic O/Fe-ratio: Insufficient to oxidize all Fe
Simple, first-order structure Mantle: Rock (silicates and oxides) Core: Fe-Ni-metal
Terrestrial planets: sizes, pressures, core fractions
Planetary accretionVariable fO2 and fH2 give variable FeO/Fe-ratios,
i.e. variable core fraction
Intermediate fO2
Large fO2
Low fO2
High FeOmantle
21% in Vesta
Interm. FeOmantle
8% in Earth
Low FeOmantle
3% in Mercury
30 wt% 65 wt%16 wt%
Mantle FeO (wt%)Mercury: 3
Venus: 7
Earth: 8
Moon: 12 (Theia: >12 ?)
Mars: 16
Vesta: 18
Melting of planetary mantles basalt volcanism
FeOmantle / FeObasalt ≈ 1
How do we know FeO in the mantle(s) ?
FeOmantle - core fraction
Oxidation state recorded by core-mantle ”bulk equilibrium”
O2 + 2 Fe = 2 FeO (Iron-Wustite buffer reaction) in core in mantle
K = aFeO2 / (fO2 * aFe
2) = 1 (at IW)
fO2 = aFeO2 / aFe
2 = (aFeO / aFe)2
log fO2 (IW) = 2 * log (aFeO/aFe)
XFeO log fO2
mantle IW
0.90 – 0.85 e.g. 0.88
Observations:- Solar nebula zoning with increasing fO2 outwards?
- Mercury’s large core: caused mostly by low fO2 ?
- Moon’s tiny core – Moon created mainly from mantles of Theia and Earth under oxidizing conditions (MantleFeO: Moon 12%, Earth: 8%)
Mercury: 0.03 -2.9Venus: 0.07 -2.2Earth: 0.08 -2.1Moon: 0.12 -1.7Mars: 0.16 -1.5Vesta: 0.18 -1.4
Exsolution of tiny amounts of kamacite
Garrick-Bethel and Weiss (2010, EPSL)
McSween (1999, Cambridge Univ. Press)
McSween (1999, Cambridge Univ. Press)
Large amounts of kamacite – small amounts of taenite
drop of troilite (FeS)
Pallasite (metal + olivine):
piece of CMB-material
Asteroid belt:
main source of meteorites
MarsCeres
Vesta
PallasBeatty et al. (1999, Cambridge Univ. Press)
Asteroid belt: mass distruibution
Asteroid belt, total mass: 1021 kg very small mass relative to the planets, only 4% of Moon, 0.05% of Earth
47% of the total mass in the 3 largest asteroids Ceres: 31% of asteroid belt Vesta: 9% Pallas: 7 %
Ceres
PallasVesta
Kirkwood gaps,Jupiter resonances
7 Iris
3 Juno
6 Hebe
8 Flora
15 Eunomia 532Herculina
Carbonaceous chond.: C + BFGD - outer belt
Ordinary chond.: S - inner belt
HED: Vesta, Vestoids: V +R
Metal, Enstatite: M +E
1 Ceres
10 Hygiea
2 Pallas (B)
250 Bettina
44 Nysa (E)
Asteroid spectral groups: link to meteorite types
Mars4 Vesta
349Dembowska (R)
16 Psyche
433 Eros
7 Iris
Juno
Hebe:H, IIE
Flora family:L, 480 Ma Eunomia
Herculina
Ceres
Hygiea
Pallas (B)
Bettina
Nysa
MarsDembowska (R)
Psyche
Eros
Vesta family:HED
Baptistina family:CM, 160 Ma
Hungaria family (3103 Eger): aubrites
Carbonaceous chond.: C-type (+ B F G D) - outer belt
Ordinary chond.: S-type - inner belt
HED: Vesta, Vestoids: V-type (+ R)
Metal, Enstatite: M-type (+ E)
Asteroid spectral groups: link to meteorite types
Primitive solar syst. materialIDP (interplanetary dust part.)
chondrites (esp. carbonaceous chondr.)
Material samples
Differentiated samplesMeteoritesVesta (HED - howardites, eucrites, diogenites)
Other asteroids (Fe-met., angrites, aubrites, etc.)
Mars (SNC-met.)
Moon (basalt, anorthosite, breccia)
Planetary samplesEarthMoon (Apollo/Luna missions)
Differentiated meteorites
Lunarbreccia
Shergottite(basalt)
Fe-meteorite
Diogenite – Vesta(HED-group)
evaporation and condensation of dust CAI
flash melting and solidification chondrules
Refractory inclusions and chondrules
Microscope images of chondrules
Strong magnetic fields from early Sun (T-Tauri phase)
Infrared image, dust and gas disk around b-pictoris
Planetary accretion (theoretical modelling)
Dust accretion bysettling and sticking:< 10 000 years
Runaway growth(gravitational):< 0.5 Ma
Late stage,giant collisions: 0.5 – 100 Ma
Planetesimals, 1 - 10 km
Planets
Planetary embryos 10 - 1000 km
Walsh et al. 2011, Nature
Saturn
0 Ma
0.3 Ma
0.07 Ma
0.1 Ma
0.6 Ma
0.5 Ma
Jupiter Uranus0.5 Ma0.1 Ma
(Hel
ioce
ntri
c di
stan
ce)
Earth
The ”grand tack” model (the Nice model) - very early formation of gas giants, Jupiter and Saturn,
- inward migration of first Jupiter and then Saturn (gas drag effect)
- then outward migration of both Jupiter and Saturn
Inner terr. planets
Cumulative result of 8 terrestrial planet simulations
Mercury
VenusEarth
Mars
Walsh et al. 2011, Nature
Start: 80 embryos of 0.026 MEa
Start: 40 embryos of 0.051 MEa
Horizontal error bars: perihelion–aphelion excursion of each planet along their orbits
17
8
5
6
Giant collision stage: large energy-production
1) Short-lived radioactivity: 26Al, 60Fe 2) Collisional heat
3) Core segregation (also exothermic process)
4) Tidal friction (important for the early Earth-Moon system)
Moon formation: giant impact strongly favoured
- Moon’s tiny core
- Moon’s volatile depletion
- Earth-Moon chemical features
- Earth-Moon dynamic coupling
Details by Diego Gonzales, on Wednesday
Results from the Apollo-program mapping and samplingRecognition of silicate magma ocean crystallization for earlyplanetary differentiation
Crystallization of the lunar magma ocean
Crust - upper mantle stratigrahy
Geochemical signs of cumulate formation in the lunar magma ocean
Mare Imbrium
OceaniusProcellarium
M. Serenitatis
M.Tranquilitatis
Mare basalt volcanism was most intense at 3.8 - 3.6 GaCaused by gravitational instabilities and turn-over in the magma ocean cumulates (?)
High-density cpx-ilmcumulates over morecommon peridotites
Source regions for mare basalts
KREEP-basalts - low Hf/W
High-Ti basalts - high Hf/W
Low-Ti basalts - intermediate Hf/W
Crust –upper mantle stratigrahy
McSween (1999, Cambridge U. Press)
High-Ti
Low-Ti
Anorthosites,pos. Eu-anomalies
Sufficiently low fO2 for Eu2+.
Eu2+ is similar to Sr2+ and both fits nicely
into the Ca-position of plagioclase
Important heatsource: the first 2-3 Ma
Chronology – radiometricU-Pb isotope systematics235U → 207Pb, Th: 0.7 Ga238U → 206Pb, Th: 4.5 Ga
206Pb / 206Pb
207 P
b / 20
4 Pb
Earth’s initial composition
m = U/Pbm=8
m=9
m=10
1 G
a
2 Ga 3 G
a
Geochron (0 Ga)
The concordia diagram (Pb*: radiogenic lead) - suitable for dating minerals that crystallize without Pb, but much U (e.g. zircon)
Partial Pb-loss when zircon is 2.5 Ga old
Further 1 Ga Pb-growth,until zircons are 3.5 Ga old
Satellite-image (175 km width) covering parts of the Pilbara craton, NW Australia.Such cratons typically contain granitic domes (here: 3.0-3.2 Ga) emplaced between dark greenstone belts comprising altered basaltic og komatiitic lavas (here: 3.2-3.5 Ga)
SEM-image of zircon crystals from a quartsite at Jack Hills, Pilbara craton, Western Australia. These are the oldest zircons found on Earth (ages in Ma)
Oldest zircon crystals on Earth, dated by U/Pb-geochronology
evaporation and condensation of dust CAI
flash melting and solidification chondrules
Refractory inclusions and chondrules
Microscope images of chondrules
Strong magnetic fields from early Sun (T-Tauri phase)
Amelin & Ireland (2013, Elements)
Age of meteorites and the EarthMostly established already in 1956 !!
Correct age, within the stated uncertainty
Amelin et al. 2002, Science
Dating refractory inclusions and chondrules
t 0 = 4567 M
a
Chondrules: t = t0 + 2 Ma
CV chondrite
CR chondrite
Bouvier et al. (2010, Nature Geoscience)Assumed constant (solar) 238U/235U-ratio
Northwest Africa 2364 CV3 chondrite
Connelly et al. (2012, Science): Used measured (variable) 238U/235U-ratio
Amelin and Ireland (2013, Elements)
CAIs: 4567.3 ± 0.16 MaChondrules: 4567 - 4564 Ma
Short-lived isotope decay
Important early heat producers:26Al → 26Mg, Th: 0.7 Ma60Fe → 60Ni, Th: 1.5 Ma
Others with low concentration orlow initial ratios:41Ca → 41K, Th: 0.1 Ma10Be → 10B, Th: 1.5 Ma182Hf → 182W, Th: 8.9 Ma146Sm → 142Nd, Th: 103 Ma
d26Mg = [(26Mg/24Mg)sample/stand -1] * 1000‰
e182W = [(182W/184W)sample/stand -1] * 104
d26Mg-excess in Al-rich minerals (plagioclase) formed early Hf: lithophile (”silicate-loving”)
W: siderophile (”iron-loving”)
WHf
Mantle: high Hf/W high eWmantle
Core: low Hf/W low eWcore
ǀeWǀ (the eW-deviation from zero):
inversely proportional to the age of core-segregation
Hf/W-ratio must be known or estimated for a good age determination
Giant collisions: largely molten planetary embryos and planets with segregated cores
Extent of core-mante re-equilibrationTwo extremes:
Minimal re-equil.: Impactor core merges directly into big core
Extensive re-equil.: Impactor core emulsifies in silicate magma ocean
The Mars case (Dauphas and Pourmand, 2011, Nature)
Mars mantle: eWMa-ma ≈ 0.4 (at least)
Hf/W-ratio of SNC-meteorites is disturbed by partial melting and fractional crystallization processes
However, the Hf/W-ratio can be estimated from: (Hf/W)Ma-ma = (Th/W)Ma-ma / (Th/Hf)Ma-ma
(Th/W)shergottites ≈ (Th/W)naklites = 0.75 ± 0.09 (independent of mineral proportions)
i.e. insignificant fractionation of Th/W
(Th/Hf)Ma-ma ≈ (Th/Hf)CHUR because both Th and Hf are refractory lithophile elements
prox
y fo
r L
u/H
f:
(Th/Hf)CHUR
Gro
wth
cur
ve, M
ars
95%
con
f. in
terv
al, M
onte
Car
lo
sim
ulat
ion
of u
ncer
tain
ties
Model simulations of embryo
growth at 1 and 3.2 AU (Mars: 1.5
AU)
M(3 Ma) / Mfinal = 80%
Mars can be considered a planetary embryo!
Growth curve, Mars
182Hf - 182W chronology (Th: 9 Ma)
- measuring small metal grains in mare basalts
eW = (182/184Wsample / 182/184Wstand -1) * 104
Original study of Kleine et al. (2005, Science):
Indication of Dt ≈ 30-50 Ma age for theLunar Magma Ocean (LMO)
Later study: Touboul et al. (2007, LPCS):
No eW-variation (KREEP, high-Ti, low-Ti)
→ LMO: Dt > 60 Ma
Pb-model (206/204Pb - 207/204Pb)
Earth: 60 – 80 Ma
W-model (182Hf-182W)Vesta, Fe-meteor.: 1- 3 MaMars: 1-3 MaEarth, Moon: > 60 Ma
Core segregation models
Measuring initial 26Al/24Al at t0
d26Mg = [(26Mg/24Mg)sample/stand -1] * 1000%
Amelin et al. 2002, Science Bouvier & Wadhwa 2010, Nature Geoscience
CAI, Northwest Africa 2364CV3 chondrite
CAI, Efremovka
Model for 26Al-heating of chondritic 60 km (diameter) object
Walter & Trønnes(2004, EPSL)
Canonical CAI-based 26Al/27Al initial ratio
The canonical, CAI-based 26Al/27Al-ratio results in a very extensive and long heating period for planetesimals
The presence of undifferentiated planetesimals then requires either: - a long period of planetesimal accretion or - a delay for the initial accretion relative to t0
in contradiction with most dynamic models
Investigation of the 26Al/27Al ratio in the angrite parent body Schiller et al. (2015, EPSL)
Early planetesimal formation and heating becomesfeasible with the lower 26Al/27Al-ratio Schilling et al (2015, EPSL)
Halliday (2008, Phil.Trans.R.Soc.):
Agreement between Rb-Sr- and U-Pb-model ages for the Moon: 4.47 ± 0.02 Ga (i.e. Dt: 100 Ma) based on Rb- and Pb-loss from Moon by volatilization
and
the oldest date of lunar anorthositic crust: 4.46 ± 0.04 Ga (Norman et al. 2003)
Earth-Theia collision and Moon formation: Dt ≈ 100 Ma
Additional time constraints on the Moon
Science,April 17, 2015
Model based on Ar-Ar and U-Pb impact agesHigh frequency of meteoritic impact ages followingthe perceived giant, Moon-forming impact (GI).
Dynamical evolution of GI ejecta:colour contours are collision velocities at about 2.5 AU (Vesta-position)
4.57 4.374.47
Few ages of4.3-4.1 Ma,but many at4.1-3.5 Ma(LHB)
Only impactheating ages
Only impactheating ages
Mixed cryst,metam, andimpact ages
4.57 4.374.47
110
Model supports giant Moon-forming impact at 80-120 Ma
Chronological summary
Hf-W systematics of core segregation:
Fe-meteorite parent bodies: < 1 Ma
Vesta: Dt = 1-2 Ma
Mars: Dt ≈ 3 Ma
Earth, Moon: Dt > 60 Ma
Important implication:
Chondrite parent bodies accreted rel. LATE: Dt > 1-2 Ma after most of the 26Al (primary heat source) had decayed
CAI: t0: 4567.3 ± 0.2 Ma
Chondrules: Dt ≈ ± 0-3 Ma
Small planetesimals, including Fe-meteorite PB: Dt < 0.5 Ma (using the low (26Al/27Al)0 ratio from the angrites)
Chondrites: Dt > 1-2 Ma (after most of the 26Al-decay)
Hf-W-models for Earth accretion and core segregation Exponentially decreasing accretion growth models
Gia
nt im
pact
Gia
nt im
pact
Equilibrium model 60% core mixing during accretion
Equilibrium model
Likely explanation:Fe-rich meteorites are formed beforeinjection of 60Fe from a late supernova ? (within 1 Ma after t0)
Main GroupPallasites
Angrite
e60Ni* = (60/58Nisample / 60/58Nistand -1) * 104
Bizzarro et al. (2007, Sci 316, 1178):
Fe-rich meteorites have variable Fe/Ni, but constant and low 60/58Ni
Correlation between stable, neutron-rich isotopes 62Ni and 54Cr (formed by common supernova processes)Earth, Mars, chondrites: highest 62/58Ni and 54/52Cr
Carb CMars
Irons
Ord C
Enst C
Pallasites
Earth
Ureilites
Angrites