Transcript Earth’s Evolution Deep Time: What Isotope Geochemistry has Taught us About

Deep Time:
What Isotope Geochemistry
has Taught us About
Earth’s Evolution
Janne Blichert-Toft
Ecole Normale Superieure de Lyon
CIDER '09 Community Workshop: May 17 20, 2009, Marconi Center, CA
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The Major Milestones….
 The Age of the Solar System (Time Zero) and, hence,
….the Age of the Earth….
 The Age of Terrestrial Core Formation
 The Age of the Giant Lunar Impact
 The Age of the Late Veneer
 The Existence and Duration of the Terrestrial Magma Ocean
 The Age and Nature of the Earliest Proto-Crust
 The Age of the First Continents
 The Time of Onset of Plate Tectonics
 Mantle-Crust Differentiation
 Depleted Mantle Evolution
 The Existence of a “Hidden” Reservoir
Tool: Long-Lived and Extinct Radioactivities
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The Major Milestones….
 The Age of the Solar System (Time Zero) and, hence,
….the Age of the Earth….
 The Age of Terrestrial Core Formation
 The Age of the Giant Lunar Impact
 The Age of the Late Veneer
 The Existence and Duration of the Terrestrial Magma Ocean
 The Age and Nature of the Earliest Proto-Crust
 The Age of the First Continents
 The Time of Onset of Plate Tectonics
 Mantle-Crust Differentiation
 Depleted Mantle Evolution
 The Existence of a “Hidden” Reservoir
Tool: Long-Lived and Extinct Radioactivities
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The Isotope Geochemist’s Right
To Be….
Thanks to the Constancy of Radioactive Decay
and Radiogenic Ingrowth, the IG has Time on
his/her Side and, by Using Chemistry and Mass
Spectrometry, can Tell When Things Happened
and What were their Nature and Origins.
The Isotopic Record
Our Best Evidence for the Details (i.e.,
Timing, Origins) of the Differentiation of the
Early Earth and Earth’s Evolution Comes
from the Isotopic Records of Lead and a
few Radiogenic, Refractory,
Lithophile/Siderophile Elements
(146,147Sm-142,143Nd,
176Lu-176Hf, 182Hf-182W)
The Age of the Solar System
It All Started 4568.5 My Ago!
1 cm
CAI
Bouvier et al. (2007)
The Age of Core Formation
Planetary Differentiation
182Hf-182W
Systematics
of the Earth
Chondrite
TCF
Hf+W
Silicate mantle
Core
W
Hf
W
Yin et al. (2002)
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f Hf/W
29.5 Ma
182Hf-182W
Systematics
of the Moon
W
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29.5 Ma
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Touboul et al. (2007)
Yin et al. (2002)
f Hf/W
The Age of the
Giant Lunar Impact
The Age of the “Late Veneer”
Several Independent Isotopic Observations Suggest the Late
Veneer Arrived on Earth ~100 ± 50 Ma After Earth’s Accretion.
The Terrestrial Magma Ocean
142Nd
Evidence From Isua for a
Terrestrial Magma Ocean
+
Boyet el al. (2003)


+
Caro et al. (2003)
More 142Nd Evidence for Very Early Mantle Differentiation: ~30
Ma After Earth’s Accretion
Boyet & carlson (2005)
Age and Nature of Earth’s
Earliest Crust
Two Fundamental Questions of Earth Evolution are:
(1) When did Growth of Continental Crust Begin,
and (2) What did the First Crust Look Like?
A Window on the Hadean Earth
Wilde et al. (2001)
Age of Jack Hills Zircon Host Rock
n = 104
2 s errors
Leached
Mean age = 4.1 ± 0.1 Ga
false-alarm level
Blichert-Toft & Albarede (2008)
Age and Geochemical Properties of Source Rock
Chondritic mantle
Depleted
mantle
176Lu/177Hf
protocrust
= 0.010
TDM = 4.36 Ga
TCHUR = 4.31 Ga
Age (Ga)
Blichert-Toft & Albarede (2008)
Age (Ga)
The first ‘KREEPy’ proto-continent(s) formed
~200 Ma after Earth’s accretion @ ~4.35 Ga.
Plate tectonics and modern-style continents
arose sometime between 4.35 and 4.1 Gy ago.
Mantle-Crust Differentiation and
Depleted Mantle Evolution
The “Hidden” Reservoir
Evidence for an Enriched “Hidden”
Reservoir from Hf and Nd Isotopes in
Terrestrial Rocks and 142Nd in Chondrites
Low Sm/Nd
Low Lu/Hf
Blichert-Toft and Albarede (1997)
Boyet and Carlson (2006)
-ve 142Nd
Where is the Hidden Reservoir ?
Buried in the Deep Mantle?
Or Lost to Space by Impacts?
Or…..the Earth didn’t start out
Chondritic!
Orgueil
House of Cards
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Timeline of Major Events in the Earliest
History of the Earth
Sm-Nd
terrestrial magma ocean
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Albarede (submitted)
Timeline Continued….
Plate tectonics
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Useful to….
• Mostly Geodynamicists (Defining Initial Conditions; Endorsing
Magma Ocean…)
• Seismologists, Mineral Physicists?
• IG Consult for Good vs Bad Data!
• Hidden Reservoir? Seismologists/Physicists
• Adequate Interpretation of Isotopes….
The End!