Heterogeneous Hadean Hafnium: Evidence of Continental Crust at 4.4 to 4.5 Ga

1
A Tale of Early Earth Told in Zircons
Yuri Amelin Science, 2005
2
Heterogeneous Hadean Hafnium Evidence of
Continental Crust at 4.4 to 4.5 Ga

• T.M Harrison, J. Blichert-Toft, W. Müller, F.
  Albarede, P. Holden, S.J. Mojzsis
• Science 2005

A Tale of Early Earth Told in Zircons
Yuri Amelin Science, 2005
3
What Are Zircons and Why are They Important to
Astrobiology?

• Rocks contain samples of zircons (250µm wide),
  which can be removed and radioactively dated
• Zircons are highly resistant to high
  temperatures and pressures, thus allowing them to
  survive periods of heavy bombardment without
  alteration

• Evidence that the Earth had water and continents
  4.34.4 G a (Mojzsis et al., Peck et al.)
• 400 Myr earlier than previously thought

4

• When did the continental crust form?
• It formed 4 billion years ago and slowly grew
  over time
• No rocks older than 4Gyr expected
• It formed very shortly after the Earth, making
  continental crust prevalent during the Hadean
  Eon.
• Several remixing events expected

5
Testing the Hypotheses

• Use zircons to trace early conditions
• Hf in zircons remains roughly constant since
  there is very little Lu in grain
• Radioactive dating using U-Pb isotopic
  compositions
• Study the Lu/Hf in grains as old as 4.37 Gyr to
  find out whether
• There was a Lu/Hf reservoir that matches that of
  the continental crust today
• OR
• There was a widespread depletion event together
  with a recycled mantle

6
The Experiment

• Deviation from bulk Earth, ?HF
• ?HF Source(Hf) - Earth(Hf)
• Bulk Earth represented by chondritic values
• High Resolution Ion Microprobe II
• 50,000 zircons from JH992
• Dated by U-Pb for time of crystallization
• Zircons with ages between 4.01-4.37 Ga analyzed
  for Lu/Hf and the Hf abundances
• Zircons that showed significant deviation from
  the bulk Earth isotopic composition were
  re-tested for different generations

7
The Experiment

• Deviation from bulk Earth, ?HF
• ?HF Source(Hf) - Earth(Hf)
• Bulk Earth represented by chondritic values
• Measured 104 zircons and obtained Hf ratios
• 44 measured by solution mass spectrometry
• Measures bulk Hf isotope composition
• Entire sample ruined during experiment
• 60 measured by laser ablation spectrometry
• Spatially resolved regions on the zircon can be
  measured
• Useful for zoned zircons
• Subsequent measurements are possible

8
Possible Contamination

• Lasar ablation for Hf isotope analysis taken from
  core
• If age from the rim entered into the age of the
  ablation pit, the Hf isotope analysis would be
  wrong
• Measuring the wrong age can lead to
• an incorrect Hf isotope composition
• an incorrect estimate of ?HF

9
Possible Contamination

• U-Pb age from the core
• Hf ratio from the entire grain
• Incorrect derivation of Hf are represented by
  these trajectories

10
The Results
Lu/Hf 0 is a forbidden region 0.01
enriched reservoir by 4.4-4.5 Ga
11
The Results
Lu/Hf 0.1 leads to quite a large deviation
today that has not been observed Possible that
continental crust recycling erased such
compositions
12
Interpretation

• Continental crust formed 4.5 Ga
• Major differentiation at 4.54.4 Ga
• Continental crust
• Depleted and enriched mantle reservoir
• Recycling of continental crust into the mantle
  occurred less than 4Ga

13

• Review of Harrison et al.
• Main result from Harrison et al. is interpreted
  global differentiation of the mantle 4.5-4.4 Ga

14

• New questions raised
• Is the time range for mantle differentiation
  inferred from ? correct?
• Layers of material making up the zircons may be
  from different generations, making ? too high or
  low
• Zircon Hf-ratio remains constant over time, while
  the U and Pb do not
• This causes the incorrect reference value in the
  primitive mantle to be assumed

15

• New questions raised
• Can the primitive mantle be represented by
  chondritic meteorites?
• Lu/Hf varies in chondrites, making the reference
  value for the primitive mantle ambiguous

16

• New questions raised
• Could the signatures of differentiation have been
  erased?
• Lunar record of heavy bombardment suggests that
  the mantle and crust of the Earth likely
  homogenized on a large scale