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


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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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slide1
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
what can be dated
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
slide3

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

conditions and assumptions
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

the rubidium strontium clock 87 rb 87 sr
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)

slide6

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)

slide7

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)

slide8

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)

the lead lead double clock
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)

short lived radioactive isotopes
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)

26 al 26 mg chronometer
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)

early solar system chronology
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

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