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Origin and Early Evolution of the Earth:a volatile elements perspective
Cider 2010Bill McDonoughGeology, University of Maryland
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A volatile rich planet?
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1897
1915
1925
1970
1935
1995
Emil Wiechert
1st order Structure of EarthRock surrounding metal
PLATE TECTONICS
CORE-MANTLE
UPPER-LOWER MANTLE
INNER-OUTER CORE
Time Line
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5 Big Questions:
- What is the Planetary K/U ratio?
- Is the mantle in-gassing or de-gassing?
- Distribution of volatiles in mantle?
- Volatiles in the core?
- Volatiles at Core-Mantle Boundary?
planetary volatility curve
secular changes
whole vs layered convection
Light element in the core
hidden reservoirs
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Role of Giant Impacts: volatiles• Earth’s volatile budget
was likely shaped by Mars-sized impacted events.
• Did the late veneer introduce HSE and volatiles?
• Differences in the volatile budget of the Earth and Moon?
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• What are the volatile elements?
• What are their abundances in the Earth?
• When did we inherit them?
• How did we inherited?
• Is there a secular variation in the volatile
elements abundances of the Earth?
More volatile questions QuickTime™ and a decompressorare needed to see this picture.
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Volatiles: defined
- H2O, CO2, N2, CH4, (i.e., H, C, N, O)
- Noble gases (group 18 elements)
- elements with half-mass condensation T <1250 K
- elements readily degassed (e.g., Re, Cd, Pb…)
- chalcogens (group 16: i.e., O, S, Se and Te)
- halides (group 17: i.e., F, Cl, Br, I)?
- alkali metals (group 1: Cs, Rb, K…)?
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Refractory >1400 K“Si, Mg, Fe, Ni… 1350 to 1250 KModerately volatile 1250 to 650 KVolatile <650 K
What are the moderately volatile elements?
classified affinity where Siderophile iron coreLithophile oxide mantleChalcophile sulfur mostly core
Redox conditions in the Solar System…..
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From Palme & Jones (2003)
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Heterogeneous mixtures of components with different formation temperatures and conditions
Planet: mix of metal, silicate, volatiles
What is the composition of the Earth? and where did this stuff come from?
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Volatiles: distribution
- Atmosphere (N2 78%, O2 21%, Ar 1%, other)
- Mantle volatiles: H2O, C(C, CO2, CO, CH4),
sulfides, etc
- Core volatiles: FeC, FeN, FeO, FeS, FeH
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0.1 μm- and 1.5 μm-sized olivine, pyroxene and quartz.
Astro-mineralogy -- determine size, crystal structure and chemistry of dust grains in space, often around protostars (observations usually at mid-infrared wavelengths (2–30 m)).
Rings around Pictoris63 light yrs away
Okamoto et al(2004, Nature)
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Mid-infrared spectroscopy (IRS) SpitzerX-ray absorption fine structure (XAFS) Chandra
<2.2% crystallinity in silicate exist in diffuse ISM.
In the Galactic ISM Si exists in the form of silicates, whereas a significant fraction of S exists in the gas phase.
ISM/solar O/Si 0:63 ± 0:17Mg/Si 1:14 ± 0:13
S/Si 1:03 ± 0:12 Fe/Si 0:97 ± 0:31
The ratio of Mg to Fe in olivine is >1.2 and 15%–37% of the total O atoms in the ISM must be contained in silicate grains.
What’s in the ISM (interstellar medium)
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Star (~1 Myr) with a clearing disk
Spitzer Space Telescope Infrared Spectrograph
D’Alessio et al (2005) ApJ
low-mass pre–main-sequence star
Glassy olivine
Cleared out
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Olivine
Pyroxene
hydrosilicate
ISM
HD142527 inner disk
Astro-Mineralogy
Von Boekel et al (2004; Nature)
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• Chondrites, primitive meteorites, are key
• So too, the composition of the solar photosphere
• Refractory elements (RE) in chondritic proportions
• Absolute abundances of RE – model dependent
• Mg, Fe & Si are non-refractory elements
• Chemical gradient in solar system
• Non-refractory elements: model dependent
• U & Th are RE, whereas K is moderately volatile
“Standard” Planetary Model
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1.E+02
1.E+03
1.E+04
1.E+05
1.E+06
1.E+07
1.E+08
1.E+02 1.E+03 1.E+04 1.E+05 1.E+06 1.E+07
Sol
ar p
hoto
sphe
re(a
tom
s S
i = 1
E6)
C1 carbonaceous chondrite(atoms Si = 1E6)
H
CN
Li
B
O
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Inner nebular regions of dust to be highly crystallized,
Outer region of one star has - equal amounts of pyroxene and olivine- while the inner regions are dominated by olivine.
Olivine-rich Ol & Pyx
Boekel et al (2004; Nature)
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Mg/Si variation in the SS
Forsterite-high temperature-early crystallization-high Mg/Si-fewer volatile elements
Enstatite-lower temperature-later crystallization-low Mg/Si-more volatile elements
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Pyroxene
Olivine P
oten
tial
tem
pera
ture
gra
dien
t
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EH
CI
H
LL L
EL
EARTH
CO
CM CV
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EH
CI
H
LL L
EL
EARTH
CO
CM CV
MARS
SS Grad
ients
-thermal-compositional-redox
Mars @ 2.5 AU Earth @ 1 AUOlivine-rich
Pyroxene-rich
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Planetary Compositional Models - Earth
1) Mg/Si -- unknown needs to be fixed
2) Hidden reservoirs -- maybe?
• 142Nd Early Earth Reservoir -- unlikely
• “Chondritic Earth” -- yes, (RLE)! but…
5) Future research -- geoneutrinos
-KamLAND, Borexnio, SNO+, etc
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Earth is “like” an Enstatite Chondrite!
1) Mg/Si -- is very different
2) shared isotopic Xi -- O, Cr, Mo,Ru, Nd,
3) shared origins -- unlikely
4) core composition -- no K, U in core.. S+
5) “Chondritic Earth” -- lost meaning…
6) Javoy’s model? -- needs to be modified
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Core
Mantle
Siderophileelements
Lithophileelements
Ca, Al, REE, K, Th & U
Fe, Ni, P, Os
Atmophilie elements
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Atomic proportions of the elements
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weight % elements
Fe
Si
Mg
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Volatility trend@ 1AU from Sun
Th & U
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Allegre et al (1995), McD & Sun (’95)Palme & O’Neill (2003)
Lyubetskaya & Korenaga (2007)
No
rmal
ized
co
nc
entr
ati
on
REFRACTORY ELEMENTS VOLATILE ELEMENTS
Half-mass Condensation Temperature
Potassiumin the core
Silicate Earth
?
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Siderophile*
and
ChalcophileChalcophile**
Core elements remaining in the Silicate Earth
*dominant chemical characteristic, but not an exclusive definition
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Abundances of element
Gases in the
Primitive Mantle
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4 most abundant elements in the Earth:Fe, O, Si and Mg
6 most abundance elements in the Primitive Mantle: - O, Si, Mg, and – Fe, Al, Ca
This result and 1st order physical data for the core yield a precise estimate for the planet’s Fe/Al ratio : 20 ± 2
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What’s in the core?
What would you like?
Constraints: density profile, magnetic field, abundances of the elements,
Insights from: cosmochemistry, geochemistry, thermodynamics, mineral physics, petrology, Hf-W isotopes (formation age)
How well do we know some elements?
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Model 1 Model 2
Core compositional models
others
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Model Core composition
(wt%) % in core rel. Earth (ug/g) % in core
rel. Earth
Fe 88.3 87 V 150 50
O 3 3 Mn 300 10
Ni 5.4 93 Cu 125 65
S 1.9 96 Pd 3.1 >98
Cr 0.9 60 Re 0.23 >98
P 0.2 93 Os 2.8 >98
C 0.2 91 Au 0.5 >98
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Earth’s D/H ratio
• Do we really know
comets
• D/H ratio of the oceans
• What do chondrites tell
us?
• Source of water and
other volatiles vs the
sources of noble
gases?Ref: Owen and Bar-Nun, in R. M. Canup and K. Righter, eds., Origin of the Earth and Moon (2000), p. 463
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Progress Report Conclusions:
Approximate concentrations
Depleted Mantle H2O 50 ppm; CO2 20 ppm; Cl 1 ppm; F 7 ppm
Enriched Mantle H2O 500 ppm; CO2 420 ppm; Cl 10 ppm; F 18 ppm
Total Mantle H2O 366 ppm; CO2 301 ppm; Cl 7 ppm; F 15 ppm
Last CIDER report on volatiles in the Earth - Saal et al 2009
• Earth: 61024 kg Oceans: 1.41021 kg• Ordinary chondritic planet -- 4 oceans• Carbonaceous chondritic planet -- 600 oceans• Enstatite chondritic planet -- ~2-4 oceans
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H/C ratio of the bulk silicate Earth is superchondritic, owing chiefly to the high H/C ratio of the exosphere.
H/C ratio of the mantle is lower than that of the exosphere, requiring significant H/C fractionation during ingassing or outgassing at some point in Earth history.
Hirschmann and Dasgupta (2009)
Volatile Budget!
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Earth’s volatiles from chondrites?
Let’s hear from what Sujoy has to say!…
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Unwary “readers” should take warning that ordinary language undergoes modification to a high-pressure form when applied to the interior of the Earth. A few examples of equivalents follow:
High-pressure form Ordinary meaning
certain dubiousundoubtedly perhapspositive proof vague suggestionunanswerable argument trivial objectionpure iron uncertain mixture of all the elements
When it comes to volatiles…. remember, always, the words of Francis Birch (1952)
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