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