HfW chronometry and the formation and early evolution of planetary bodies

1
Hf-W chronometry and the formation and early
evolution of planetary bodies

• Thorsten Kleine
• Department of Earth Sciences, Institute for
  Isotope Geology,
• Swiss Federal Institute of Technology Zürich,
  Switzerland

2
Outline

• An introduction to Hf-W chronometry
• Accretion of the first planetesimals iron
  meteorites vs. primitive chondrites
• Thermal metamorphism ordinary chondrites and
  eucrites
• Accretion and differentiation of the Earth and
  Moon

3
Hf-W chronometry
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Hf-W properties

• Both Hf and W are refractory
• Hf/W of bulk planets is chondritic
• Hf is lithophile, W is siderophile
• Hf-W fractionation during metal-silicate
  separation (e.g., core formation)
• W is more incompatible than Hf
• Hf-W fractionation during melting/crystallization
  in silicate mantles (e.g., during crystallization
  of the lunar magma ocean)

5
Hf-W isochrons
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Hf-W model ages
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eW - notation
eW (182W/184W)sample/(182W/184W)standard - 1
x 10,000 eW (Earths mantle) 0
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Outline

• An introduction to Hf-W chronometry
• Accretion of the first planetesimals iron
  meteorites vs. primitive chondrites
• Thermal metamorphism ordinary chondrites and
  eucrites
• Accretion and differentiation of the Earth and
  Moon

9
Hf-W isochron for Allende CAIs
(Kleine et al. 2005, GCA in press)
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W isotopes in iron meteorites
(Kleine et al. 2005, GCA in press)
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W isotopes in iron meteorites and CAIs

• Magmatic irons have eW values similar to or even
  slightly below the initial eW of Allende CAIs
• Real time intervals
• Burnout of W isotopes in iron meteorites
• Alteration of Allende CAIs

12
Hf-W isochron for Allende CAIs
(Kleine et al. 2005, GCA in press)
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Are the Hf-W sytematics of Allende CAIs affected
by secondary effects?

• Thermal metamorphism
• Mobilization of radiogenic W from silicates into
  metals requires temperatures in excess of 600C
• Aqueous alteration
• Average carbonaceous chondrites plot on the
  isochron
• Bulk CAIs and separates from an individual CAI
  plot on one well-defined isochron

14
Hf-W isochron for Ste. Marguerite
(Kleine et al. 2002, Nature 418)
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Ste. Marguerite age comparison
16
Burnout of W isotopes
(Kleine et al. 2005, GCA in press)
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Chondrules-irons formation interval
(Kleine et al. 2005, GCA in press)
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Summary I

• Core formation in iron meteorite parent bodies
  occurred within the first 1.5 Myr of the solar
  system
• Hence, core formation in some asteroids predates
  chondrule formation, which occurred 2-3 Myr
  after CAI formation
• The early evolution of planetesimals appears to
  be controlled by the decay of 26Al, which was
  sufficiently abundant to melt early-formed
  planetesimals, but produced too little heat to
  cause differentiation in the chondrite parent
  bodies
• The parent asteroids of chondrites may have
  accreted farther from the Sun and, hence, later
  than those of the magmatic iron meteorites. This
  could account for the heliocentric zoning of the
  asteroid belt (Grimm and McSween, 1993)

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Outline

• An introduction to Hf-W chronometry
• Accretion of the first planetesimals iron
  meteorites vs. primitive chondrites
• Thermal metamorphism ordinary chondrites and
  eucrites
• Accretion and differentiation of the Earth and
  Moon

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Thermal metamorphism of ordinary chondrites

• Unequilibrated ordinary chondrites (type 3)
• Metals have relatively low W contents (300 ppb)
• Equilibrated ordinary chondrites (types 4-6)
• Metals have high W contents (0.6-1.4 ppm)
• Mobilization of W from silicates into metal
  during metamorphism

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Hf-W isochron for Julesburg
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Hf-W isochrons for tpye 6 ordinary chondrites
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Hf-W isochrons for tpye 6 ordinary chondrites
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Summary of Hf-W ages
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Eucrites whole-rock isochron
(Kleine et al. 2004, GCA 68)
26
Thermal metamorphism of eucrites
(Kleine et al. 2005, EPSL 231)
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Thermal metamorphism of eucrites

• Metamorphism occurred 16 Myr after igneous
  activitiy, i.e., too late to be caused by
  internal heating
• Disturbed Hf-W systematics of eucrites indicate a
  short re-heating event
• Metamorpism most likely caused by an impact

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Summary II

• Formation of metal in unequilibrated chondrites
  2-3 Myr after CAI formation, i.e., coeval with
  chondrule formation
• Thermal metamorphism in equilibrated ordinary
  chondrites 8-10 Myr after CAIs
• Thermal metamorphism of eucrites 20 Myr after
  CAIs caused by impact heating

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Outline

• An introduction to Hf-W chronometry
• Accretion of the first planetesimals iron
  meteorites vs. primitive chondrites
• Thermal metamorphism ordinary chondrites and
  eucrites
• Accretion and differentiation of the Earth and
  Moon

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Accretion and differentiation of the Earth and
Moon
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Accretion and differentiation of the Earth and
Moon

• W isotopic composition of carbonaceous chondrites
• Hf-W models of core formation in Earth
• Hf-W age of the lunar magma ocean

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W isotopes in carbonaceous chondrites

• Two-stage model age 30 Myr

(Kleine et al. 2002, Nature 418 2004, GCA 68)
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More realistic models

• Two-stage model
• Assumes core formation as one event at a
  well-defined point in time
• Continuous core formation
• Radiogenic ingrowth in the high Hf/W mantle
• Addition of newly accreted, chondritic material

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Endmember models

• Core merging
• Metal cores of newly accreting planetesimals
  combine with Earths core without any
  re-equilibration
• Full equilibration
• All newly accreted material re-equilibrates with
  Earths mantle before entering the core

35
W isotope evolution during core formation
(Kleine et al. 2004, EPSL 228)
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Degree of metal-silicate equilibration
(Kleine et al. 2004, EPSL 228)
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Summary III

• Earths mantle has an excess of 182W relative to
  chondrites, indicating core formation during the
  effective life-time of 182Hf
• Formation of Earths core must have involved
  substantial re-equilibration of newly accreted
  metal with Earths mantle because otherwise
  Earths mantle would have highly radiogenic 182W
• Determing an excact age of core formation
  requires knowledge of the degree of
  metal-silicate equilibration during core formation

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Hf-W chronometry of the Moon

• 182Hf decays to 182W (half-life 9 Myr)
• Strong Hf-W fractionation amongst the products of
  the lunar magma ocean

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Lunar magma ocean
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Hf-W chronometry of the Moon

• 182Hf decays to 182W (half-life 9 Myr)
• Strong Hf-W fractionation amongst the products of
  the lunar magma ocean
• Application of Hf-W chronometry to the Moon
  hindered by cosmogenic 182W-production via
  neutron-capture of 181Ta (Leya et al., 2000 Lee
  et al., 2002)

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Cosmogenic 182W production
(Calculated using equations from Leya et al.,
2003)
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Approach

• Determine the indigenous W isotope composition of
  lunar samples by investigating phases that do not
  contain Ta that could have been converted to 182W
• Almost all lunar rocks contain some metal, which
  is highly enriched in W and contains no Ta

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Lunar metals
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W isotopes in lunar metals
(Kleine et al. 2005, Science 310)
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Burnout of W isotopes in metals
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Indigenous 182W/184W in lunar rocks
(Kleine et al. 2005, Science 310)
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Hf-W age of differentiation
(Kleine et al. 2005, Science 310)
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Summary IV

• Lunar mantle reservoirs have distinct W isotope
  compositions
• These indicate lunar differentiation during the
  effective life-time of 182Hf
• Crystallization of the lunar magma ocean 30-50
  Myr after the start of the solar system
• The age of magma ocean crystallization provides a
  minimum age for the formation of the Moon
• As such the Hf-W age of 30-50 Myr also defines
  the end of the major stages of Earths accretion

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Conclusions

• Rapid core formation in iron meteorite parent
  bodies (lt1.5 Myr after CAIs) predated chondrule
  formatoin
• Early evolution of planetesimals controlled by
  26Al decay, which caused early core formation but
  provided too little heat to melt chondrite parent
  bodies
• Metals in unequilibrated ordinary chondrites
  formed coeval with chondrules 2-3 Myr after CAIs
• Thermal metamorphism of ordinary chondrites 8-10
  Myr after CAIs
• Thermal metamorphism of eucrites 20 Myr after
  CAIs, caused by impact heating
• W model ages for core formation in Earth depend
  on the degree of metal-silicate equilibration
  during core formation
• The only slightly enhanced 182W abundance of
  Earths mantle relative to chondrites indicates a
  high degree of metal-silicate equilibration, most
  likely in a magma ocean
• Crystallization of the lunar magma ocean 30-50
  Myr after CAIs. This age provides the currently
  best estimate for the age of the Earth and Moon