Origin and Early Evolution of the Earth:a volatile elements perspective

Cider 2010Bill McDonoughGeology, University of Maryland

QuickTime™ and a decompressor

are needed to see this picture.

Support from:

A volatile rich planet?

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

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

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?

• 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.

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…)?

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…..

From Palme & Jones (2003)

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?

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

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)

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)

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

Olivine

Pyroxene

hydrosilicate

ISM

HD142527 inner disk

Astro-Mineralogy

Von Boekel et al (2004; Nature)

• 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

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

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)

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

Pyroxene

Olivine P

oten

tial

tem

pera

ture

gra

dien

t

EH

CI

H

LL L

EL

EARTH

CO

CM CV

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

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

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

Core

Mantle

Siderophileelements

Lithophileelements

Ca, Al, REE, K, Th & U

Fe, Ni, P, Os

Atmophilie elements

Atomic proportions of the elements

weight % elements

Fe

Si

Mg

Volatility trend@ 1AU from Sun

Th & U

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

?

Siderophile*

and

ChalcophileChalcophile**

Core elements remaining in the Silicate Earth

*dominant chemical characteristic, but not an exclusive definition

Abundances of element

Gases in the

Primitive Mantle

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

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?

Model 1 Model 2

Core compositional models

others

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

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

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

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!

Earth’s volatiles from chondrites?

Let’s hear from what Sujoy has to say!…

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)