Cosmochemistry Cosmochronology

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CosmochemistryCosmochronology
Elements and their ages

• Trevor Ireland

GES 163
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Cosmochemistry

• deals with the distribution of the elements in
  the universe
• Dust and gas
• resulting from
• production ratios (nucleosynthesis)
• thermal differentiation (e.g. melting,
  volatility)
• physical differentiation (grain survival,
  density)
• chemical differentiation (e.g. ƒO2, minerals)
• Ashes to ashes, dust to solar system
• Stars, planets, leftovers

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Production Ratios

• The abundance curve of the nuclei in the solar
  system comprises an average of different
  processes from different stars
• on average, it appears to be a good
  representation of a galactic average as well.
• All planets dominated by (12C),16O, 24Mg, 28Si
• The reference state for all elemental abundances
  is the production ratio in stars
• astrophysical models of nucleosynthesis
• solar system must be an average

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Stellar nucleosynthesis

• Production (nuclear reactions)
• Ejection (winds, explosions)
• Recycling (dust)
• ash acts as seed nuclei for different reactions
• Temporal constraints
• inferred (degree of metallicity increases)
• direct (isotope abundances)

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Nuclear Cosmochronology

• The age of the elements
• Use of radioactive decay to date nucleosynthesis
• how long before the solar system was formed were
  the elements themselves produced?

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Scenario

• Production (instantaneous at t0 )
• or continuous, episodic, etc
• Decay Interval (?t)
• (transport to solar nebula)
• Solar system formation (t1)
• lasts about 1-10 Ma
• May be many solutions for different isotopes

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Cosmochronology

• Shortlived radionuclides
• now extinct
• abundances indicate nucleosynthesis shortly
  before solar system formation
• 26Al (t1/2 0.75 Ma)
• 53Mn (3.7 Ma)
• 60Fe (0.1 Ma)
• 129I (17 Ma)
• chronology of solar system formation

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Cosmochronology

• Long lived nuclides
• 244Pu (8.2 x 107 a)
• 235U (7.1 x 108 a)
• 40K (1.2 x 109 a)
• 238U (4.5 x 109 a)
• 232Th (1.4 x 1010 a)
• 187Re (4 x 1010 a)
• 87Rb (4.9 x 1010 a)

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Radioactive decay

• Radioactive nuclei
• Allow calculation of an age
• N N0 e-?t
• N no. of atoms (now)
• No no. of atoms originally
• ? decay constant (t½ ln2/?)

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Calculating ages

• Ratio of decay products
• Parent/daughter (e.g. 238U/206Pb)
• Parent1/Parent2
• If initial abundance is known (e.g. 232Th/238U)
• Daughter1/Daughter2 (e.g. 207Pb/206Pb)
• Resetting of Parent, daughter abundances

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Age of what?

• Resetting/Closure
• Terrestrial geochronology
• Nucleosynthetic Production
• When were the elements made?
• Cosmochronology of stars, galaxy
• T Period of nucleosynthesis before SS
• ?T free decay interval (isolation)

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TransU Decay Schemes

• Heavy elements
• breakdown by fission or a decay
• a? 4He particle (Z-2, N-2)
• Fission
• Spontaneous fragmentation of nucleus
• fission parameter Z2/A
• limits nuclei to A ca 256
• Superheavy elements?
• a-decay dominant decay mode
• Below mass 256

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Transuranic decay schemes

• a decay
• loss of four mass units/decay
• Only 4 decay chains
• 4n 232Th n58
• 4n1 237Np n59
• 4n2 238U n59
• 4n3 235U n58

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Progenitors

• Can calculate r process contributions
• to a given mass
• e.g. 232Th
• Maximum A256 (fission above this)
• 252, 248, 244
• Decay products pile up at long-lived 244Pu
• For long times, 244Pu -gt 232Th
• joined by r-process contributions
• at 240, 236, 232
• 6 progenitors for 232Th

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Progenitors

• Number of progenitors
• 232Th - 6
• 237Np - 5
• 238U - 3
• 235U - 6
• Initial abundances
• 235U/238U 2
• 232Th/238U 2

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What This Means

• 235U/238U production ratio
• Actually 1.4 - 1.8
• allowing for more details, odd/even
• Terrestrial (now) 0.00723
• Solar System Fmn (4.56 Ga) 0.31
• Solve decay equation
• Get to production ratio at 7 Ga
• for single nucleosynthetic event

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U nucleosynthesis

• Single event unlikely for all of galaxy
• Supernova frequency in galaxy decreasing
• Endmembers continuous, sudden
• continuous model - 10 Ga before SS
• sudden model down to 2 Ga before SS
• Uncertainty in production, modelling
• inexact estimate

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Th-U

• Adds another constraint
• Problems with chemical fractionation
• Production 232Th/238U 1.7 (0.3)
• Th/U estimated at
• 3.8 for terrestrial (now)
• 2.33 for 4.56 Ga
• indicates prior decay history

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r-process nucleosynthesis

• Best solution
• Began about 11 Ga ago
• decreased to about 1/3 by SS formation

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187Re

• long half life
• little change in parent abundance
• more conventional Re-Os dating
• 187Os s-process only
• predictable abundance relative to 186Os
• remainder due to cosmogenic decay of 187Re

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Re-Os age

• 187Os/Os 0.0132, 186Os/Os 0.0159
• 187Re/Re 0.65
• s(186)/s(187) 0.4 0.1
• Os/Re 11.3
• 187Osc/187Re 187Os/Os - s186/s187
  (186Os/Os)(Re/187Re)(Os/Re)
• 12 Re has decayed
• 11 Ga for sudden nucleosynthesis
• 18 Ga for uniform

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Interpretation

• Galaxy can be considered a system
• Elemental production can be constrained
• Model dependent
• History of galaxy from present isotope abundances

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Selection

• After stellar nucleosynthesis, material must be
  ejected into ISM so that it is available for next
  generation of stars.
• AGB, SN, WR (Wolf-Rayet) stars biggest sources of
  dust
• Condensation of gas into grains
• first thermal fractionation

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Thermal Processing

• Evaporation/Condensation
• Fractionation of elements with volatility
• Isotopic mass fractionation
• lighter isotopes have greater velocity (for same
  E)
• residue always heavier than starting material
• condensate starts lighter but becomes
  progressively heavier with fractionation of
  reservoir

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Thermal Processing

• Melting
• redistribution of elements according to
  distribution coefficients

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Physical processing

• Mineralogy of grains dictates survival
• physical processing in SN shock waves, sputtering
  by cosmic rays etc
• average dust size in ISM ca. 100 nm ( oom)
• magnetic properties
• Collection of dust in clouds
• sweep up rate size dependent
• dust incorporated into next generation of stars

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Chemical Fractionation

• Usually accompanying melting (kinetic)
• siderophile-chalcophile-lithophile
• especially in planetary bodies
• ƒO2 can affect
• valence
• volatility

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Stellar processing

• Nucleosynthesis
• Condensation of grains
• Dispersal to ISM

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Reassembly

• Aggregation in molecular clouds
• Assembly in protoplanetary disks

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Tadpoles

• Fractionation of gas and dust
• Concentration

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Lighting up

• Gravitational collapse

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Home
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References

• D. D. Clayton - Principles of stellar evolution
  and nucleosynthesis (Chicago) 1st ed 1968, 2nd ed
  1983
• Nuclear cosmochronology - P591-606
• A. P. Dickin - Radiogenic Isotope Geology
  (Cambridge) (1995)
• Nucleosynthesis - P1-14