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Isotope chronology of meteorites and oxygen isotopes Part I: Radiometric dating methods Esa Vilenius 13.2.2006

Isotope chronology of meteorites and oxygen isotopes Part I: Radiometric dating methods Esa Vilenius 13.2.2006. Outline Introduction Rubidium-Strontium chronometer Problems of radiometric chronometers Lead-lead method Short-lived isotopes Chronology of early Solar System.

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Isotope chronology of meteorites and oxygen isotopes Part I: Radiometric dating methods Esa Vilenius 13.2.2006

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  1. Isotope chronology of meteorites and oxygen isotopesPart I: Radiometric dating methodsEsa Vilenius 13.2.2006 • Outline • Introduction • Rubidium-Strontium chronometer • Problems of radiometric chronometers • Lead-lead method • Short-lived isotopes • Chronology of early Solar System

  2. What can be dated? • Formation age of solid material • Formation intervals (relative to other meteorites) • Reheating events (metamorphic ages) • Cosmic ray exposure age (meter-sized objects) • Terrestrial age

  3. What changes isotopic abundances? radioactive decay and its effects on neighboring nuclides bombardment by high-energy particles (cosmic rays) fractionation (= differentiation between isotopes) - example 1: binding energy of D2 is lower than H2 - example2: evaporation of water favors lighter isotopes of H and O in the gas phase, and heavier in the liquid phase

  4. Conditions and assumptions • Decay constant of parent nuclide accurately known. • Several samples of the rock are available, with variation in parent/daughter ratios. • Material has been a closed system w. r. t. parent and daughter nuclides. • Initial isotopic composition of the daughter element was homogeneous in all samples. • Radiogenic component of the daughter nuclide can be distinguished from the initial, • nonradiogenic component. radiogenic nuclide = product of radioactive decay

  5. The Rubidium-Strontium clock (87Rb -> 87Sr) • 87Rb -> 87Sr + e- + anti ne • 86Sr is the nonradiogenic nuclide. • CASE 1: Caused by melting, Rb and Sr ions floated freely in a homogeneous liquid. • At the time of crystallization Rb and Sr ions are squeezed into minerals, where they occur as impurities. Rb+ typically replaces K+ and Sr2+ typically replaces Ca2+. • CASE 2: In the primordial solar system Rb and Sr were well-mixed in the gas. The ratio Rb/Sr is different in the gas and solid phases, because Rb+ has a tendency for substitution in minerals with low melting temperatures. Examples of K- and Ca-bearing minerals: orthoclase (KAlSi3O8), anorthite (CaAl2Si2O8)

  6. The 87Rb -> 87Sr clock (2) Freshly formed rock The different minerals in a rock have the same 87Sr/86Sr ratio (same size of ions). 87Rb/86Sr ratio is different for different minerals (host mineral depends on ion size). Old rock (87Rb/86Sr)t = (87Rb/86Sr)o exp(-lt), decay constantl=ln(2)/t, half-lifet = 5*1010years. The amount of the daughter nuclide at time t is (87Sr)t = (87Sr)o + [ (87Rb)o - (87Rb)t ] = (87Sr)o + (87Rb)t [exp(lt) -1] => (87Sr/86Sr )t = (87Sr/86Sr )o + (87Rb/ 86Sr)t[exp(lt) -1] -> Measure (87Sr/86Sr )t and (87Rb/ 86Sr)t for at least 2 minerals, then solve t and (87Sr/86Sr )o A schematic plot of the ratio 87Sr/86Sr vs. 87Rb/86Sr of four minerals, where 86Sr is a stable, non-radiogenic nuclide. (Cowley 1995)

  7. The 87Rb -> 87Sr clock (3) Example of results1: H-group chondrites Whole-rock Rb-Sr isochron of 16 H-chondrite meteorites => Common formation age 4.69±0.07 Gyr. Example of results2: formation intervals Initial 87Sr/86Sr ratios from isochrons of 6 meteorites. Kaushal and Wetherill (1969)

  8. Contamination and isochrons System not closed w. r. t. daughter nuclide -> loss of colinearity System not closed w. r. t. parent nuclide -> loss of colinearity Daughter nuclide partially homogenized -> partial reset of isochron -> colinear, but wrong age Graphics from Stassen (1998)

  9. The lead-lead double clock • Two systems: 235U -> 207Pb 0.7*109 years • 238U -> 206Pb 4.5*109 years • Nonradiogenic nuclide 204Pb • Slope of the isochron: R1 =207Pb/204Pb R2 = 206Pb/204Pb k = 238U/235U CAIs are 2.5 Myears older than chondrules (Amelin et. al. 2002)

  10. Short-lived radioactive isotopes • Parent nuclides extinct • Excess amount of daughter nuclides • A stable isotope of the parent is used in measurements • Uniform initial concentration of parent nuclides • Differences in concentration => relative crystallization ages • Inclusions containing 26Al must have been cool enough to prevent isotopic exchange within Myears following the production in a supernova => samples of interstellar grains McKeegan and Davis (2002)

  11. 26Al -> 26Mg chronometer • Half-life 720 000 years • Ratio (26Al / 27Al) at the formation time of rock • A low ratio indicates that decay of 26Al predates solar-system formation (26Mg / 24Mg) = (26Mg / 24Mg)o + (26Al / 27Al)*(27Al / 24Mg) slope -> (26Al / 27Al)

  12. Early Solar System chronology • At 4568 Ma a supernova triggers gravitational collapse. • CAIs are the first solid material (aluminium-26 relative ages) • Formation of CAIs 4567.2 ± 0.6 Ma (lead-lead isochron). • Formation of chondrules 4564.7 ± 0.6 Ma (lead-lead isochron), • lasting 1-2 Myears. • CAIs join chondrules forming chondrites at 4565 - 4564 Myears, • melting and differentiation of meteorite parent bodies. www.spacedaily.com Allende CV3, 200x zoom www.zeiss.com www.spaceflightnow.com

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