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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CAI
Iron
Allende
Carbonaceous Chondrite
Stony Iron
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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Formation of Solar System and Abundances of Elements
• 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!
• Rb-Sr and
Pb-Pb
isochron
diagrams for
Meteorites
The age of the Earth:
Terrestrial sediments
Fall 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 EarthMoon 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!
Electronegativity
Continuity, 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
inner+outer core
mantle
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
–
Layered Earth
Lithophile
Chalcophile
Siderophile
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