Formation of Solar System and Abundances of Elements

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1 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 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 (10 6 yr scale) Planetary embryos form (10 8 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)

2 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)

3 CAI Allende Carbonaceous Chondrite Iron 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)

4 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

5 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 235 U and 238 U to 207 Pb and 206 Pb 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

6 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)

7 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)

8 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 (SiO 4 ) 4- Ca, Al, Mg (low E) rapidly used up to form Mg 2 SiO 4 (olivine) and Mg 2 Si 2 O 6 (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

9 Le Recipe Use up Oxygen Si + O form (SiO 4 ) 4- Ca, Al, Mg (low E) rapidly used up to form Mg 2 SiO 4 (olivine) and Mg 2 Si 2 O 6 (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 km diameter near base of magma ocean, percolated downward to form core

10 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