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Formation of Solar System and Abundances of Elements• Composition of Earth cannot be understood in isolation

– Sun and meteorites are closely linked• Solar system formed in Milky Way galaxy @ Big Bang 15 Ma

– Nucleosynthesis in stars, H+He ejected > rotating gas/dust cloud– Material in compressed disk heats, volatilizes, cools

• Most refractory dust particles cooled first– Accretion in several stages:

• Planetesimals 10 m to 1000 km diameter form (10 kyr time scale)• Planetesimals grow by collisions/intersecting orbits (106 yr scale)• Planetary “embryos” form (108 yr time scale)

– Embryos collided to form planets– Earth-Moon system may reflect such a collision

– Sun’s composition gives best estimate for that of Solar Nebula• Mainly H + He• Relative abundances of other elements nearly identical to meteorites

Brown & Mussett (1993)

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Faure (Geochemistry)

Formation of Solar System and Abundances of Elements• Meteorites

– Samples of extraterrestrial material from asteroid belt– Two broad categories:

• Differentiated– Irons, stony irons, achondrites

• Undifferentiated– Chondrites (most abundant falls)

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Iron

Stony IronAllendeCarbonaceous Chondrite

CAI

Formation of Solar System and Abundances of Elements• Meteorites

– “Fossils” of various stages of accretion process• Chondrites similar in composition to the Sun• Chondrites are more “primitive” than differentiated meteorites

– Carbonaceous chondites are most primitive of all (still contain volatiles)

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• Meteorites– Ages of meteorites determined from isotopic abundances

• Crystallization ages (from Rb-Sr, Sm-Nd, U-Pb, and Pb-Pb isochrons)

• Exposure ages• Formation interval (from extinct radionuclides)

– Crystallization ages of chondrites are between 4500 and 4600 Ma

• Pb-Pb isochron has a slope of 4550 Ma

• Oldest known inclusions in these meteorites are 4.56 Ga• The similar age connects most chondrites to a common source

body• Terrestrial sediments lie on the Pb-Pb isochron

– Thus Earth is connected to this source as well!

Formation of Solar System and Abundances of Elements

• Rb-Sr and Pb-Pb isochrondiagrams for Meteorites

The age of the Earth:Terrestrial sedimentsFall on isochron with meteorites

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Accretion and Layering of Terrestrial Planets• The question is: What processes concentrated heavy elements comprising

only 2% of solar nebula into terrestrial planets?

• Meteorites give best clues– Inner terrestrial planets are close to chondritic in bulk composition

• Chondritic Earth Model (CEM)• But there are important differences

• The first 100 m.y.– To understand processes, we need a time scale

• Radioisotopic decay of 235U and 238U to 207Pb and 206Pb yields Pb-Pb isochron

• Oldest material is tiny high-Temp. inclusions in Allende: 4559 ± 4 Ma• 30 m.y. older than other meteorites• 100 m.y. older than oldest lunar crust

– Thus, ca. 100 m.y. between initial condensation of particles and Earth-Moon system

Accretion and Layering of Terrestrial Planets• The first 100 m.y.

(continued)– Initial segregation &

condensation of elements driven by Pressure, Temperature, Density such that heating and vaporization took < 1 m.y.

– Accumulation of planetesimals, planetary growth and development over ca. 100 m.y.

– The variable chemical composition of Meteorites provides important evidence of this sequence

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Accretion and Layering of Terrestrial Planets• How a “solid-earth” geochemist thinks

– K is a volatile element (low melting T), U is a refractory element (high melt. T)

– Both K and U have similar periodic properties (radius, charge) such that they behave nearly identically during crystallization or melting.

– Meteorites and planets have huge range in K/U (and concentration of K)• Implies separation of K from U early and rapidly while K was still volatile• Implies compositional gradients across solar nebula• Melting and differentiation during post accretion time required to increase K contents

and form surface rocks (of you guessed it: BASALT)

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Accretion and Layering of Terrestrial Planets• Post-Accretional Chemical Processes

– Shift from low-P processes to higher P processes on planetary surfaces and within interiors

– Element segregation: Geochemical rules based on:• Electronic configuration• Types of crystalline bonds

– Clear groupings of elements:1. Lithophile (oxygen, oxides, silicate minerals, Greek lithos=stone)2. Chalcophile (sulphides, Greek Khalkos=copper)3. Siderophile (metallic, Greek sideros=iron)

– Electronegativity, E• Dimensionless parameter; scale 0-4 (Linus Pauling)• Ability of atom to attract electrons and become negatively charged anion• Governs nature of bonding to other atoms

– Recall: ionic, covalent, metallic bonding!

ElectronegativityContinuity, overlap between bonding types•Lithophile (large contrast in E: ionic bonds)•Chalcophile (E 1.6 to 2.0 share electrons: covalent bonds)•Siderophile (E 2.0 to 2.4 transition elements: metallic bonds)

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Accretion and Layering of Terrestrial Planets• Layering in the Terrestrial Planets

– Two fundamental sets of questions• What chemical reactions determined elements that formed cores and

mantles?• How and when was the energy necessary for melting delivered?

– Relative proportions of Major Elements govern extent that particular reactions occurred

mantle

inner+outer core

Accretion and Layering of Terrestrial Planets• Le Recipe

– Use up Oxygen• Si + O form (SiO4 )4-

• Ca, Al, Mg (low E) rapidly used up to form Mg2SiO4 (olivine) and Mg2Si2O6 (pyroxene)

• Oxygen is so abundant that some is left over when Si, Mg, Ca, Al used up

– Use up Sulfer• Fe combines with S• Leftover Fe remains as metal

• Thus,– O content determines size of planet’s lithophile silicate layer– S content determines size of chalcophile layer– Excess cation-forming atoms not used in above layers determine

the size of the siderophile layer• Hence we observe 3 layers in the terrestrial planets

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Accretion and Layering of Terrestrial Planets• Le Recipe

– Use up Oxygen• Si + O form (SiO4 )4-

• Ca, Al, Mg (low E) rapidly used up to form Mg2SiO4 (olivine) and Mg2Si2O6 (pyroxene)

• Oxygen is so abundant that some is left over when Si, Mg, Ca, Al used up

– Use up Sulfer• Fe combines with S• Leftover Fe remains as metal

• Thus,– O content determines size of planet’s lithophile silicate layer– S content determines size of chalcophile layer– Excess cation-forming atoms not used in above layers determine

the size of the siderophile layer

Accretion and Layering of Terrestrial Planets• Hence we observe 3 layers in the terrestrial planets

– If temperature high enough to melt, layers form in order of increasing density with depth

– Separation was inefficient and incomplete• e.g., we still find gold, platinum and sulfer in small quantities at the

surface

• Heat sources and melting– Planetary embryos became hot enough to melt and differentiate– Heat from accretion process:

• Kinetic energy (gravity driven) converted to heat• Important as bodies grew in size and velocity/attraction increased• Radiation chilled crust• Produced high temperatures near surface• Produced subsurface magma ocean

– Iron droplets formed, accumulated to blobs 10-100 km diameter near base of magma ocean, percolated downward to form core

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Accretion and Layering of Terrestrial Planets• Embryo-embryo collisions, giant impacts continue for

100 m.y.– Redistributed and segregated layers chaotically– Led to incremental development of iron rich core

• Sinking of dense metal released gravitational energy– Heat generated raised T by 1000 K– Self compression raised T by another 2000 K– Core Formation evened out heat distribution as interior heated up

• Short-lived radioisotopes = heating• Combined heat sources led to 7000 K at earth’s center,

5500 K at core-mantle boundary (CMB)

• Controversial new research here at UW-Madison suggests that the magma ocean may not have existed for very long–

LithophileLayered Earth

ChalcophileSiderophile