Hf-W chronometry
of planetary accretion
T. Kleine1, M. Touboul1, B. Bourdon1, F. Nimmo2
1Institute
for Isotope Geochemistry and Mineral Resources, ETH Zürich
2Department
of Earth & Planetary Sciences, UC Santa Cruz
© ETH Zürich | Taskforce Kommunikation
Dating planetary accretion
 Need a chronometer that is set by a process, which is directly
linked to planetary accretion
 This process is core formation because the separation of a
metal core from a silicate mantle should occur during or briefly
after accretion:

For larger bodies such as the Earth the energy required for
differentiation is provided by the accretion process itself

Early-formed bodies melted due to heating by 26Al decay
 Hence, dating core formation provides information on the
timescale of accretion
182Hf-182W
chronometry of core formation
 Dating techniques using radioactive decay usually date the
time of chemical fractionation of parent and daughter elements
 Core formation results in fractionation of lithophile (i.e.,
"silicate-loving") and siderophile ("metal-loving") elements
 Thus, the ideal chronometer for core formation consists of a
lithophile-siderophile pair:

Hf is lithophile, W is siderophile

Hf-W fractionation during core formation and the decay of 182Hf to
182W (t ~9 Ma) results in variable 182W abundances
1/2
ε
182


W  




182
182
W/
W/
184
184

W
W
sample
standard


 1  10,000


182Hf-182W
chronometry of core formation
Martian meteorites
Lunar samples
Basaltic achondrites
Terrestrial samples
Iron meteorites
W isotope evolution of chondrites –
reference for bulk planets
 Both Hf and W are refractory and should therefore occur
in chondritic relative proportions in bulk planets
 The W isotope composition of chondrites thus equals
that of any bulk planet. It is defined by:

The initial e182W and 182Hf/180Hf of the solar system
 can be determined from Ca-Al-rich inclusions (CAIs)

The present-day 182W/184W of chondrites
 can be directly measured on chondrites
W isotope evolution of chondrites
(Kleine et al. 2004, GCA 68)
(Burkhardt et al. 2008, GCA subm.)
182Hf-182W
chronometry of core formation
Martian meteorites
Lunar samples
Basaltic achondrites
Terrestrial samples
Iron meteorites
Hf-W systematics of bulk mantles and
cores of differentiated planets
W isotopes in iron meteorites
Kleine et al. (2005) GCA 69; Kleine et al. (2008) GCA subm.
Rapid accretion and early differentiation
of iron meteorite parent bodies
 Hf-W ages for iron
meteorites indicate accretion
and differentiation of their
parent bodies in less than
~1 Ma after CAI formation
 Melting and differentiation
due to heating by decay of
abundant 26Al
 Iron meteorites are older
than chondrite parent
bodies, that formed more
than 2-3 Ma after CAIs
Kleine et al. (2008) GCA subm.
Hf-W systematics of Mars
 Hf-W fractionation does not only occur during core formation but also
during silicate melting (e.g., magma ocean crystallization)
 (142Nd data from Caro et al. (2008), Nature)
Model age of the Martian core
 Two-stage model ages range from 0 to 8 Ma after CAI formation
 Accretion scenarios involving large impacts permit core formation
timescales of up to ~20 Ma (Nimmo and Kleine 2007, Icarus)
Hf-W chronometry of Earth's core
 For large bodies such as the Earth, core formation is a
continuous process. So, it is important to define to what stage
of core formation an age refers:

Mean age of core formation = 63%

End of core formation
 W isotope composition of Earth's mantle depends on:

Timescale of accretion

Process of accretion, occurrence of large collisions

Degree of metal-silicate equilibration during core formation

Changes in the partition behaviour of W (i.e., Hf/W): oxidation
state, dependence on pressure

Accretion and differentiation history of the impactors
Metal-silicate equilibration
f=1
f=0
(after Nimmo and Agnor (2006), EPSL)
Exponentially decreasing accretion
Wetherill, 1980
Metal-silicate equilibration
Kleine et al. (2004) EPSL 228
Effects of giant impacts
Kleine et al. (2008) GCA subm.
Effects of giant impacts
Kleine et al. (2008) GCA subm.
Summary
 Hf-W ages for the Earth's core are sensitive to assumptions
regarding the degree of metal-silicate equilibration and the
occurrence of giant impacts
 Hf-W model ages for Earth's core range from ~30 Ma to >100
Ma after CAI formation are sensitive to assumptions regarding
metal-silicate equilibration and the accretion process
 An alternative approach for determining the age of the Earth is
determining the timing of the Moon-forming impact because
this event might have been the last event in Earth's accretion
W isotopes in lunar samples
Kleine et al. (2005), Science 310; Touboul et al. (2007), Nature 450
Hf-W age of the lunar magma ocean
Touboul et al. (2007), Nature 450
Age of the Moon
Touboul et al. (2007), Nature 450
Conclusions
 Iron meteorite parent bodies accreted and differentiated within
less than ~1 Ma after CAI formation
 Hf-W ages for the Martian core range from 0-20 Ma. Major
source of uncertainty are uncertainties in Hf/W and e182W of
the Martian mantle
 Hf-W ages for core formation in Earth range from ~30 to >100
Ma. Calculated ages are sensitive to the degree of metalsilicate equilibration, occurrence of giant impacts, and much
more...
 W isotopes in lunar samples combined with the age of the
oldest known lunar samples provide an age for the giant
Moon-forming impact: 100±50 Ma
W isotopes in accretion simulations
Kleine et al. (2008) GCA subm.